Abstract:
Apparatus for assessing the electrical properties of patient-electrode interfaces has a carrier signal source injecting two carrier signals comprising an AC signal with a DC offset to the electrodes. The carrier signals are out of phase. The outputs from the electrodes are formed into electrocardiographic lead signals in a pre-amplifier circuit. Signal processing circuit is coupled to the pre-amplifier circuit and provides a first signal comprising the AC carrier signal contained in an ECG lead signal and a second signal containing a DC offset signal. The first and second signals are provided to a microprocessor to obtain an output indicative of the electrical properties of electrode interfaces for the ECG lead signal.

Description:
FIELD OF THE INVENTION 
   This invention relates to the evaluation of the electrical characteristics or “quality” of the connection of one or more biomedical electrodes to a patient. 
   BACKGROUND OF THE INVENTION 
   The collection of biopotential signals, such as electrocardiographic (ECG), electromyographic (EMG), and electroencephalographic (EEG) signals, is commonly used in minimally invasive techniques for obtaining diagnostic and patient monitoring data. These techniques are performed by placing a plurality of biomedical electrodes in electrical contact with the patient&#39;s skin. A patient connection system includes the plurality of electrodes, arranged in different standard configurations depending on the specific biopotential signals to be collected, and lead wires attached to the electrodes. The electrodes sense the electrical signals generated by the patient&#39;s heart, muscles, or neural pathways. For example, in ECG, electrical potentials generated in the heart are collected by a system of three, five, or ten electrodes. 
   Biopotential electrodes are typically single-use and disposable. They rely on a layer of conductive and adhesive gel to both create the electrical connection with the patient&#39;s skin and removably affix the electrode to the patient. The adhesive properties of these electrodes deteriorate over the duration of use thereby diminishing the conductive properties as well. This deterioration in conductive properties occurs in two general situations: first by compression and flexion due to the motion of the electrode site if the patient is active, or secondly, when the electrodes have been attached to the patient for an extended period of time, as is found in many long-term hospital and critical care situations. 
   Therefore, there are clinical advantages to a system that monitors the quality of the connection of electrodes to a patient&#39;s skin as, for example, that shown in Simon et al. U.S. Pat. No. 4,577,639. Furthermore, a system that detects a poor connection or disconnected electrode and sends a signal to clinicians to warn them to replace the electrode is also desired. Additionally, or alternatively, the signal could activate a lead-switching network, to select a proper configuration from those electrodes having good connections and maintain a valid measurement with this new configuration, as is also shown in Simon et al. 
   Most electrode connection quality measurement techniques currently in use measure the resistance or the impedance of the electrode connection. Electrode connection impedance is a composite measurement of many sources of impedance. These include minor contributors such as the impedance of the electrode itself (˜100 Ohms), the patient&#39;s internal impedance (˜100 Ohms), and the impedance of the conductive material in the electrode (˜1 KOhm). But the electrode impedance is primarily the electrode-skin interface impedance. This is usually stated to be between 15KΩ and 1MΩ. See  Med Instr. Application  &amp;  Design , Webster, Ed et al. (1998) p. 198. 
   The epidermal layer of the skin behaves electrically as a parallel RC circuit, and therefore the impedance of the electrode-skin interface is frequency dependent. This is characterized by an epidermal impedance that ranges from approximately 200 KOhms at 1 Hz to 200 Ohms at 1 MHz. (Webster et al.) As the connection between the electrode and/or the skin deteriorates, and should the electrode detach, the impedance of the electrode-skin interface increases. 
   Electrode connection quality measurement devices are combined with lead switching technology to provide a continuous high quality biopotential measurement. A lead-switching network selects the proper combination of well connected electrodes to produce the desired biopotential signal or signals. An example of such a system is disclosed in Simon &#39;639. An additional example of the electrode connection quality measurement is depicted in FIG. 1 of Marriott U.S. Pat. No. 5,020,541. 
