Patent Publication Number: US-8989832-B2

Title: Photoplethysmography with controlled application of sensor pressure

Description:
This application is a continuation application of U.S. patent application Ser. No. 12/543,908, entitled “Photoplethysmography with Controlled Application of Sensor Pressure”, filed Aug. 19, 2009, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates generally to non-invasive diagnostic measurements dependent on pulse spectra and, more particularly, to photoplethysmographic measurements taken with a controlled application of pressure. 
     This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art. 
     Diagnostic measurements, such as pulse oximetry and non-invasive measurements of total hemoglobin, may be determined from pulse spectrum measurements at varying wavelengths of light. For example, pulse oximetry may involve measurements at wavelengths of approximately 660 nm and 900 nm, and non-invasive measurements of total hemoglobin may involve measurements of wavelengths of approximately 1320 nm and 800-900 nm. In operation, conventional two-wavelength photoplethysmographic sensors may emit light from one or more emitters (e.g., light emitting diodes (LEDs) or fiber optic cables to one or more remote light sources) into a pulsatile tissue bed and collect the transmitted light with a detector (e.g., a photodiode or fiber optic cables to a remote photodetector). The detected light may then be utilized to estimate, for example, a level of oxygen saturation in the blood that is present in the tissue bed. The emitters and detector may be positioned in various orientations. In a transmission-type photoplethysmographic sensor, the emitters and detector are positioned substantially opposite one another (e.g., on opposite sides of a patient&#39;s finger), while in a reflectance-type photoplethysmographic sensor, the emitters and detector are placed adjacent to one another. 
     Signals from a photodetector of a photoplethysmographic sensor may be decoded to ascertain a plethysmographic waveform, which may be due to the cycling light attenuation caused by the varying amount of arterial blood that the light from the emitters passes through. Various factors may cause diminished signal quality or cause inconsistent or unreliable plethysmographic waveform readings. Specifically, the presence of excessive extravascular fluid or venous blood in a tissue bed of interest may interfere with the detection of arterial blood, producing inaccurate or inconsistent plethysmographic waveforms. The quantity of extravascular fluid or venous blood in a tissue bed of interest may vary from patient to patient or from time to time for the same patient. 
     SUMMARY 
     Certain aspects commensurate in scope with the originally disclosed embodiments are set forth below. It should be understood that these aspects are presented merely to provide the reader with a brief summary of certain forms the embodiments might take and that these aspects are not intended to limit the scope of the presently disclosed subject matter. Indeed, the embodiments may encompass a variety of aspects that may not be set forth below. 
     The present disclosure relates to systems, methods, and devices for obtaining consistently reproducible diagnostic measurements with a photoplethysmographic sensor. In one embodiment, a method for obtaining such a diagnostic measurement includes applying a pressure between a photoplethysmographic sensor and a patient, increasing the pressure until the photoplethysmographic sensor outputs a plethysmographic waveform of minimal amplitude, decreasing the pressure by a predetermined fraction, and obtaining the diagnostic measurement using the photoplethysmographic sensor. The pressure may be applied using a pressure device that includes, for example, a clip, a wrap, an inflatable balloon or bladder, an inflatable cuff, or any combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the presently disclosed subject matter may become apparent upon reading the following detailed description and upon reference to the drawings in which: 
         FIG. 1  is a plot of pulse spectra amplitude at varying levels of applied pressure; 
         FIG. 2  is a perspective view of a photoplethysmographic system, in accordance with an embodiment; 
         FIG. 3  is a block diagram of a photoplethysmographic system, in accordance with an embodiment; 
         FIG. 4  is a flowchart describing a method of determining and applying a pressure with a photoplethysmographic sensor, in accordance with an embodiment; 
         FIG. 5  is a perspective view of a photoplethysmographic sensor having an adhesive for applying pressure against a patient, in accordance with an embodiment; 
         FIG. 6  is a perspective view of a photoplethysmographic sensor having an inflatable balloon or bladder for applying pressure to a patient, in accordance with an embodiment; 
         FIG. 7  is a perspective view of a photoplethysmographic sensor having an inflatable cuff for applying pressure on a patient, in accordance with an embodiment; 
