Patent Publication Number: US-10314546-B2

Title: Capacitance enhanced physiological measurements

Description:
RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 14/273,764 that was filed on May 9, 2014, and is entitled “CAPACITANCE ENHANCED PHYSIOLOGICAL MEASUREMENTS,” which is also related to and claims priority to U.S. Provisional Patent Application No. 61/912,998, titled “CAPACITANCE ENHANCED OPTICAL PHYSIOLOGICAL MEASUREMENTS,” filed Dec. 6, 2013, and which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     Aspects of the disclosure are related to the field of medical devices, and in particular, measuring physiological parameters of tissue. 
     TECHNICAL BACKGROUND 
     Various medical devices can non-invasively measure parameters of blood in a patient. Pulse oximetry devices are one such non-invasive measurement device, typically employing solid-state lighting elements, such as light-emitting diodes (LEDs) or solid state lasers, to introduce light into the tissue of a patient. The light is then detected to generate a photoplethysmogram (PPG). These photoplethysmography systems can also measure changes in blood volume of tissue of a patient and calculate various parameters such as heart rate, respiration rate, and oxygen saturation. 
     However, some conventional optical pulse oximetry devices only measure certain limited blood parameters, and lack the ability to measure other patient physiological parameters. Some optical pulse oximetry devices are also subject to patient-specific noise and inconsistencies which limit the accuracy of such devices. For example, monitoring infants or patients in intensive care units can be difficult. Motion of the patient and other incidental factors can lead to noise and inaccuracies of optical-based measurements. Some optical measurement systems are sensitive to shifts in venous blood volumes, introducing errors into arterial blood measurements. 
     Capacitive sensing has been employed to measure some physiological parameters by applying electric fields directly through tissue using two-plate capacitors having individual plates positioned on different sides of the tissue. This two-plate capacitive sensing can be combined with optical measurement to determine changes in volume of tissue due to cardiac pulsing. 
     OVERVIEW 
     Systems, methods, apparatuses, and software for measuring and determining physiological parameters of a patient are presented. In one example, a physiological measurement system includes an optical system configured to emit one or more optical signals into tissue of a patient, and detect the one or more optical signals after propagation through the tissue. The physiological measurement system includes a capacitance system configured to apply one or more electric field signals to a single side of the tissue to determine a capacitance signal based at least on changes in the one or more electric field signals, and a processing system configured to process at least the one or more detected optical signals and the capacitance signal to determine a physiological parameter of the patient. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. While several embodiments are described in connection with these drawings, the disclosure is not limited to the embodiments disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents. 
         FIG. 1A  is a system diagram illustrating a physiological measurement system. 
         FIG. 1B  is a system diagram illustrating a physiological measurement system. 
         FIG. 2  is a flow diagram illustrating a method of operating a physiological measurement system. 
         FIG. 3  is a system diagram illustrating a physiological measurement system. 
         FIG. 4  is a flow diagram illustrating a method of operating a physiological measurement system. 
         FIG. 5  is a system diagram illustrating a physiological measurement system. 
         FIG. 6  is a flow diagram illustrating a method of operating a physiological measurement system. 
         FIG. 7  is a system diagram illustrating a physiological measurement system. 
         FIG. 8  is a flow diagram illustrating a method of operating a physiological measurement system. 
         FIG. 9  is a diagram illustrating measurement of physiological parameters. 
         FIG. 10  is a diagram illustrating measurement of physiological parameters. 
         FIG. 11A  is a diagram illustrating measurement of physiological parameters. 
         FIG. 11B  is a diagram illustrating measurement of physiological parameters. 
         FIG. 12  is a diagram illustrating measurement of physiological parameters. 
         FIG. 13  is a system diagram illustrating a physiological measurement system. 
         FIG. 14  is a system diagram illustrating a physiological measurement system. 
         FIG. 15  is a diagram illustrating measurement pads for measurement of physiological parameters. 
         FIG. 16  is a system diagram illustrating a physiological measurement system. 
         FIG. 17  is a system diagram illustrating a physiological measurement system. 
         FIG. 18  is a block diagram illustrating a measurement system. 
         FIG. 19  is a diagram illustrating various capacitance configurations. 
     
    
    
     DETAILED DESCRIPTION 
     The examples discussed herein include systems, apparatuses, methods, and software for enhanced measurement of physiological parameters in patients. When optical measurements are employed to detect physiological parameters in tissue of a patient, optical signals associated with the optical measurement can be subjected to various interference and noise, such as patient motion artifacts and patient-specific variability. Capacitance-based sensing can be employed to enhance or supplement the optical measurements to provide corrections, data stabilization, or additional sensing capabilities to optical-only measurement systems. In other examples, capacitance measurements are employed to detect physiological parameters in tissue of a patient without relying upon optical measurements. 
     In addition to the advantages and applications of the capacitance-enhanced optical and capacitance-based systems below, other applications such as fitness band monitors to monitor pulse, breathing rate, heart rate, sweating levels, oxygen levels, or other parameters of subjects performing athletic activities can include these capacitance and optical monitoring systems. For example, such systems are described below with respect to  FIGS. 7-9 . Other examples include cuff pressure and moisture monitoring systems for tracheal tubes or for monitoring of premature babies. 
     The physiological parameters measured or determined by the capacitance-enhanced systems can include various plethysmograph (pleth) information, such as photoplethysmograms (PPG), PPG parameters, and temporal variability of PPG parameters (such as pleth morphology and pulse information). The physiological parameters measured or determined by the capacitance-enhanced systems can also include electrocardiography (ECG) information via capacitive sensing, pulse rate, respiratory rate, respiratory effort, blood pressure, oxygen concentrations, hemoglobin concentrations, total hemoglobin concentration (tHb), saturation of peripheral oxygen (SpO 2 ), SpO 2  variability, regional oxygen saturation (rSO 2 ), apnea conditions, arrhythmia, and saturation pattern detection among other parameters and characteristics, including combinations and variations thereof. Physiological measurements can be performed using the various examples herein. Some of these include determining respiration rate from a finger, pulse rate from a finger, motion of patient, continuous non-invasive blood pressure measurement (CNIBP), deltaPOP (a measurement of the variability of the pleth pulses), variability of optical pleth to determine vessel elasticity, dehydration, apnea detection and monitoring, and auto-regulation of patients. In addition, enhancements to measurement include body location detection to determine where sensors are applied on patient, smart blood pressure cuffs, blood pressure measurement triggered when blood pressure changes beyond thresholds, enhanced cuff position and accuracy detection, and skin type and color detection. Also, the measurement systems described herein can provide various improvements to conventional optical pulse oximetry using optical signals separately or in tandem with electric field signals. Some examples include motion correction due to patient movement, sensor on/off tissue or finger, ECG lock for pleth signals, checking assumptions and correlations of optical-only measurements, reducing time to post results to a doctor or patient, correction of measurements for DC shifts, sensor pressure correction on tissue, signal quality improvements, sensor fault detection, optical signal fault detection, signal processing tailoring to skin/blood types and conditions, detecting and correcting changes in skin properties due to moisture/sweat or elasticity, improved signal analysis using wavelet analysis of signals, pleth morphology, or FFT (Fast Fourier Transform), and power saving by turning off optical sensors/emitters when not applied on tissue properly. 
     In yet further examples, the capacitance signal can be employed to reduce noise or enhance measurement of other non-optical physiological sensors. Breathing monitors can be coupled to a capacitive sensor on a fingertip, and motion of the patient as determined by the capacitive sensor can scale or modify the readings captured by the breathing monitors. The capacitance signal can be used to cross-check other physiological sensors. For example, if a first sensor observes a low breathing rate or lack of breathing, a capacitive signal can be monitored to determine if the breathing sensor is faulty or reading an actual low breathing rate event. In infant monitoring environments, such as a neonatal intensive care unit (NICU), breathing monitors can be difficult to apply and maintain proper positioning on a moving infant. Capacitance based sensors, as discussed herein, can monitor breathing or pulse rate of the infant to determine if the breathing as monitored by the breathing monitor is valid or if the sensor has fallen off or been mis-aligned. Furthermore, capacitance based sensing can be used to replace bulky breathing monitor equipment for infants and instead use only a small adhesive pad with a capacitive sensor or a clip-on sensor with a capacitive element. Capacitance sensing can be used in a NICU to monitor activity and movement, such as to ensure that an infant is moving regularly. 
     As a first example of a measurement system for measuring physiological parameters of a patient,  FIG. 1A  is presented.  FIG. 1A  is a system diagram illustrating measurement system  100 . Elements of measurement system  100  measure one or more physiological parameters of tissue  140 . In the example shown in  FIG. 1A , measurement system  100  includes single plate capacitor  182  on a single side of tissue  140 , using tissue  140  as a dielectric. 
     Measurement system  100  includes processing system  110 , physiological sensor system  120 , and capacitance system  130 , with sensor node  190  and capacitance node  182  applied to tissue  140 . Processing system  110  and physiological sensor system  120  communicate over link  170 . Processing system  110  and capacitance system  130  communicate over link  171 . Physiological sensor system  120  and sensor node  190  are coupled over link  160 . Capacitance system  130  and capacitance node  182  are coupled over link  162 . In some examples, processing system  110 , physiological sensor system  120 , and capacitance system  130  are included in measurement equipment  101 . In addition to the capacitance and physiological sensors shown in  FIG. 1A , measurement system  100  can also include further sensors, such as accelerometers, temperature sensors, moisture sensors, blood pressure cuffs, or other physiological and environmental sensors. 
     In operation, physiological sensor system  120  is configured to receive instructions and signals from processing system  110  over link  170 . In some examples, physiological sensor system  120  is configured to perform signal detection and processing for signals received over link  160  and transfer information related to these signals to processing node  110  over link  170  for further processing and analysis. Physiological sensor system  120  can measure one or more physiological parameters of tissue  140  using sensor node  190 . Physiological sensor system  120 , along with sensor node  190 , can comprise at least one of a pulse oximetry system, an ECG system, an acoustic physiological parameter measurement system, a breathing monitor, and a pulse rate monitor, among other physiological measurement systems, including combinations thereof. Physiological sensor system  120  or processing system  110  can also receive physiological parameters or other physiological information from other equipment and systems, such as other monitoring and detection equipment not shown in  FIG. 1A . Capacitance system  130  is configured to receive instructions and signals from processing system  110  over link  171  to generate signals for emission as electric field signal  151 . Capacitance system  130  is also configured to perform signal detection and processing for electric field signals monitored over link  162  and transfer information related to these signals to processing node  110  over link  171  for further processing and analysis. 
     In some examples, physiological sensor system  120  includes optical-based measurement equipment and systems.  FIG. 1B  is a system diagram illustrating an example configuration of measurement system  102 . Elements of measurement system  102  in  FIG. 1B  emit and detect optical and electric field signals in tissue  140  of a patient for measuring one or more physiological parameters of tissue  140 . 
     In the example shown in  FIG. 1B , measurement system  102  includes a single plate capacitor positioned proximate to a single side of tissue  140 . The single plate capacitor is included in capacitance node  180  in  FIG. 1B . The single plate capacitor of capacitance node  180  can be placed on tissue  140  in some examples. However, in other examples, such as shown in  FIG. 1B , the single plate capacitor of capacitance node  180  can have an air gap or have one or more layers of non-tissue material such as air, clothing, coatings, polymers, or other materials. Thus, in  FIG. 1B , the single plate capacitor is in a non-contact configuration with regard to tissue  140 . Also, the optical measurement system of  FIG. 1B  is shown as a reflectance pulse oximetry configuration with optical emitter and detector equipment positioned on a same side of tissue  140 . Further examples can instead be employed, such as transmission pulse oximetry with optical emitter and detector equipment located on different sides of tissue  140 . 
     Measurement system  102  includes processing system  110 , optical system  121 , and capacitance system  130 , with optical nodes  180 - 181  and capacitance node  182  applied to tissue  140 . Processing system  110  and optical system  121  communicate over link  170 . Processing system  110  and capacitance system  130  communicate over link  171 . Optical system  121  and optical node  180  are coupled over link  160 . Optical system  121  and optical node  181  are coupled over link  161 . Capacitance system  130  and capacitance node  182  are coupled over link  162 . In some examples, processing system  110 , optical system  121 , and capacitance system  130  are included in measurement equipment  101 . In addition to capacitance and optical sensors, measurement system  102  can also include further sensors, such as accelerometers, temperature sensors, moisture sensors, blood pressure cuffs, or other physiological and environmental sensors. 
     In operation, optical system  121  is configured to receive instructions and signals from processing system  110  over link  170  to generate signals for emission as optical signal  150 . Optical system  121  is also configured to perform signal detection and processing for signals received over link  161  and transfer information related to these signals to processing node  110  over link  170  for further processing and analysis. Capacitance system  130  is configured to receive instructions and signals from processing system  110  over link  171  to generate signals for emission as electric field signal  151 . Capacitance system  130  is also configured to perform signal detection and processing for electric field signals monitored over link  162  and transfer information related to these signals to processing node  110  over link  171  for further processing and analysis. Capacitance system  130  can measure a capacitance signal which is described below in  FIG. 2 . 
     As a first example operation of any of measurement systems  100  and  102 ,  FIG. 2  is provided.  FIG. 2  is a flow diagram illustrating a method of operation of a measurement system. The operations of  FIG. 2  are referenced below parenthetically. In  FIG. 2 , physiological sensor system  120  detects ( 201 ) a physiological signal representative of one or more physiological parameters of a patient. As discussed in  FIG. 1A , physiological sensor system  120  can comprise any of a pulse oximetry system, an ECG system, an acoustic physiological parameter measurement system, a breathing monitor, and a pulse rate monitor, among other physiological measurement systems, including combinations thereof. Thus, physiological sensor system  120  can measure physiological signals related to physiological parameters of tissue  140  using at least sensor node  190 , or physiological sensor system  120  can monitor patient physiological parameters (in breathing monitor examples) to identify the physiological parameters. 
     Alternatively, physiological sensor system  120  can receive physiological parameters or other physiological information from other equipment and systems, such as external monitoring systems. In  FIG. 2 , a physiological parameter can be measured by another system not included in  FIG. 1A , such as a capnometer, breathing rate measurement system, blood pressure system, pulse monitoring system, among others, including combinations thereof. Sensor system  120  can receive one or more physiological parameters from an external system instead of measuring signals related to physiological parameters. 
     Measurement system  102  of  FIG. 1B  illustrates an optical measurement configuration to detect physiological signals.  FIG. 1B  shows a pulse oximetry measurement system, namely optical system  121  and associated optical nodes  180 - 181 . Optical system  121  emits signals over link  160  for emission as optical signal  150  into tissue  140 , and optical node  180  emits optical signal  150  into tissue  140 . In some examples, link  160  is a wired or wireless signal link, and carries a measurement signal to optical node  180  which converts the measurement signal into an optical signal and emits optical signal  150  into tissue  140 . The optical signal can be emitted using a laser, laser diode, light emitting diode (LED), or other light emission device. In other examples, link  160  is an optical link, and carries an optical signal to optical node  180 . Optical node  180  can comprise tissue interface optics, such as lenses, prisms, or other optical fiber-to-tissue optics, which interface optical link  160  to tissue  140  for emission of optical signal  150 . One or more optical wavelengths can be used by optical node  180  to measure tissue  140 , and the one or more optical wavelengths can be selected based on various physiological factors, such as isosbestic wavelengths associated with blood components of tissue  140 . In a particular example, wavelengths such as 660 nm and 808 nm are employed. 
     Continuing in the pulse oximetry example of  FIG. 1B , optical system  121  detects the optical signals after propagation through the tissue. Optical node  181  receives optical signal  150  after propagation through tissue  140 . Optical system  121  receives signals over link  161  from optical node  181  representative of optical signal  150  after propagation through tissue  140 . In some examples, link  161  is a wired or wireless signal link, and carries a detection signal from optical node  181  which converts a received optical signal  150  into a detection signal after detecting optical signal  150  from tissue  140 . Optical signal  150  can be detected using a photodiode, avalanche photodiode, or other optical detection device, along with any associated tissue interface optics. In other examples, link  161  is an optical link, and carries an optical signal from optical node  181 . Optical node  181  can comprise tissue interface optics, such as described above for node  180 , which interface optical link  161  to tissue  140  for capture of optical signal  150  after propagation through tissue  140 . 
     Capacitance system  130  applies ( 202 ) electric field signal  151  to tissue  140  to determine a capacitance signal. Capacitance system  130  generates one or more electric signals over link  162  to generate electric field signal  151  in tissue  140 . Link  162  is an electric signal link which drives capacitance node  182  to emit electric field  151  in tissue  140 . Electric field signal  151  can be applied by a capacitor portion of capacitance node  182 . In some examples, capacitance node  182  includes a single plate capacitor which uses tissue  140  as a dielectric. Tissue interface elements, such as pads, adhesives, clamps, and the like, can be included in capacitance node  182 . In many examples, electric field signal  151  is a modulated electric signal. 
