IMPLANTABLE BIOCOMPATIBLE ELECTRODE FOR BOTH ELECTRICALLY AND MECHANICALLY STIMULATING THE TISSUE

Abstract: An implantable electrode for stimulating tissue comprises a biocompatible piezoelectric electrode adapted to be in direct electrical contact with tissue for electrically and mechanically stimulating the tissue and for detecting electrical and mechanical evoked response of the stimulated tissue. There is further an implantable lead and an implantable stimulation device comprising such an electrode and further comprising means for making a diagnosis of the heart condition using such an electrode.

Description: IMPLANTABLE BIOCOMPAΗBLE ELECTRODE FOR BOTH ELECTRICALLY AND MECHANICALLY STIMULATING THE TISSUE Technical Field of the Invention The invention relates generally to an electrode for implantable stimulation devices such as heart pacemakers or defibrillators. The invention relates further to implantable leads and stimulation devices such as heart pacemakers or defibrillators which comprise such an electrode. Moreover, the invention relates to the use of the electrode for diagnosing the condition of stimulated tissue. In particularly, the invention relates to an electrode which electrically and mechanically transfers stimulation energy to tissue, to an electrode which electrically and mechanically receives electrical and mechanical evoked response of the tissue to which stimulation energy has been transferred and to an electrode which is a combination of the two. Background of the Invention The life span of most pacemakers is dictated by the rate at which their batteries drain. Thus, a substantial effort has been directed toward minimising the amount of energy used by pacemakers, while ensuring that the devices continue to deliver effective therapy. For example, demand pacemakers effectively reduce the battery drain by delivering pacing pulses only when required, i.e. if the pacemaker has not detected any spontaneous activity. Another way to reduce the current consumption is to minimise the amplitude and/or the duration of the stimulation pulse to a value just above the threshold. However, there are for example times when the heart emits an electrical signal, without providing a corresponding mechanical contraction (electromechanical dissociation). However, the pacemaker detects and interprets the electrical signal as an intrinsic beat or an evoked response. There are also times when the heart does not respond normally with increased cardiac output for increased stimulation rate as for example for patients with coronary artery disease during angina pectoris. A way of minimising the amount of energy needed for defibrillation, while ensuring that the defibrillators continue to deliver effective therapy, is disclosed in US-A-5 433 731 to Hδgnelid et al. wherein is disclosed a defibrillator comprising means for supplying the heart with a mechanical shock instead of an electrical shock. One embodiment discloses an electrode for supplying a defibrillation pulse, whereby the electrode is provided with an element on its distal exterior, which presses against the heart tissue and converts the electrical energy into mechanical energy. The element can for example be a piezoelectric element. US-A-5 304 208 to Inguaggiato et al. discloses a cardiostimulator device comprising an electrode including an acceleration sensor for detecting the acceleration to which the cardiac mass is subjected as a reaction to any contraction whatsoever of the said cardiac mass. The acceleration sensor is solely sensitive to inertial forces and can therefore be located in an entirely rigid capsule and consequently be entirely insensitive to the pressure in the ventricle or the atrium, and to pressure which the cardiac wall can exert, particularly on the distal electrode. It is the object of the invention to overcome the disadvantages brought about with the above referenced prior art documents. Summary of the Invention This invention is embodied in an electrode according to claim 1, 2 or 3, a related electrode lead according to claim 18 and a related implantable stimulation device according to claim 19 for effectively both electrically and mechanically stimulating tissue and/or detecting evoked response therefrom. An advantage of the invention is that it is possible to more reliably stimulate heart tissue and to detect a heart contraction. As a result a lower energy consumption is ensured. According to the invention, the piezoelectric electrode comprises a biocompatible piezoelectric material adapted to be in direct contact with the tissue, it surprisingly having been found that the conductive layer hitherto believed necessary can be excluded. In one embodiment the piezoelectric electrode is the tip electrode and in another embodiment the piezoelectric electrode is the ring electrode. In a preferred embodiment the stimulation pulse generator supplies the electrode with a chopped stimulation pulse. Preferred embodiments are set forth in the dependent claims. Brief Description of the Drawings Fig. 1A and IB are schematic drawings of two embodiments of a tip electrode in accordance with the invention, for both electrically and mechanically stimulating tissue and detecting evoked response; Fig. IC is a schematic drawing of an