Abstract:
An improved apparatus and method for determining the cardiac output of a living subject. The improved apparatus generally comprises one or more electrode assemblies or patches affixed to the skin of the subject in the vicinity of the thoracic cavity. The terminals of each electrode patch are in contact with an electrolytic gel, and are spaced a predetermined distance from one another within the patch. This predetermined spacing allows for more consistent measurements, and also allows for the detection of a loss of electrical continuity between the terminals of the patch and their associated electrical connectors in the clinical environment. The method generally comprises generating and passing a stimulation current through the terminals and the thoracic cavity of the subject, and measuring the impedance as a function of time. This impedance is used to determine cardiac muscle stroke volume, which is then used in conjunction with the subject&#39;s cardiac rate (also detected via the electrode patches) to determine cardiac output. A method of detecting a loss of electrical continuity in one or more of the terminals of the electrode patch is also disclosed.

Description:
This application is a Request for Continued Examination (RCE) of U.S. patent application Ser. No. 09/613,183 of the same title filed Jul. 10, 2000, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the field of biomedical analysis, and particularly to an apparatus and method for non-invasively determining the cardiac output in a living subject using impedance cardiography. 
     2. Description of Related Technology 
     Noninvasive estimates of cardiac output (CO) can be obtained using impedance cardiography. Strictly speaking, impedance cardiography, also known as thoracic bioimpedance or impedance plethysmography, is used to measure the stroke volume of the heart. As shown in Eq. (1), when the stroke volume is multiplied by heart rate, cardiac output is obtained. 
     
       
         CO=stroke volume×heart rate.  (1) 
       
     
     The heart rate is obtained from an electrocardiogram. The basic method of correlating thoracic, or chest cavity, impedance, Z T (t), with stroke volume was developed by Kubicek, et al. at the University of Minnesota for use by NASA. See, e.g., U.S. Reissue Patent No. 30,101 entitled “Impedance plethysmograph” issued Sep. 25, 1979, which is incorporated herein by reference in its entirety. The method generally comprises modeling the thoracic impedance Z T (t) as a constant impedance, Z o , and time-varying impedance, ΔZ (t), as illustrated schematically in FIG.  1 . The time-varying impedance is measured by way of an impedance waveform derived from electrodes placed on various locations of the subject&#39;s thorax; changes in the impedance over time can then be related to the change in fluidic volume (i.e., stroke volume), and ultimately cardiac output via Eqn. (1) above. 
     Despite their general utility, prior art impedance cardiography techniques such as those developed by Kubicek, et al. have suffered from certain disabilities. First, the distance (and orientation) between the terminals of the electrodes of the cardiography device which are placed on the skin of the subject is highly variable; this variability introduces error into the impedance measurements. Specifically, under the prior art approaches, individual electrodes  200  such as that shown in FIGS. 2 a  and  2   b , which typically include a button “snap” type connector  202 , compliant substrate  204 , and gel electrolyte  206  are affixed to the skin of the subject at locations determined by the clinician. Since there is no direct physical coupling between the individual electrodes, their placement is somewhat arbitrary, both with respect to the subject and with respect to each other. Hence, two measurements of the same subject by the same clinician may produce different results, dependent at least in part on the clinician&#39;s choice of placement location for the electrodes. It has further been shown that with respect to impedance cardiography measurements, certain values of electrode spacing yield better results than other values. 
     Additionally, as the subject moves, contorts, and/or respirates during the measurement, the relative orientation and position of the individual electrodes may vary significantly. Electrodes utilizing a weak adhesive may also be displaced laterally to a different location on the skin through subject movement, tension on the electrical leads connected to the electrodes, or even incidental contact. This so-called “motion artifact” can also reflect itself as reduced accuracy of the cardiac output measurements obtained using the impedance cardiography device. 