     The capacitor portion, such as the single-plate capacitor employed in  FIGS. 1A and 1B , can apply electric field  151  proximate to tissue  140 , which can include application of electric field signal  151  without contact of any capacitor plate portion of capacitance node to tissue  140 . In other examples, any associated capacitor plate portion of capacitance node  182  is positioned to contact tissue  140 . A contact example is shown in  FIG. 1A , while a non-contact example is shown in  FIG. 1B . Electric field signal  151  can comprise a modulated signal produced by circuitry of capacitance system  130  and transferred onto link  162  for application of electric field signal  151  to tissue  140  by capacitance node  182 . 
     Capacitance system  130  detects changes in electric field signal  151  to identify a capacitance signal. These changes in electric field signal  151  can be measured over link  162 . The capacitance signal can correspond to a change in capacitance of a capacitor portion of capacitance node  182  that can be detected by capacitance system  130 . The change in capacitance can be monitored as electric field signal  151  is applied to tissue and the capacitance signal can reflect the change in capacitance. Electric field signal  151  can comprise a modulated signal, such as a sine wave signal. Modulation circuitry used to produce electric field signal  151  can include a capacitor portion of capacitance node  182 . Changes in a capacitance value of a capacitor used to apply electric field signal  151  to tissue can be detected by capacitance system  130  as a change in modulation frequency or a change in power draw of the capacitor or associated modulation circuitry, among other detection methods. These changes in electric field signal  151  can also be measured by monitoring changes in a noise level, current draw, or other characteristics of electric field signal  151  as detected by capacitance system  130 . Capacitance system  130  can comprise capacitance-to-digital converter circuitry. The capacitance signal can be monitored concurrent with other physiological parameter monitoring, such as that described in operation  201 . 
     Electric field signal  151  can experience changes due to a change in the dielectric environment into which electric field signal  151  is applied. The dielectric environment can include any materials that are proximate to a capacitor plate which applies electric field signal  151 . Example materials in the dielectric environment include tissue  140 , clothing, air gaps, pads, coatings, casings, gels, or adhesives, among other materials. The changes can be due to physiological changes in tissue  140  or changes in the physiological environment of tissue  140 . The changes in electric field signal  151  can be caused by motion of tissue  140 , where the motion is caused by venous blood movement within tissue  140 , physical movement of tissue  140 —such as movement of a limb associated with tissue  140 , or due to changes in pressure/orientation of capacitance node  182  on tissue  140 . 
     In further examples, tissue  140 , or the associated patient, is connected electrically to an electrical reference potential, such as a ground potential. A low-resistance connection of tissue of the patient to the reference potential can be employed, such as a metallic bracelet worn by the patient, an electrical connection used for other physiological measurements, such as an ECG lead, or though tissue interface portions of system  100 . The tissue interface portions can include clamp-on probes, adhesive pads, conductive foams, or conducting gels which interface electrically to tissue of the patient. The electrical grounding or reference potential connection can be employed to enhance signal measurement of the single-plate capacitor. Some measurements of the capacitance signal can be affected by unwanted influence from the environment around tissue  140 , such as nearby objects, nearby people, things the patient is presently touching or contacting, among other influences. A reference or ground connection of tissue  140  or the associated patient can reduce unwanted noise and improve measurement signal amplitude. 
     Processing system  110  reduces ( 203 ) a noise level in the physiological signal based on at least the capacitance signal. In the example shown in  FIG. 1B , an optical signal is measured which might include noise or undesired signal artifacts. Physiological parameters determined from the physiological signal might be inaccurate due to this noise. However, in this example, measured physiological signals can have various noise or undesired signal artifacts reduced or removed using the capacitance signal. For example, various signal elements of the capacitance signal can be isolated and used to remove unwanted noise from the physiological signals and determine physiological parameters. The physiological signals can include those identified in operation  201 , such as by monitoring pulse oximetry-based physiological signals in  FIG. 1B , among others. In one example, the capacitance signal can be monitored to detect motion associated with tissue  140 . Signal elements of the capacitance signal due to motion of tissue  140  can be isolated and used to subtract unwanted motion-based noise artifacts from the pulse oximetry-based physiological signals. In examples where physiological parameters are received from external systems or equipment, processing system  110  can correct the physiological parameters using at least the capacitance signal to determine corrected physiological parameters of the patient. 
     In addition to correction of measured physiological signals using the capacitance signal, other enhanced measurements can be employed by measurement system  100 . For example, measurement system  100  can provide enhanced measurement beyond the optical measurement of  FIG. 1B  due to the combination of optical and capacitance-based signals applied to tissue  140 . These enhancements can compensate optical readings for variation due to movement of tissue  140 , deformation of tissue  140 , and pressure of sensor portions applied to tissue  140 . In some examples, capacitance system  130  can detect when optical sensor portions, such as optical nodes  180 - 181  are or are not sufficiently applied to tissue  140 . Responsive to optical nodes  180 - 181  not being sufficiently applied to tissue, measurement system  100  can suspend measurement of optical signal  150 , prevent processing of optical signal  150  to determine physiological parameters, or alert an operator of measurement system  100  to an improper sensor application condition, among other operations. In other examples, a pressure-based compensation can be performed on optical signal  150  based on electric field signal  151  to provide more accurate and stable measurements of optical signal  150 , as described below in  FIG. 10 . 
     In addition to capacitance system  130  being used to compensate for variability in optical signals in tissue  140 , capacitance system  130  can be employed to add additional measurement capability to an optical-only measurement system. A pulse oximetry measurement to detect SpO 2  can be improved using capacitance system  130  and a single plate capacitor configuration, such as capacitance node  182 . An optical-only hemoglobin measurement of blood of tissue  140  can be improved, or an optical or electrical plethysmograph can be measured, by adding a single plate capacitor configuration, such as capacitance node  182 . A capacitive measurement can add additional measured parameters and can be employed in various hemoglobin and SpO 2  measurement methods to reduce a number of variables in associated blood parameter equations. For example, both optical and capacitive sensing can be used to reduce a number of unknown equation variables compared to an optical-only system. In some examples, respiratory rate of tissue  140  can be determined from heart rate changes detected using optical system  121  and capacitance system  130 . 
     In another example, a multi-level measurement process can be employed. A high level of accuracy using both optical and capacitive measurements can be employed to measure physiological parameters of tissue  140 , and a threshold condition can be established on one or more of the physiological parameters. If the threshold condition is not met, then a lower level of accuracy using only the capacitive measurements can be employed to monitor for the threshold condition. If the threshold condition is met during capacitive-only measurements, then optical system  121  can be engaged to provide the high level of accuracy for physiological measurement. As an example, a drop in oxygen saturation level is likely to trigger an increase in heart rate in a patient. A pulsatile signal indicative of heart rate can be detected from a capacitive measurement. Accordingly, a capacitive signal can be used to monitor heart rate while a pulse oximetry device is turned off to save power or reduce data collection requirements, without missing significant oxygen desaturation events. A pulse oximetry device can power down one or more emitters, such as LEDs, to save power. The capacitance signal can continue to track the heart rate and optionally cycle the LEDs on to confirm the heart rate and to check oxygen saturation. Examples of this multi-level measurement are discussed below regarding  FIG. 11B . In further examples, automatic gain control elements of a measurement system can use a capacitance signal to track phases of cardiac cycles and adjust brightness of associated optical emitters to increase signal-to-noise characteristics during desired segments of the cardiac cycle. 
     In yet further examples, a reduction in time to post results to a doctor or patient can be achieved using capacitance-enhanced measurements. For example, a capacitance-based measurement of a physiological signal can be monitored which can give a doctor or patient a quick and rough estimate of a physiological parameter. If a more detailed or more accurate measurement is desired, then another measurement system, such as an optical system, can be enabled. 
     Further examples of these various enhancements using capacitance-based measurements are described herein, such as in  FIGS. 3-8 . As a first example,  FIG. 3  is a system diagram illustrating measurement system  300 . Measurement system  300  includes at least capacitance nodes  183 - 184  and links  163 - 164 . Measurement system  300  is also shown including some similar elements as found in  FIGS. 1A and 1B , although variations are possible. It should be understood that other physiological measurement elements can be included in system  300 . In  FIG. 3 , capacitance nodes  183 - 184  are shown located an exemplary distance apart. This distance can be established based on calibration of the capacitance signal to a specific spacing, among other spacing. In  FIG. 3 , a two-plate capacitor configuration is shown, with both capacitor plates located on the same side of tissue  140 . 
     Measurement system  300  emits and detects optical and electric field signals in tissue  140  of a patient for measuring one or more physiological parameters of tissue  140 . Measurement system  300  includes processing system  110 , optical system  121 , and capacitance system  130 , with optical nodes  180 - 181  and capacitance nodes  183 - 184  applied to tissue  140 . Processing system  110  and optical system  121  communicate over link  170 . Processing system  110  and capacitance system  130  communicate over link  171 . Optical system  121  and optical node  180  are coupled over link  160 . Optical system  121  and optical node  181  are coupled over link  161 . Capacitance system  130  and capacitance node  183  are coupled over link  163 . Capacitance system  130  and capacitance node  184  are coupled over link  164 . In some examples, processing system  110 , optical system  121 , and capacitance system  130  are included in measurement equipment  101 . 
     In operation, optical system  121  is configured to receive instructions and signals from processing system  110  over link  170  to generate signals for emission as optical signal  150 . Optical system  121  is also configured to perform signal detection and processing for signals received over link  161  and transfer information related to these signals to processing node  110  over link  170  for further processing and analysis. Capacitance system  130  is configured to receive instructions and signals from processing system  110  over link  171  to generate signals for emission as electric field signal  151 . Capacitance system  130  is also configured to perform signal detection and processing for electric field signals monitored over links  163 - 164  and transfer information related to these signals to processing node  110  over link  171  for further processing and analysis. 
     As an example operation of measurement system  300 ,  FIG. 4  is provided.  FIG. 4  is a flow diagram illustrating a method of operation of measurement system  300 . The operations of  FIG. 4  are referenced below parenthetically. In  FIG. 4 , optical system  121  emits ( 401 ) optical signal  150  into tissue  140  of the patient. Optical system  121  emits signals over link  160  for emission as at least optical signal  150  into tissue  140 , and optical node  180  emits optical signal  150  into tissue  140 . In some examples, link  160  is an electric signal link, and carries an electrical signal to optical node  180  which converts the electrical signal into an optical signal and emits optical signal  150  into tissue  140 . The optical signal can be emitted using a laser, laser diode, light emitting diode (LED), or other light emission device. In other examples, link  160  is an optical link, and carries an optical signal to optical node  180 . Optical node  180  can comprise tissue interface optics which interface optical link  160  to tissue  140  for emission of optical signal  150 . 
     Optical system  121  detects ( 402 ) the optical signals after propagation through the tissue. Optical node  181  receives optical signal  150  after propagation through tissue  140 . Optical system  121  receives signals over link  161  from optical node  181  representative of optical signal  150  after propagation through tissue  140 . In some examples, link  161  is an electric signal link, and carries an electrical signal from optical node  181  which converts a received optical signal  150  into an electrical signal after detecting optical signal  150  in tissue  140 . Optical signal  150  can be detected using a photodiode, avalanche photodiode, or other optical detection device, along with any associated tissue interface optics. In other examples, link  161  is an optical link, and carries an optical signal from optical node  181 . Optical node  181  can comprise tissue interface optics which interface optical link  161  to tissue  140  for capture of optical signal  150  after propagation through tissue  140 . 
     Capacitance system  130  applies ( 403 ) electric field signal  152  to tissue  140 . Capacitance system  130  generates one or more electric signals over links  163 - 164  to generate electric field signal  152  in tissue  140 . Links  163 - 164  are electric signal links which drive capacitance nodes  183 - 184  to emit electric field  152  in tissue  140 . In this example, capacitance nodes  183 - 184  form a two-plate capacitor positioned on a single side of tissue  140 , and uses tissue  140  as a dielectric. Tissue interface elements can be included in capacitance nodes  183 - 184 . In many examples, electric field signal  152  is a modulated electric signal. 
     Capacitance system  130  detects ( 404 ) changes in electric field signal  152 . Capacitance system  130  measures changes in electric field signal  152  over links  163 - 164 . As the environment of tissue  140 , the internals of tissue  140 , and tissue  140  itself changes, associated changes in electric field signal  152  can be monitored by capacitance system  130 . These changes can be reflected in a change in capacitance of a capacitor formed by capacitance nodes  183 - 184 . In some examples, the changes in electric field signal  152  are due to motion of tissue  140 , where the motion is caused by venous blood movement within tissue  140 , physical movement of tissue  140 , such as movement of a limb associated with tissue  140 , or due to pressure/orientation of capacitance nodes  183 - 184  on tissue  140 . In further examples, the changes in electric field signal  152  are due to changes in a capacitance value associated with capacitance nodes  183 - 184  due to variation in the dielectric environment of capacitance nodes  183 - 184 . These changes can also be changes in a noise level, current draw, power level, or other characteristics of electric field signal  152  as detected by capacitance system  130 . 
     Processing system  110  processes ( 405 ) optical signal  150  and the changes in electric field signal  152  to determine the physiological parameters of the patient. In some examples, processing the changes in electric field signal  152  includes detecting motion or noise induced by tissue  140 , such as by motion of tissue  140 , motion of biological elements within tissue  140 , environmental noise, signal noise, or other effects. These effects can be processed to correct for noise or motion artifacts of optical signal  150  to determine physiological parameters. 
     The processing performed in operation  405  can include different processing techniques. In a first example, processing system  110  compares ( 406 ) changes in electric field signal  152  against measured optical signal  150  to determine the physiological parameters. For example, signal components of electric field signal  152  can be compared to signal components of measured optical signal  150  to improve measured optical signal  150  and determine physiological parameters from the improved optical signal  150 . A capacitance signal or changes in electric field signal  152  can be used to identify noise in a portion of measured optical signal  150 , and then that portion can be de-weighted when processing measured optical signal  150  to determine a physiological parameter. A capacitance signal or changes in electric field signal  152  can be used to identify a period of noise in the capacitance signal or the changes in electric field signal  152 , and this period of noise can be used to drop or exclude a correlating timewise portion of measured optical signal  150  during processing to determine any physiological parameters. Thus, noisy periods or portions of measured optical signal  150  can be excluded from calculation of physiological parameters. 
     In a second example, processing system  110  subtracts ( 407 ) signal components of electric field signal  152  from measured optical signal  150  to determine the physiological parameters. For example, signal components of electric field signal  152  can be subtracted from signal components of measured optical signal  150  to reduce noise in measured optical signal  150  and determine physiological parameters from the noise-reduced optical signal  150 . A scaled or filtered portion of a capacitance signal or changes in electric field signal  152  can be subtracted from a correlated portion of measured optical signal  150 . 
     In a third example, processing system correlates ( 408 ) changes in electric field signal  152  to measured optical signal  150  to determine the physiological parameters. For example, signal components of electric field signal  152  can be correlated to signal components of measured optical signal  150  to improve measured optical signal  150  and determine physiological parameters from the improved optical signal  150 . Other processing techniques can be employed, such as those discussed herein, including combinations and variations thereof. 
     Measurement system  300  can provide enhanced measurement beyond optical measurement due to at least the combination of optical and capacitance-based signals applied to tissue  140 . These enhancements can compensate optical readings for variables in movement of tissue  140 , deformation of tissue  140 , and pressure of sensor portions applied to tissue  140 . In some examples, capacitance system  130  can detect when optical sensor portions, such as optical nodes  180 - 181  are sufficiently applied to tissue  140 . Responsive to optical nodes  180 - 181  not being sufficiently applied to tissue, measurement system  100  can prevent measurement by optical signal  150  or prevent processing of at least optical signal  150  to determine physiological parameters. In other examples, a pressure-based compensation can be performed on optical signal  150  based on electric field signal  151  to provide more accurate and stable measurements of optical signal  150 . 
     In addition to capacitance system  130  being used to identify and compensate for variability in the detected optical signals, capacitance system  130  can be employed to add additional measurement capability to an optical measurement system. A pulse oximetry measurement to detect SpO 2  can be improved using capacitance system  130  and a dual-plate, single-sided capacitor configuration, such as capacitance nodes  183 - 184 . An optical hemoglobin measurement of blood of tissue  140  can be improved, or an optical or electrical plethysmograph can be measured, by adding a dual-plate, single-sided capacitor configuration, such as capacitance nodes  183 - 184 . In some examples, respiratory rate of tissue  140  can be determined from heart rate changes detected using optical system  121  and capacitance system  130 . 