         FIG. 8  illustrates a perspective view of a system for obtaining a diagnostic measurement using a photoplethysmographic sensor, in accordance with an embodiment; and 
         FIG. 9  is a flowchart describing a method of obtaining a diagnostic measurement using the system of  FIG. 8 , in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     One or more specific embodiments of the present disclosure will be described below. In an effort to provide a concise description of these embodiments, not all features of an actual implementation are described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     Present embodiments may apply to a variety of photoplethysmographic diagnostic measurements based on pulse spectra detected from patient tissue. For example, pulse oximetry and non-invasive measurements of total hemoglobin may be determined from measurements of pulse spectra on a patient tissue at varying wavelengths of light. Pulse oximetry may involve measurements at wavelengths of approximately 660 nm and 900 nm, and non-invasive measurements of total hemoglobin may involve measurements of wavelengths of approximately 1320 nm and 800-900 nm. As disclosed herein, photoplethysmographic amplitudes were found to vary significantly at certain wavelengths of pulse spectra depending on the amount of pressure with which the sensor is applied to patient tissue. Thus, the present disclosure describes various embodiments of systems, methods, and devices for improving the reliability and reproducibility of measurements taken with photoplethysmographic sensors. Such diagnostic measurements may include pulse oximetry measurements or non-invasive measurements of total hemoglobin. 
     In experiments carried out to measure pulse spectra on five human subjects, plethysmographic amplitudes were found to vary significantly by pressure. The experiments were carried out using a fiber optic reflectance sensor having a 5 mm diameter ring of illumination fibers surrounded by a bundle of detection fibers. The illumination fibers were illuminated by a 90 W quartz-halogen bulb, and the detection fibers were routed to two different spectrometers to enable the pulse spectra to be measured across the visible and near infer red regions. The first spectrometer (the “Si spectrometer”) included an f/8 monochromator (Acton, Model 275) with a grating of 150 grooves/mm blazed at 500 nm and a linear 512 element silicon array (Hamamatsu C5964-0900). The second spectrometer (the “InGaAs spectrometer”) included an f/2.8 monochromator (American Holographics, Model 492.85) and a 256-element InGaAs linear array (Sensors Unlimited, Model SU256LX-1.7). Long pass filters with cutoff wavelengths of 475 nm and 900 nm were placed at the entrance ports of the Si and InGaAs spectrometers, respectively, to reduce effects due to higher order grating diffraction. The spectral resolutions of the Si and InGaAs Spectrometers were 10 nm and 18 nm, respectively, and the time resolutions of the spectra acquired by the Si and InGaAs Spectrometers were 84 ms and 23 ms, respectively. 
     Pulse spectra for five healthy, human subjects who were breathing room air were measured at varying levels of pressure. Raw tissue spectra observed on the patients were converted to absorbance spectra by subtracting the spectrum measured with the light source turned off and dividing by the spectrum measured on a solid reflectance standard (Teflon), and subsequently computing the negative logarithm (base 10) of the result. The absorbance spectra were decimated by wavelength, such that the spacing between channels corresponded to approximately one half of the spectral resolution. The absorbance spectra were then temporally bandpass filtered with lower and upper frequency cutoffs of 0.6 and 4.0 Hz, respectively. Additionally, the absorbance spectra were Fourier phase filtered using wavelengths of 500 nm and 1000 nm, respectively, as reference signals for the pulse spectra detected at the Si and InGaAs Spectrometers. Pulse spectra were constructed by computing the slope of a least-squares linear fit to the absorbance at each wavelength versus a reference wavelength. 
     Measurements of the pulse spectra were collected on the middle or ring finger of five volunteer subjects at three different pressures: “low,” “medium,” and “high.” “Low” pressure was a pressure only just sufficient to contact the finger with the sensor, occurring at approximately 5 mm Hg. “High” pressure was a pressure just below the pressure required to fully extinguish the photoplethysmographic waveform at a certain reference wavelength, such as 900 nm, occurring at approximately 125 mm Hg. “Medium” pressure was a pressure of approximately one-half of the “high” pressure, occurring at approximately 60 mm Hg. Three replicate measurements were performed at each pressure, with pressures being measured using a piezo-resistive sensor (Flexiforce B201, Tekscan) shaped to surround the illumination fiber ring. Table 1 below summarizes the average and standard deviation of the pressures applied to the tissues of the patient, which varied from patient to patient. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Pressures applied to the tissue at the sensor site (mm Hg) 
               
