APPARATUS AND METHOD FOR MEASURING BLOOD VESSEL AND CARDIAC CHARACTERISTICS

Abstract: A sensor (10)includes a catheter (12) with a sensor end (14) and an instrumentation end (16), at least one pair of electrodes (24, 26) with associated leads (20, 22) mounted near the sensor end and spaced apart by a predetermined distance and a conductivity measuring and processing network (30) selectively connected to the leads at the instrumentation end to provide signals representative of blood vessel characteristics, e.g. continuous cardiac output, stroke volume on a beat-to-beat basis.

Description: APPARATUSANDMETHODFORMEASURINGBLOODVESSEL ANDCARDIACCHARACTERISTICS The present invention is in the field of medi¬ cal instrumentation, and more particularly relates to instrumentation for the iji vivo measurement of cardiac characteristics. In many applications, it is desired to have accurate continuous measurement of cardiac output, for example, during stress tests, surgery, or during onset of vasodilator therapy. There have been a number of techniques developed in the prior art for measuring car¬ diac output, including those referred to in the prior art as dye dilution, thermal dilution and PO2 respiration (using the Fick method). These dilution techniques require the injection of a fluid in a blood vessel leading to the heart and a time interval for measurement, and thus are based on measurement of blood changes rather than flow, or attempt to compute flow from aortic pressure, or from the impedance plethysmogram. None of these techniques accommodate substantial stroke-to-stroke changes in vascular bed brought on by exercise or various theraputic interven¬ tions, but rather rely on intermittent and time-averaged cardiac output determinations, which are also par¬ ticularly inaccurate during arrhythmias. Prior art continuous output techniques include use of a magnetic flowmeter (which must be surgically implanted) , or imaging (which requires injection of radio-opaque fluid), or doppler technique (non- invasive) . However, none of these techniques provide desired accuracy and safety to patient. It is an object of the present invention to provide a system for continuously measuring the cardiac output. O Pl Another object is to provide an improved system for measuring stroke volume on a beat-to-beat basis. SUMMARY OF THE INVENTION Briefly, the present invention is directed to a cardiac sensor including a catheter. The catheter extends along a central axis and has a sensor (distal) end and an instrumentation (proximal) end. The catheter includes at least one pair of electrodes near its sensor end. A pair of conductive leads are mechanically coupled to and extend along the catheter between the sensor and instrumentation ends. Each lead is coupled to one of the electrodes at its end nearest the sensor end of the catheter. The electrodes of each pair of leads are separated by a predetermined distance along the central axis. The proximal end of each lead, i.e. the end nearest the instrumentation end of the catheter, is adapted for electrical coupling to measurement 'instrumentation. In one form of the invention, the electrodes of each pair of leads are aligned along a reference axis parallel to the central axis, wherein the reference axes of the respective pairs are substantially equidistant from the central axis and uniformly disposed about that central axis. Measurement instrumentation coupled to the instrumentation end of the leads provides continuous measurement of cardiac characteristics such as cardiac output, and also provides measurement of stroke volume on a beat-to-beat basis. In accordance with this inven- tion, the cardiac sensor is adapted for ill vivo place¬ ment within a chamber of the heart. That chamber is then utilized as part of a conductivity cell. The contraction and expansion of the heart chamber walls causes a change in the total impedance (or conductance) for the region of the chamber between and surrounding the two electrodes. The sensor may also be positioned within a blood vessel. TITUTE In one form of the invention, measurement instrumentation coupled to the sensor leads provides a signal representative of this conductivity variation. The instrumentation further provides a signal represent- ative of the peak-to-peak variation of the conductance measured over a stroke (or beat) , corresponding to a measure of the stroke volume. The average of the stroke volume values is multiplied by a signal representative of the heart rate to provide a signal representative of the continuous cardiac output. In some forms of the invention, a temperature compensation thermistor may also be positioned on the end of the catheter within the conductivity cells so that changes in patient tempera¬ ture may also be incorporated in the measurements for better accuracy. Generally, since the conductivity of the blood depends on the packed cell volume, a factor representa¬ tive of this packed cell volume may be determined initially, and used for calibration. This factor may be monitored by an additional pair of electrodes coupled to the catheter and upstream of the pair of conductivity electrodes attached to the principle measurement leads, and preferably out of the vessel being measured and in the flow of a lesser vessel. Since a lower packed cell volume increases conductivity and since the blood flow in a small vessel increases in conductivity with increasing pulsitile flow (cells accumulate in the axial stream surrounded by plasma) , the readout from the addi¬ tional pair of electrodes is representative of the packed cell volume. Information from this pair of electrodes is used to periodically update the calibra¬ tion factor for the conductivity measurements of the conductivity electrodes. Similarly, information is pro¬ vided