     Further examples of these various enhancements using capacitance-based measurements are described herein, such as in  FIG. 5 .  FIG. 5  is a system diagram illustrating measurement system  500 . Measurement system  500  includes at least capacitance nodes  185 - 186  and links  165 - 166 . Measurement system  500  is also shown including some similar elements as found in  FIGS. 1A and 1B , although variations are possible. It should be understood that other physiological measurement systems can be included in system  500 .  FIG. 5  includes two single-plate capacitors, with each single plate capacitor positioned on a single side of tissue  140 . Also, each single plate capacitor can be applied onto tissue  140 , or can include an air gap and other materials that separate each single plate capacitor from tissue  140 .  FIG. 5  shows an exemplary gap between tissue  140  and each single plate capacitor. 
     Measurement system  500  emits and detects optical and electric field signals in tissue  140  of a patient for measuring one or more physiological parameters of tissue  140 . Measurement system  500  includes processing system  110 , optical system  121 , and capacitance system  130 , with optical nodes  180 - 181  and capacitance nodes  185 - 186  applied to tissue  140 . Processing system  110  and optical system  121  communicate over link  170 . Processing system  110  and capacitance system  130  communicate over link  171 . Optical system  121  and optical node  180  are coupled over link  160 . Optical system  121  and optical node  181  are coupled over link  161 . Capacitance system  130  and capacitance node  185  are coupled over link  165 . Capacitance system  130  and capacitance node  186  are coupled over link  166 . In some examples, processing system  110 , optical system  121 , and capacitance system  130  are included in measurement equipment  101 . 
     In operation, optical system  121  is configured to receive instructions and signals from processing system  110  over link  170  to generate signals for emission as optical signal  150 . Optical system  121  is also configured to perform signal detection and processing for signals received over link  161  and transfer information related to these signals to processing node  110  over link  170  for further processing and analysis. Capacitance system  130  is configured to receive instructions and signals from processing system  110  over link  171  to generate signals for emission as electric field signals  153 - 154 . Capacitance system  130  is also configured to perform signal detection and processing for electric field signals monitored over links  165 - 166  and transfer information related to these signals to processing node  110  over link  171  for further processing and analysis. 
     As an example operation of measurement system  500 ,  FIG. 6  is provided.  FIG. 6  is a flow diagram illustrating a method of operation of measurement system  500 . The operations of  FIG. 6  are referenced below parenthetically. In  FIG. 6 , optical system  121  emits ( 601 ) optical signal  150  into tissue  140  of the patient. Optical system  121  emits signals over link  160  for emission as optical signal  150  into tissue  140 , and optical node  180  emits optical signal  150  into tissue  140 . In some examples, link  160  is an electric signal link, and carries an electrical signal to optical node  180  which converts the electrical signal into an optical signal and emits optical signal  150  into tissue  140 . The optical signal can be emitted using a laser, laser diode, light emitting diode (LED), or other light emission device. In other examples, link  160  is an optical link, and carries an optical signal to optical node  180 . Optical node  180  can comprise tissue interface optics which interface optical link  160  to tissue  140  for emission of optical signal  150 . 
     Optical system  121  detects ( 602 ) the optical signals after propagation through the tissue. Optical node  181  receives optical signal  150  after propagation through tissue  140 . Optical system  121  receives signals over link  161  from optical node  181  representative of optical signal  150  after propagation through tissue  140 . In some examples, link  161  is an electric signal link, and carries an electrical signal from optical node  181  which converts a received optical signal  150  into an electrical signal after detecting optical signal  150  in tissue  140 . Optical signal  150  can be detected using a photodiode, avalanche photodiode, or other optical detection device, along with any associated tissue interface optics. In other examples, link  161  is an optical link, and carries an optical signal from optical node  181 . Optical node  181  can comprise tissue interface optics which interface optical link  161  to tissue  140  for capture of optical signal  150  after propagation through tissue  140 . 
     Capacitance system  130  applies ( 603 ) a first electric field as electric field signal  153  to tissue  140 . Capacitance system  130  generates one or more electric signals over link  165  to generate electric field signal  153  in tissue  140 . Link  165  is an electric signal link which drives capacitance node  185  to emit electric field  153  in tissue  140 . In this example, capacitance node  185  forms a single-plate capacitor positioned on a single side of tissue  140 , and uses tissue  140  as a dielectric. Tissue interface elements can be included in capacitance node  185 . In many examples, electric field signal  153  is generated by a modulated electric signal. 
     Capacitance system  130  detects ( 604 ) changes in electric field signal  153 . Capacitance system  130  measures changes in electric field signal  153  over link  165 . As the environment of tissue  140 , the internals of tissue  140 , and tissue  140  itself changes, associated changes in electric field signal  153  can be monitored by capacitance system  130 . In some examples, the changes in electric field signal  153  are due to motion of tissue  140 , where the motion is caused by venous blood movement within tissue  140 , physical movement of tissue  140 , such as movement of a limb associated with tissue  140 , or due to pressure/orientation of capacitance node  185  on tissue  140 . In further examples, the changes in electric field signal  153  are due to changes in a capacitance value associated with capacitance node  185  due to variation in the dielectric environment of capacitance node  185 . These changes can also be changes in a noise level, current draw, power level, or other characteristics of electric field signal  153  as detected by capacitance system  130 . 
     Capacitance system  130  applies ( 605 ) a second electric field as electric field signal  154  to tissue  140 . Capacitance system  130  generates one or more electric signals over link  166  to generate electric field signal  154  in tissue  140 . Link  166  is an electric signal link which drives capacitance node  186  to emit electric field  154  in tissue  140 . In this example, capacitance node  186  forms a single-plate capacitor positioned on a single side of tissue  140 , and uses tissue  140  as a dielectric. Capacitance node  186  is positioned on a different side of tissue  140  than capacitance node  185 . In some examples, each of capacitance nodes  185 - 186  are positioned on opposite sides of tissue  140 , such as a top and bottom of a finger or digit. In other examples, each of capacitance nodes  185 - 186  are positioned on adjacent sides of tissue  140 , such as a top and adjacent side of a finger or digit. Tissue interface elements can be included in capacitance node  186 . In many examples, electric field signal  154  is a modulated electric signal. 
     In some examples, capacitance system  130  is configured to emit electric field signal  153  and electric field signal  154  simultaneously, using each of capacitance node  185  and capacitance node  186  as separate single-plate capacitors. Single-plate operation can be achieved in some examples by using isolation circuitry, such as separate measurement and drive circuitry, transformers, opto-isolators, or other isolation elements to ensure single-plate operation. When simultaneous operation is performed, each of capacitance node  185  and capacitance node  186  can use similar or different modulation frequencies for the respective electric field signals. For example, non-interfering modulation frequencies can be selected for each of electric field signal  153  and electric field signal  154 . Frequency hopping, chirping, or spread spectrum techniques can also be employed to minimize interference of simultaneous measurement using electric field signal  153  and electric field signal  154 . In further examples, the modulation frequency can be swapped after a first measurement is taken to perform a second measurement using simultaneous electric field signal  153  and electric field signal  154 . In yet further examples, the modulation frequency of an associated electric field signal can be selected to minimize interference with other measurement devices, such as other physiological measurement equipment monitoring the patient. Non-simultaneous emission of electric field signal  153  and electric field signal  154  can also be employed, such as when using a similar modulation frequency for each of electric field signal  153  and electric field signal  154 . For example, a sequential measurement using electric field signal  153  and electric field signal  154  can be employed. 
     Capacitance system  130  detects ( 606 ) changes in electric field signal  154 . Capacitance system  130  measures changes in electric field signal  154  over link  166 . As the environment of tissue  140 , the internals of tissue  140 , and tissue  140  itself changes, associated changes in electric field signal  154  can be monitored by capacitance system  130 . These changes can be reflected in a change in capacitance of a capacitor formed by capacitance node  186  due to changes in the dielectric environment of capacitance node  186 . The change in capacitance can be reflected in a capacitance signal monitored by capacitance system  130 . In some examples, the changes in electric field signal  154  are due to motion of tissue  140 , where the motion is caused by venous blood movement within tissue  140 , physical movement of tissue  140 , such as movement of a limb associated with tissue  140 , or due to pressure/orientation of capacitance node  186  on tissue  140 . In further examples, the changes in electric field signal  154  are due to changes in a capacitance value associated with capacitance node  186  due to variation in the dielectric environment of capacitance node  186 . These changes can also be changes in a noise level, current draw, power level, or other characteristics of electric field signal  154  as detected by capacitance system  130 . 
     Processing system  110  processes ( 607 ) optical signal  150 , the changes in first electric field signal  153 , and changes in second electric field signal  154  to determine the physiological parameters of the patient. In some examples, processing the changes in electric field signals  153 - 154  includes detecting motion or noise induced by tissue  140 , such as by motion of tissue  140 , motion of biological elements within tissue  140 , environmental noise, signal noise, or other effects. These effects can be used to correct for noise or motion artifacts of optical signal  150  to determine the physiological parameters. The processing performed in operation  607  can include different processing techniques, such as those described in  FIG. 4 . 
     Further examples of these various enhancements using capacitance-based measurements are described herein, such as in  FIG. 7 .  FIG. 7  is a system diagram illustrating measurement system  700 . Measurement system  700  is also shown including some similar elements as found in  FIGS. 1A and 1B , although variations are possible. Measurement system  700  also omits optical measurement elements as discussed in the previous examples. It should be understood that other physiological measurement systems can be included in system  700 . 
       FIG. 7  shows two example capacitor arrangements, namely arrangement  790  and  791 . Capacitor arrangement  790  illustrates one or more capacitance nodes  187 - 188  positioned on the same side of tissue  140 . In a first example of arrangement  790 , a single plate capacitor of capacitance node  187  is employed on a single side of tissue  140 . In a second example of arrangement  790 , a two-plate capacitor is employed via capacitance nodes  187 - 188 —with both capacitor plates of the two-plate capacitor positioned on a single side of tissue  140 . Capacitor arrangement  791  illustrates a single plate capacitor of capacitance node  189  that wraps around tissue  140 . Capacitance node  189  can comprise a rigid or flexible material to achieve the wrap-around feature illustrated in  FIG. 7 . In some examples, capacitor plates associated with capacitance nodes  187 - 189  are positioned on tissue  140 , while in other examples, an air gap or other material separates the associated capacitor plates from tissue  140 . 
     Measurement system  700  emits and detects electric field signals in tissue  140  of a patient for measuring one or more physiological parameters of tissue  140 . Measurement system  700  includes processing system  110  and capacitance system  130 , with capacitance nodes  187 - 189  applied to tissue  140 . Processing system  110  and capacitance system  130  communicate over link  171 . Capacitance system  130  and capacitance node  187  are coupled over link  167 . Alternatively, capacitance system  130  and capacitance node  189  are coupled over link  167 . Capacitance system  130  and optional capacitance node  188  are coupled over link  168 . In some examples, processing system  110  and capacitance system  130  are included in measurement equipment  101 . Capacitance nodes  187 - 188  are shown as located an exemplary distance apart. This distance can be arbitrary, or can be established based on calibration of the capacitance signal to a specific spacing, among other spacing. 
     In operation, capacitance system  130  is configured to receive instructions and signals from processing system  110  over link  171  to generate signals for emission as electric field signal  155 . Alternatively, capacitance system  130  can be configured to receive instructions and signals from processing system  110  over link  171  to generate signals for emission as electric field signal  156 . Capacitance system  130  is also configured to perform signal detection and processing for electric field signals monitored over links  167 - 168  and transfer information related to these signals to processing node  110  over link  171  for further processing and analysis. 
     In further examples, one or more portions of measurement system  700  can be incorporated into a wearable device. For example, at least capacitance node  187  can be incorporated into a fitness wristband for monitoring of physiological parameters during fitness activities. This fitness wristband can include moisture and sweat protection to isolate elements of measurement system  700  from environmental exposure. Capacitance measurements for fitness can include breathing rate, heart rate, sweat levels, electrolyte loss rate, running pace, and changes thereto. In some examples, all elements of measurement system  700  are included in the fitness wristband, with capacitance node  187  configured to be located next to tissue of the fitness participant when worn, such as contacting skin of a wearer. It should be understood that one capacitor plate (such as capacitance node  187  or  189 ) or two capacitor plates (such as capacitance nodes  187 - 188 ) can be incorporated into the fitness wristband, as discussed above  FIG. 7  and below for  FIG. 8 . In some examples, such as in capacitor arrangement  791 , a flexible or bendable capacitor plate can be employed. 
     As an example operation of measurement system  700 ,  FIG. 8  is provided.  FIG. 8  is a flow diagram illustrating a method of operation of measurement system  700 . The operations of  FIG. 8  are referenced below parenthetically. In  FIG. 8 , a physiological measurement begins ( 801 ) and a selection of measurement style is selected based on a capacitor plate arrangement ( 802 ). In first capacitor plate arrangement, such as illustrated in arrangement  790 , one or more capacitor plates are positioned on a single side of tissue  140 . In a second capacitor plate arrangement, such as illustrated by arrangement  791 , a single capacitor plate is positioned on more than one side of tissue  140 , such as by wrapping around a finger or limb of a patient. Tissue interface elements can be included in capacitance nodes  187 - 189 , such as adhesives, clamps, pads, gels, and the like. 
     If a single side arrangement  790  is employed, capacitance system  130  applies ( 803 ) applies at least electric field signal  155  to a single side of tissue  140  of the patient. Capacitance system  130  generates one or more electric signals over link  167  and optionally link  168  to generate electric field signal  155  in tissue  140 . Links  167 - 168  are each electric signal links which drives associated capacitance nodes  187 - 188  to emit electric field  155  in tissue  140 . In a first example, capacitance node  187  is employed and capacitance node  188  is omitted. When only capacitance node  187  is employed, capacitance node  187  forms a single plate capacitor to emit electric field signal  155  into tissue  140 . In a second example, both capacitance node  187  and capacitance node  188  are employed. When both capacitance nodes  187 - 188  are employed, capacitance nodes  187 - 188  comprise a two-plate capacitor applied to the same side of tissue  140 . Capacitance nodes  187 - 188  can use tissue  140  as a dielectric. In many examples, electric field signal  155  is a modulated signal. 
     If a multi-side arrangement  791  is employed, capacitance system  130  applies ( 804 ) applies at least electric field signal  156  to more than one side of tissue  140  of the patient. Capacitance system  130  generates one or more electric signals over link  167  to generate electric field signal  156  in tissue  140 . Capacitance node  189  forms a single plate capacitor to emit electric field signal  156  into tissue  140 . Capacitance node  189  can use tissue  140  as a dielectric, as well as any associated air gaps or other materials. In many examples, electric field signal  156  is a modulated signal. 
     Capacitance system  130  detects ( 805 ) changes in electric field signal  155 , or alternatively electric field signal  156 . Capacitance system  130  measures changes in electric field signal  155 / 156  over one or more of links  167 - 168 . As the environment of tissue  140 , the internals of tissue  140 , and tissue  140  itself changes, associated changes in electric field signal  155 / 156  can be monitored by capacitance system  130 . These changes can be reflected in a change in capacitance of a capacitor formed by one or more of capacitance nodes  187 - 189  due to changes in the dielectric environment of an associated capacitance node  187 - 189 . These changes can be changes in a noise level, current draw, power level, or other characteristics of electric field signal  155 / 156  as detected by capacitance system  130 . 
     Processing system  110  processes ( 806 ) the changes in electric field signal  155  to determine the physiological parameters of the patient. In this example, the physiological parameter identified is a hemoglobin concentration of blood in tissue  140  of the patent. The hemoglobin concentration is measured using a single plate capacitor when capacitance node  187  or  189  are employed alone, or using a two-plate single-side capacitor when both capacitance nodes  187 - 188  are employed. Further physiological parameters can be measured, such as ECG information. The processing performed in operation  806  can include different processing techniques, such as those described in  FIG. 4 . 
     Returning back to the elements of  FIGS. 1-8 , processing system  110  comprises communication interfaces, computer systems, microprocessors, circuitry, non-transient computer-readable media, or other processing devices or software systems, and may be distributed among multiple processing devices. Processing system  110  can be included in the equipment or systems of optical system  121  or capacitance system  130 , or can be included in separate equipment or systems. Examples of processing system  110  may also include software such as an operating system, logs, utilities, drivers, databases, data structures, processing algorithms, networking software, and other software stored on non-transient computer-readable media. 
     Physiological sensor system  120  can comprise at least one of a pulse oximetry system, an ECG system, an acoustic physiological parameter measurement system, a breathing monitor, a blood pressure monitoring system, and a pulse rate monitor, among other physiological measurement systems, including combinations thereof. Physiological sensor system  120  can also receive physiological parameters or other physiological information from other equipment and systems, such as other monitoring and detection equipment not shown. For example, a separate monitoring device can be employed and live physiological data can be transferred by the separate monitoring device for receipt by physiological sensor system  120 . Physiological sensor system  120  can comprise transceivers, network interfaces, data links, and the like to receive this physiological data from the separate monitoring equipment. In another example, physiological sensor system  120  can instead monitor the physiological parameters itself, and include the associated monitoring equipment mentioned above. In further examples, physiological sensor system  120  can comprise environmental sensing equipment and systems, such as temperature sensing equipment, accelerometers, clocks, timers, chemical sensors, pressure sensors, or other sensing equipment to supplement patient monitoring equipment. 