            
           
           
               
               
               
               
            
               
                   
                 Pressure 
                 Average 
                 Between subj. stand. dev. 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 High 
                 126 
                 21 
               
               
                   
                 Medium 
                 60 
                 9 
               
               
                   
                 Low 
                 5 
                 4 
               
               
                   
                   
               
            
           
         
       
     
     As noted above, pulse spectra may be employed for use in various photoplethysmographic diagnostic measurements, such as pulse oximetry and non-invasive total measurement of hemoglobin. In the case of pulse oximetry, the pulse spectrum may be measured at approximately 660 nm and at approximately 900 nm. As such, pulse oximetry may benefit from a consistent relationship between the measured amplitudes at approximately 660 nm and at approximately 900 nm. The pulse spectra experimentally collected from the five human subjects were compared at wavelengths of 660 nm and 900 nm, the results of which are shown below in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Pulse amplitude measured at 660 nm (normalized 
               
               
                 to 900 nm) as a function of pressure 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Mean 
                 Within subj. 
                 Between subj. 
               
               
                   
                 Pressure 
                 amplitude 
                 std. dev. 
                 std. dev. 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Low 
                 0.478 
                 0.030 
                 0.069 
               
               
                   
                 Medium 
                 0.444 
                 0.022 
                 0.042 
               
               
                   
                 High 
                 0.514 
                 0.039 
                 0.048 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 relates the amplitude of the measured pulse spectra at 660 nm to that of 900 nm. As indicated by Table 2, the mean amplitude at 660 nm may be dependent on the amount of pressure applied to the sensor. In particular, application of “medium” pressure results in the lowest standard deviation of the mean amplitude on a particular human subject, which is noted as “within subj.,” as well as across the group of subjects, noted at “between subj.” Since all of the subjects were healthy and breathing room air, their arterial oxygen saturation percentages were all expected to be near 100%. Within-subject and between-subject variations were therefore expected to be indicative of the reproducibility of the measurement. In both cases, the “medium” pressure measurement proved to be the most reproducible of those tested. However, a fraction of the “high” pressure other than the “medium” pressure may be determined to produce more reproducible results. For example, with further experimentation, it may be determined that a pressure equivalent to approximately one-quarter of the “high” pressure or three-quarters of the “high” pressure may produce results more reproducible than results produced using the tested “medium” pressure. 
     For the purpose of measuring total hemoglobin in blood non-invasively, a pulse amplitude at 1320 nm relative to that at 800-900 nm, as well as other possible wavelengths, may be useful. As such, non-invasive measurement of total hemoglobin may also benefit from a consistent relationship between the measured amplitudes at approximately 1320 nm and at approximately 800-900 nm. The pulse spectra experimentally collected from the five human subjects were compared at wavelengths of 1320 nm and 900 nm, the results of which are shown below in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Pulse amplitude measured at 1320 nm (normalized 
               
               
                 to 900 nm) as a function of pressure 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                 Mean 
                 Within subj. 
                 Between subj. 
               
               
                   
                 Pressure 
                 amplitude 
                 std. dev. 
                 std. dev. 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Low 
                 0.129 
                 0.061 
                 0.137 
               
               
                   
                 Medium 
                 −0.009 
                 0.031 
                 0.038 
               
               
                   
                 High 
                 0.059 
                 0.046 
                 0.119 
               
               
                   
                   
               
            
           
         
       