   Two approaches have developed in the art to determine electrode connection quality. Each approach has its advantages and its limitations. One approach, shown in Simon &#39;639, teaches the injection of a constant DC current to the electrodes. This current will create a DC bias voltage across the electrode-skin interface which is directly proportional to the resistance of the electrode connection. A threshold voltage is set that is indicative of a disconnected electrode and once this threshold is met a “leads off” condition is indicated to the clinician via an alarm or a visual display. Additionally, the lead switching network monitors which electrodes are still well connected and determines the optimal combination of the remaining electrodes to produce a quality ECG signal. This determination is made by referring to a predetermined set of lead switching alternatives or preset optimal combinations based upon which ECG leads are currently in use and which ECG electrode has been disconnected. 
   The advantage of using a DC current is that no additional frequency component is injected on the patient. Additional frequency components can interfere with the monitoring of other physiological signals, such as EEG and EMG. The disadvantage of the DC method is that the varying DC bias or offset voltage, that provides the measurement of electrode quality, corrupts the biopotential signal and must later be taken out or compensated for to provide an accurate measurement of the biopotential signal. Also, the voltage drop resulting from the DC current may be hard to distinguish from other DC offsets that are present in ECG measurement. Another disadvantage of the DC method is that a DC current will only produce a voltage correlated to the resistance of the electrode connection. The electrode, however, is not purely resistive and to provide an accurate analysis of the electrode connection quality other frequency dependent properties, such as capacitance, must also be taken into account. 
   The approach of utilizing an AC signal for the on-off determination of electrode connection is taught by Morgan in U.S. Pat. No. 4,619,265. This approach is similar in method to the DC approach, except that an AC signal is injected and the impedance of the electrode is measured instead of the resistance. The Marriott &#39;541 patent, noted above, depicts such use of an AC signal for an ECG electrode quality determination system. Marriott &#39;541 discloses the injection of two out of phase AC carrier signals. One signal is sent to a reference lead and the other signal is sent to a plurality of collection leads. The different combinations of ECG electrode signals are supplied to differential amplifiers wherein the amplitudes of two AC carrier signals are compared to determine a differential voltage measurement that corresponds to the impedance of the electrode interface for a given ECG lead. This measurement can be analyzed by downstream components to detect and indicate electrode connection quality or a leads off condition. 
   The advantage of utilizing AC signals is that the impedance of the electrode connection can be measured to provide a more accurate picture of the entire connection over that provided by the DC method that measures only resistance. The disadvantage of utilizing AC signals is that they inject a frequency component onto the patient. This can interfere with the concurrent measurement of other physiological parameters of the patient by other pieces of monitoring equipment, especially equipment such as EEG and EMG equipment. 
   While the foregoing describes the collection of biopotential signals from a patient, the electrical quality of a patient-electrode interface is also important when electrical energy is applied to a patient, as by a defibrillator. 
   SUMMARY OF THE INVENTION 
   The present invention provides an apparatus for the improved detection and measurement of electrode connection or interface quality is provided. The electrode interface quality so determined may be displayed in a useful manner and used to carry out electrode lead switching. 
   The electrode connection quality detection apparatus employs a carrier signal source, a signal collection preamplifier, a digital signal processor, and a lead switching network. The carrier signal source injects both AC and DC carrier signals onto the patient. These signals are collected along, with the biopotential signals from biopotential electrodes applied to the skin of the patient and supplied to differential amplifiers in the preamplifier. The analog signals are filtered and digitized in the digital signal processor. The desired biopotential signals are extracted and can be displayed or otherwise used for diagnostic purposes. 
   The injected carrier signal data is sent to a microprocessor in the digital signal processor which determines the electrode connection quality. The electrode connection quality may also be displayed. Also, if the microprocessor detects a failed or disconnected electrode, a signal is sent to the lead switching network which in turn selects a viable configuration of electrodes to enable the collection of the biopotential signals to continue. 
   The present invention combines the use of both AC and DC impedance measurement techniques simultaneously to provide a graduated measurement of electrode connection quality. This graduated measurement can be displayed in real time to provide an early warning before lead failure occurs. 