     A second disability associated with prior art impedance cardiography techniques relates to the detection of a degraded electrical connection or loss of electrical continuity between the terminals of the electrode and the electrical leads used to connect thereto. Specifically, as the subject moves or sweats during the measurement, the electrolyte of the electrode may lose contact with the skin, and/or the electrical leads may become partially or completely disconnected from the terminals of the electrode. These conditions result at best in a degraded signal, and at worst in a measurement which is not representative of the actual physiological condition of the subject. 
     Another significant consideration in the use of electrodes as part of impedance cardiographic measurements is the downward or normal pressure applied to the subject in applying the electrode to the skin, and connecting the electrical leads to the electrode. It is desirable to minimize the amount of pressure needed to securely affix the electrode to the subject&#39;s skin (as well as engage the electrical lead to the electrode), especially in subjects whose skin has been compromised by way of surgery or other injury, since significant pressure can result in pain, and reopening of wounds. 
     Based on the foregoing, there is a need for an improved apparatus and method for measuring cardiac output in a living subject. Such improved apparatus and method ideally would allow the clinician to repeatedly and consistently place the electrodes at the optimal locations. Additionally, such an improved apparatus and method would also permit the detection of degraded electrical continuity between the electrode terminal and skin, or the electrode terminal and electrical leads of the measurement system, and be adapted to minimize the normal pressure on the subject&#39;s tissue when applying the electrodes and electrical leads. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies the aforementioned needs by providing an improved method and apparatus for measuring the cardiac output of a living subject. 
     In a first aspect of the invention, an improved apparatus for determining the cardiac output of a living subject is disclosed. The apparatus generally comprises: a plurality of electrode assemblies having a plurality of terminals, at least two of the plurality of terminals being spaced from one another by a predetermined distance; a current source capable of generating a substantially constant current; a plurality of electrical leads connecting the current source with individual ones of the terminals of the electrode assemblies, a circuit for measuring the difference in voltage at the terminals resulting from the flow of current through the subject and the terminals; and a circuit for measuring ECG potentials from at least one of the electrode assemblies. In one exemplary embodiment, the subject is a human being. Cardiac stroke volume is measured by applying a constant current to the stimulation electrodes, measuring the resulting voltage differential, and determining the stroke volume from the measured voltage and a predetermined relationship describing intra-thoracic impedance. 
     In a second aspect of the invention, an improved cardiac electrode apparatus is disclosed. The apparatus generally comprises: a substrate having a plurality of apertures formed therein, at least two of the apertures being formed a predetermined distance apart; a plurality of terminals disposed within respective ones of the apertures, at least a portion of each of the terminals being capable of conducting an electrical current; and at least one gel element being adapted to transfer electrical current between the skin of the subject and at least one of the plurality of terminals. In one exemplary embodiment, the electrode apparatus comprises a pair of “snap” terminals disposed a predetermined distance apart within the substrate and which can be readily and positively connected to using jaw-type connectors. The electrode apparatus is adapted to mate uniformly with the skin of the subject, and maintain the desired contact with the skin as well as the predetermined spacing between the electrode terminals. 
     In a third aspect of the invention, an improved method of measuring the cardiac output of a living subject is disclosed. The method generally comprises: providing a plurality of electrode arrays each having a plurality of terminals, at least two of the terminals being spaced a predetermined distance apart; positioning the electrode arrays at respective locations in relation to the thoracic cavity of the subject; generating an electrical current, the current passing from a first electrode of at least one of the electrode arrays through the subject and to a second electrode of at least one of the arrays; measuring the voltage at the second electrode; determining stroke volume from the measured voltage; and determining cardiac output based at least in part on the stroke volume. In one exemplary embodiment, four electrode pairs are utilized, each having predetermined terminal spacing. The electrode pairs are placed at various locations above and below the thoracic cavity of the subject, on both sides of the cavity. Both differential voltage and cardiac rate are measured via the electrode pairs. 