     Optical system  121  can include electrical to optical conversion circuitry and equipment, optical modulation equipment, and optical waveguide interface equipment. Optical system  121  can include direct digital synthesis (DDS) components, CD/DVD laser driver components, function generators, oscillators, or other signal generation components, filters, delay elements, signal conditioning components, such as passive signal conditioning devices, attenuators, filters, and directional couplers, active signal conditioning devices, amplifiers, or frequency converters, including combinations thereof. Optical system  121  can also include switching, multiplexing, or buffering circuitry, such as solid-state switches, RF switches, diodes, or other solid state devices. Optical system  121  also can include laser elements such as a laser diode, solid-state laser, or other laser device, along with associated driving circuitry. Optical couplers, cabling, or attachments can be included to optically mate to links  160 - 161 . Optical system  121  can also include light detection equipment, optical to electrical conversion circuitry, photon density wave characteristic detection equipment, and analog-to-digital conversion equipment. Optical system  121  can include one or more photodiodes, phototransistors, avalanche photodiodes (APD), or other optoelectronic sensors, along with associated receiver circuitry such as amplifiers or filters. Optical system  121  can also include phase and amplitude detection circuitry and processing elements. 
     Capacitance system  130  can comprise modulation circuitry, digital to analog conversion circuitry, analog to digital conversion circuitry, capacitor to digital conversion circuitry, amplifiers, impedance matching circuitry, analog switches, transceivers, processing circuitry, and other circuitry, including combinations thereof. Capacitance system  130  receives instructions from processing system  110  to drive electric field signals in tissue. Capacitance system  130  detects electric field properties of tissue and the environment around a patient and sensor equipment, as monitored by associated capacitance nodes. Capacitance system  130  can process the electric field properties from an analog format to a digital format for transfer to processing system  110 . In some examples, capacitance system  130  comprises a capacitance detector, which can detect changes in capacitance of associated capacitance nodes. Capacitance system  130  can include capacitance measurement components such as Analog Devices AD7745. Capacitance system  130  can include touch screen controllers or associated integrated processing devices, such as Kinetis K10 devices (Freescale Semiconductor, Inc.). 
     Tissue  140  illustrates a portion of the tissue of a patient undergoing measurement of a physiological parameter, and is represented by a rectangular element for simplicity in  FIGS. 1, 3, 5, and 7 . It should be understood that tissue  140  can represent a finger, fingertip, toe, earlobe, chest, foot, arm, leg, head, limb, forehead, or other tissue portion of a patient undergoing physiological parameter measurement. Tissue  140  can comprise muscle, fat, blood, vessels, bone, or other tissue components. The blood portion of tissue  140  can include tissue diffuse blood and arterial or venous blood. In some examples, tissue  140  is instead a test sample or representative material for calibration or testing of system  100 . The patient undergoing measurement can be any individual organism or group or organisms. 
     Links  160 - 161  can comprise optical links, wired electrical links, or wireless links. In examples where ones of links  160 - 161  comprise optical links, links  160 - 161  each comprise one or more optical waveguides, and use glass, polymer, air, space, or some other material as the transport media for transmission of light, and can each include multimode fiber (MMF) or single mode fiber (SMF) materials. A sheath or loom can be employed to bundle each of links  160 - 161  together for convenience. One end of each of links  160 - 161  mates with an associated component of optical system  121 , and the other end of each of links  160 - 161  is configured to interface with tissue  140  through an associated optical node  180 - 181 . Link  160  is configured to emit light via optical node  180  into tissue  140 , while link  161  is configured to receive light via optical node  181  from tissue  140 . Also, in examples where links  160 - 161  comprise optical links, optical nodes  180 - 181  can comprise one or more optical interfacing elements to interface the waveguide portions of links  160 - 161  to tissue  140 . In examples where ones of links  160 - 161  comprise wired electrical links, links  160 - 161  each comprise one or more wired for carrying electrical to and from ones of optical nodes  180 - 181 . In examples where ones of links  160 - 161  comprise wireless links, links  160 - 161  can include wireless signaling for exchanging communications between optical nodes  180 - 181  and optical system  121 . 
     Optical node  180  can comprise can include laser elements such as a laser diodes, solid-state lasers, light emitting diodes (LED), or other light emitting devices, along with associated driving circuitry and electrical-to-optical conversion circuitry. Optical node  181  can include light detection equipment, optical to electrical conversion circuitry, photon density wave characteristic detection equipment, and analog-to-digital conversion equipment. Optical node  181  can include one or more photodiodes, phototransistors, avalanche photodiodes (APD), or other optoelectronic sensors, along with associated receiver circuitry such as amplifiers or filters. Optical couplers, cabling, lenses, prisms, or attachments can be included in optical nodes  180 - 181  to optically mate tissue  140  to links  160 - 161 . In examples where links  160 - 161  are wireless links, optical nodes  180 - 181  can include wireless transceivers and antennas. 
     In  FIGS. 1, 3, 5, and 7 , link  160  and link  161  are shown coupled to optical nodes  180  and  181  which are located an exemplary distance apart, but can be located on the surface of tissue  140  at predetermined locations or distances. Although the term ‘optical’ is used herein for convenience, it should be understood that the optical measurement signals are not limited to visible light, and can comprise any light wavelength, such as visible, infrared, ultraviolet, or other signals. 
     Capacitance links  162 - 168  each comprise one or more electrical links for emitting or detecting an electric field in the environment of tissue  140 . In one example, link  162  is driven by a modulated electrical signal which produces a similarly modulated electric field  151 . Further figures include similar features. Capacitance links  162 - 168  can include wires, shields, coaxial links, twisted pair links, or other electrical links, including combinations thereof. 
     Capacitance nodes  182 - 189  comprise elements to induce associated electric field signals  151 - 155  in the environment of tissue  140  and detect an electric field in the environment of tissue  140 . In some examples, each capacitance node comprises a capacitor plate. Capacitance nodes  182 - 189  can be co-planar, single plate, fringe field or other capacitor styles using at least tissue  140  as a dielectric or as a plate of a capacitor system. Electric field signals  151 - 155  typically comprise one or more modulated signals which are induced as a variable electrostatic field in the environment and tissue  140 . The modulation frequency can be 10 kHz-50 GHz, among others, and include sine wave, square wave, or other signal characteristics. Electric field signals  151 - 155  can be a modulated signal selected based on the environment around tissue  140 , anticipated or detected interference, patient parameters (skin type, size, shape), maximum sensitivity to motion of patient/sensor, or other factors. In some example, the modulation signal of electric field signals  151 - 155  can be selected to not interfere with the optical drive frequency used in modulation of optical signal  150  or to not interfere with other physiological measurement equipment monitoring the patient. The modulation signal electric field signals  151 - 155  can be selected to interfere constantly with the optical signal  150  by driving one at a multiple of the other frequency. Alternatively, capacitive measurement using electric field signals  151 - 155  can be performed when an optical measurement is not being presently performed. In further examples, the modulation signal of electric field signals  151 - 155  can be swept through a range of modulation frequencies, such as to find an optimal frequency or to reduce dependency of the measurement of electric field signal  151  on various forms of interference. Frequency hopping, chirping, or spread spectrum techniques can also be employed to minimize interference of simultaneous measurement using multiple electric field signals. 
     In the examples of capacitance nodes, the associated capacitor plates can be positioned on tissue  140 , or located proximate and separated by a gap or distance from tissue  140 . The gaps between tissue  140  and ones of capacitance nodes can include air, dielectric materials, pads, coatings, adhesives, gels, clothing of the patient, or other dielectric materials. 
     In further examples, ones of capacitance nodes  182 - 189  comprise a Faraday shield or electromagnetic interference (EMI) shield associated with link  160  or  161 . The shield of link  160  or  161  can be repurposed as a plate of a capacitor and thus driven to create or monitor an electric field signal. Changes in electric field signals can also be detected using the shield. In further examples, optical node  180  or  181  includes a Faraday shield or EMI shield which surrounds associated optical or electrical elements of optical node  180  or  181 . This Faraday shield or EMI shield associated with optical node  180  or  181  can be used as a capacitor plate to generate electric field signals and detect changes in electric field signals. The patient under measurement can also be electrically grounded in some single-plate capacitor examples. Further examples of a Faraday shield and other twisted pair arrangements are discussed in  FIGS. 16 and 17 . 
     In some examples of capacitance nodes, a single-plate capacitor is employed and positioned on one side of tissue  140 . The return path or ground connection of a single-plate single-side capacitor configuration is typically tissue  140  and the earth or nearby structural environment around tissue  140 . In other examples, capacitance nodes comprise two plate elements positioned next to each other but on the same side of tissue  140 . In two-plate, single-side configurations, plates of capacitance nodes can be positioned on the same side of the fingertip as each other. One plate of a capacitance node can be the ‘positive’ or driven portion, while the other plate of a capacitance node can be the ‘negative’ or return/ground/reference portion. Other configurations of signal polarity can be employed. In yet other examples, such as  FIGS. 5-6 , a two separate single-plate capacitors which are positioned on a different sides of tissue  140  are employed. A first measurement can be performed using a two-plate, two-side measurement with a first plate of a capacitance node and a second plate of another capacitance node. A second measurement can be performed using a single-plate, single-side measurement with the first plate of a capacitance node. A third measurement can be performed using a single-plate, single-side measurement with the second plate of another capacitance node. 
     In further examples of capacitance nodes, a capacitive touch surface or touch screen can be employed. A capacitive touch surface or touch screen can detect pressure and touch of tissue on the touch screen and can be used to detect sensor on/off conditions, among assisting with optical measurement of physiological parameters. 
     Links  170 - 171  each use metal, glass, optical, air, space, or some other material as the transport media, and comprise analog, digital, RF, optical, modulated, or power signals, including combinations thereof. Links  170 - 171  can each use various communication protocols or formats, such as Serial Peripheral Interface (SPI), Synchronous or Asynchronous Serial Ports, external bus interface, Controller Area Network (CAN) bus, Inter-Integrated Circuit (I2C), 1-Wire, Radio Frequency Identification (RFID), optical, circuit-switched, Internet Protocol (IP), Ethernet, wireless, Bluetooth, communication signaling, or some other communication format, including combinations, improvements, or variations thereof. Links  170 - 171  can each be direct links or may include intermediate networks, systems, or devices, and can each include a logical network link transported over multiple physical links. 
     Links  160 - 168  and  170 - 171  may each include many different signals sharing the same associated link, as represented by the associated lines in  FIGS. 1, 3, 5, and 7 , comprising channels, forward links, reverse links, user communications, overhead communications, frequencies, wavelengths, carriers, timeslots, spreading codes, logical transportation links, packets, or communication directions. 
     The measurement systems discussed above in  FIGS. 1-8  can be applied to further examples. Some of the examples are discussed below in  FIGS. 9-19 , although it should be understood that different measurement systems and associated elements can be employed in  FIGS. 9-19 . 
       FIG. 9  is a diagram illustrating measurement of physiological parameters.  FIG. 9  includes measurement environment  900 , graph  950 , and graph  960 . Measurement environment  900  includes measurement system  910 , capacitor  911 , and measurement link  912 , with capacitor  911  configured to measured properties of tissue  920 . In operation, measurement system  910  drives a modulated electrical signal onto link  912  to drive capacitor  911  and measure tissue  920 . In this example capacitor  911  is a single-side, single-plate capacitor, although it should be understood that a two-plate capacitor or a multi-side, single-plate capacitor can instead be employed. 
     Graph  950  illustrates an example capacitance signal as found on link  912 . The capacitance signal can be representative of a changing capacitance as monitored by measurement system  910 , such as due to changes in an electric field signal applied to tissue  920 . In one example, capacitor  911  can be included in an oscillator circuit along with circuit elements of measurement system  910  to establish a modulated electric field signal in tissue  920 . The capacitance measured for capacitor  911  by measurement system  910  can vary in time due to a corresponding change in the dielectric environment of capacitor  911 . The dielectric environment of capacitor  911  can change due to changes in many factors, such as tissue  920 , the environment of tissue  920 , motion of tissue  920 , and variation in internal elements of tissue  920 , among others. These capacitance changes can be seen in graph  950  as a change in oscillation frequency of the capacitance signal. Graph  950  is simplified to emphasize changes in frequency or wavelength of the measurement signal. In other examples, noise, amplitude variations, rapid frequency changes, and other signal variations can be observed. To identify graph  950 , a capacitance monitoring circuit or system can be employed, such as a capacitance to digital converter circuit. In other examples, a power or current draw of a circuit that includes capacitor  911  is monitored to determine the change in capacitance and graph  950 . Other techniques can be employed, such as monitoring transmission line capacitive coupling. 
     Graph  960  illustrates the changes in capacitance over time as detected by measurement system  910 . Graph  960  can be derived from graph  950  by determining changes in capacitance based on the frequency changes in graph  950 . Link  912  indicates a first capacitance for time t 1 , a second capacitance for time t 2 , and a third capacitance for time t 3 . Although the changes in capacitance in this example are highly simplified, real-world measurements can include changes in noise, dynamic AC components, DC shifts, among other factors. These various changes in capacitance can be used to extract characteristics of the capacitance signal to determine physiological parameters, such as hemoglobin measurements, and also to aid in physiological measurements made with optical sensing equipment, such as PPG measurements. Further examples of capacitance enhanced measurements are discussed below. 
     The various signal components of the measured capacitance signal can include components that are representative of physiological parameters, or can be used to calculate physiological parameters, such as hemoglobin parameters. For example, AC components and DC components of the capacitance signal can be compared or correlated to determine hemoglobin parameters or changes in hemoglobin parameters of a patient. In further examples, AC components and DC components of the capacitance signal can be processed to determine breathing rates, pulse rates, ECG information, and other physiological parameters discussed herein. 
     In further examples, one or more portions of system  900  can be incorporated into a wearable device. For example, at least capacitor  911  can be incorporated into a fitness wristband for monitoring of physiological parameters during fitness activities. This fitness wristband can include moisture and sweat protection to isolate elements of system  900  from environmental exposure. Capacitance measurements for fitness can include breathing rate, heart rate, sweat levels, electrolyte loss rate, running pace, and changes thereto. In some examples, all elements of system  900  are included in the fitness wristband, with capacitor  911  configured to be located next to tissue of the fitness participant when the wristband is worn. 
       FIG. 10  is a diagram illustrating measurement of physiological parameters.  FIG. 10  includes graph  1000  and associated pressure configurations  1001 - 1003 .  FIG. 10  illustrates capacitance-based identification of pressure changes that may affect PPG measurements, although the identification of pressure changes can be applied to other physiological measurements.  FIG. 10  includes three different sensor configurations, namely a “sensor not applied” configuration  1001 , a “sensor partially applied” configuration  1002 , and a “sensor fully applied” configuration  1003 . Each pressure configuration  1001 - 1003  corresponds to a different region on graph  1000 , as indicated by the dotted lines. Graph  1000  includes an x-axis which represents “time” (T) and a y-axis which represents “capacitance” which can be measured in Farads, such as picofarads (pF). Three capacitance thresholds are indicated in graph  1000 , namely threshold  1  (TH 1 ), threshold  2  (TH 2 ), and threshold  3  (TH 3 ), and will be discussed in greater detail below. 
     Each configuration  1001 - 1003  includes optical sensor  1010  and capacitance sensor  1011  as applied to finger  1020 . Optical sensor  1010  is configured to emit and detect optical signal  1030  in finger  1020  to determine a PPG waveform. Capacitance sensor  1011  is configured to emit and detect electric field signal  1031  in proximity to finger  1020 . Optical sensor  1010  can include elements discussed herein for optical nodes  180 - 181  in  FIGS. 1-8 , although variations are possible. Capacitance sensor  1011  can include elements as discussed herein for capacitance nodes  182 - 189  in  FIGS. 1-8 , although variations are possible. In this example, capacitance sensor  1011  comprises a single plate capacitor, and uses at least finger  1020  as a dielectric for the single plate capacitor. In further examples, two-plate capacitors can be employed. 
     Optical sensor  1010  and capacitance sensor  1011  can be coupled together physically, and share a common structural support. The common structural element can allow for contact of sensor elements with finger  1020 . This common structural support can comprise a rigid carrier, such as clip-on finger probe, or can include a flexible carrier such as an adhesive pad that can be fit onto finger  1020 . Further examples of pads are discussed in  FIGS. 15 and 16 . 