to monitor the effect of infusions, infections (increase in white blood cells and other factors related to the packed cell volume) . The present invention may also be used in conjunction with instrumentation for Q.y-Pi > , - • W1P analyzing the waveforms representative of the measured conductivity variation to detect the start of fibrilla¬ tion and arrhythmias. The system may also be used to profile a vessel by slowly withdrawing the lead while continuously recording the conductivity on a char recorder. This would provide information relative to the diagnosis of arteriosclerosis, thrombosis, and the like. In yet other forms of the invention, a plurality of pairs of leads (such as n, where n is an integer) may be coupled to the catheter, where each pair has an associated pair of conductivity electrodes separated by a predetermined distance, D, along the central axis of the catheter, with the electrodes of each pair being aligned along a reference axis which is parallel to the central axis. In this form of the invention, the * reference axes for the vario.us pairs of leads may be uniformly disposed about the central axis. With this configuration, similar measurements to those described above may be made, where each measurement is representative of a l/n angular volume segment of the conductivity cell. The information from the n pairs of leads may then be readily processed to provide a three- dimensional mapping of the region within the vessel, i.e. the conductivity cell. This latter form of the invention is particularly useful in identifying certain cardiac characteristics, such as valve disorders, septal defects, and the like. The invention in this form may also be used for a number of other functions, such as roviding EKG signals, temporary cardiac pacing, and detecting region wall movement to identify dead tissue. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing and other objects of this inven¬ tion, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompp-.lying drawings in which: OMPI τ « ϊ-ττr c *--τ ~ τ ~ r - Fig. 1 shows a catheter and measuring system in accordance with the present invention; Fig. 2 shows in block diagram form, an exemplary form of the measuring system of Fig. 1; Fig. 3 shows in schematic form, an exemplary form of the measuring system of Fig. 2y Figs. 4 and 5 show alternative forms for the catheter of Fig. 1; and Fig. 6 shows in block diagram form another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Fig. 1 shows an exemplary cardiac sensor 10 in accordance with the present invention. Sensor 10 includes a catheter 12 which has a sensor end 14 and instrumentation end 16, and which extends along a central axis 18. By way of example, the catheter 12 may generally have the form of a DUCOR catheter, manufac¬ tured by Cordis Corporation, Miami, Florida. A pair of electrically conductive leads 20 and 22 are mechanically coupled to catheter 12 (e.g. through a lumen) and extend along the central axis 18. ..In the illustrated embodi¬ ment of Fig. 1, the leads 20 and 22 are positioned within the outer wall of catheter 12 and thus are not shown in that figure. At the ends of leads 20 and 22 which are closest to the sensor end 14, each lead is terminated by a respective one of conductivity probes 24 and 26. In the present embodiment each of electrodes 24 and 26 is a smooth annular ring of thin wall stainless steel tubing having an outer diameter matching the outer diameter of catheter 12. In fabrication, leads 20 and 22 are brought out to the exterior of, and looped around catheter 12. Then catheter 12 is heated in boiling water and stretched so that the electrodes can be slipped into the desired position. The respective electrodes and associated leads are then cemented together using a silver-loaded (conductive) epoxy. Finally the catheter is exposed to dry heat, returning the diameter of the catheter to its original size. The probes 24 and 26 are separated by a prede¬ termined distance D along axis 18. In the preferred form of the invention, the distance D is selected to be in the range 4-8 centimeters, depending for example on the size heart in which the distal end is to be inserted. Other separations may also be used. As shown, the electrode 26 is spaced by a distance T from the distal end of catheter 10. This spacing ensures that the electrode 26 is spaced from the end wall of the conductivity cell (minimizing the possibility that signals in leads 20 and 22 may themselves provide unde¬ sirable cardiac stimulation. The ends of leads 20 and 22 which are opposite the ends coupled to electrodes 24 and 26 are adapted for electrically coupling to measurement instrumentation, which is represented by a measurement system 30 in Fig. 1. In general, the measurement network 30 provides circuitry for continuously measuring cardiac characteristics of the heart, or beat-to-beat stroke characteristics of a patient's heart. The network 30 may continuously monitor the electrical conductivity between the probes of each of the pairs, or alter¬ natively may periodically monitor that conductivity on a ultiplexed basis. Generally, this network 30 includes a circuit 30a for generating a first signal representative of the time varying electrical conductivity between the probes of at least one of the pairs of probes, and also a cir- BSTITUTΞ SHCΞT cuit (not shown) for generating a second signal repre¬ sentative of the heart rate of the heart in which the sensor end of the catheter is inserted. The heart rate signal is determined by generating a signal having 5pulses corresponding to peaks, or maxima, of the first (or conductivity) signal. An associated computer 52 processes the conductivity variation signal by generating an SV signal representative of the peak-to- peak value of that conductivity variation