     In a fourth aspect of the invention, a method of monitoring the electrical continuity of a plurality of electrodes in an impedance cardiography system is disclosed. The method generally comprises: providing a plurality of electrically conductive terminals; disposing the terminals in relation to the thoracic cavity of a subject; generating a current between a first of the terminals and a second of the terminals, the current passing through at least a portion of the thoracic cavity; obtaining an impedance waveform from the second terminal; and comparing the impedance waveform to a similar waveform obtained from another of the terminals; wherein the difference between the waveforms is used to evaluate the electrical continuity of the terminals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic diagram illustrating the parallel column model of the impedance of the thoracic cavity of a human being. 
     FIGS. 2 a  and  2   b  are perspective and cross-sectional views, respectively, of a prior art impedance cardiography electrode assembly. 
     FIG. 3 a  is a plane view of a typical human thorax illustrating an exemplary placement of the electrode arrays of the present invention during cardiac output measurement. 
     FIG. 3 b  is a schematic diagram illustrating the measurement of cardiac output using the electrode arrays and current source of the present invention. 
     FIG. 4 is graph of the derivative of the time-variant component ΔZ (t) of thoracic impedance as a function of time, illustrating the systole “peak” used in determining ventricular ejection time (VET). 
     FIG. 5 is a logical flow diagram illustrating one exemplary embodiment of the method of measuring cardiac output within a living subject according to the invention. 
     FIG. 6 is a logical block diagram illustrating one exemplary embodiment of the cardiac output measurement system of the present invention. 
     FIG. 7 a  is an assembly diagram illustrating the construction of a first embodiment of the electrode array of the present invention. 
     FIG. 7 b  is a cross-sectional view detailing the shape of the electrode terminals of the electrode array of FIG. 7 a , and the construction thereof. 
     FIG. 7 c  illustrates top and bottom perspective views of the electrode array of FIG. 7 a  when fully assembled. 
     FIG. 7 d  is a perspective view of a second embodiment of the electrode array of the invention. 
     FIG. 8 is a perspective view of a third embodiment of the electrode array of the invention. 
     FIG. 9 is perspective view of one embodiment of a biased-jaw electrical connector as used in conjunction with the present invention. 
     FIG. 10 is a logical flow diagram illustrating one exemplary embodiment of the method of evaluating electrical lead continuity according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is now made to the drawings wherein like numerals refer to like parts throughout. 
     It is noted that while the invention is described herein in terms of an apparatus and method for determining cardiac output suitable for use on the thorax of a human subject, the invention may also conceivably be embodied or adapted to monitor cardiac output at other locations on the human body, as well as monitoring cardiac output on other warm-blooded species. All such adaptations and alternate embodiments are considered to fall within the scope of the claims appended hereto. 
     Methodology 
     Referring now to FIGS. 3 a - 5 , the general methodology of measuring cardiac output in a living subject according to the invention is described. 
     As previously discussed, the thoracic impedance Z T (t) of a living subject may be modeled as comprising a constant impedance, Z o , and time-varying impedance, ΔZ (t). According to the well known “parallel-column” model of the thorax, this change in thoracic impedance, ΔZ (t), is related to the pulsatile blood volume change. In this model, illustrated in the form of a schematic diagram in FIG. 1 herein, effectively constant tissue impedances such as bone, muscle, and fat are modeled as a conducting volume Z o    102  in parallel with the pulsatile impedance of the blood ΔZ (t)  104 . This second impedance  104  is a time-varying fluid column with resistivity, ρ, cylindrical length, L, and a time-varying cross-sectional area that oscillates between zero and a value A, the latter which correlates to the stroke volume V. When the pulsatile volume is at a minimum in the cardiac cycle, all the conducting tissues and fluids are represented by Z o . During the cardiac cycle, the cylinder cross-sectional area increases from zero until the cylinder&#39;s volume equals the blood volume change. 