     
     Table 3 relates the amplitude of the measured pulse spectra at 1320 nm to that of 900 nm. As shown in Table 3, the experimental data may indicate that the pulse amplitude at 1320 nm is strongly dependent on the pressure applied to the sensor. The data may also indicate that by applying a “medium” pressure, the amplitude at 1320 nm may be reproducible as measured both on a particular subject and across a group of subjects. 
       FIG. 1  is a plot  2  of average amplitudes obtained for the three pressures “low,” “medium,” and “high” along a spectrum from 600 nm to 1400 nm. As described above, “low” pressure represents a pressure only just sufficient to contact the finger with the sensor, occurring at approximately 5 mm Hg; “high” pressure represents a pressure just below the pressure required to fully extinguish the photoplethysmographic waveform at a certain reference wavelength, such as 900 nm, occurring at approximately 125 mm Hg; and “medium” pressure represents a pressure of approximately one-half of the “high” pressure, occurring at approximately 60 mm Hg. The ordinate of the plot  2  indicates AC absorbance averaged across all tests as reflected in the photoplethysmographic amplitude (normalized to 900 nm), and the abscissa of the plot  2  indicates wavelength of the spectrum in units of nanometers (nm). 
     In the plot  2  of  FIG. 1 , a curve  4  represents averaged AC spectra obtained at the “low” pressure, a curve  6  represents averaged AC spectra obtained at the “medium” pressure, and a curve  8  represents averaged AC spectra obtained at the “high” pressure. Error bars associated with each curve  4 ,  6 , and  8  represent the standard deviation between subjects for pulse spectra obtained at the “low,” “medium,” and “high” pressures, respectively. As is apparent from the plot, the “medium” pressure curve  6  has the least variability across substantially the entire the range of disclosed wavelengths. Thus, it should be understood that a “medium” pressure applied on a plethysmographic sensor may provide the greatest reproducibility between subjects across wavelengths of interest in plethysmographic diagnostic measurements. 
     With the foregoing in mind,  FIG. 2  illustrates a perspective view of a photoplethysmography system  10  in accordance with present embodiments for obtaining consistent and reproducible diagnostic measurements. The system  10  may be employed to observe the blood constituents of a patient&#39;s arterial blood by emitting light at particular wavelengths into tissue and detecting the light after dispersion and/or reflection by the tissue. For example, diagnostic measurements for pulse oximetry may involve photoplethysmographic measurements at approximately 660 nm and at approximately 900 nm, and diagnostic measurements for non-invasively measuring total hemoglobin may involve photoplethysmographic measurements at approximately 1320 nm and at approximately 800-900 nm. 
     The system  10  may include a patient monitor  12  that communicatively couples to a photoplethysmographic sensor  14 . The patient monitor  12  may include a display  16 , a memory, a processor, and various monitoring and control features. The patient monitor  12  may be configured to perform pulse oximetry measurements, calculations, and control algorithms using high precision values in accordance with present embodiments. The photoplethysmographic sensor  14  may include a sensor cable  18 , a connector plug  20 , and a sensor assembly or body  22  configured to attach to a patient (e.g., a patient&#39;s finger, ear, forehead, or toe). In the illustrated embodiment, the sensor assembly is configured to attach to a finger and to apply a pressure sufficient to exclude extraneous extravascular fluid while permitting arterial blood flow in the pulsatile tissue of the finger. The system  10  may include a separate display feature  24  that is communicatively coupled with the patient monitor  12  to facilitate presentation of plethysmographic data and that may display a plethysmogram, pulse oximetry information, non-invasive measurement of total hemoglobin, and/or related data. 
     The photoplethysmographic sensor  14  may include an emitter  28  and a detector  30 . When attached to patient tissue, the emitter  28  may transmit light at different wavelengths into the tissue and the detector  30  may receive the light after it has passed through or is reflected by the tissue. The amount of light that passes through the tissue and other characteristics of light waves may vary in accordance with the changing amount of certain blood constituents in the tissue and the related light absorption and/or scattering. For example, the system  10  may emit light from two or more LEDs or other suitable light sources, such as lasers or incandescent light sources guided by fiber optics, into the pulsatile tissue. The reflected or transmitted light may be detected with the detector  30 , such as a photodiode or photo-detector, after the light has passed through or has been reflected by the pulsatile tissue. 
     The photoplethysmographic sensor  14  may facilitate certain diagnostic measurements by specifically examining responses by the tissue at certain wavelengths. For example, to conduct pulse oximetry measurements, the emitter  28  of the photoplethysmographic sensor  14  may emit light of wavelengths of approximately 660 nm and 900 nm. To conduct non-invasive measurements of total hemoglobin, the emitter  28  of the photoplethysmographic sensor  14  may emit light of wavelengths of approximately 1320 nm and 800-900 nm. Because the ratio of amplitudes for measurements obtained at one wavelength to another wavelength may vary with pressure, a pressure device  32  may apply an optimum amount of pressure between the photoplethysmographic sensor  14  and the patient tissue, enhancing the reproducibility of measurements taken at the various wavelengths. In the embodiment of  FIG. 2 , the photoplethysmographic sensor  14  may be shaped as a clip and the pressure device  32  may cause the sensor body  22  to fold or compress around the patient&#39;s finger. The pressure device  32  may be controlled manually or automatically by the patient monitor  12 . 