     Although omitted for clarity in  FIG. 10 , optical sensor  1010  can be coupled to further measurement equipment and systems, such as processing system  110  or optical system  121  in  FIGS. 1-8 , although variations are possible. Also, although omitted for clarity in  FIG. 10 , capacitance sensor  1011  can be coupled to further measurement equipment and systems, such as processing system  110  or capacitance system  130  in  FIGS. 1-8 , although variations are possible. Finger  1020  is shown as representing tissue under measurement by sensors  1010 - 1011 . Other tissue portions can instead be included, such as a forehead, chest, toe, limb, or other tissue of a patient, including combinations thereof. 
     A capacitance level measured by capacitance sensor  1011  can relate to a pressure of capacitance sensor  1011  on finger  1020 .  FIG. 10  indicates three discrete pressure configurations of the sensors onto finger  1020  as correlated to three different capacitance thresholds, TH 1 , TH 2 , and TH 3 . Although various random noise might be detected by capacitance sensor  1011 , a DC component can be correlated to a pressure of capacitance sensor  1011  on finger  1020  and a pressure of likewise optical sensor  1010  on finger  1020 . Also, motion noise based at least on movement of finger  1020  or elements internal to finger  1020  can be measured by capacitance sensor  1011 . However, this motion noise typically leads to AC components of the capacitance signal measured by capacitance sensor  1011 . Further examples of motion detection are discussed in at least  FIG. 11A  below. 
     Configuration  1001  shows a “sensor not applied” condition. In configuration  1001 , capacitance sensor  1011  measures a first capacitance level, as indicated in graph  1000 . This first capacitance level might have a certain amount of random noise in it, but on average, the first capacitance level is below a first capacitance threshold (TH 1 ). When the average capacitance level measured by capacitance sensor  1011  is below TH 1 , then an associated measurement system can determine that optical sensor  1010  has not been properly applied to finger  1020 . The capacitance level can be affected by finger  1020  due to finger  1020  being too far away from the capacitor portion of capacitance sensor  1011  or the sensor portions completely off finger  1020 , thus providing a first level of dielectric influence for the capacitor portion of capacitance sensor  1011 . 
     When the capacitance level is below TH 1 , then the measurement system associated with optical sensor  1010  can at least prevent optical signal  1031  from being emitted. The measurement system can turn off optical emitter elements of optical sensor  1010 , such as removing power from a laser diode or LED portion of optical sensor  1010  that emits optical signal  1031 . By turning off the power to the emitter portion of optical sensor  1010 , the measurement system can save power and allow associated equipment to operate in a low power mode, or a battery-save mode. Additionally, the measurement system can prevent measurement and determination of physiological parameters when optical sensor  1010  is not properly applied to finger  1020 , preventing erroneous or inaccurate measurement results when optical sensor  1010  is not properly applied. Furthermore, various alarms for healthcare professionals or measurement systems that are related to physiological measurement of finger  1020  can be modified or disabled due to optical sensor  1010  not properly on finger  1020 . Configuration  1001  can prompt an associated measurement system to alert or signal an operator of the measurement system than the sensor has not been applied properly. Configuration  1001  can prompt an associated measurement system to alert or signal that any associated optical measurements taken are potentially inaccurate. 
     Configuration  1002  shows a “sensor partially applied” condition. In configuration  1002 , capacitance sensor  1011  measures a second capacitance level, as indicated in graph  1000 . This second capacitance level might have a certain amount of random noise in it, but on average, the second capacitance level is below a second capacitance threshold (TH 2 ) and above the first capacitance level (TH 1 ). When the average capacitance level measured by capacitance sensor  1011  is below TH 2 , but above TH 1 , then an associated measurement system can determine that optical sensor  1010  has been only partially applied to finger  1020 . The capacitance level can be affected by finger  1020  due to finger  1020  being too far away from the capacitor portion of capacitance sensor  1011 , not having enough pressure applied by the sensor portions onto finger  1020 , or the surface area of the sensor portions not contacting finger  1020  a desired amount, and thus providing a second level of dielectric influence for the capacitor portion of capacitance sensor  1011 . 
     When the capacitance level is below TH 2 , but above TH 1 , then the measurement system associated with optical sensor  1010  can adjust measurement of physiological parameters, and can act in a few different ways. In a first example, the measurement system can at least prevent optical signal  1031  from being emitted, prevent optical measurement from occurring, or prevent an alarm from being produced, much like when the average capacitance level is below TH 1 . In a second example, the measurement system can instead perform optical measurement using optical sensor  1010  but modify the measurement based on the capacitance level. For example, the measurement system can identify an average capacitance level of a capacitor of capacitance sensor  1011  and scale optical measurement by optical sensor  1010  based on at least the capacitance level. The scaling can encompass selecting calculation coefficients for calculating physiological parameters based on optical signal  1031 , such as calculations used in determining a PPG. The scaling can encompass changing an amplification level associated with signal processing elements used in optical detection and optical measurement using optical signal  1031 . In a third example, an intensity level of optical signal  1031  can be modified based on the capacitance level, so that at a lower capacitance level a higher intensity of optical signal  1031  is employed and at a higher capacitance level a lower intensity is employed for optical signal  1031 . As with configuration  1001 , configuration  1002  can prompt an associated measurement system to alert or signal an operator of the measurement system than the sensor has not been applied properly. Configuration  1002  can prompt an associated measurement system to alert or signal that any associated optical measurements taken are potentially inaccurate. 
     Configuration  1003  shows a “sensor fully applied” condition. In configuration  1003 , capacitance sensor  1011  measures a third capacitance level, as indicated in graph  1000 . This third capacitance level might have a certain amount of random noise in it, but on average, the third capacitance level is above a third capacitance threshold (TH 3 ). When the average capacitance level measured by capacitance sensor  1011  is above TH 3 , then an associated measurement system can determine that optical sensor  1010  has been fully or properly applied to finger  1020 . The capacitance level above TH 3  can occur when the sensors are applied at a desired pressure and contact area to finger  1020 , and thus providing a third level of dielectric influence for the capacitor portion of capacitance sensor  1011 . 
     When the capacitance level is above TH 3 , then the measurement system associated with optical sensor  1010  can emit optical signal  1031  into finger  1020 . The measurement system can turn on optical emitter elements of optical sensor  1010 , such as providing power to a laser diode or LED portion of optical sensor  1010  that emits optical signal  1031 . By turning on the power to the emitter portion of optical sensor  1010  only above TH 3 , the measurement system can save power and allow associated equipment to operate in a low power mode until sensor portions are properly applied to finger  1020 . Additionally, the measurement system can now measure and determine physiological parameters, such PPG measurement, when optical sensor  1010  is properly applied to finger  1020 , preventing erroneous or inaccurate measurement results when optical sensor  1010  is not properly applied. Configuration  1003  can prompt an associated measurement system to alert or signal an operator of the measurement system than the sensor has been applied properly. Configuration  1003  can prompt an associated measurement system to alert or signal that any associated optical measurements taken are accurate. 
     In further examples, the measurement system can process the properties of one or more electric field signals to determine a where on the body of a patient a capacitor plate has been placed or located. This determination can help to identify an improper location of the plate on the patient. In some examples, a user interface included in the measurement system is configured to alert an operator of the measurement system when the placement comprises the at least one capacitor plate improperly applied to the tissue of the patient. For example, different locations on the body, such as a finger, ear, nose, forehead, or other location, exhibit different electric field properties, capacitance, change in capacitance, or capacitance measurements. These differences may be measured and noted, such as by measuring them at different modulation frequencies, to first characterize various body locations. Later, when sensor portions are placed on tissue, the capacitance system can measure these properties to identify where the sensor has been placed. For example, the system can determine that the sensor has been placed on a forehead instead of a finger. Measurement parameters can be modified based on the body location and placement, such as loading correct calculation coefficients based on the placement or body location. An error message or alert can be provided to an operator if a sensor that is designed for one or more body locations is applied to a different or incompatible body location. 
     In addition to placement of a sensor on a particular body part of a patient, properties of the tissue can be detected. For example, a skin type, skin moisture content, skin elasticity, or other tissue parameters can be identified with a capacitance signal. Measurements of physiological signals, such as optical measurements, can be corrected or adjusted based on the skin or tissue parameters identified by the capacitance signal, such as scaling calculation coefficients, adjusting intensity of measurement signal sources, adjusting sensitivity or gain of detection elements, among other adjustments, including combinations thereof. 
       FIG. 10 , discussed above, changes in average or “DC” capacitance levels due to pressure of a capacitance sensor on tissue.  FIGS. 11A-11B , in contrast, discusses dynamic changes or “AC” capacitance levels due to dynamic conditions, such as movement or motion of tissue undergoing measurement ( FIG. 11A ) or due to changing physiological characteristics of the tissue or patient undergoing measurement ( FIG. 11B ), among other dynamic conditions, including combinations thereof. 
       FIG. 11A  is a diagram illustrating measurement of physiological parameters.  FIG. 11A  includes graph  1100  and two different motion configurations, namely a “sensor in motion” configuration  1105 , and a “sensor not in motion” configuration  1106 . Each motion configuration  1105 - 1106  corresponds to a different region on graph  1100 , as indicated by the dotted lines. Graph  1100  includes an x-axis which represents “time” (T) and a y-axis which represents “capacitance” which can be measured in Farads, such as picofarads (pF). A capacitance threshold region is indicated in graph  1100 , namely the region between threshold  1  (TH 1 ) and threshold  2  (TH 2 ), and will be discussed in greater detail below. 
     Each sensor configuration  1105 - 1106  includes optical emitter  1110 , optical detector  1112 , and capacitance node  1111  as applied to finger  1120  or other patient tissue. Optical emitter  1110  is configured to emit optical signal  1130  in finger  1120 . Optical detector  1112  is configured to detect optical signal  1130  after propagation in finger  1120 . Capacitance node  1111  is configured to emit and detect electric field signal  1131  in proximity to finger  1120 . Optical emitter  1110  and optical detector  1112  can include elements discussed herein for optical nodes  180 - 181  in  FIGS. 1-8 , although variations are possible. Capacitance node  1111  can include elements as discussed herein for capacitance nodes  182 - 189  in  FIGS. 1-8 , although variations are possible. In this example, capacitance node  1111  comprises a single plate capacitor, and uses at least finger  1120  as a dielectric for the single plate capacitor. In further examples, two-plate capacitors can be employed. 
     Optical emitter  1110 , optical detector  1112 , and capacitance node  1111  can be coupled together physically, and share a common structural support. The common structural element can allow for contact of sensor elements with finger  1120 . This common structural support can comprise a rigid carrier, such as clip-on finger probe, or can include a flexible carrier such as an adhesive pad that can be fit onto finger  1120 . Further examples of pads are discussed in  FIGS. 15-17 . 
     Although omitted for clarity in  FIG. 11A , optical emitter  1110  and optical detector  1112  can be coupled to further measurement equipment and systems, such as processing system  110  or optical system  121  in  FIGS. 1-8 , although variations are possible. Also, although omitted for clarity in  FIG. 11A , capacitance node  1111  can be coupled to further measurement equipment and systems, such as processing system  110  or capacitance system  130  in  FIGS. 1-8 , although variations are possible. Finger  1120  is shown as representing tissue under measurement by sensors  1110 - 1112 . Other tissue portions can instead be included, such as a forehead, chest, toe, limb, or other tissue of a patient, including combinations thereof. 
     Configuration  1105  shows a “sensor in motion” condition. In configuration  1105 , capacitance node  1111  measures a capacitance signal  1101 , as indicated in graph  1100 . This capacitance signal might include large variations, such as AC noise due to motion of finger  1120  or motion of components within finger  1120 . These large variations can be considered to be a noise component of the capacitance signal. The variation can be periodic and correlated to a movement of finger  1120 , such as when finger  1120  is being “waved” regularly or tapped against a surface. The variation can be quasi-random, such as when finger  1120  is being moved in a random manner. Motion of components within finger  1120  can also contribute to an AC noise signal, such as due to sloshing of blood in vessels of finger  1120 . Each peak or valley in signal  1101  of graph  1100  might correspond to an impulse of movement experienced by finger  1120 , such as sudden change in direction or motion. Motion artifacts can also be related to changes in sensor contact pressure with body tissue. The capacitive sensor of  FIG. 11A  also detects movements of the sensor relative to finger  1120 , such as due to changes in sensor contact pressure. 
     As shown in graph  1100 , capacitance signal  1101  has a variation level that exceeds a noise threshold range (TH 1 -TH 2 ). That is, the amplitude of the AC portion of signal capacitance  1101  is larger than a threshold, such as the difference between two capacitance levels TH 1  and TH 2 . As with  FIG. 9 , the capacitance level can be affected by finger  1120  due to finger  1120  being too far away from the capacitor portion of capacitance node  1111 , a sensor mis-application, or sensor contact pressure variation. It should be understood that operations in  FIG. 9  can be combined with operations in  FIG. 11A . 
     When the capacitance level measured by capacitance node  1111  rises above and falls below the threshold range TH 1 -TH 2 , or when the AC amplitude of capacitance signal  1101  exceeds a threshold amount, then an associated measurement system can determine that too much noise due to motion will also occur for any optical measurement performed by optical detector  1112 . Accordingly, the associated measurement system can disable measurement of optical signals in finger  1120  when the capacitance level exceeds the threshold range. Optical emitter  1110  can have power turned off when the capacitance level or noise exceeds the threshold range, such as indicated by “optical power off” signal  1103  in graph  1100 . The measurement system removes power from a laser diode or LED portion of optical emitter  1110  that emits optical signal  1130 . By turning off the power to optical emitter  1120 , the measurement system can save power and allow associated equipment to operate in a low power mode. Additionally, the measurement system can prevent measurement and determination of physiological parameters when the threshold range is exceeded, preventing erroneous or inaccurate measurement results when motion of finger  1120  can introduce undesirable noise or artifacts into measurement. Alternatively, when the threshold range is exceeded, an associated measurement system can alert or signal an operator of the measurement system than the sensor is experiencing noise or being moved too rapidly. An associated measurement system can alert or signal an operator that any associated optical measurements taken are potentially inaccurate. Although a threshold capacitance range is discussed, in other examples a magnitude of noise in signal  1101  or an amplitude of signal  1101  might exceed a threshold level. 
     When the capacitance level measured by capacitance node  1111  falls within the threshold range, then an associated measurement system can enable measurement of optical signals in finger  1120 . Signal  1102  of graph  1100  can indicate a low motion or low noise condition. Optical emitter  1110  can have power turned on when the capacitance level or noise falls within the threshold range, such as indicated by “optical power on” signal  1104  in graph  1100 . The measurement system supplies power to a laser diode or LED portion of optical emitter  1110  that emits optical signal  1130 . 
     The measurement system can perform measurement and determination of physiological parameters when the capacitance signal is within the threshold range, preventing erroneous or inaccurate measurement results when motion of finger  1120  can introduce undesirable noise or artifacts into measurement. An associated measurement system can alert or signal an operator that any associated optical measurements taken are accurate regarding motion-based noise. For example, when an optical measurement is experiencing a noisy condition, such as due to motion-based noise or other noise sources, a display on the measurement system can indicate to an operator that the optical measurement is currently noisy or exceeds a noise threshold for accurate measurement. In further examples, a capacitance signal, such as measured by capacitance node  1111 , can be used as a trigger for displaying physiological parameters on a monitor or display when a threshold condition is met. 
     When the capacitance level measured by capacitance node  1111  rises above and falls below the threshold range TH 1 -TH 2 , or when the AC amplitude of capacitance signal  1101  exceeds a threshold amount, then an associated measurement system can detect that the tissue under measurement is in motion. This capacitance-based motion detection can be employed to monitor movement of a patient, such as movement of a baby in a NICU environment, or to monitor movement of a comatose or bed-ridden patient. When the capacitance level measured by capacitance node  1111  falls within the threshold range, then an associated measurement system can detect that the tissue under measurement is not in motion. Multiple body parts can be monitored using multiple capacitive motion sensors to determine when various limbs, head, body, or other body parts are moved. Additionally, the patterns of capacitance signals  1101 - 1102  can be characterized. This characterization can include identifying patterns or ‘fingerprints’ in the motion-based noise. For example, a finger tapping can be characterized as a first pattern, while a hand waving can be characterized by a second pattern. The patterns can comprise patterns in amplitude, frequency, phase, or other characterizations. Measured patterns can be compared against a database of previously determined patterns to establish the particular movement or motion type. Motion of the patient or the tissue under measurement can be used to alert medical personnel or logged by the measurement system. The alerts or logs can indicate what pattern of motion is occurring or which limb is being moved by the patient. 