     Because Z o  is much greater than ΔZ(t), the relationship of Eqn. (2) holds:                SV   =       ρ        (       L   2       Z   0   2       )          VET               Z        (   t   )                t   min             ,           (   2   )                                
     where L is the distance between the measurement electrodes in cm (FIG. 3 a ), VET is the ventricular ejection time in seconds, and               Z        (   t   )                t   min                              
     is the magnitude of the largest negative derivative of the impedance change occurring during systole in ohms/s. Often, the impedance derivative  400  is purposely inverted as shown in FIG. 4 so that the original negative minimum change will appear as a positive maximum  402 ,                 Z        (   t   )                t   max         ,                          
     in a manner more familiar to clinicians. 
     The ventricular ejection time (VET) is estimated from features in the impedance waveform, which is obtained from the measurement terminals of the electrode arrays  302 ,  304 ,  306 ,  308  placed on various locations of the subject&#39;s thorax as illustrated in FIGS. 3 a  and  3   b . In the present embodiment, a value of 150 ohm-cm is used for the resistivity of the blood, although it will be recognized that other values may be substituted as appropriate. 
     It is noted that the description of the volume of participating tissue may be modified. Rather than model the thorax as a cylinder as shown in FIG. 1 above, the thorax may instead be modeled as a truncated cone (as first described by Sramek and Bernstein). This approach results in a modified stroke volume calculation as in Eqn. (3):              SV   =         L   3       4.25        Z   0            VET                 Z        (   t   )                t   min         .               (   3   )                                
     With either of the two aforementioned approaches (i.e., cylindrical or truncated cone), the pulsatile impedance is estimated using Ohm&#39;s law, which is well known in the electrical arts. Specifically, current from a constant current source, I T (t), is applied, and the resulting voltage, V T (t), is measured in order to calculate the ratio of Eqn. (4):                  Z   T          (   t   )       =           V   T          (   t   )           I   T          (   t   )         .             (   4   )                                
     In the selected frequency range (i.e., 68 kHz), the typical impedance associated with a human subject&#39;s skin is 2 to 10 times the value of the underlying thoracic impedance Z T (t). To aid in eliminating the contribution from skin and tissue impedance, the present invention uses at least two, and typically four electrode arrays  302 ,  304 ,  306 ,  308  for measurement, as shown in FIG. 3 a . The physical construction and these electrode arrays is described in detail with reference to FIGS. 7 a - 8  herein. 
     In a simple application, one electrode array  302  comprising a stimulation electrode terminal  310  and a measurement electrode terminal  312  is applied above the thorax  300  of the subject, while a second electrode array  304  (having stimulation electrode terminal  314  and measurement electrode terminal  316 ) is applied below the thorax  300 . The AC current from the current source is supplied to the stimulation electrode terminals  310 ,  314 . As shown in FIG. 3 b , current flows from each stimulation electrode terminal  310 ,  314  through each constant skin impedance, Z sk1 , or Z sk4 , each constant body tissue impedance, Z b1  or Z b1 , and each constant skin impedance, Z sk2  or Z sk3 , to each measurement electrode terminal  312 ,  316 . The voltages at the measurement electrode terminals  312 ,  316  are measured and input to a differential amplifier to obtain the differential voltage, V T (t). The desired thoracic impedance, Z T (t), is then obtained using the relationship of Eqn. (4). 
     As shown in FIG. 3 a , two sets of electrode arrays may advantageously be used to monitor the impedance associated with the left and right portion of the thorax  300  in the present invention. When eight electrode terminals (four arrays  302 ,  304 ,  306 ,  308 ) are used in this manner, the four measurement arrays are also used to obtain an electrocardiogram (ECG), based on one of four vectors modified from Lead I, I, III, or IV. The resulting electrocardiograms are based on the original Lead configurations, but are not of diagnostic quality. Regardless of the modified Lead configuration used, the Q wave of the ECG QRS interval is used to determine the heart rate and to trigger measurements of VET within the               Z        (   t   )              t                            
     waveform. 