   In a further embodiment of the present invention the AC measurement component may be turned off to reduce the adverse effects of the electrode connection quality measurement on various other pieces of biopotential monitoring equipment due to interference. This allows the present invention to operate with input portions in a low emission DC “quiet” mode so that the additional frequency component does not interfere with the measuring of other biopotentials by other monitoring equipment. 
   The AC carrier signal may comprise a single AC signal or may comprise two signals that are of equal amplitude but are out of phase and injected onto different electrodes. 
   The invention may also find use in automatic portable defibrillators. Defibrillation requires the proper low impedance connection of the defibrillation electrodes to the patient. An improved technique for measurement of electrode connection quality would potentially provide increased safety for the caregiver or operator and clinical efficacy benefits to the patient. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
       FIG. 1  is a schematic diagram of an embodiment of a dual mode AC/DC impedance measurement apparatus for biopotential electrode interface connection quality of the present invention. 
       FIG. 2  is a partial schematic diagram of a portion of the apparatus showing an alternative embodiment; and 
       FIG. 3  is a partial schematic diagram of a portion of the apparatus showing a further embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic diagram of a dual mode AC/DC impedance measurement apparatus for assessment of electrode interface connection quality. The apparatus is exemplarily shown in an electrocardiographic environment. 
   Referring now to  FIG. 1 , electrodes  10   a - e  are shown applied to the skin of patient  12 .  FIG. 1  shows the conventional designation for such electrodes, such as LA (left arm), LL (left leg), V (chest), etc. Electrodes  10  are connected to corresponding conductors  14   a - e  of a patient connection system  16  that connects the electrodes to an electrical device, such as an electrocardiograph or cardiac monitor. Selected pairs of conductors  14  form electrocardiographic leads. While one chest electrode is shown in  FIG. 1  for illustrative purposes, conventional systems may utilize a plurality of chest electrodes applied to the patient. 
   The impedance measurement apparatus comprises the operational blocks of a carrier signal source  18 , an ECG preamplifier circuit  20 , signal processor  22 , and a lead switching network  24 . The carrier signal source  18  operates to provide AC and DC carrier signals to the electrodes applied to patient  12  through the conductors  14  of patient connection system  16 . The ECG preamplifier circuit collects the ECG signals and the carrier signal for each desired ECG lead using amplifiers such as  26   a ,  26   b ,  26   c , and  26   d  which signals are then sent to signal processor  22 . The signal processor comprises a plurality of anti aliasing filters  28   a - 28   g , a single multiplexed A/D converter  30 , a digital signal processor (DSP)  32  and a microprocessor  34 . The signal processor  22  serves to convert the signals from the preamplifier circuit  20  to digital signals so that the DSP  32  can extract the AC carrier, DC carrier, and ECG signals. These signals are sent to microprocessor  34  and used to determine electrode connection quality, which is then displayed on display  38 . 
   Upon detection of a leads off condition, microprocessor  34  sends a signal via digital signal processor  32  and conductors  72  and  76  to lead switching network  24  comprising lead selector  40  and reference electrode selector  42  whereby lead switching network  24  selects a functional set of ECG leads and a reference electrode to maintain the collection of the ECG signal by the biopotential electrodes. 
   Carrier signal source  18  is connectable to an AC signal source at  44  and a DC signal source at  46 . The AC signal that is supplied to carrier signal source  18  is typically in a range of 240 Hz to 800 Hz. The peak magnitude of the AC signal and DC signal from the signal sources is controlled by variable resistors  48  and  50  respectively. The AC signal is sent to the inverting input of operational amplifier  52  through resistor  54 . The DC signal is sent through a voltage divider formed by resistors  50  and  53  to the non-inverting inputs of operational amplifiers  52  and  56 . The output of amplifier  52  is connected to the inverting input of operational amplifier  56  through resistor  58 . Operational amplifiers are implemented in a conventional manner with feedback being provided via resistors  55  and  57  respectively. This combination of operational amplifiers produces two AC signals with a positive DC offset that are 180° out of phase with each other. One signal is provided in conductor  59 . The other signal is provided in conductor  61 . 