       FIG. 3  is a block diagram of an embodiment of the monitoring system  10  that may be configured to implement the techniques described herein. By way of example, embodiments of the system  10  may be implemented with any suitable patient monitor, such as those available from Nellcor Puritan Bennett LLC. The system  10  may include the patient monitor  12  and the photoplethysmographic sensor  14 , which may be configured to obtain a plethysmographic signal from patient tissue at certain predetermined wavelengths at an optimum pressure. The photoplethysmographic sensor  14  may be communicatively connected to the patient monitor  12  via a cable or wireless device. When the system  10  is operating, light from the emitter  28  may pass into a patient  36  and be scattered and detected by the detector  30 . The patient monitor  12  may include a microprocessor  38  connected to an internal bus  40 . Also connected to the bus  40  may be a RAM memory  42  and a display  44 . A time processing unit (TPU)  46  may provide timing control signals to light drive circuitry  48  which may control when the emitter  28  is illuminated, and if multiple light sources are used, the multiplexed timing for the different light sources. The TPU  46  may also control the gating-in of signals from the detector  30  through an amplifier  50  and a switching circuit  52 . These signals may be sampled at the proper time, depending upon which of multiple light sources is illuminated, if multiple light sources are used. The received signal from the detector  30  may be passed through an amplifier  54 , a low pass filter  56 , and an analog-to-digital converter  58 . The digital data may then be stored in a queued serial module (QSM)  60 , for later downloading to the RAM  42  as the QSM  60  fills up. In one embodiment, there may be multiple parallel paths of separate amplifier, filter and A/D converters for multiple light wavelengths or spectra received. 
     In an embodiment, the photoplethysmographic sensor  14  may also contain an encoder  62  that provides signals indicative of the wavelength of one or more light sources of the emitter  28  to allow the patient monitor  12  to select appropriate calibration coefficients for calculating a physiological parameter such as blood oxygen saturation. By way of example, present embodiments may be implemented with any suitable photoplethysmographic sensor, such as those available from Nellcor Puritan Bennett LLC. The encoder  62  may, for instance, be a coded resistor, EEPROM or other coding devices (such as a capacitor, inductor, PROM, RFID, a barcode, parallel resonant circuits, or a colorimetric indicator) that may provide a signal to the processor  38  related to the characteristics of the photoplethysmographic sensor  14  that may allow the processor  38  to determine the appropriate calibration characteristics for the photoplethysmographic sensor  14 . Further, the encoder  62  may include encryption coding that prevents a disposable part of the photoplethysmographic sensor  14  from being recognized by a processor  38  that is not able to decode the encryption. For example, a detector/decoder  64  may be required to translate information from the encoder  62  before it can be properly handled by the processor  38 . 
     In various embodiments, based at least in part upon the value of the received signals corresponding to the light received by detector  30 , the microprocessor  38  may calculate a physiological parameter using various algorithms. These algorithms may utilize coefficients, which may be empirically determined, corresponding to, for example, the wavelengths of light used. These may be stored in a ROM  66 . In a two-wavelength system, the particular set of coefficients chosen for any pair of wavelength spectra may be determined by the value indicated by the encoder  62  corresponding to a particular light source in a particular sensor  14 . For example, the first wavelength may be a wavelength that is highly sensitive to small quantities of deoxyhemoglobin in blood, and the second wavelength may be a complimentary wavelength. Specifically, for example, such wavelengths may be produced by orange, red, infrared, green, and/or yellow LEDs. Different wavelengths may be selected with control inputs  68 . The control inputs  68  may be, for instance, a switch on the monitor, a keyboard, or a port providing instructions from a remote host computer. 
     The patient monitor  12  may be connected to a network via a network interface  70 . The network interface  70  may implement any suitable networking technology or protocol, such as Ethernet, wireless Ethernet, and so forth. The network interface  70  may be connected to a network port  72  via a network cable or via a wireless connection. Additionally, the patient monitor  12  may include a non-volatile memory  74  that may store caregiver preferences, patient information, or any other information useful for configuring the patient monitor  12 . The software for performing the configuration of the patient monitor  12  and retrieval of information over the network interface  70  may also be stored on the memory  74 , or may be stored on the ROM  66 . 