     FIG. 5 illustrates the logical flow of the method of measuring cardiac output according to the invention. As shown in FIG. 5, the method  500  generally comprises first providing a plurality of electrode “arrays” of the type previously described herein per step  502 . The electrode arrays are positioned at predetermined locations above and below the thoracic cavity per step  504 , as illustrated in FIG. 3 a  herein. In one embodiment of the method, these locations are chosen to be on the right and left sides of the abdomen of the subject, and the right and left sides of the neck. These locations, with prior art band electrodes, were first used by Kubicek. Other locations and/or combinations of arrays may be substituted with equal success. 
     Next, a substantially constant AC current is generated in step  506 , and the current applied to the stimulation electrode terminal  310 ,  314  of each of the electrode arrays in step  508 . The voltage generated at the measurement electrode terminal  312 ,  316  of each electrode array is next measured in step  510 . As previously discussed, this voltage is generally reduced from that applied to the stimulation electrode by virtue of the impedance of, inter alia, the thoracic cavity. Note that the measured voltage may be absolute, or relative (i.e., a differential voltage) as desired. Next, in step  512 , the cardiac stroke volume from the measured voltage, using for example the relationship of Eqn. (3) above. Cardiac rate (step  514 ) is also determined by using the measurement electrodes to sense the ECG potentials generated by the heart of the subject. Lastly, in step  516 , cardiac output is determined based on the stroke volume determined in step  512  and the cardiac rate in step  514  using the relationship of Eqn. 1 above. 
     Apparatus 
     Referring now to FIG. 6, the apparatus for measuring cardiac output using the above-described technique is disclosed. In addition to the four electrode arrays  302 ,  304 ,  306 ,  308  previously discussed, the system  600  generally comprises an alternating current (AC) current source  604  capable of generating a substantially constant current, a plurality of electrical leads in the form of a multi-ended lead assembly  606  for connecting the instrument monitor  607  to the individual terminals of the electrode arrays  302 ,  304 ,  306 ,  308 , a processor  608  with associated algorithms capable of running thereon for performing analysis of the signals measured from the measurement terminals, data and program memory  609 ,  610  in data communication with the processor  608  for storing and retrieving program instructions and data; an I/O interface  611  (including analog-to-digital converter) for interfacing data between the measurement electrodes and the processor  608 ; a mass storage device  612  in data communication with the processor for storing and retrieving data; a display device  614  (with associated display driver, not shown) for providing an output display to the system operator, and an input device  616  for receiving input from the operator. It will be recognized that the processor  608 , memory  609 ,  610 , I/O interface  611 , mass storage device  612 , display device  614 , and input device  616  (collectively comprising the instrument monitor  607 ) may be embodied in any variety of forms, such as a personal computer (PC), hand-held computer, or other computing device. The construction and operation of such devices is well known in the art, and accordingly is not described further herein. 
     The applied current derived from the current source  604  is a 70 kHz sine wave of approximately 2.5 mA peak-to-peak. The measured voltage associated with the aforementioned sine wave is on the order of 75 mV peak-to-peak. These values are chosen to advantageously minimize electric shock hazard, although it will be appreciated that other frequencies, currents, or voltages may be substituted. The construction and operation of AC current sources is well known in the electronic arts, and accordingly is not described further herein. 
     The electrode lead assembly  606  of the illustrated embodiment contains a ten wire assembly (two wires are left unused) that branches to eight individual connectors  606   a-h . The conductors  610   a-h  of the lead assembly are fashioned from electrically conductive material such as copper or aluminum, and are insulated using a polymer-based insulation having the desired dielectric strength as is well known in the electrical arts. The length of the conductors may also be controlled so as to match the impedance of each individual conductor to that of the others within the assembly  606 . 