   The output of amplifier  52  is connected to the patient at the patient&#39;s right arm (RA) electrode  10   d  via conductor  59 . This signal sees an impedance shown diagrammatically in  FIG. 1  as Z RA  which is a combination of the lead wire  14   d  impedance, the electrode-to-skin impedance, the patient&#39;s internal impedance, the impedance of electrode  10   d , and the impedance of the conductive component of the electrode. The output of amplifier  56  is connected in the same manner via conductor  61  to the patient&#39;s left arm (LA) at electrode  10   a , chest (V) at electrode  10   e , left leg (LL) at electrode  10   b , and right leg (RL) at electrode  10   c . This signal sees impedances that are similar to Z RA , which are designated as Z LA , Z V , Z LL , and Z RL . It is understood that an alternative design may be implemented with the output of amplifier  52  connected to electrodes  10   a ,  10   b ,  10   c , and  10   e  while the output of amplifier  56  is connected to electrode  10   d.    
   In preamplifier circuit  20 , the ECG signals combined with the injected carrier signals are collected by amplifiers  26   a ,  26   b ,  26   c , and  26   d  to provide the necessary combination of signals from the electrodes to provide ECG lead LI, lead LII, lead LIII and lead LV signals at the outputs of this preamplifier circuit. The lead LI ECG signal is produced by signals collected from the right arm electrode  10   d  and the left arm electrode  10   a , the lead LII ECG signal is produced from the signals from the right arm electrode  10   d  and left leg electrode  10   b , the LIII ECG signal is produced from the signals from the left arm electrode  10   a  and left leg electrode  10   b , and the lead LV ECG signal is produced from the ECG signal from right arm electrode  10   d  and the ECG signal from chest electrode  10   e.    
   The electrode and lead portion of the circuitry shown in  FIG. 1  also include driver  60  which is a common-mode driver amplifier with its input connected to lead selector  40  and its output connected to the appropriate reference electrode by reference electrode selector  42  for common mode interference or noise reduction. 
   The injected AC carrier signal supplied to right arm electrode  10   d  is 180° out of phase with the AC carrier signal that is supplied to electrodes  10   a ,  10   e , and  10   b  so that when the different combinations of electrode signals are processed by preamplifier circuit  20 , there is always a differential AC signal for amplifiers  26   a ,  26   b ,  26   c , and  26   d  during multilead operation. This produces the desired ECG lead signals along with the AC carrier amplitude and DC signal level offsets resulting from the injected carrier signals and the impedances of each of the electrode connections. These ECG lead signals from ECG preamplifier  20  are sent to the signal processor  22 . 
   The ECG LI, LII, LIII and LV lead signals from the ECG preamplifier  20  are substantially similar in nature so the further signal processing of these signals is described, in detail, with respect to the ECG LI lead signal from amplifier  26   a  in conductor  62 . The signal in conductor  62  is sent to an anti-aliasing filter  28   a . Additionally, a reference signal from the reference electrode selected by reference electrode selector  42  and indicated at  64 , is sent to anti-aliasing filter  28   e . A suitable anti-aliasing filter would be a 2 pole low pass filter with a comer frequency at 360 Hz. These signals from the anti-aliasing filters are sent to a multiplexed A/D converter  30  which samples the signals and digitizes them to be processed further by digital signal processor  32 . Digital signal processor  32  employs appropriate filters to extract the AC carrier amplitude, DC signal level, and ECG lead LI signal information. These signals are provided to microprocessor  34  for further analysis. Additionally, microprocessor  34  provides the ECG lead LI signal data through conductor  35  for display on ECG signal display  36 . 
   Digital signal processor  32  provides AC carrier amplitude  66  and DC signal level  68  to microprocessor  34  which uses these signals in conjunction with an appropriate algorithm to generate a quantification of the electrode connection quality for the ECG LI lead. The amplitude of the AC signal in conductor  66  is indicative of the impedance of the electrode connection. The level of the DC signal is used in determining connection or disconnection (failure) of the electrode to the skin. For this reason, this aspect of the signal analysis is more binary in nature. 