     The photoplethysmographic sensor  14  may include the pressure device  32  and/or the pressure sensor  34 , which may operably connect to the patient monitor  12 . Specifically, the pressure device  34  may be controlled by a pressure device controller  76  in the patient monitor  12 , which may increase or decrease the pressure of the photoplethysmographic sensor  14  on the patient  36  to achieve a desired pressure. The pressure device controller  76  may transmit an electronic signal or a signal of supplied liquid or gas to control the pressure device  34 . Based on routines stored in RAM  42 , ROM  66 , and/or nonvolatile memory  74  that may be executed by the microprocessor  38 , the pressure device controller  76  may increase or decrease pressure until an optimum pressure for a desired diagnostic measurement is obtained. The pressure sensor  34  may provide an indication of the current pressure to a sensor pressure decoder  78  in the patient monitor  12 . As the pressure device controller  76  instructs the pressure device  32  to increase or decrease the pressure applied to the patient  36 , sensor pressure decoder  78  may provide the microprocessor  38  with data indicating the current applied pressure. Such data may be used, for example, to provide closed-loop feedback to the microprocessor  38 . Based on such closed-loop feedback, the microprocessor  38  may suitably control the applied pressure with a PID controller or a PID control algorithm, which may be implemented in software running on the microprocessor  38 . 
     In one embodiment, the pressure device controller  76  may instruct the pressure device  32  to maintain the pressure of the photoplethysmographic sensor  14  against the patient  36  at a low level when plethysmographic diagnostic measurements are not being obtained. When such plethysmographic diagnostic measurements are being obtained, (e.g., just prior to and during measurement of the pulse amplitude of the patient  36 ), the pressure device controller  76  may instruct the pressure device  32  to increase the pressure against the patient  36  to an optimal value of pressure (e.g., approximately half of a maximal value). In this way, the electronic patient monitor  12  may obtain reproducible results across a range of subjects and time periods, and the effect of the pressure applied by the photoplethysmographic sensor  14  on the blood circulation in the tissue of the patient  36  may be minimized. 
       FIG. 4  is a flowchart  80  of an embodiment of a method for performing a desired photoplethysmographic diagnostic measurement at an optimum pressure. At the optimum pressure, the photoplethysmographic sensor  14  may provide reproducible photoplethysmographic waveforms across a range of patients and times, which may translate into reproducible diagnostic measurements. Steps of the flowchart  80  may be performed manually by a medical practitioner or automatically by the photoplethysmography system  10 , as identified in the discussion below. 
     The flowchart  80  may begin with a first step  82 , when a medical practitioner may attach the photoplethysmographic sensor  14  with minimal pressure to a tissue site, such as a finger, on the patient  36 . Such a minimal pressure may be, for example, approximately 1-10 mm Hg. In step  84 , the medical practitioner may manually increase the pressure or the patient monitor  12  may control the pressure device  32  to increase the pressure against the patient  36  while observing a photoplethysmographic waveform for a predetermined wavelength of light. The predetermined wavelength of the photoplethysmographic waveform may be chosen based on the type of diagnostic measurement that is intended. For example, if the desired diagnostic measurement includes pulse oximetry, the predetermined wavelength of the photoplethysmographic waveform may be approximately 660 nm or 900 nm. If the desired diagnostic measurement includes non-invasive measurement of total hemoglobin, the predetermined wavelength of the photoplethysmographic waveform may be approximately 1320 nm or 900 nm. As the pressure is increased in the step  86 , the earliest point at which the photoplethysmographic waveform reaches a minimum at the predetermined wavelength may represent a maximal sensor pressure. Such a maximal pressure is believed to cause extravascular fluid to exit the tissue site and arterial blood flow to substantially cease when applied against the tissue site of the patient  36 , and may be, for example, approximately 100-150 mm Hg. 
     In step  86 , the medical practitioner or the patient monitor  12  may record the maximal pressure using the pressure sensor  34 . In step  88 , the medical practitioner or the patient monitor  12  may cause the pressure of the photoplethysmographic sensor  14  against the patient  36  to be a predetermined fraction of the maximal pressure recorded in step  86 . For example, the pressure may be decreased to approximately half of the maximal pressure, since a “medium” pressure has been experimentally shown to be reproducible across a range of subjects and time periods. At such a medium pressure, it is believed that most extravascular fluid is excluded from the tissue site while most arterial blood continues to flow. It should be appreciated, however, that other predetermined fractions of the maximal pressure recorded in step  86  may be applied in step  88 . Such predetermined fractions may be any fraction of the maximal pressure greater than the minimal pressure and less than the maximal pressure, and may be, for example, approximately one-quarter, one-third, two-thirds, or three-quarters of the maximal pressure. For example, if the pressure recorded in step  86  is approximately 100-150 mm Hg, the medium pressure applied in step  88  may be approximately 50-70 mm Hg for patients with normal blood pressure. 