embodiment of a ring electrode in accordance with the invention, for both electrically and mechanically stimulating tissue and detecting evoked response; Fig. ID is a schematic drawing of an electrode lead in accordance with an embodiment of the invention; Fig. IE is a schematic equivalent circuit of the piezoelectric electrode in accordance with an embodiment of the invention; Fig. 2 is a pulse diagram of the detector input signal generated by the electrode in accordance with an embodiment of the invention and comprising the stimulation pulse, the electrical evoked response (A) and the mechanical evoked response (B); Fig. 3 is a schematic drawing of a heart pacemaker comprising an electrode in accordance with the invention; Fig. 4 is a schematic circuit diagram of a heart pacemaker in accordance with a first embodiment of the invention; Fig. 5 is a schematic circuit diagram of a heart pacemaker in accordance with a second embodiment of the invention. Fig. 6 is a schematic drawing of a single lead, whereby a piezoelectric electrode in accordance with an embodiment of the invention is placed at the tip of the lead and an intravascular defibrillation electrode is placed behind the same. Description of the Preferred Embodiment With reference now to the drawings, and in particular Figs. 1 A and IB, there is shown an electrode 10 for a heart pacemaker 100. The electrode 10 comprises a conductor 20 enclosed by an insulator 30, e.g. silicon rubber. The conductor 20 is at one end in contact with an electrically conductive core 40, which is covered with a piezoelectric material 50. So as to obtain a high capacitance usually of the order 10- 100 nF, the layer of piezoelectric material is very thin (0,1-5 n) .The piezoelectric material 50 is biocompatible. The metal core 40 and the piezoelectric layer 50, i.e. the piezoelectric electrode, form the tip of the electrode lead 11. Fig. 1A and IB show a hemispherical and a plane embodiment of the tip respectively, the plane embodiment being more sensitive to how it is placed with respect to the myocardial tissue. In a preferred embodiment the conductor 20 is made of the commonly used alloy MP35 and the conductive core 40 of e.g. graphite, titanium, platinum or iridium. The size of the electrode is about the same as for standard electrodes and may for instance vary between 1 - 10 mm^. It should be noted that the term biocompatible should be read as comprising all materials that may be in direct contact with the tissue without adverse effects. Thus, the piezoelectric material PZT, a material that at least in some compositions contains lead and therefore sometimes is considered to be not biocompatible, possibly could be termed biocompatible when used in minute amounts. The amounts that would be used in the above preferred embodiment could be termed minute in view of the maximal amounts of lead that in the worst case could be released from the piezoelectric layer. According to yet another embodiment of the invention, Figs. IC and ID show a coaxial stimulating and sensing piezoelectric electrode 40,50. The coaxial piezoelectric electrode 40,50 is positioned about 1 to 15 cm behind the tip having an endocardial stimulation electrode 200. This embodiment may e.g. be used in a single lead DDD pacemaker system as disclosed in US-A-5 476 499. The tip is thereby screwed into the atrial myocardium and a loop descends into the ventricle and makes contact with the ventricular wall. The design of the lead 11 is such that the ring 10 of the lead 11 is found in the contact area and the ring 10 comprises the coaxial piezoelectric electrode 40,50 . The lead 11 must have two conductors in this case. One conductor 12 is connected to the tip and atrial part of the DDD pacemaker. The other conductor 11 is connected to the piezoelectric electrode 40,50 and the ventricular circuits of the pacemaker. The block schematics in Figs. 3, 4 and 5 are thus still valid. The interactions between the atrial and ventricular parts of the DDD pacemaker are well known to the person skilled in the art of pacemakers. Fig. IE shows a schematic equivalent circuit 45 of the electrode in accordance with an embodiment of the invention, whereby the piezoelectric electrode 40,50 comprises a voltage source V« and a capacitor C«. The electrode 10 is further characterised by the tip surface 50. The conductor 13, 20 electrically connects the electrode to the electronics of the pacemaker. A stimulation pulse delivered to the electrode 10 and thus to the piezo electrode 45, will change the thickness of the piezoelectric material during the pulse and two pressure waves will be emitted therefrom, there being one pressure wave for each slope of the stimulation wave. The capacitor C« of the piezoelectric electrode 40,50 transmits the electrical stimulation pulse to the heart cells. To avoid charging of the piezoelectric material, the material can be doped or contaminated with a conducting material such as carbon. It is conceivable to have different time constants for the charging. A short time constant, for instance 10 - 100 ms, entails that the charge has been dissipated before the mechanical response arrives. In this case, only fast events can be monitored/detected (> 20 Hz). An alternative is to provide a slow