signal over a 10 stroke of the heart. This SV signal is representative of the stroke volume of the heart for that stroke, or beat, and is displayed on display 54. The average of a suc¬ cession of the SV signals is multiplied by the heart rate signal to provide a CO signal. In the preferred 15 form, the computer 52 includes a microprocessor which is adapted to perform these various functions in a conven¬ tional fashion. The measurement system 30-..for the present embodiment includes a two probe, time-varying 20impedance/conductivity measuring circuit 30a shown in block diagram form in Fig. 2. Th s circuit includes a positive bridge 32 coupled to the leads 20 and 22. Bridge 32 is also coupled by way of pulse transformers 34 to an oscillator 38 and by way of pulse transformer 2542 to rectifier/mixer 44 and amplifier 48. With this configuration, the system 30, in effect, alternately applies a pulse to the two probe electrodes 24 and 26 of the conductivity cell (i.e. the region surrounding electrodes 24 and 26), and then reads the resultant 30change in current through that cell, providing a voltage signal at the output -terminal 49 of amplifier 48 which is representative of the time-varying conduct conduc¬ tivity of that cell. By way of example, Fig. 3 shows a schematic diagram for an exemplary form of the circuit 3530a of Fig. 2, where oscillator 38 is a 25 kHz oscilla¬ tor. 0MPI In measurement systems particularly suited for cardiac monitoring, system 30 may also include a circuit for generating a signal representative of the trigger signals for the heart, where that signal includes a pulse each time the heart is to beat. In this form, the trigger signal as well as the signal from amplifier 48 may be displayed for visual analysis on a chart record 56 (or a CRT display) . Arryth ias may readily be detected by identifying trigger signal pusles for which there is no corresponding variation in the conductivity signal from leads 20 and 22. With this illustrated two probe impedance/ conductivity form of measurement system 30, the entire field in the conductivity cell is monitored by the lectrodes, rather than just the field between two sen¬ sor electrodes, as in the case where a pair of excita¬ tion electrodes might be used'to generate drive a.c. current between two points in the cell and the two sensor electrodes positioned between those excitation electrodes might be used to detect voltage which is representative of conductivity changes between those sensor electrodes. As a direct result of the two probe form for system 30, one of the electrodes may be posi¬ tioned near the valve in the heart with the result that the conductivity variation signal reflects detailed operation of the valve, permitting detection of valve disorders from visual analysis of the conductivity variation waveform. Fig. 4 shows the distal end of an alternative embodiment for the cardiac sensor of Fig. 1 which is particularly adapted for four pairs of electrically con¬ ductive leads and associated conductivity electrodes. In Fig. 4, elements corresponding to similar elements in Fig. 1 are identified by the same reference designa- tions. The catheter in Fig. 4 shows the portion of catheter 12 near the sensor end 24. The sensor of Fig. 4 includes four pairs of electrically conductive leads, where the leads of the first pair are coupled to conductivity electrodes 24a and 26a, respectively, the leads of the second pair are coupled to electrodes 24b and 26b, respectively, the leads of the third pair coupled to electrodes 24c and 26c, respectively, and the leads of the fourth pair are coupled to electrodes 24d (not shown) and 26d (not shown) , respectively. The electrodes for each of the pairs of leads are positioned along an associated reference axis which is parallel to the central axis 18. In Fig. 4, electrodes 24a and 26a are shown to be posi¬ tioned along reference axis 18a, and electrodes 24c and 26c are shown positioned along reference axis 18c. In order to provide this configuration, the end of catheter 12 is constructed of an expandable mesh material and associated inflatable balloon portions associated with portions of the catheter 12 underlying the conductivity electrodes. With this configuration, the catheter 12 may be inserted into a patient's heart, for example, with the balloons deflated, so that the catheter has a relatively small radial dimension. Wnen the sensor end of the catheter is in place, the balloons underlying the conductivity electrodes may be inflated in a conventional manner so that a desired radial separation of the electrodes 24a-d and electrodes 26a-d may be attained. Fig. 5 shows an alternative embodiment of the configuration of Fig. 4 for a non-mesh catheter. In Fig. 5, elements having corresponding parts to those shown in Fig. 4 are identified by identical' reference designations. In this configuration, the end of catheter 12 underlying the conductivity electrodes has longitudinal slits in its outer surface, with underlying inflatable balloons 32 and 34 (shown inflated in Fig. 5). " πιen the catheter is in place, the balloons provide expansion of the region supporting the electrodes 24a-d and 26a-d to provide a similar electrode geometry as that shown in the embodiment of Fig. 4. In yet other embodiments, the number of pairs of conductive leads may differ, for example, there may be two pairs of leads or there may be in general n pairs, where n is an integer. The electrodes for each pair are separated by a predetermined distance D. The conductivity electrodes for these n pairs are uniformly distributed about the central axis 18 with each of the conductivity electrodes being substantially equidistant from the central axis. The catheters of Figs. 4 and 5 as well as those more generally having n pairs of electrodes, may be