       FIG. 11B  is a diagram illustrating measurement of physiological parameters.  FIG. 11B  includes some similar elements at  FIG. 11A , such as elements  1110 - 1112 ,  1120 , and  1130 - 1131 , although variations are possible.  FIG. 11B  includes graph  1140  and two different optical sensor power configurations, namely an “optical power off” configuration  1147 , and an “optical power on” configuration  1148 . Each configuration corresponds to a different region on graph  1140 , as indicated by dotted lines below graph  1140 . Graph  1100  is a graph of a physiological parameter and includes an x-axis which represents “time” (T) and a y-axis which represents “capacitance” which can be measured in Farads, such as picofarads (pF). 
     Each configuration  1147 - 1148  includes optical emitter  1110 , optical detector  1112 , and capacitance node  1111  as applied to finger  1120  or other patient tissue. Optical emitter  1110  is configured to emit optical signal  1130  in finger  1120 . Optical detector  1112  is configured to detect optical signal  1130  after propagation in finger  1120 . Capacitance node  1111  is configured to emit and detect electric field signal  1131  in proximity to finger  1120 . Optical emitter  1110  and optical detector  1112  can include elements discussed herein for optical nodes  180 - 181  in  FIGS. 1-8 , although variations are possible. Capacitance node  1111  can include elements as discussed herein for capacitance nodes  182 - 189  in  FIGS. 1-8 , although variations are possible. In this example, capacitance node  1111  comprises a single plate capacitor, and uses at least finger  1120  as a dielectric for the single plate capacitor. In further examples, two-plate capacitors can be employed. Although omitted for clarity in  FIG. 11B , optical emitter  1110  and optical detector  1112  can be coupled to further measurement equipment and systems, such as processing system  110  or optical system  121  in  FIGS. 1-8 , although variations are possible. Also, although omitted for clarity in  FIG. 11B , capacitance node  1111  can be coupled to further measurement equipment and systems, such as processing system  110  or capacitance system  130  in  FIGS. 1-8 , although variations are possible. Finger  1120  is shown as representing tissue under measurement by sensors  1110 - 1112 . Other tissue portions can instead be included, such as a forehead, chest, toe, limb, or other tissue of a patient, including combinations thereof. 
     Capacitance node  1111  measures a capacitance signal  1141 , as indicated in graph  1140 . This capacitance signal might include variations due to a physiological parameter, such as a pulse, breathing rate, or heart rate, among others. Noise and motion artifacts can also be included in capacitance signal  1141 , such as discussed in  FIG. 11A , but these artifacts are omitted in this example for clarity. 
     Optical power is applied to optical emitter  1110  as-needed to ensure a targeted level of detail in measurement of the physiological parameter monitored by capacitance node  1111 . Capacitance node  1111  can monitor the physiological parameter, but the associated capacitance signal might have a lower accuracy or lower resolution than an optical system. Thus, capacitance node  1111  is employed to provide a less accurate measurement of a physiological parameter until a more accurate measurement is desired. In this example, if the patient is in a steady-state or stable condition at a certain time, then the optical power can be in an ‘off’ condition for optical emitter  1110  to reduce power consumption and reduce any associated data logging requirements. At some point, the physiological parameter that is monitored by capacitance node  1111  crosses a parameter threshold and a higher accuracy reading is desired. Responsive to the physiological parameter crossing the physiological threshold, a measurement system can enable measurement using an optical signal, namely optical signal  1130 . Once the physiological parameter has dropped below a physiological threshold, then the optical power can be removed again and optical measurement can be suspended. 
     In the example shown in  FIG. 11B , graph  1140  illustrates a rhythmic capacitance signal that indicates a rhythmic physiological parameter, such as a pulse rate, heart rate, breathing rate, and the like. The rhythm of physiological parameter can vary in frequency over time, such as due to health or stability changes in the patient being monitored. Notably, during time T 1  the capacitance signal increases in frequency and during time T 2  the capacitance signal decreases in frequency. A rising frequency threshold is reached at point  1142  which triggers optical measurement using optical signal  1130 . This optical measurement continues for the physiological parameter until another falling frequency threshold is reached at point  1143  which halts optical measurement using optical signal  1130 . Steady-state measurement using the capacitance signal can continue after the optical measurement has ceased. 
     It should be noted that the measurement or non-measurement using optical signal  1130  can be achieved in several ways. In a first example, such as shown in graph  1140 , power to optical emitter  1110  or optical detector  1112  can be selectively provided or removed to enable or disable the associated optical measurement elements. An associated measurement system can supply or remove power to a laser diode or LED portion of optical emitter  1110  that emits optical signal  1130 . In a second example, power can remain active for optical emitter  1110  or optical detector  1112 , but a measurement system can ignore measurements using the optical systems when not desired. 
     Multiple body parts can be monitored using multiple capacitive motion sensors to determine when more accurate measurement of physiological parameters for various limbs, head, body, or other body parts are desired. In some examples, the patterns of capacitance signal  1141  can be characterized. This characterization can include identifying patterns or ‘fingerprints’ in the physiological parameters monitored by capacitance node  1111 . The patterns can comprise patterns in amplitude, frequency, phase, or other characterizations that trigger a measurement signal to monitor the physiological parameter optically, or using other measurement apparatuses. Measured patterns can be compared against a database of previously determined patterns to establish the particular threshold or pattern. Enabling or disabling of the optical measurement can also alert medical personnel to a change in patient condition or can be logged by the measurement system. 
       FIG. 12  is a diagram illustrating measurement of physiological parameters.  FIG. 12  includes three graphs, namely optical signal graph  1200 , capacitance signal graph  1210 , and processed graph  1220 .  FIG. 12  illustrates one example process to identify a corrected plethysmograph of tissue of a patient. Data from optical and capacitance measurements are processed to identify a plethysmograph with motion and noise artifacts removed. The processing of the various data can be performed by any measurement system or processing system discussed herein, such as processing system  110  of  FIGS. 1-8 . Processing node  1230  is also included to illustrate an example processing system that processes the various signals in  FIG. 12  to identify a corrected plethysmograph. Processing node  1230  can include elements of any processing system as discussed herein. 
     Graph  1200  illustrates optical signal  1201  formed from data derived from an optical measurement of tissue. In typical examples, graph  1200  represents a PPG which comprises an optically measured plethysmograph, such as performed using optical system  121  of  FIG. 1B . However, optical signal  1201  includes various noise elements, such as due to motion or other noise sources. In graph  1200 , the plethysmograph signal dominates, with motion noise layered on top of the plethysmograph signal. 
     Graph  1210  illustrates capacitance signal  1211  formed from data derived from a capacitance-based measurement of tissue. In typical examples, graph  1210  represents a timewise changing capacitance signal, such as performed using capacitance system  130  of  FIGS. 1-8  based on changes in electric field signals or capacitance. However, capacitance signal  1211  includes various noise elements, such as due to motion or other noise sources, as well as signal elements of a plethysmograph signal. In graph  1210 , the noise artifacts dominate, with a plethysmograph signal layered on top of the motion noise signal. It should be understood that different variations of noise and plethysmograph signals can be measured than those shown in  FIG. 12 . 
     Graph  1220  illustrates a processed signal, taking into account signal elements of optical signal  1201  and capacitance signal  1211 . A processing system can adjust the optical plethysmograph using the capacitance signal to reduce a noise level in the optical plethysmograph. A processing system can correct the optical plethysmograph using the capacitance signal to reduce a noise level in the optical plethysmograph. In some examples, optical signal  1201  and capacitance signal  1211  are processed to determine common noise elements, such as motion artifacts, due to motion of the tissue under measurement. Common noise elements that are correlated between both signals can be removed from the optical plethysmograph signal by subtracting the capacitance signal, or a scaled or filtered version or portion of the capacitance signal, from signal  1201  to form a smooth PPG signal  1221 . This noise cancellation or noise rejection for optical signals can allow for determination of an adjusted PPG  1220 . This adjusted PPG can be used to determine physiological parameters to provide enhanced detection of deltaPOP, respiratory rate, respiratory effort, SpO 2 , heart rate, or other physiological parameters. 
     Processing node  1230  can be configured to process changes in the one or more electric field signals to determine at least one noise component in the changes in the one or more electric field signals caused by the motion of the tissue of the patient. Processing node  1230  can process the at least one noise component in the changes in the one or more electric field signals to reduce at least one noise component in the one or more optical signals to determine corrected physiological parameters of the patient, such as a corrected plethysmograph. Processing node  1230  can find correlated noise components to select components of the electric field signals to reduce noise in the optical signals. These noise components can be correlated in time, such as correlating transient events due to movement, or can be correlated in frequency, such as correlating certain noise frequencies of the capacitance signal to similar noise frequencies in the optical signal to cancel the noise frequencies out of the optical signal. Wavelet or frequency domain processing can be employed to identify common noise elements between the optical signals and the capacitance signals. These correlated, noisy portions of the capacitive signal can be used to identify and remove noise from the optical signal, such as by subtracting noise components from the optical signal, as discussed herein. 
     Instead of or in addition to removing noise from the optical signal, processing node  1230  can assign a weighting or quality indicator of the optical signals when the capacitance signals indicated large transient noise events. For examples, processing node  121  can process the one or more optical signals with a first, lower, processing weight when the noise level of the capacitance signal exceeds a noise threshold and process the one or more optical signals with a second, higher, processing weight when the noise level of the capacitance signal does not exceed the noise threshold. Processing node  1230  can identify noisy periods using the capacitance signal and avoid optical measurements during noisy periods, or indicate poor quality of measurements by changing a light or indicator based on a confidence level of the optical measurements. In other examples, where physiological parameters are being logged, a confidence indication can accompany any optical measurements to indicate noisy periods so a user or processing system can give appropriate weighting to optical measurement data. 
     The capacitance signal can be compared to the optical signal to determine physiological events. These events can include sudden changes or movements of the patient which can indicate a seizure or other transient movement event. Another event includes blood loss by the patient, which can be identified when a capacitance signal remains roughly unchanged but an optical signal deviates greatly, which may occur if a patient is bleeding profusely. 
     Further signal processing can be performed utilizing both the capacitance based measurements and the optical based measurements. For example, processing node  1230  can determine severity of motion artifacts in a PPG or power of artifacts at certain frequencies of a PPG to determine at what frequencies of a PPG waveform might be corrupt or poor quality. Processing node  1230  can determine impulse response from the capacitance signal and apply this impulse response to a PPG waveform. The impulse response can be employed in a finite impulse response (FIR) filter, where the capacitance signal is input to the FIR, and the FIR output is subtracted from a PPG. Filter coefficients can be adapted over time based on measurements of the capacitance signals. 
     A fundamental frequency of a PPG can be determined by processing system  1230 , and a spectrum of a noise signal can be derived from capacitance signal with a high pass or bandpass filter. The PPG waveform can be filtered to remove noise that is at one or more fundamental frequencies of the noise waveform. In some examples, the fundamental frequencies of the PPG waveform correlate to a heart rate, and noise can be removed from other frequencies to smooth out a signal that contains the remaining fundamental frequencies. A comb filter can be employed by processing system  1230  to extract only heart rate harmonics of a heart rate from a PPG, and eliminate an Nth harmonic of the heart rate signal if the Nth harmonic is found to be excessively noisy as determined by the capacitance signal. The time epoch to calculate the spectrum of noise from the capacitance signal may vary in duration depending on what frequency of noise is to be eliminated. Other methods such as correlations, independent component analysis (ICA), pleth morphology, FFT, or wavelet analysis may be used for noise mitigation or subtraction. Correction of measurements for DC shifts can also be improved by comparing the capacitance signal to a measured physiological signal. 
     Furthermore, processing node  1230  can process the capacitance signal to provide an input to an ensemble averaging algorithm. This input, which may be referred to as a lock signal, can be employed in ensemble averaging as a clock signal to lock a PPG signal to the clock to improve the ensemble averaging and establish a consistent periodic averaging. In ensemble averaging, the capacitance signal can also be employed to indicate a confidence level for each individual averaging of the ensemble based on a measured noise level. The capacitance signal can be used to determine an instantaneous noise level which can indicate a confidence level of the optical measurement. When the optical measurements are used in an ensemble averaging process, when a high instantaneous noise is monitored for a portion of the optical signal, then that portion of the optical signal can receive a lower weighting in the ensemble average. Less noisy portions of the optical signal can be given a higher weighting in the ensemble average. The weighting can be correlated to a noise level measured by the capacitance signal. In another example, an ensemble average weight can be modified based on likelihood of noise in a physiological parameter, such a PPG. The likelihood of noise can be established by monitoring a capacitance signal and identifying when noise of the capacitance signal indicates a concurrently measured physiological parameter is subject to similar noise. For example, an optical signal used to measure a PPG can have an ensemble average weight modified when a concurrent capacitance measurement indicates noise. 
       FIG. 13  is a system diagram illustrating physiological measurement system  1300 . System  1300  includes measurement system  1310  and pressure system  1311 , with capacitive sensors  1320 - 1322 , and pressure cuff  1330  applied to tissue  1340 . Measurement system  1310  and capacitive sensors  1320 - 1322  communicate over link  1350 , which can include one or more links for each capacitive sensor. 
     Pressure system  1311  supplies air pressure to pressure cuff  1330  over pneumatic link  1351 . Pressure system  1311  and pressure cuff  1330  can be included in sphygmomanometer equipment. Tissue  1340  comprises tissue of a patient under measurement of physiological parameters. Tissue  1340  can comprise an arm, leg, limb, finger, or other tissue element. Measurement system  1310  can include elements described herein for measurement equipment  101  in  FIGS. 1-8 , although variations are possible. 
     Each of capacitive sensors  1320 - 1322  are distributed across pressure cuff  1330  to lie in proximity with tissue  1340  and emit associated electric fields into the surrounding environment of each sensor. When pressure cuff  1330  is placed over tissue  1340  for measurement of blood pressure, sensors  1320 - 1322  are also placed in proximity to tissue  1340 . 
     Capacitance of each of capacitive sensors  1320 - 1322  can be employed to detect correct orientation, application, or alignment of pressure cuff  1330  on tissue  1340 . For example, pressure cuff  1330  might be applied too loosely, too tightly, be of an incorrect size for the patient, or have poor contact with tissue  1340 . As the capacitance measured by each of capacitive sensors  1320 - 1322  is monitored, these configurations of pressure cuff  1330  can be monitored and alerted. Pressure modulations due to blood pumping in tissue  1340  can induce capacitance modulations in capacitive sensors  1320 - 1322 . These capacitance modulations can be monitored to determine proper magnitudes or amplitudes to establish if correct pressure is being applied by pressure cuff  1330  to tissue  1340 . 
     In other examples, capacitive sensors  1320 - 1322  can detect where pressure modulations are greatest, and alert an operator of measurement system  1310  to reposition cuff  1330 . Greatest modulations are typically preferred in the center of cuff  1330 , and capacitive sensors  1320 - 1322  can be employed by an operator to ensure correct positioning and pressure of cuff  1330  on tissue  1340 . 
     Further examples include measurement system  1310  producing a warning or alarm if an incorrect cuff size or cuff position is used based on cuff pressure measured by capacitive sensors  1320 - 1322 . Measurement system  1310  can illuminate one or more indicator lights, such as LEDs, on cuff  1330  to indicate where the largest pressure modulation or oscillation is presently found, based on pressure measured by capacitive sensors  1320 - 1322 , and an operator can reposition cuff  1330  to align the largest oscillation to the center of cuff  1330  using the indicator lights as a guide. Previous measurements of capacitive sensors  1320 - 1322  can be stored for later use when re-applying cuff  1330  to help guide cuff inflation or positioning. 
     Measurement of blood pressure of tissue  1340  can also be enhanced by capacitive sensors  1320 - 1322 . Specifically, a capacitance-based pressure measurement can be determined by each of capacitive sensors  1320 - 1322 . This capacitance-based pressure measurement can be used to adjust pressure of a multi-chamber cuff to adjust pressure of each chamber based on a desired pressure and the capacitance-based measurement of current pressure. Cuff size can be variable based on pressure, and capacitive sensors  1320 - 1322  can indicate when a desired pressure or size is achieved. Correction of various measurement parameters can also occur based on capacitance-based pressure measurement. For example, regions of cuff  1330  with higher pressure can have a first level of measurement correction included, while regions of cuff  1330  with lower pressure can have a second level of measurement correction included. The correction can include scaling values measured for systolic, diastolic, or mean arterial pressure based on pressure measurements found at each of capacitive sensors  1320 - 1322 . In addition, pressure measurements of cuff  1330  performed by capacitive sensors  1320 - 1322  can indicate confidence values for the blood pressure measurements, leading to a quality score that scales based on a desired pressure of cuff  1330  and an actual pressure of cuff  1330 . Sensors may also aid in smarter cuff inflation and deflation profiles, leading to faster or more accurate measurements with less patient discomfort. 
