Dual indifferent electrode

Abstract: An endocardial lead (40) having a sensing electrode (28) coupled to a sense amplifier (14) and a carrier signal driving electrode (30). An oscillator (22), providing alternating current carrier signals (22) of a predetermined frequency, is coupled to the driving electrode (30). A third electrode (12) is in electrical contact with body tissues. A pacer can (10) functions as a sensing electrode and has a plastic top wherein the third electrode is located. The pacer can (10) is coupled to the sense amplifier (14). A demodulator and filters circuit (18) for demodulating the modulated carrier signal and recovering the modulating signal therefrom is connected to the output of the sense amplifier (14) and develops a control signal, which is applied to the pulse generator (44) to control the rate of stimulating pulses to the heart. In a modified tripolar embodiment, excluding the sensing electrode (28), sensing is carried out by the pacer can (10) and the electrode (30) carried by the lead. In an alternative tripolar embodiment, sensing is carried out by the electrode carried by the lead and the third electrode (30,12) and the oscillator (22) is connected to the pacer can (10).

Description: FIELD OF THE INVENTION This invention relates broadly to the art of implantable medical devices and, more particularly, to apparatus having dual indifferent electrodes which allow the implementation of an implanted tetrapolar impedance system that requires only a bipolar pacing lead. Such an apparatus finds use in a tetrapolar impedance system that provides a stroke volume signal and a ventilatory signal using any bipolar pacing lead. In a further aspect, the dual indifferent electrode of the invention also facilitates a tripolar impedance technique using only a unipolar endocardial lead. BACKGROUND OF THE INVENTION The stroke volume of the heart has been recognized as providing a useful signal to control the timing circuit of a demand-type cardiac pacer. In such a system, the pacer pulse generator will output stimulating pulses in accordance with the physiologic demand indicated by stroke volume changes in the patient's heart. In US Patent 4,686,987 to Salo, et al., entitled "Biomedical Method and Apparatus for Controlling the Administration of Therapy to a Patient in Response to Changes in Physiologic Demand", a biomedical apparatus capable of sensing changes in the heart's ventricular volume or stroke volume is disclosed. The apparatus changes the operating performance of the device as a function of stroke volume. The teachings of US Patent 4,686,987 are hereby incorporated by reference. Salo, et al. teaches that a relatively low frequency signal (under 5 KHz) is applied between spaced electrodes disposed in the heart. The beating action of the heart serves to modulate the signal due to changes in impedance being sensed between these or other electrodes within the heart. The modulated carrier signal is processed to remove R-Wave and other electrical artifacts and then demodulated to remove the carrier frequency component, leaving an envelope signal which is proportional to instantaneous ventricular volume. This envelope signal then contains stroke volume and ventricular volume information which can be used by the biomedical apparatus to vary its operating parameters. For example, a current proportional to changes in the stroke volume may be injected into the timing circuit of a demand-type cardiac pacer pulse generator whereby the interpulse interval of the pulse generator is varied as a function of stroke volume. US-A-5137019 recognizes that the ventilatory signal also appears as a component of the impedance signal. Because the intrathoracic pressure is directly related to ventilation (ie pressure drops during inspiration and increases during expiration), the amplitude of the variation in intrathoracic pressure during a ventilatory cycle is directly related to the depth of ventilation (ie respiration). US-A-5137019 provides an impedance system for measurement of right ventricular (or atrial) volume or a pressure transducer for measurement of right ventricular (or atrial) pressure, a signal processing means to extract one of the volume or pressure parameters on a beat-by-beat basis to thereby yield a signal varying at the ventilatory rate and with a peak-to-peak amplitude proportional to ventilatory depth. Referring again to the Salo, et al. patent, for example, a cardiac lead having two sensing electrodes and a stimulating electrode is used. Often, in the case of a cardiac pacer replacement, a bipolar lead having only two electrodes has previously been implanted in the heart. In such cases, since it is desirable to use the already implanted lead with a new pacemaker system in the case of, for example, replacing a worn-out pacemaker, the three electrode lead as used by Salo, et al. is often not available. In such cases, only three electrodes are typically available, namely, the pulse generator case or can, a lead ring on the endocardial lead and a tip electrode on the endocardial lead. Prior approaches to implementing an intracardiac impedance system with only three electrodes available have used at least one electrode as a simultaneous drive and sense electrode, since two drive and two sense points are required. Such approaches have several disadvantages. One disadvantage of prior art techniques results from a high current density region being sensed at the "common" electrode (ie, the electrode being used as both a drive and sense electrode) making it very sensitive to local effects such as, for example, mechanical motion. Another disadvantage of prior art systems results from the interface impedance at the common electrode which presents a large DC offset when sensed, yielding a lower modulation index relative to that experienced with tetrapolar impedance. Yet another drawback of prior art systems is that if the common electrode is on the pacemaker lead, either the ring or the tip, system