     In step  90 , one or more diagnostic measurements of interest may be taken while the predetermined fraction of the maximal pressure is being applied. For example, while the predetermined fraction of the maximal pressure is being applied, the photoplethysmographic system  10  may take a pulse oximetry reading based on, for example, wavelengths of approximately 660 nm and 900 nm. Additionally or alternatively, the photoplethysmographic system  10  may take measurements of total hemoglobin based on, for example, wavelengths of approximately 1320 nm and 800-900 nm. 
     Steps  84 - 88  above may be performed only when the photoplethysmographic sensor  14  is first placed on the patient  36 . Alternatively, to further increase measurement reproducibility, steps  84 - 88  may be repeated each time a diagnostic measurement is to be obtained, and after the measurement has been obtained, the pressure may be reduced to the minimal pressure. In another embodiment, steps  84 - 86  may be performed at a periodic interval (e.g., once every hour, half hour, 15 minutes, etc.) or after a predetermined number of diagnostic measurements have been obtained, and step  88  may be performed each time a diagnostic measurement is to be obtained. 
       FIGS. 5-7  represent various alternative embodiments of the photoplethysmographic sensor  14  for practicing the embodiment of the method of the flowchart  80  of  FIG. 4 . In particular,  FIG. 5  illustrates a photoplethysmographic sensor with a manually-controlled wrap-based pressure device  32 ,  FIG. 6  illustrates a photoplethysmographic sensor with an inflatable-balloon-based pressure device  32 , and  FIG. 7  illustrates a photoplethysmographic sensor with an inflatable-cuff-based pressure device  32 . Turning first to  FIG. 5 , a wrap-based photoplethysmographic sensor  94  may enable a medical practitioner to manually perform the method described in flowchart  80 . The sensor  94  may include any underlying photoplethysmographic sensor  96 , which may include, for example, an emitter  28  and a detector  30  in either a transmission-type or reflectance-type configuration. The sensor  96  may be formed from a flexible material, such as cloth or soft plastic, and may include adhesive straps  98  to attach the sensor  96  to the patient  36 . 
     A medical practitioner may use a wrap-based pressure device  32 , illustrated in  FIG. 5  as a foam adhesive  100 , to maintain pressure on the sensor  96 . Additionally or alternatively, the foam adhesive  100  may be or include any material that may be wrapped around the sensor  96 , such as gauze, tape, elastic bands, bands of Velcro-type hook-and-loop fasteners, and so forth. A pressure sensor  34  may be placed beneath or attached to the sensor  96  to monitor the amount of pressure applied. To carry out the steps of the flowchart  80  using the wrap-based photoplethysmographic sensor  94 , a medical practitioner may first loosely attach the sensor  96  to the patient  36  by attaching the adhesive straps  98 ; the pressure sensor  34  should indicate a minimal pressure of approximately 1-10 mm Hg. Next, the medical practitioner may apply pressure on the sensor  96  at the locations of the admitter  28  and detector  30 , observing the plethysmographic waveform displayed on the patient monitor  12  at the predetermined wavelength. When the plethysmographic waveform extinguishes to a minimum, the medical practitioner may record the maximal pressure indicated by the pressure sensor  34 . The medical practitioner may subsequently attach the foam adhesive  100  tightly enough to achieve a pressure of approximately half of the maximal pressure before taking a diagnostic measurement of interest. 
       FIG. 6  illustrates a photoplethysmographic sensor  102  with an inflatable-balloon-based pressure device  32 , which may represent another embodiment of the photoplethysmographic sensor  14  and which may also enable the method of the flowchart  80 . The sensor  102  may include, for example, the flexible sensor  96  having the emitter  28  and detector  30 , which may attach to a tissue site of the patient  36  using the adhesive straps  98 . As in the sensor  94  of  FIG. 5 , the pressure sensor  34  beneath the sensor  96  may enable monitoring of the pressure applied to the patient  36 . The pressure device  32  of the photoplethysmographic sensor  102  may be include an inflatable balloon or bladder  104  attached to an inflation tube  105  and surrounded by an inflexible casing  106 . The inflatable balloon or bladder  104  may be formed from any flexible material capable of holding a gas or liquid supplied via the inflation tube  105 , such as latex or flexible plastic. The inflexible casing  106  may include, among other things, rigid plastic or metal to hold the inflatable balloon or bladder  104  against the patient  36  while inflated. 
     The sensor  102  of  FIG. 6  may be used to carry out the method of the flowchart  80  of  FIG. 4  in a variety of manners. For example, the inflatable balloon or bladder  104  may be pumped manually by a medical practitioner, who may carry out the method of the flowchart  80  in a manner similar to that described above with reference to  FIG. 5 . Additionally or alternatively, the patient monitor  12  may automatically control the sensor  102  to carry out the steps  84 - 90  of the flowchart  80 . For example, the pressure device controller  76  of the patient monitor  12  may be configured to inflate the balloon or bladder  104 . When a plethysmographic waveform signal from the photoplethysmographic sensor  102  extinguishes to a minimum at a predetermined wavelength, the patient monitor  12  may record the pressure indicated by the pressure sensor  34  and as determined by the sensor pressure decoder  78 , storing the pressure value in the RAM  42  or the nonvolatile memory  74 . The pressure device controller  76  may next cause the balloon or bladder  104  to deflate to approximately half of the stored pressure value. Having obtained an approximately optimum “medium” pressure, the patient monitor  12  may next obtain a diagnostic measurement of interest, such as a pulse oximetry measurement or a non-invasive measurement of total hemoglobin. 