discharge during for instance 1 - 10 seconds, which prevents a cumulative charge, but which permits a relatively low cut-off frequency fg. If the total resistance against leaking over the piezo-material is termed R and the total capacitance is termed R, the following examples can be given: Example 1, fast tip. C = 10 nF, R = 500 kohm => τ = RC = 5 ms => fg = 32 Hz Example 2, slow tip. C = 100 nF, R = 500 Mohm => τ = RC = 0.5 s => fg - 0.32 Hz Fig. 2 shows a pulse diagram of the detector input signal generated by the electrode in accordance with an embodiment of the invention and comprising the stimulation pulse, the electrical evoked response A and the electrical signal B corresponding to the mechanical evoked response. Consequently, a successful heart stimulation will be sensed as two electrical signals by the detector 110 shown in Fig. 3. First the muscle cells close to the electrode will immediately after the stimulation pulse generate an electrical signal A related to the trigged ion transport. Then the global heart muscle contraction will exert a mechanical pressure on the piezo electrode 45 which generates the second electrical signal B. The electrical signal B arrives within a time window C after a certain time D of the electrical signal A. The time interval D depends on the location of the electrode and on the activity of the autonomic nervous system. However, the time interval D is substantially constant for each individual. The time interval D is approximately 5 to 100 ms if the electrode is located in the ventricle. Furthermore, the electrical signal B appears in a relatively narrow time window C, which is approximately 50 ms if the electrode is located in the ventricle. By letting a control unit 130, e.g. a microprocessor, comprise means for analysing the detected electrical signals A and B and how they relate to each other and to the stimulation pulse, information regarding the condition of the heart can be obtained. This information can therefore be used as a diagnostic tool for analysing the condition of the heart. The control unit 130 may obtain information from the dual sensing detector for analysing the evoked response signals. It is e.g. often difficult to handle fusion beats in pacemakers comprising an autocapture function. A fusion beat is a cardiac depolarisation (atrial or ventricular) resulting from two foci. In pacing it typically refers to the ECG waveform which results when an intrinsic depolarisation and a pacemaker output pulse occur simultaneously and both contribute to the electrical activation of that chamber. Another difficulty when analysing evoked response signals is related to the declining electrode polarisation after the stimulation pulse. If the polarisation artefact is large, compared to the electrical signal generated by the heart, the control unit 130 may interpret the polarisation as a capture. A capture is at hand when the stimulation results in a heart contraction. Using this electrode, a new possibility for the control unit 130 to verify capture has been created. If the electrical signal B does not fall within the time interval C, the heart contraction is probably not related to the stimulation pulse. If the electrical signal B arrives before the time window C, a fusion beat is present, or the QRS detector sensitivity is set too low, so that the pacemaker does not inhibit the pacing pulse. If the electrical signal B arrives after the time window C, there is a loss of capture followed by a spontaneously released heart beat. If only the electrical signal A is present, the detector either senses the polarisation artefact due to the sensitivity being too high and should be adjusted, i.e. evoked response oversensing, or the patient has a beat with electromechanical dissociation. By analysing the morphology, i.e. duration and amplitude, of the electric signal B, information regarding the heart contractility can be obtained. For patients with coronary artery disease during angina pectoris, the contractile behaviour is changed. With the electrode according to the invention it is possible for the pacemaker to detect this adverse situation and start therapy. The pacing rate should be reduced until the attack is over. This function is especially important for physiologically rate controlled pacemakers such as the ones being controlled by the venous oxygen contents. Certain patients have a prolonged or varying time between the atrial stimulation A and the atrial evoked electrical response. By letting the control unit 130 start the A-V timer in a two chamber pacing system after the detection of the electrical signal B corresponding to the mechanical evoked response, instead of after the evoked electrical response, these patients will obtain a more stable heart function. The A-V timer is the timer keeping track of the time elapsed between the atrial stimulation A and the ventricular stimulation V. There are times when the heart in response to a stimulation pulse emits an electrical signal, but does not actually contract (electromechanical dissociation). However, the pacemaker detects and interprets the electrical signal as an evoked response. Since the electrode according to the invention registers both electrical and mechanical evoked response, it can distinguish