   The measurement and quantification of electrode interface quality is aided in the present invention because, since this system for measuring electrode interface connection quality utilizes both AC and DC carrier signals, a better estimate of electrode quality is possible, including a gradation system categorically rating the electrode connection quality. This measurement of electrode connection quality may be sent to electrode connection quality display  38  in conductor  70  so that a real time measurement of electrode connection quality is readily available to the attending clinician. Display  38  may display the value as a categorical gradation of the electrode connection quality. For example, the categorical gradation may include the levels of excellent, good, fair and poor. This visual indication of the electrode connection quality, will enable the clinician to observe any deterioration of electrode connection quality over time. This presentation of electrode connection quality could provide the beneficial effect of providing an advance warning of electrode connection insufficiency which, if good estimates of electrode connection quality can be trended, could translate into 30 minutes to 2 hours of advance warning before a “leads off” condition occurs. 
   Upon detection of a “leads off” or disconnection condition, as determined through analysis of the DC carrier signal, microprocessor  34  utilizes an electrode selection algorithm to select an appropriate combination of the remaining electrodes to maintain proper ECG signal collection. This selection data is sent via conductor  72  to digital signal processor  32 , which in turn sends it to lead switching network  24  via conductor  76 . Reference electrode selector  42  also receives the signal in conductor  76  and appropriately switches the reference electrode in the system. Alternatively, the electrode “leads off” detection may be performed by digital signal processor  32  and the electrode selection performed by microprocessor  34 . The AC and DC signal conditions appearing at the reference electrode  64  are sent along conductor  74  to anti-aliasing filter  28   e  and on to A/D converter  30  to aid in signal analysis by digital signal processor  32 . The lead switching signal sent from microprocessor  34  along conductors  72  and  76  is also received by lead selector  40 . Lead selector  40  selects the active lead to be used by driver  60  in conjunction with the selected reference electrode for common mode noise reduction in the collection of ECG signal data. 
     FIG. 2  is a partial schematic diagram of an alternative embodiment of the dual mode AC/DC impedance measuring apparatus of the present invention. Specifically, in  FIG. 2 , the carrier signal source  118  includes the same basic components as depicted in  FIG. 1 , and have been labeled with analogous numbers. However, a switch  182  is included between the AC voltage source  144  and the variable resistor  148 . Switch  182  allows the clinician to turn off the injected AC signal from source  144  to enable the apparatus to operate in a low emissions DC “quiet” mode. As noted above, an injected AC signal can interfere with the measurement of other concurrently monitored biopotentials such as EEG and EMG. Giving a clinician the ability to deactivate the injected AC signal, when desired, will eliminate or lessen interference with the other concurrently monitored biopotentials while still providing a reliable assessment of electrode connection based on the injected DC signal. 
     FIG. 3  is a partial schematic diagram of a further embodiment of the dual mode AC/DC impedance measuring apparatus. Specifically, in  FIG. 3 , the carrier signal source  218  is shown as substantially the same as the carrier signal source shown in  FIG. 2 , and has been labeled with analogous numbers. However, a switch  285  is disposed between operational amplifier  256 , operational amplifier  252  and conductor  230  which provides the injected carrier signal to the RA electrode  210   d . Switch  185 , which may comprise a 2 to 1 multiplexer, allows the phase of the AC carrier signal injected to the RA electrode  210   d  to be switched, thus allowing the signal source for the Z RA  impedance to be connected as shown in  FIGS. 1 and 2  or to be driven from the same carrier signal source as the other ECG electrodes. This allows the AC carrier component supplied to right arm electrode RA to be 180° out of phase with the others for a normal multi-lead operating mode or in phase with the others when single lead vector operation is required, for example, when the RL electrode connection has failed or is not present and data is collected using only the RA, LA, and LL electrodes. This operation mode is also applicable in other single lead monitoring situations, such as when the RA and RL electrodes are connected and either the LA or LL lead has become disconnected. 
   It is recognized that other equivalents, alternatives, and modifications in addition to those expressly stated, are possible including the replacement of components described herein with their electronic or software counterparts. Various alternatives and embodiments are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter regarded as the invention.