used in a system 60 having the form shown in Fig. 6. System 60 includes n measurement circuits (denoted 30ι-30 n ) , each of which is similar to circuit 30a and is coupled to a pair of leads from an n-pair catheter simi¬ lar to that shown in Fig. 2. The circuits 30ι-30 n are coupled by way of a multiplexer 62 to a computer 64 having an associated display 66. Generally, circuits 30ι-30 n provide waveform signals representative of the conductivity of an angular segment of the region interior to a conductivity cell, such as a heart chamber or blood vessel. These signals are multiplexed conven¬ tionally and passed to computer 64, which in turn may provide drive signals to display (on display 66) a con¬ ductivity profile of a transverse slice of the cell. In yet other embodiments, the catheter 10 may include additional electrodes along each of the respec¬ tive reference axes. The conductivity between various pairs of these electrodes may be read out in a multiplexed fashion to provide a pluarality of data points representative of a three-dimensional, mapping of the conductivity within the vessel. These data points E ay be displayed using conventional CRT graphics display techniques. By way of example, one such display would provide an on-line three dimensional perspective display of the interior region of the beating heart. The invention may be embodied in other speci¬ fic forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustra¬ tive and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. -

Claims: CLAIMS 1. A sensor comprising: A. a catheter extending along a central axis and having a sensor end and an instrumentation end, B. at least one pair of electrodes and associated electrically conductive leads, said electrodes being positioned at the lead ends nearest said sensor end of said catheter and on the exterior surface of said catheter, and said leads being mechanically coupled to and extending along said catheter between said sensor and instrumentation ends, each lead including means for electrically coupling said lead to measurement instrumentation at its end nearest said instrumentation end of said catheter, wherein the electrodes of each of said pair of leads are separated by a prede¬ termined distance along said central axis, and wherein the electrode closest to said sensor end is spaced from said sensor end by a prede¬ termined distance along said central axis. 2. A sensor according to claim 1 including n pairs of said leads wherein the probes for each pair are aligned along a reference axis parallel to said central axis, and wherein the reference axes of said n pairs are substantially equidistant from said central axis, and uniformly disposed about said central axis. 3. A sensor according to claim 2 further including one or more additional electrodes associated with each of said pairs, said additional electrodes being substan¬ tially aligned with the reference axis of their associ¬ ated pair and being spaced apart along said reference -SIFREA " , OMPI TITUTE -I-. ' -- axis by predetermined distances between each other and from the electrodes of said pair. 4. A sensor according to claim 2 where n is an integer in the range 2-10. 55. A sensor according to claim 3 where n is an integer in the range of 2-10. 6. A sensor according to claim 1 or 2 or 3 wherein said sensor end of said catheter is adapted for in vivo insertion into a blood vessel and further comprising: 0 conductivity measuring means associated with at least one of said pairs of electrodes and including circuit means coupled to said one pair of electrodes for alter¬ nately exciting said electrodes and generating a response signal therefrom, said response signal being representative of the electrical conductivity between . said one pair of electrodes. 7. A sensor according to claim 6 wherein said circuit means further includes means for generating a waveform representative of the electrical * conductivity between said one pair of electrodes. 8. A sensor according to claims 1 or 2 or 3 wherein said sensor end of said catheter is adapted for in vivo insertion into a heart chamber, further comprising: means for generating a waveform signal representative of the electrical conductivity between the electrodes of one or more of said pairs. 9. A sensor according to claim 8 further comprising: means for generating a SV signal representative of the peak-to-peak variation of said waveform signal over a OMPI stroke of said heart, said variation corresponding to the stroke volume of said heart for said stroke. 10. A sensor according to claim 9 further including means responsive to said SV signal and a rate signal representative of the rate of said heart to generate a CO signal representative of the cardiac output of said heart, said CO signal having the product of the average of said SV signals and said rate signal. 11. A sensor according to claims 2 or 3 or 4 or 5 wherein the diameter of the exterior surface of said catheter underlying said electrodes is selectively adjustable at least between the diameter f said catheter near said sensor end and twice t e distance aetween said central axes and one of said reference axes. 12. A sensor according to claim 11 wherein said exterior surface of said catheter underlying and between said electrodes is an expandable mesh material, and wherein said mesh material overlies a selectively inflatable member. 13. A sensor according to claim 12 wherein said exterior surface of said catheter between said electrodes is slit in the direction of said central axis, and wherein that exterior surface of said catheter underlying and between said electrodes overlies a selectively inflatable member.

Inventors: STOREY L
Applicants: CORDIS CORP, STOREY L
Application year: 1980
Main classification: A61B5
IPC codes: A61B5, A61M25