     Capacitive sensors  1320 - 1322  can comprise thin metallic plates, metallic grids, metallic patches, or other capacitor plate materials. Although capacitive sensors  1320 - 1322  are included in  FIG. 13 , other sensor types could instead be employed to detect variable pressure of cuff  1330  on tissue  1340 , such as impedance sensors, resistance sensors, inductance sensors, or pressure sensors, including combinations thereof. 
       FIG. 14  is a system diagram illustrating physiological measurement system  1400 . System  1400  includes measurement system  1410 , with optical emitter  1420 , optical detector  1421 , and inductive coils  1430 - 1431  applied to tissue  1440 . Measurement system  1410  and inductive coil  1430  communicate over link  1450 . Measurement system  1410  and inductive coil  1431  communicate over link  1451 . Measurement system  1410  and optical emitter  1420  communicate over link  1452 . Measurement system  1410  and optical detector communicate over link  1453 . 
     Measurement system  1410  can include elements described for measurement equipment  101  in  FIGS. 1-8 , although variations are possible. Likewise, optical emitter and optical detector  1420 - 1421  can include elements described for optical emitter and detector  180 - 181  in  FIG. 1B , although variations are possible. Links  1450 - 1451  are electric links for carrying a time-based electrical signal to ones of coils  1430 - 1431 . Links  1452 - 1453  comprise elements described herein for links  160 - 161 , although variations are possible. 
     An electrical schematic representation of inductive coils  1430 - 1431  and tissue  1440  is shown at the bottom of  FIG. 14 . In operation, at least one of coils  1430 - 1431  are powered by a modulated electrical signal to create a transformer, with tissue  1440  acting as a pseudo-core to the transformer. In one example, coil  1430  acts as a transmitter and coil  1431  acts as a receiver, although the opposite configuration is possible. When operating, magnetic fields will be induced in tissue  1440  by the coils. Tissue  1440  will have a variable magnetic permeability, such as due to changes in blood components, pulsing of arterial or venous blood, among other variations. As the magnetic permeability of tissue  1440  changes with time, parameters such as pulsing of blood, hydration, blood oxygenation, hemoglobin movement, and other physiological measurements can be extracted from the changes in magnetic permeability. 
     In a first example measurement, variations in magnetic permeability can be detected by measurement system  1410  and these variations can represent a pulsatile waveform. The pulsatile waveform can be indicative of a pulse in tissue  1440 . Measurement system  1410  can use this pulsatile waveform to determine physiological parameters, or to supplement and enhance other physiological measurements. For example, optical emitter  1420  emits optical signal  1450  into tissue  1440  and optical detector  1421  detects optical signal  1450  after propagation in tissue  1440 . The pulsatile waveform detected using inductive coils  1430 - 1431  can be used to cross-check a PPG monitored by optical sensors  1420 - 1421 . This cross-check can verify that a PPG signal is valid, or reduce the number of false alarms associated with a PPG monitoring system due to optical sensors improperly applied to tissue  1440  or entirely off of tissue  1440 . To verify that a PPG signal is valid, a pulsatile waveform detected using inductive coils  1430 - 1431  can be correlated or aligned to a PPG measured by optical sensors  1420 - 1421 . If a pulsatile waveform detected using inductive coils  1430 - 1431  correlates to a PPG measured by optical sensors  1420 - 1421 , then the PPG can be considered valid. However, if the PPG measured by optical sensors  1420 - 1421  does not correlate or align to that measured using inductive coils  1430 - 1431 , then the PPG can be considered suspect or invalid, and an operator can be notified of a possible discrepancy or error in the measurements. 
       FIG. 15  is a diagram illustrating measurement pads for measurement of physiological parameters.  FIG. 15  includes  5  pad configurations, namely pads  1510 ,  1520 ,  1530 ,  1540 , and  1550 . Each of pads  1510 ,  1520 ,  1530 ,  1540 , and  1550  can comprise any bio compatible material for interfacing with tissue of a patient. Adhesive can be included in any of pads  1510 ,  1520 ,  1530 ,  1540 , and  1550  to attach to tissue of a patient, or other mechanical attachment elements can be included, such as clips, springs, bands, and the like. Each of pads  1510 ,  1520 ,  1530 ,  1540 , and  1550  can be used for the various capacitive node or optical node sensor elements herein, and other variations are possible. Each measurement pad may be attached to or integrally incorporated into a sensor that is configured to attach to or interface with the tissue to be measured. 
     As shown in system  1560 , an example finger as tissue  1562  has an exemplary pad  1561  applied thereto. Link  1563  supplies any signaling and shielding appropriate for measurement or sensor elements of pad  1561 . Pad  1561  can be applied to one side or more than one side of tissue  1562 , such as a top and bottom of a finger as shown. Capacitor elements can be included in pad  1561  to lie in proximity to one or both sides of tissue  1562 . 
     Turning now to each of pads  1510 ,  1520 ,  1530 ,  1540 , and  1550  each pad is intended to be folded over an extremity, such as a finger or toe, as indicated by the dotted fold line on each pad. Alternatively, each pad can be applied generally flat to tissue such as a forehead, chest, leg, arm, and the like. Although not shown for clarity in  FIG. 15 , each capacitive element can include an associated electrical link for connection with a measurement system, along with any associated shielding and ground pads. Likewise, optical measurement elements can be included in each pad, with each pad indicating two optical apertures. A first of the optical apertures can be employed for one or more optical emitters, while a second of the optical apertures can be employed for one or more optical detectors. When multiple emitters are included in a pad, more than one optical wavelength can be employed. In some examples, both an optical emitter and detector are employed in each optical aperture, with each optical emitter-detector pair dedicated to a different wavelength of light. 
     Pad  1510  includes two conductive plates  1513 - 1514  as capacitor plates. Capacitor plates  1513 - 1514  are positioned on either side of optical aperture  1512 . Capacitor plates  1513 - 1514  can be flexible and metallic for conforming to tissue once applied with pad  1510 . Capacitor plates  1513 - 1514  can comprise thin metallic sheets, metallic plates, or metallic grids, along with other configurations. Each of capacitor plates  1513 - 1514  can be employed as a single plate capacitor, or both plates can be combined into a two-plate capacitor. Pad  1510 , when wrapped around a finger or toe, or when placed flat onto tissue, can provide two single-plate capacitors on a single side of tissue, or one two-plate capacitor on a single side of tissue. 
     Pad  1520  includes conductive plate  1523  as a capacitor plate. Capacitor plate  1523  is positioned around both of optical apertures  1521 - 1522 . Capacitor plate  1523  can be flexible and metallic for conforming to tissue once applied with pad  1520 . Capacitor plate  1523  can comprise a thin metallic sheet, metallic plate, or metallic grid, along with other configurations. Capacitor plate  1523  can be employed as a single plate capacitor. Pad  1520 , when wrapped around a finger or toe, can provide a single-plate capacitor that extends around two sides of tissue. Alternatively, when placed flat onto tissue, pad  1520  can provide a single-plate capacitor that is positioned on one side of tissue. 
     Pad  1530  includes capacitor plate  1533 . Capacitor plate  1533  is positioned around both of optical apertures  1531 - 1532  in a serpentine fashion. Capacitor plate  1533  can be flexible and metallic for conforming to tissue once applied with pad  1530 . Capacitor plate  1533  can comprise a thin metallic wire or narrow flat sheet (such as a circuit trace), along with other configurations. Capacitor plate  1533  can be employed as a single plate capacitor. Pad  1530 , when wrapped around a finger or toe, can provide a single-plate capacitor that extends around two sides of tissue. Alternatively, when placed flat onto tissue, pad  1530  can provide a single-plate capacitor that is positioned on one side of tissue. 
     Pad  1540  includes four conductive plates  1543 - 1546  as capacitor plates. Capacitor plates  1543 - 1544  are positioned on either side of optical aperture  1542 . Capacitor plates  1545 - 1546  are positioned on either side of optical aperture  1541 . Capacitor plates  1543 - 1546  can be flexible and metallic for conforming to tissue once applied with pad  1540 . Capacitor plates  1543 - 1546  can comprise thin metallic sheets, metallic plates, or metallic grids, along with other configurations. Each of capacitor plates  1543 - 1546  can be employed as a single plate capacitor, or in pairs to form two-plate capacitors around each associated optical aperture. Pad  1540 , when wrapped around a finger or toe, can provide four single-plate capacitors on two sides of tissue, or two dual-plate capacitors on two sides of tissue. Alternatively, when placed flat onto tissue, pad  1540  can provide various capacitor plate configurations positioned on one side of tissue. 
     Pad  1550  includes two conductive wires  1553 - 1554  as capacitor plates. Capacitor plates  1553 - 1554  are positioned between both of optical apertures  1551 - 1552  in a spiral fashion. Capacitor plates  1553 - 1554  can be flexible and metallic for conforming to tissue once applied with pad  1550 . Capacitor plates  1553 - 1554  can comprise thin metallic wires or narrow flat sheets (such as a circuit traces), along with other configurations. Each of capacitor plates  1553 - 1554  can be employed as a single plate capacitor, or both plates can be combined into a two-plate capacitor. Pad  1550 , when wrapped around a finger or toe, can provide a single-plate capacitor that extends around two sides of tissue. Alternatively, when placed flat onto tissue, pad  1550  can provide a single-plate capacitor that is positioned on one side of tissue. 
       FIG. 16  is a system diagram illustrating physiological measurement system  1600 . System  1600  illustrates usage of an electromagnetic shield or faraday shield of a pulse oximetry sensor as a capacitive plate. System  1600  includes pulse oximetry probe  1610 , optical emitter  1620 , optical detector  1621 , faraday shield  1622 , measurement link  1650 , and shield interface  1660 , applied to tissue  1640 . Measurement link  1650  is a coaxial link that further includes outer shield  1652 , inner shield  1651 , detector link  1653 , and emitter link  1654 . Although one link per emitter/detector is shown in  FIG. 16 , more the one link can instead be employed. Further measurement equipment, such as processing systems, optical systems, and capacitance systems, are omitted for clarity from  FIG. 16 . 
     Pulse oximetry probe  1610  comprises a structural element to position sensing portions of measurement system  1600  onto tissue  1640 , such as clips, pads, bands, springs, and the like. In  FIG. 16 , an example pad  1611  is used as the structural element. Pulse oximetry probe  1610  includes optical emitter  1620  and optical detector  1621  for emitting optical signals  1630  into tissue  1640  for measurement of physiological parameters, such as pulse and SpO 2  of blood. In some examples, pulse oximetry probe  1610  comprises a modified MAX-A pulse oximetry probe available from Covidien LP (Boulder, Colo.). 
     To provide a level of environmental and electromagnetic shielding for at least emitter  1620  and detector  1621 , faraday shield  1622  is provided to surround at least emitter  1620  and detector  1621 . Faraday shield  1622  comprises a metal or metallic surround or enclosure with associated optical apertures  1623  to allow emission or detection of optical signal  1630 . Faraday shield  1622  can comprise a braided shield or wire mesh configuration. 
     In this example, inner shield  1651  is electrically connected to faraday shield  1622 . In some examples, inner shield  1651  is extended to create faraday shield  1622  around at least emitter  1620  and detector  1621 . Inner shield  1651  also includes and surrounds emitter link  1654  and detector link  1653 . Outer shield  1652  surrounds inner shield  1651 , along with any additional wires or links that might be included in link  1650 . In this example, outer shield  1652  is not electrically connected to inner shield  1651  and is isolated by dielectric  1655 , such as a sheathing or coaxial insulator. Furthermore, outer shield  1652  is not electrically connected to faraday shield  1622  in this example. 
     In operation, optical signals  1630  are emitted by optical emitter  1620  into tissue  1640  after probe  1610  is applied to tissue  1640 . Optical detector  1621  detects optical signal  1630  after propagation through tissue  1640 . Optical emitter  1620  and optical detector  1621  communicate with measurement equipment, not shown for clarity, over associated links  1653 - 1654 . Optical emitter  1620  includes LED or laser diode equipment and receives an electrical signal over link  1654 . Likewise, optical detector  1621  includes optical detection elements, such as photodiodes, photodetectors, or other optical sensors, and transfers electrical signals representative of the optical signals detected to measurement equipment over link  1653 . Inner shield  1651  is provided as an electromagnetic interference (EMI) shield for not only optical emitter  1620  and optical detector  1621 , but for also links  1653 - 1654 . In some examples, faraday shield  1622  does not surround optical emitter  1620  and instead surrounds optical detector  1621  and associated links. 
     Faraday shield  1622  can be employed as a single plate capacitor when placed in proximity to tissue  1640 . An ambient electric field can be detected by faraday shield  1622  which can be indicative of proximity to tissue  1640 . When an ambient signal is used, then a calibration routine can be performed which establishes measurement thresholds for using faraday shield  1622  as a capacitive plate. These thresholds can indicate when probe  1610  is placed on tissue  1640  or off of tissue  1640 . The thresholds can be determined by having an operator take a first measurement when probe  1610  is not on tissue  1640  and a second measurement once probe  1610  is properly placed on tissue  1640 . In this manner, measurement equipment can detect when probe  1610  has fallen off or been removed from tissue  1640  and alert monitoring personnel accordingly. 
     Alternatively, an electrical signal  1631  can be driven onto faraday shield  1622  by inner shield  1651  to emit an electrical signal into tissue  1640  as discussed herein for single plate capacitors. When electrical signals are driven onto faraday shield  1622  by measurement equipment, then processes as discussed herein can be employed to detect when probe  1610  is placed on or is off of tissue  1640 . As with the ambient measurement above, appropriate thresholds for probe on/off conditions can be determined. Measurement equipment can then detect when probe  1610  has fallen off or been removed from tissue  1640  and alert monitoring personnel accordingly. 
     Using faraday shield  1622  as a capacitor plate can introduce noise onto the EMI shielding for at least optical detector  1621 . To mitigate this noise, shield interface  1660  can be employed. Shield interface  1660  can include one or more analog switches, digital-to-analog converters, or other high-impedance interface circuitry to isolate inner shield  1651  from measurement equipment attempting to detect sensor on/off conditions. When a measurement for sensor on/off is desired, then measurement equipment can be electrically connected by enabling an analog switch to drive a signal onto inner shield  1651  using a lower impedance connection. Likewise, a normally off high-impedance output of a digital-to-analog converter can be enabled to drive a signal onto inner shield  1651 . The measurement signal driven onto inner shield  1651  can be driven by a general purpose  110  pin of a microprocessor as a pulse-width modulated signal, with or without analog filtering. 
     For example, if an optical measurement using optical emitter  1620  and optical detector  1621  indicates an error condition for the patient undergoing physiological measurement, then before an alert is generated based on the optical measurement, the capacitive measurement using faraday shield  1622  is performed. Shield interface  1660  can drive a measurement signal onto inner shield  1651  which drives electrical field signal  1631  from faraday shield  1622 . Based on monitoring this measurement signal a determination can be made whether or not probe  1610  is actually on tissue  1640  or has been removed or fallen off. Alerts for medical personnel or medical logs can be adjusted based on whether probe  1610  is on tissue  1640  or not. 
     Further methods can be employed to mitigate unwanted interference caused by driving an active signal onto faraday shield  1622  and inner shield  1651 . In a first example, inner shield  1651  is driven at more than one modulation frequency at different times, so that resistance and capacitance can be derived from different measurements. In another example, the driven signal is frequency hopped or employs spread spectrum techniques to minimize emitting electromagnetic interference by faraday shield  1622 . In yet another example, both inner shield  1651  and outer shield  1652  are driven actively when a capacitance measurement is desired, but each of inner shield  1651  and outer shield  1652  are driven with differential/complementary signals to minimize interference. In further examples, a dummy wire is present in link  1650 , or outer shield  1652  is employed as a reference wire, to monitor and cancel environmental interference, such as EMI from lighting, measurement equipment, patients touching conductive objects, ambient EMI, or other sources of EMI. A dummy wire is discussed further in  FIG. 17 . Regardless of the method, measurement times are typically kept short to minimize interference and to minimize displacement of optical measurements. 
       FIG. 17  is a system diagram illustrating physiological measurement system  1700 . System  1700  illustrates usage of spiral capacitance plate  1720  along with a differential measurement link arrangement of link  1750 . System  1700  includes spiral capacitance plate  1720 , optical emitter/detector  1722 , measurement link  1750 , and measurement system  1760 . Spiral capacitance plate  1720  can be applied to tissue, such as a finger or forehead. Measurement link  1750  is a coaxial link with an internal twisted pair of wires. Measurement link  1750  includes outer shield  1752 , inner shield  1751 , capacitance link  1753 , and reference link  1754 . Measurement link  1750  can include further capacitance links to interface with plate  1720 . Measurement link  1750  can also include optical links or other electrical links used for optical emitter/detector  1722 , but these are omitted for clarity in  FIG. 17 . In some examples, measurement system  1760  comprises a capacitance to digital converter to convert signals detected over link  1753  into a digital format. Further measurement equipment, such as processing systems, optical systems, and capacitance systems, are also omitted for clarity from  FIG. 17 . 