performance will vary as a function of electrode material, effective surface area, geometry and various other electrode characteristics. EP-A-0249819 discloses a cardiac pacer, wherein the pacing rate is controlled by the respiration of the patient. The pacer has a pacing electrode and a metallic housing which acts as a common electrode and defines both the indifferent electrode and the sensing electrode. The present invention overcomes the problems of using common electrodes by providing a variable rate cardiac pacer apparatus as set out in Claims 1, 2 and 3. The precharacterising parts of Claims 1, 2 and 3 are based on EP-A-0249819, and the distinguishing features of the present invention are set out in the characterising parts of these claims. One embodiment of the present invention uses tetrapolar impedance techniques and overcomes the above described disadvantages of prior art devices. Since this embodiment of the present invention effectively implements a tetrapolar impedance system that provides a stroke volume signal using any bipolar pacing lead, the quality of the sensed stroke volume signal equals that of a tetrapolar system using a pulse generator can and a tripolar pacing lead. In a further aspect, the signal sensed with the present invention contains a lower frequency component due to ventilation. This component may be extracted as it is related to tidal volume and may be used as another rate controlling parameter. The present invention also affords an advantage even when used on a unipolar pacing lead. Although a tetrapolar method is not possible for intra-cardiac use in such a case, the dual indifferent method possible with the apparatus of the invention allows a tripolar technique. This has the advantages of reduced motion artifact at the pacer can, as well as a lower DC offset. The present invention provides a modification of the invention of EP-A-0450229. The present invention provides apparatus for use in a variable rate pacer apparatus responsive to the metabolic needs of the patient. In carrying out pacing with the apparatus of the present invention and in accordance with a first embodiment, an endocardial lead having first and second electrodes resides in a patient's heart. The first electrode is a sensing electrode and the second electrode is a carrier signal driving electrode. The lead has conductors coupling a source of alternating current carrier signals of a predetermined frequency to the second electrode. A third electrode is in electrical contact with body tissues. The pacer can functions as a fourth electrode and has a plastic top wherein the third electrode is located. The third electrode is coupled to the carrier signals and acts in cooperation with the second electrode to form a pair of driving electrodes. The first electrode and the can may comprise a sensing electrode pair. The sensing electrode pair is coupled to a sense amplifier means for receiving and amplifying modulated electrical signals developed across the sensing electrode pair. A demodulator and filters circuit means for demodulating the modulated carrier signal and recovering the modulating signal therefrom is connected to the output of the sense amplifier means. The modulating signal contains components proportional to instantaneous stroke volume of the patient's heart and the patient's ventilatory tidal volume, and the demodulator and filters circuit develops control signals therefrom called stroke volume and ventilation signals respectively. The control signals may be applied individually or in combination to the pulse generator so as to control the rate of stimulating pulses in accordance with a predetermined rate control algorithm. It is one object of the invention to provide an implantable tetrapolar impedance system that requires only a bipolar endocardial lead. It is another object of the invention to provide an implantable tripolar impedance system that requires only a unipolar endocardial lead. It is an object of a preferred embodiment of the invention to provide a button electrode electrically isolated from a pacemaker can and having a surface area on the same order as that for a lead ring electrode of an endocardial lead. It is yet another object of the invention to provide an apparatus for effective implementation of a tetrapolar impedance system that provides a stroke volume signal using any bipolar pacing lead wherein the quality and pulsatile morphology of the signal equals that of a tetrapolar system using a pulse generator can and a tripolar pacing lead as electrodes. It is yet another object of the invention to provide an apparatus for an effective implementation of a tetrapolar impedance system that provides a ventilation signal free from can motion artifact using any bipolar pacing lead. Other objects, features and advantages of the invention will become apparent to those skilled in the art through the description claims and drawings herein. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows a pacer apparatus having a dual indifferent electrode apparatus; Figure 2 schematically shows an embodiment of a dual indifferent electrode apparatus; Figure 3 schematically shows an alternative embodiment of a dual indifferent electrode apparatus employing a unipolar endocardial lead; and Figure 4 schematically shows an alternative embodiment of a dual indifferent electrode apparatus for a pacer in which the drive voltage is applied between the pacer can and a button, or spot, electrode. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figure 1 there is diagrammatically shown a side view of a pacemaker apparatus 2 comprised of a conductive metal can 10 and an insulating top or header 11. Mounted in the top 11 and isolated from the metal can 10 is a button electrode 12. Contained within the can 10 is electronic circuit 100 which is explained in more detail below and which comprises the dual indifferent electrode circuitry. Now, referring to Figure 2, the circuit 100 is shown in more detail. An endocardial lead 40 is connected to a pulse generator 44 which is contained within the pacemaker 2. The lead 40 includes electrodes 28 and 30 located within one of the chambers of the heart 50. Electrode 30 may be, for example, a stimulating tip electrode on a catheter type lead while electrode 28 may be, for example, a ring electrode. Insulator lead body 42 mechanically supports electrodes 28 and 30. An oscillator 22 serves as a carrier current source and is arranged to produce an alternating current carrier signal at a frequency which is quite high compared to the heart rate. Typically the carrier signal is in the range of from about 500 to 20000 Hz. Button electrode 12 has a surface area typically on the same order of magnitude as the surface area of ring electrode 28 and is advantageously disposed on the plastic top 11 of the implantable pacemaker 2. The oscillator 22 is connected between the tip electrode 30 and the button electrode 12. Sensing takes place between the conductive metal can 10 and the ring electrode 28. The ring electrode 28 is connected via lead 32 to a first input of a differential amplifier 14 and a conductor 52 connects the can 10 to a second input of the amplifier. The output of differential amplifier 14 is carried via conductor 16 into demodulator and filters circuit 18. The demodulator and filters circuit 18 is connected by line 20 to the pulse generator. The demodulator and filters circuit 18 may include signal processing circuits as are shown in US Patent 4,686,987, as well as filtering means to separate the higher frequency stroke volume signal from the lower frequency ventilation signal as shown in US-A-5137019 . In operation, the pulse generator 44 provides stimulating pulses to stimulating electrodes in a well known manner to pace the heart. The ring electrodes 28 and the can 10 sense electrical impedance variations in the thoracic cavity which may be due to the pumping action of the heart or other physiological signals of interest. The signals fed into the sense amplifier 14 reflect impedance changes in the body tissue present between the location of the can 10 in the chest wall and the ring electrode 28 which would typically be in the right atrium of the heart. This modulation is removed by the demodulator and filters 18, resulting in a ventilatory signal component on line 21 (VENT SIGNAL) and a stroke volume related signal on line 20 (SV SIGNAL). These signals may be used individually or may be combined together in a predetermined algorithm with the resulting control signal used to adjust the rate at which the pulse generator 44 emits pacing pulses to the tip electrode 30. Now referring to Figure 3, an alternative embodiment of the invention is shown as employed with a unipolar endocardial lead 40A. In this embodiment, it will be understood that circuit 100A is similar to circuit 100 except that it is modified to accommodate unipolar pacing and sensing techniques. The endocardial lead 40A is connected to the pulse generator 44 which is contained within the pacemaker 2. Lead 45 connects the pulse generator to the can 10 which, in this case, serves as a stimulating electrode. The endocardial lead 40A includes an electrode 30 located within one of the chambers of the heart 50. The electrode 30 may be, for example, a tip electrode on a catheter type lead. The oscillator 22 is connected between the tip electrode 30 and the button, or spot, electrode 12, and is arranged to produce an alternating current carrier signal at a frequency which is quite high compared to the heart rate, in the range of from 500 to 20000 Hz. Button electrode 12 has a surface area typically on the same order of magnitude as the surface area of electrode 30 and is advantageously disposed on the plastic top 11 of the implantable pacemaker 2. The first input to the differential sense amplifier 14 is connected by a conductor 54 to the metal can 10 while a conductor 32A joins the second input of the differential sense amplifier 14 to the tip electrode 30. The output of differential amplifier 14 is carried via conductor 16 into demodulator and filters circuit 18. The demodulator and filters circuit 18 are connected by lines 20 and 21 to the pulse generator. The circuit 18 is configured as described above with reference to Figure 2. In operation, the pulse generator 44 provides stimulating pulses to stimulating electrodes in a well known manner to pace the heart. The tip electrode 30 and the can 10 sense stroke volume impedance signals or other physiological signals of interest. The signals are fed into the differential amplifier 14 which provides a differential signal to the circuit 18. The demodulator and filters circuit operates as described above with reference to Figure 2. In the embodiment shown in Figure 4, the oscillator 22 is coupled between the pacer can 10 and the button electrode 12. Sensing occurs between the monopolar tip electrode 30 on the pacing lead 42 and the button electrode 12. When a high frequency signal which may range between 500 Hz and 20000 Hz is applied between the can 10 and button electrode 12, the signal developed between the button electrode 12 and the tip electrode 30 and applied to the sense amplifier 14 is found to be modulated primarily by ventilatory activity rather than systolic activity. Thus, the output from the demodulator and filters circuit 18 on line 21 is significantly more dominating than the stroke volume signal present on line 20. The control signal proportional to stroke volume change may be used in conjunction with an implantable infusion pump for administering such drugs as dobutamine, isoproterenol or nitroprusside whereby stroke volume may be maintained at a desired value. Alternatively, the demodulated waveform or control signal may be used directly by other diagnostic equipment. By appropriately utilizing the information derived from the ventricular impedance, it would be possible to measure stroke volume without having to resort to thermal dilution or other techniques.