     Using one of four modified Lead configurations, the body surface potential is measured between two measurement electrodes. This time-varying voltage reflects the electrical activity of the heart, and contains one QRS interval per cardiac cycle. The biopotential is analyzed to identify each QRS complex. The frequency of QRS complexes determines the heart rate. The Q wave within the QRS complex is then used to trigger identification of VET within the               Z        (   t   )              t                            
     waveform, as the opening of the aortic valve (the beginning of VET) occurs after the appearance of the Q wave. 
     Referring now to FIGS. 7 a - 7   c , the electrode arrays  302 ,  304 ,  306 ,  308  of the invention are described in detail. As illustrated in FIG. 7a, each array comprises a flexible substrate  704  having a plurality of apertures  706 ,  708  formed therein. In the illustrated embodiment, two terminals  310 ,  312  are disposed through the apertures such that the top portions  716 ,  718  of the terminals project above the plane of the substrate  704 . The two terminals  310 ,  312  comprise a stimulation terminal  310  and measurement terminal  312  as previously described with respect to FIG. 3 a . The stimulation terminal  310  is used to apply the potential necessary to generate the current flowing through the thoracic cavity of the subject. It will be noted that despite designation of one terminal as a “stimulation terminal” and one as a “measurement” terminal, the role of these terminals may be reversed if desired, since they are functionally and physically identical but for the potential applied thereto (or measured therefrom). It is noted that the asymmetric shape of the substrate  704  of the embodiment of FIGS. 7 a - 7   c  may be used to assist the clinician in rapidly determing which electrode is the stimulation electrode and which the measurement electrode, such as by assigning a convention that the end of the array having a given shape always contains the stimulation electrode. Additionally, the substrate may be shaped to adapt to certain physical features of the patient, such as by using a substrate having a broader width so as to better conform to the generally cylindrical shape of the subject&#39;s neck. Any number of different substrate shapes may be employed; FIG. 7d illustrates one such alternative shape. 
     As shown in FIGS. 7 a - 7   c , The terminals  310 ,  312  are firmly held in place within the substrate  704  at a predetermined distance  705  by a mounting element  707  or any one of a variety of other constructions as will be described in greater detail below. The distance (measured centerline-to-centerline on the terminals  310 ,  312 ) is approximately 5 cm in the embodiment of FIG. 7 a , although it will be recognized that other distances may be substituted. Desired distances may be determined through experimentation, anecdotal observations, calculations, or any other suitable method; however, experimental evidence obtained by the Applicant herein indicates that a distance of 5 cm is optimal for impedance cardiography measurements. 
     The substrate  704  in the embodiment of FIG. 7 a  is formed from a Polyethylene foam, although other materials such as cloth or vinyl may be substituted. The polyethylene foam is chosen for its compliance and flexibility, thereby allowing it to conform somewhat to the contours of the subject&#39;s anatomy, while still maintaining sufficient rigidity for maintaining the terminals  312 ,  314  in the desired position and orientation. 
     As shown in FIG. 7 b , the terminals  310 ,  312  of each array comprise a generally cylindrical shaped sidewall portion  720  having a first diameter  722 , and a top portion  724  having a second diameter  726 , the second diameter  726  being greater than the first diameter  722  in order to assist in retaining a connector mated to the terminal  310 ,  312  as described in greater detail below. The outer wall  721  of the sidewall portion  720  is essentially vertical in orientation (i.e., parallel to the central axis  725  of the terminal  310 ,  312 ), while the top portion is progressively rounded as shown. The terminals may be manufactured from an extruded metal such nickel, with a coating of brass, or may be molded from carbon. Alternatively, the terminals may be molded of plastic, and coated with a metal such as gold or impregnated with carbon. The extruded metal possesses the advantage of low cost, while the molded plastic impregnated with carbon possesses the advantage of radiolucency. A terminal molded of plastic and coated with gold may possess low noise artifact. 