     Spiral capacitance plate  1720  comprises structural elements to position sensing portions of measurement system  1700  onto tissue  1740 , such as clips, pads, bands, springs, and the like. Spiral capacitance plate  1720  comprises one or more wires, such as circuit traces, that comprise individual capacitor plates. The spiral capacitance wires of plate  1720  allow for larger sensor surface area to sense various physiological parameters in tissue. Plate  1720  can include a single spiral wire or can include multiple, such as two shown in  FIG. 17 . Plate  1720  includes a central aperture through which optical emitter/detector  1722  can emit/detect optical signals  1730  into tissue for measurement of physiological parameters, such as pulse and SpO 2  of blood. 
     In this example, inner shield  1751  creates a faraday shield around at least links  1753 - 1754 . Inner shield  1751  can be electrically connected to an AC signal source. Outer shield  1752  surrounds inner shield  1751 , along with any additional wires or links that might be included in link  1750 . Also in this example, outer shield  1752  creates a faraday shield around at least links  1753 - 1754  and inner shield  1751 . Outer shield  1752  is not electrically connected to inner shield  1751  and is isolated by dielectric  1758 , which can include sheathing  1755  or coaxial insulator material. Furthermore, outer shield  1752  can be optionally connected to ground or a reference potential. 
     Measurement link  1750  comprises a two shield system ( 1751 - 1752 ) with a twisted pair wire inside. In the twisted pair, one wire ( 1753 ) acts as a conductor for a single plate capacitor  1720  and the second wire ( 1754 ) acts as a ‘dummy’ reference conductor wire. When a person grabs both wires simultaneously, both wires indicate a change in capacitance of a similar magnitude, which can be subtracted out as a common mode signal, leaving behind the desired signal measured using capacitance link  1753 . When only capacitance link  1753  or capacitance plate  1720  are contacted or proximate to tissue, then the common mode signal will be smaller and less of a factor in measurement processing. Higher twists per inch in the twisted pair can aid to keep the wires electromagnetically coupled. An electrically driven shield around the twisted pair, such as inner shield  1751 , acts as a shield but also can minimize electromagnetic coupling onto the twisted pair, in part because energy will not typically flow between two wires of the same potential. An optional second shield (outer shield  1752 ) around inner shield  1751  should be grounded to act as a mitigation of radiated emissions from inner shield  1751  when inner shield  1751  is electrically driven, such as by an AC signal. In another example, both inner shield  1751  and outer shield  1752  are driven actively when a capacitance measurement is desired, but each of inner shield  1751  and outer shield  1752  are driven with differential/complementary signals to minimize interference. In further examples, reference link  1754  can be used to monitor and cancel environmental interference, such as EMI from lighting, measurement equipment, patients touching conductive objects, ambient EMI, or other sources of EMI. 
     In operation, optical signals are emitted by optical emitter/detector  1722  into tissue. Optical emitter/detector  1722  optical signals after propagation through tissue. Optical emitter/detector  1722  communicate with measurement equipment, not shown for clarity, over associated links. Optical emitter/detector  1722  include LED or laser diode equipment and optical detection elements, such as photodiodes, photodetectors, or other optical sensors, and transfers electrical signals representative of the optical signals detected to measurement equipment. Plate  1720  can be employed as a single plate capacitor or two plate capacitor when placed in proximity to tissue. An electrical signal can be driven onto plate  1720  by capacitance link  1753  to emit an electrical signal into tissue as discussed herein for single plate capacitors or two plate capacitors. Measurement system  1760  can monitor changes in an electric field or changes in a capacitance signal associated with the electric field emitted by plate  1720  to determine physiological parameters. Measurement system  1760  can use the capacitance signals to modify or correct measurements performed using the optical elements, among other operations as discussed herein. 
       FIG. 18  is a block diagram illustrating measurement system  1800 , as an example of elements of measurement equipment  101  in  FIGS. 1-8 , measurement system  910  in  FIG. 9 , measurement system  1310  in  FIG. 13 , or measurement system  1410  in  FIG. 14 , although these can use other configurations. Measurement system  1800  includes optical system interface  1810 , processing system  1820 , user interface  1840 , capacitance system interface  1850 , and optionally optical system  1870  and capacitance system  1880 . Optical system interface  1810 , processing system  1820 , user interface  1840 , and capacitance system interface  1850  are shown to communicate over a common bus  1860  for illustrative purposes. It should be understood that discrete links can be employed, such as communication links or other circuitry. Measurement system  1800  may be distributed or consolidated among equipment or circuitry that together forms the elements of measurement system  1800 . 
     Optical system interface  1810  comprises a communication interface for communicating with other circuitry and equipment, such as with optical system  1870 . Optical system interface  1810  can include transceiver equipment exchanging communications over one or more of the associated links  1861 - 1862 . It should be understood that optical system interface  1810  can include multiple interfaces, pins, transceivers, or other elements for communicating with multiple external devices. Optical system interface  1810  also receives command and control information and instructions from processing system  1820  or user interface  1840  for controlling the operations of optical system interface  1810 . Links  1861 - 1862  can each use various protocols or communication formats as described herein for links  180 - 181 , including combinations, variations, or improvements thereof. 
     Processing system  1820  includes storage system  1821 . Processing system  1820  retrieves and executes software  1830  from storage system  1821 . In some examples, processing system  1820  is located within the same equipment in which optical system interface  1810 , user interface  1840 , or capacitance system interface  1850  are located. In further examples, processing system  1820  comprises specialized circuitry, and software  1830  or storage system  1821  can be included in the specialized circuitry to operate processing system  1820  as described herein. Storage system  1821  can include a non-transitory computer-readable medium such as a disk, tape, integrated circuit, server, flash memory, or some other memory device, and also may be distributed among multiple memory devices. 
     Software  1830  may include an operating system, logs, utilities, drivers, networking software, tables, databases, data structures, and other software typically loaded onto a computer system. Software  1830  can contain application programs, server software, firmware, processing algorithms, or some other form of computer-readable processing instructions. When executed by processing system  1820 , software  1830  directs processing system  1820  to operate as described herein, such as instruct optical or capacitance systems to generate optical or electrical signals for measurement of physiological parameters of patients, receive signals representative of optical or capacitance measurements of patients, and process at least the received signals to determine physiological parameters of patients, among other operations. 
     In this example, software  1830  includes generation module  1831 , detection module  1832 , and signal processing module  1833 . It should be understood that a different configuration can be employed, and individual modules of software  1830  can be included in different equipment in measurement system  1800 . Generation module  1831  determines parameters for optical or capacitance signals, such as modulation parameters, signal strengths, amplitude parameters, voltage parameters, on/off conditions, or other parameters used in controlling the operation of optical systems and capacitance systems over ones of links  1861 - 1864 . Generation module  1831  directs optical system  1870  and capacitance system  1880  to perform physiological measurements, and can selectively apply or remove power from various detection sensors, emitters, capacitors, and other sensor elements. Detection module  1832  receives characteristics of optical and capacitance signals as detected by external circuitry. Signal processing module  1833  processes the received characteristics of optical and capacitance signals to determine physiological parameters, among other operations. 
     User interface  1840  includes equipment and circuitry to communicate information to a user of measurement system  1800 , such as alerts, measurement results, and measurement status. Examples of the equipment to communicate information to the user can include displays, indicator lights, lamps, light-emitting diodes, haptic feedback devices, audible signal transducers, speakers, buzzers, alarms, vibration devices, or other indicator equipment, including combinations thereof. The information can include blood parameter information, waveforms, summarized blood parameter information, graphs, charts, processing status, or other information. User interface  1840  also includes equipment and circuitry for receiving user input and control, such as for beginning, halting, or changing a measurement process or a calibration process. Examples of the equipment and circuitry for receiving user input and control include push buttons, touch screens, selection knobs, dials, switches, actuators, keys, keyboards, pointer devices, microphones, transducers, potentiometers, non-contact sensing circuitry, or other human-interface equipment. 
     Capacitance system interface  1850  comprises a communication interface for communicating with other circuitry and equipment, such as with capacitance system  1880 . Capacitance system interface  1850  can include transceiver equipment exchanging communications over one or more of the associated links  1863 - 1864 . It should be understood that capacitance system interface  1850  can include multiple interfaces, pins, transceivers, or other elements for communicating with multiple external devices. Capacitance system interface  1850  also receives command and control information and instructions from processing system  1850  or user interface  1840  for controlling the operations of capacitance system interface  1850 . Links  1863 - 1864  can each use various protocols or communication formats as described herein for links  170 - 171 , including combinations, variations, or improvements thereof. 
     Bus  1860  comprises a physical, logical, or virtual communication link, capable of communicating data, control signals, and communications, along with other information. In some examples, bus  1860  is encapsulated within the elements of measurement system  1800 , and may be a software or logical link. In other examples, bus  1860  uses various communication media, such as air, space, metal, optical fiber, or some other signal propagation path, including combinations thereof. Bus  1860  can be a direct link or might include various equipment, intermediate components, systems, and networks. 
     Optical system  1870  can include electrical to optical conversion circuitry and equipment, optical modulation equipment, and optical waveguide interface equipment. Optical system  1870  can include direct digital synthesis (DDS) components, CD/DVD laser driver components, function generators, oscillators, or other signal generation components, filters, delay elements, signal conditioning components, such as passive signal conditioning devices, attenuators, filters, and directional couplers, active signal conditioning devices, amplifiers, or frequency converters, including combinations thereof. Optical system  1870  can also include switching, multiplexing, or buffering circuitry, such as solid-state switches, RF switches, diodes, or other solid state devices. Optical system  1870  also can include laser elements such as a laser diode, solid-state laser, or other laser device, along with associated driving circuitry. Optical couplers, cabling, or attachments can be included to optically mate to links  1871 - 1872 . Optical system  1870  can also include light detection equipment, optical to electrical conversion circuitry, photon density wave characteristic detection equipment, and analog-to-digital conversion equipment. Optical system  1870  can include one or more photodiodes, phototransistors, avalanche photodiodes (APD), or other optoelectronic sensors, along with associated receiver circuitry such as amplifiers or filters. Optical system  1870  can also include phase and amplitude detection circuitry and processing elements. Links  1871 - 1872  can each use various signal formats as described herein for links  161 - 162 , including combinations, variations, or improvements thereof. 
     Capacitance system  1880  comprises one or more electrical interfaces for applying one or more electric field signals to tissue of a patient over electrical links  1881 - 1882 . In some examples, capacitance system  1880  drives one or more capacitor plates that are placed in proximity to tissue of a patient. Capacitance system  1880  can include transceivers, amplifiers, modulators, capacitance monitoring systems and circuitry, impedance matching circuitry, human-interface circuitry, electrostatic discharge circuitry, and electromagnetic shield interface circuitry, including combinations thereof. It should be understood that capacitance system interface  1850  can include multiple interfaces, pins, transceivers, or other elements for communicating with multiple external devices. Capacitance system interface  1880  also can receive command and control information and instructions from capacitance system interface  1850  for controlling the operations of capacitance system  1880 . Links  1881 - 1882  can each use various signal formats as described herein for links  162 - 168 , including combinations, variations, or improvements thereof. 
       FIG. 19  is a diagram illustrating various capacitance configurations.  FIG. 19  includes four different capacitance measurement configurations, namely single plate configuration  1901 , coplanar two-plate configuration  1902 , parallel two-plate configuration  1903 , single plate with apertures configuration  1904 , and single plate, multi-side configuration  1905 . Other capacitor plate configurations are possible, such as discussed herein. 
     Single plate configuration  1901  includes capacitor plate  1910  which is separated from tissue  1990  by dielectric  1920 . In some examples, dielectric  1920  is omitted. Dielectric  1920  can comprise a dielectric material or an air gap, including combinations thereof. Capacitor plate  1910  is driven by drive signal  1930  to produce electric field  1940  in proximity of tissue  1990 . In this configuration, field  1940  comprises an electric field which can extend to infinity or can use tissue  1990  as a second capacitive plate. In some examples, tissue  1990  is connected to a ground potential. Configuration  1901  can be an example of capacitance node  182  of  FIGS. 1A-1B , capacitance nodes  185 - 186  in  FIG. 5 , or capacitance node  187  or  189  in  FIG. 7 , among others, including variations thereof. In the example shown in  FIG. 19 , capacitor plate  1910  is positioned on one side of tissue  1990 . In further examples, single capacitor plate  1910  can wrap around more than one side of tissue  1990 , such as capacitor plate  189  in  FIG. 7  or pad  1530  of  FIG. 15 . 
     Coplanar two-plate configuration  1902  includes capacitor plates  1911 - 1912  which are separated from tissue  1991  by dielectric  1921 . In some examples, dielectric  1921  is omitted. Dielectric  1921  can comprise a dielectric material or an air gap, including combinations thereof. Capacitor plate  1911  is driven by drive signal  1931  to produce electric field  1941  in proximity of tissue  1991 , with capacitor plate  1912  connected to a ground potential. In this configuration, field  1941  comprises an electric field between capacitor plates  1911 - 1912 . The portion of field  1941  that penetrates into tissue  1991  comprises a fringe field of a two-plate capacitor formed by capacitor plates  1911 - 1912 . In some examples, tissue  1991  is connected to a ground potential. Configuration  1902  can be an example of capacitance nodes  183 - 184  of  FIG. 3  or capacitance nodes  187 - 188  in  FIG. 7 , among others, including variations thereof. In the example shown in  FIG. 19 , capacitor plates  1911 - 1912  are positioned on the same side of tissue  1990 . 
     Parallel two-plate configuration  1903  includes capacitor plates  1913 - 1914  which are separated from tissue  1992  by associated dielectric  1922 - 1923 . In some examples, dielectric  1922 - 1923  are omitted. Dielectric  1922 - 1923  can comprise a dielectric material or an air gap, including combinations thereof. Capacitor plates  1913 - 1914  are driven by drive signal  1932  to produce electric field  1942  in proximity of tissue  1992 , with capacitor plate  1914  connected to a ground potential. In this configuration, field  1942  comprises an electric field between capacitor plates  1913 - 1914 . The portion of field  1942  that penetrates into tissue  1992  comprises a field of a two-plate capacitor formed by capacitor plates  1913 - 1914 . In some examples, tissue  1992  is connected to a ground potential. Configuration  1903  can be an example of pad  1540  of FIG.  15 , among others, including variations thereof. In the example shown in  FIG. 19 , capacitor plates  1913 - 1914  are positioned on different, opposing, sides of tissue  1990 . 
     Single plate with apertures configuration  1904  includes capacitor plate  1915  which is separated from tissue  1993  by dielectric  1924 . In some examples, dielectric  1924  is omitted. Dielectric  1924  can comprise a dielectric material or an air gap, including combinations thereof. Capacitor plate  1915  is driven by drive signal  1933  to produce electric field  1943  in proximity of tissue  1993 . In this configuration, field  1943  comprises an electric field which can extend to infinity or can use tissue  1993  as a second capacitive plate. In some examples, tissue  1993  is connected to a ground potential. Configuration  1904  can be an example of pad  1520  of  FIG. 15  or pad  17  of  FIG. 17 . Configuration  1904  can also be an example of capacitance node  182  of  FIGS. 1A-1B , capacitance nodes  185 - 186  in  FIG. 5 , or capacitance node  187  or  189  in  FIG. 7 , among others, when one or more apertures are included. In the example shown in  FIG. 19 , capacitor plate  1915  is positioned on one side of tissue  1993 . In further examples, single capacitor plate  1915  can wrap around more than one side of tissue  1993 , such as capacitor plate  189  in  FIG. 7 , pad  1520  of  FIG. 15 , or configuration  1905  in  FIG. 19 . 
     Single plate, multi-side configuration  1905  includes capacitor plate  1916  which is separated from tissue  1994  by dielectric  1925 . In some examples, dielectric  1925  is omitted. Dielectric  1925  can comprise a dielectric material or an air gap, including combinations thereof. Capacitor plate  1916  is driven by drive signal  1934  to produce electric field  1944  in proximity of tissue  1994 . In this configuration, field  1944  comprises an electric field which can extend to infinity or can use tissue  1994  as a second capacitive plate. In some examples, tissue  1994  is connected to a ground potential. Configuration  1905  can be an example of capacitance node  189  of  FIG. 7 , or pads  1520  or  1530  of  FIG. 15 , among others. In the example shown in  FIG. 19 , capacitor plate  1916  wraps around more than one side of tissue  1994 . 
     The included descriptions and drawings depict specific embodiments to teach those skilled in the art how to make and use the best mode. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these embodiments that fall within the scope of the invention. Those skilled in the art will also appreciate that the features described above can be combined in various ways to form multiple embodiments. As a result, the invention is not limited to the specific embodiments described above.