Claims: A variable rate cardiac pacer apparatus responsive to metabolic needs of a patient, wherein the cardiac pacer apparatus includes: a) a pulse generator (44); b) a source (22) of alternating current carrier signals of a predetermined frequency; c) an endocardial lead (40) having a first electrode (28) disposed therein; d) a pacer housing including a conductive pacer can (10); e) sense amplifier means (14) having first and second inputs and an output (16), said inputs of the amplifier means (14) being individually connected to the first electrode (28) and to the pacer can (10), for providing at the output of the amplifier means a modulated signal proportional to the instantaneous tissue impedance within the thoracic cavity, f) demodulator and filters circuit means (18) for demodulating said modulated signal and recovering a demodulated signal therefrom, said demodulated signal also being proportional to instantaneous impedance within the thoracic cavity wherefrom the demodulator and filters circuit means (18) develops a control signal consistent with the instantaneous impedance in the thoracic cavity; and g) means (20,21) for applying said control signal to said pulse generator (44) for modifying the pacing rate thereof; characterised by: h) a second electrode (30) disposed in the lead (40) and i) a third electrode (12) mounted on said pacer housing and insulated from the pacer can, the third electrode (12) being contactable with body tissues of the patient, and the source of alternating current carrier signals being connected between the second electrode (30) and the third electrode (12). A variable rate cardiac pacer apparatus responsive to metabolic needs of a patient, wherein the cardiac pacer apparatus includes: a) a pulse generator (44); b) a source (22) of alternating current carrier signals of a predetermined frequency; c) a unipolar lead (40A) having a first electrode (30); d) a pacer housing including a conductive pacer can (10); e) sense amplifier means (14) having first and second inputs and an output (16), said inputs of the amplifier means (14) being individually connected to the first electrode (30) of the unipolar lead and to the pacer can (10) for providing at the output of the amplifier means a modulated signal proportional to the instantaneous tissue impedance within the thoracic cavity; f) demodulator and filters circuit means (18) for demodulating said modulated signal and recovering a demodulated signal therefrom, said demodulated signal also being proportional to instantaneous impedance within the thoracic cavity wherefrom the demodulator and filters circuit means (18) develops a control signal consistent with the instantaneous impedance in the thoracic cavity; and g) means (20,21) for applying said control signal to said pulse generator (44) for modifying the pacing rate thereof; characterised by h) a second electrode (12) mounted on said housing and insulated from the pacer can (10), the second electrode being contactable with body tissues of the patient, and the source (22) of alternating current carrier signals being connected between the first electrode (30) and the second electrode (12). A variable rate cardiac pacer apparatus responsive to metabolic needs of a patient, wherein the cardiac pacer apparatus includes: a) a pulse generator (44), b) a source (22) of alternating current carrier signals of a predetermined frequency; c) an endocardial lead (40) having a first electrode (30) disposed thereon; d) a pacer housing including a conductive pacer can (10); e) sense amplifier means (14) having first and second inputs and an output; f) demodulator and filter circuit means (18) for demodulating a modulated signal and recovering a demodulated signal therefrom, said demodulated signal being proportional to instantaneous impedance within the thoracic cavity wherefrom the demodulator and filters circuit means (18) develops a control signal consistent with the instantaneous impedance in the thoracic cavity; and g) means (20,21) for applying said control signal to said pulse generator (44) for modifying the pacing rate thereof; characterised by h) a second electrode (12) mounted on said housing and insulated from the pacer can (10), the second electrode being contactable with body tissues of the patient, the source of alternating current carrier signals being connected between said conductive pacer can (10) and said second electrode (12) and said first and second inputs of said sense amplifier means (14) being individually connected to said first electrode (30) and to said second electrode (12) for giving at the output of said sense amplifier said modulated signal, the modulated signal being proportional to the instantaneous tissue impedance within the thoracic cavity. The apparatus of Claim 1, 2 or 3 wherein the modulating signal includes a ventilatory signal component, the apparatus having means following demodulation to recover the ventilatory signal component and, said signal being proportional to instantaneous ventilation, to develop a heart rate control signal consistent with instantaneous or time-averaged ventilation. The apparatus of any preceding claim, wherein the pacer apparatus includes an insulated top on said can (10) and said electrode (12) that is mounted on said pacer housing resides exposed through the insulated top. The apparatus of Claim 5, wherein the electrode mounted on the pacer housing is a button electrode (12). The apparatus of any preceding claim wherein the predetermined carrier signal frequency is in a range from about 500 to 20000 Hz.

Inventors: HAUCK JOHN A, PEDERSON BRIAN D, HAUCK, JOHN A., PEDERSON, BRIAN D.
Applicants: CARDIAC PACEMAKERS INC, CARDIAC PACEMAKERS, INC.
Application year: 1991
Main classification: A61N1
IPC codes: A61N1