     The terminals  310 ,  312  of the electrode array comprise a two piece construction, having an upper terminal element  730  and a lower terminal element  732  as shown in FIGS. 7 a  and  7   b . The post  734  of the lower terminal element  732  is adapted to be frictionally received within the cavity  736  of the upper terminal element when the two components are mated. In this fashion, the upper and lower elements  730 ,  732  form a single unit when assembled, with the mounting element  707  being frictionally held or “pinched” between the lower surface  740  of the upper element  730  and the upper surface  742  of the lower element  732 . The post  734  of the lower element perforates the mounting element  707 , or alternatively penetrates through a pre-existing aperture  738  formed therein. The lower elements  730 ,  732  of the electrode array terminals  310 ,  312  are coated with Ag/AgCl, although other materials with the desirable mechanical and electrochemical properties such as Zinc Chloride may be used if desired. 
     The electrolytic element  750  of each electrode array comprises an electrolytic gel of the type well known in the bio-electrical arts; in the present embodiment, the gel comprises an ultraviolet (UV) cured potassium chloride (KCl) gel, although it will be recognized that other types of compounds or materials may be used. UV curing of the gel allows the element  750  to have a more solidified consistency and improved mechanical properties, thereby preventing excessive spreading or thinning of the element when the array is applied to the subject while still maintaining its overall adhesiveness and electrolytic properties. As shown in FIGS. 7 b  and  7   c , the element  750  is sized so as to encompass the edges  752  of the respective aperture  706 ,  708  in the substrate  704  over which it is placed when assembled, although other configurations may be used. The top portion  755  of the element  750  fits at least partially within the aperture  706 ,  708  and conforms substantially thereto, thereby effecting contact with the bottom surface  760  of the bottom terminal element  732 . In this way, ions are passed between the skin of the subject and the terminals of the array via the gel element  750 . The gel also provides for adhesion of the array to the skin of the subject, although the array of the present embodiment also includes a separate adhesive  762  which is applied to the bottom surface of the substrate  704 , as shown in FIG. 7 c.    
     Since the placement of the electrolytic element  750  with respect to the terminals  310 ,  312  of the array may in certain cases affect the ultimate measurements of cardiac output obtained with the system, the gel of the element  750  is advantageously placed in the embodiment of FIGS. 7 a-c  so as to be symmetric with respect to the terminal  310 ,  312 . It will be recognized, however, that the element(s)  750  may also be placed so as to produce certain desired electrolytic conditions. Similarly, the element  750  may be split into two or more component parts if desired. 
     Furthermore, it is noted that while the embodiment of FIGS. 7 a-c  employs two fixed terminals that are effectively immovable within the substrate, means for allowing adjustment or change of the relative position of the terminals may be substituted. For example, as illustrated in FIG. 8, a terminal array having three terminal posts may be used, the second post  802  being spaced a first distance  804  from the first post  806 , and the third post  810  being spaced a second distance  808  from the first post  806 , such that the clinician can select one of two terminal spacings as desired. 
     As illustrated in FIG. 9, each electrode lead assembly connector  606   a-h  is designed to mitigate the downward force required to mate the connector with its respective electrode array terminal. Specifically, each connector  606   a-h  contains two spring-biased conductive jaws  902  that are spread apart by the cam surface  904  of an actuator button  906  disposed on the front  907  of the connector body  908 . The connector jaws  902  and bias mechanism are designed to allow the upper and sidewall portions  724 ,  720  of the electrode terminal  310 ,  312  (FIG. 7 b ) to be received within the recess  910  of the jaws  902  when the button  906  is fully depressed. In this fashion, effectively no downward force is required to engage the connector to its respective terminal. The jaws  902  are contoured to engage substantially the entire surface of the sidewall portion  720  of the terminal when the actuator button  906  is released. Since the sidewall portion  720  of the terminal is effectively circular in cross-section, the connector may advantageously rotate around the axis of the terminal  310 ,  312  when lateral tension is applied to the conductor attached to that connector. U.S. Pat. No. 5,895,298 issued Apr. 20, 1999, entitled “DC Biopotential Electrode Connector and Connector Condition Sensor,” and incorporated herein by reference in its entirety, describes a bias jaw electrical connector of the type referenced above in greater detail. 