       FIG. 7  illustrates a photoplethysmographic sensor  108  with an inflatable-cuff-based pressure device  32 , which may represent another embodiment of the photoplethysmographic sensor  14  and which may also enable the method of the flowchart  80 . As shown in  FIG. 7 , the photoplethysmographic sensor  108  may include the flexible sensor  96 , over which an inflatable cuff  110  may be placed. Additionally or alternatively, the inflatable cuff  110  may incorporate the elements of the flexible sensor  96 , such as the emitter  28  and the detector  30 , which may be sewn into the inflatable cuff  110 . The inflatable cuff  110  may function in a substantially similarly manner as a blood pressure cuff, by inflating or deflating in response to a supplied liquid or gas from an inflation tube  111 . Like the inflatable balloon or bladder  104  shown in  FIG. 6 , the inflatable cuff  110  may be controlled manually by a medical practitioner or automatically via the patient monitor  12 . 
       FIGS. 8 and 9  relate to an alternative method of determining an optimal “medium” pressure to achieve reproducible photoplethysmographic data across patients and times. Specifically,  FIGS. 8 and 9  describe embodiments that determine an optimum pressure by proxy using measured thicknesses of a photoplethysmographic sensor against patient tissue. As shown in  FIG. 8 , a thickness-measurement-based photoplethysmographic sensor system  112  may include a photoplethysmographic sensor  114 , which may be a finger clip photoplethysmographic sensor similar to the photoplethysmographic sensor  14 , and a thickness-measuring device  116 , which may be a micrometer. The thickness measuring device  116  may apply pressure at a point  118 , representing an approximate location of an emitter  28  and detector  30  in the sensor  114 . In some embodiments, the thickness-measuring device  116  may be electronic and controllable by the patient monitor  12  in the same general manner as the pressure control device  32 , and may additionally or alternatively provide thickness measurements to the patient monitor  12  in the same general manner as the pressure sensor  34 . A medical practitioner or the patient monitor  12  may generally obtain an approximately optimum predetermined fraction of the maximal pressure with the thickness-measurement-based photoplethysmographic sensor system  112  using an embodiment of a method described with reference to  FIG. 9 . 
     A flowchart  120 , shown in  FIG. 9 , describes an embodiment of a method for obtaining a reproducible diagnostic measurement using the thickness-measurement-based photoplethysmographic sensor system  112  of  FIG. 8 . The embodiment of the method of  FIG. 9  may be particularly useful when a pressure sensor  34  is not available. The flowchart  120  may begin with a first step  122 , when the medical practitioner may attach the photoplethysmographic sensor  114  with minimal pressure. Using the thickness-measuring device  116 , the medical practitioner or the patient monitor  12  may ascertain and record a first thickness around the sensor  114 . In step  126 , the medical practitioner or the patient monitor  12  may increase the pressure of the photoplethysmographic sensor  114  on the patient  36 , which may result in a decreased thickness. In step  128 , at the point at which a plethysmographic waveform signal from the photoplethysmographic sensor  114  reaches a minimum, the medical practitioner or the patient monitor  12  may record a second thickness of the photoplethysmographic sensor  114  using the thickness-measuring device  116 . 
     In step  130 , the medical practitioner or the patient monitor  12  may allow pressure to decrease by setting the thickness to be approximately equal to a predetermined fractional distance between the first thickness and the second thickness. For example, the thickness may be set to be approximately midway between the first recorded thickness and the second recorded thickness. Such a thickness may generally approximate an optimum “medium” pressure against the patient  36 . While continuing to apply the pressure provided by the thickness applied in step  130 , the medical practitioner or the patient monitor  12  may begin taking a diagnostic measurement of interest in step  132 . The diagnostic measurement of interest may include, for example, a pulse oximetry measurement or a non-invasive measurement of total hemoglobin. 
     Steps  124 - 128  above may be performed each time that the photoplethysmographic sensor  14  is first placed on the patient  36 . Alternatively, to increase measurement reproducibility, steps  124 - 128  may be repeated each time a diagnostic measurement is to be obtained, and after the measurement has been obtained, the pressure may be reduced to the minimal pressure. In another embodiment, steps  124 - 126  may be performed at a periodic interval (e.g., once every hour, half hour, 15 minutes, etc.) or after a predetermined number of diagnostic measurements have been obtained, and step  128  may be performed each time a diagnostic measurement is to be obtained. 
     While the embodiments set forth in the present disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the disclosure is not intended to be limited to the particular forms disclosed. The disclosure is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure as defined by the following appended claims.