     When used with the four two-terminal electrode arrays  302 ,  304 ,  306 ,  308  shown in FIG. 3 a , each connector  606   a-h  is fastened to one of the two terminals  310 ,  312  of an electrode array. The 68 kHz constant current is applied from the current source to four electrode terminals (i.e., one terminal per array). Hence, complete circuits are formed between the current source and the I/O device  611  of the system  600  via the electrical conductors and connectors associated with the stimulation electrode terminals, the stimulation electrode terminals themselves, the thorax of the subject, the measurement terminals, and the electrical conductors and connectors associated with the measurement terminals. 
     Method of Evaluating Electrical Continuity 
     Referring now to FIG. 10, the method of evaluating the electrical continuity of one or more leads within the system is described. Note that while the following description is based on the two-terminal array configuration (FIGS. 7 a - 7   c ) and the use of four arrays as shown in FIG. 3 a , the method may be applied to many alternate configurations with equal success. 
     First, in step  1002 , the electrode arrays are disposed on the skin of the subject. The position at which the electrode arrays are disposed on the subject are measured in relation to the thoracic cavity as illustrated in FIG. 3 a , or alternatively may be inferred by the weight and height of the subject. Next, a current is generated between the stimulation electrodes and the measurement electrodes of the respective arrays in step  1004 . As previously discussed, the current passes through at least a portion of the subject&#39;s thoracic cavity, encountering a time-variant impedance therein. 
     An impedance waveform is then measured from two or more of the measurement terminals of the arrays in step  1006 . The waveforms comprise measurements of impedance as a function of time, which is well known in the cardiographic arts. These measured waveforms are then compared to one another in step  1008  to detect changes or variations between them. In the present embodiment, two waveforms are differenced by way of a simple differencing algorithm resident on the processor  608  of the system  600  (FIG.  6 ), although it will be recognized that other approaches may be used. For example, the base impedance may be calculated for the left and right sides. The larger base impedance may then be subtracted from the smaller base impedance, with this difference then divided by the smaller impedance. The resulting percentage ratio, when greater than a predetermined threshold value, may represent the presence of detached or loose electrodes. While some variation between the waveforms is normal, significant variations are indicative of either a degraded electrical connection, such as between the electrode array terminal and its respective connector, or between the electrolytic gel and the skin of the patient, or even the gel and the terminal of the array or between the cable and connector. A threshold value is determined and set by the operator of the system in step  1010  such that when the threshold “difference” is exceeded as determined by the aforementioned algorithm (step  1012 ), the operator will be alerted to the degraded condition such as by a visual or audible alarm in step  1014 . 
     It is noted that the use of the multi- terminal electrode arrays having predetermined and substantially equal terminal spacing as previously described allows such comparisons between electrode waveforms to be made; errors resulting from uncontrolled spacing of the terminals are effectively eliminated. Using prior art electrodes, the aforementioned method would be largely ineffective, since these error sources would force the threshold value to be set artificially high, thereby potentially masking conditions of degraded electrical continuity which could affect the ultimate accuracy of and cardiac output estimation made by the system. 
     It will be recognized that while certain aspects of the invention have been described in terms of a specific sequence of steps of a method, these descriptions are only illustrative of the broader methods of the invention, and may be modified as required by the particular application. Certain steps may be rendered unnecessary or optional under certain circumstances. Additionally, certain steps or functionality may be added to the disclosed embodiments, or the order of performance of two or more steps permuted. All such variations are considered to be encompassed within the invention disclosed and claimed herein. 
     While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the invention. The foregoing description is of the best mode presently contemplated of carrying out the invention. This description is in no way meant to be limiting, but rather should be taken as illustrative of the general principles of the invention. The scope of the invention should be determined with reference to the claims.