e.g. hemodynamically stable tachycardias at exercise from a pathological situation. Consequently, the electrode according to the invention is suitable for therapy when using an implantable cardiac defibrillator. Fig. 3 shows the schematic drawing of a heart pacemaker 100 for tissue stimulation. The heart pacemaker 100 comprises a stimulation pulse generator 120 that has its output side connected via a lead 11 to an electrode 10 applied in the ventricle of the heart for delivering stimulation pulses to the heart. Of course, even if Fig. 3 shows the electrode 10 to be located in the ventricle, the invention also covers the electrode 10 being located in the atrium. The stimulation pulse generator 120 can be activated to deliver a stimulation pulse via a control line, which is connected to a corresponding output of a control unit 130, e.g. a microprocessor. The stimulation pulse generated by the stimulation pulse generator 120 may be anyone of the stimulation pulses known to the skilled person. The duration of the each stimulation pulse as well as the amplitude thereof are set by the control unit 130. In the illustrated preferred embodiment, the control unit 130 has access to a memory 140 wherein a program that execute all functions of the heart pacemaker 100 via the control unit 130 is stored. The pacemaker 100 comprises a telemetry unit 150 connected to the control unit 130 for programming and for monitoring the functions of the pacemaker 100 and of parameters acquired therewith on the basis of data exchange with an external programming and monitoring device (not shown). In order to be able to acquire the reaction of the heart given a stimulation, the pacemaker 100 comprises a detector unit 110 which has an input side connected via the lead 11 to the electrode 10 for acquiring the electrical potential in the heart tissue. This arrangement is simple because only a single electrode 10 is required both for stimulating the heart and for acquiring the reaction thereof. Of course, the electrode according to the invention may be used only as stimulation electrode for stimulating tissue or a measuring electrode for acquiring the evoked response for e.g. operating in the VDD stimulating mode. In such cases either the stimulation generator 120 is programmed not to deliver stimulation pulses or the detector unit 110 not to register any evoked response (not shown). The control unit 130 further comprises means for evaluating the electrical signals received by the detector 110 for making a diagnosis of the condition of the heart depending on e.g. the morphology of the electrical signal B or how the two electrical signals A and B relate to each other and/or to the stimulation pulse, and possibly for starting a therapy based on the made diagnosis. Fig. 4 shows a schematic circuit diagram of a pacemaker in accordance with a first embodiment of the invention. The stimulation pulse generator comprises a charge pump 121, a capacitor C\, e.g. 1 F, and a switch SI which, when closed, charges the capacitor to a voltage of e.g. 20 V. When the stimulation pulse generator 120 rapidly transfers charge to the electrode 10, the thickness of the piezoelectric material 50 changes and pressure waves are emitted to the heart tissue. It is known that mechanical irritation of the endocardium can start a heart contraction, the mechanical stimulation may decrease the threshold for the electrical stimulation or may by itself initiate a heart contraction. Because the piezoelectric electrode 40, 50 functions as a capacitor as well, electrical current is transferred to the tissue when closing the switch S2- Since the capacitance Cp of the piezoelectric material preferably is 10 to 100 nF, a relatively high voltage of about 5 to 25 volt is needed during a very short time of about 10 to 100 ^s for reaching the stimulation threshold. This voltage may be generated inductively or capacitively and then be stored on C] . The voltage is higher than the voltage at conventional electrodes. The total energy used is, however, about the same as with conventional electrodes since the pulse width is small. The detector unit 110 comprising a detector 111 and a charge amplifier 112 detects both electric signals A and B corresponding to the electrical and mechanical evoked response respectively registered by the piezo electrode 45. An alternative embodiment of the schematic circuit diagram of Fig. 4 is shown in Fig. 5. In order to influence the stimulation threshold, the stimulation pulse generator 120 may generate a stimulation pulse which is chopped with a high frequency of e.g. 10 to 100 kHz. The chopped stimulation frequency may be obtained by opening and closing the switch S2- Due to the chopped stimulation pulse, the piezo sensor generates a series of pressure waves. Since the high frequency improves the electrical transmission through the piezo capacitor C«, more normal pulse amplitudes may be used. The piezoelectric electrode 10 may be used together with a defibrillation electrode 300, either as two separated electrodes, i.e. two leads, or in combination on a single lead, whereby the piezoelectric electrode is placed at the tip of the lead and the intravascular defibrillation electrode 300 is placed behind the piezo electrode 10 as is shown in Fig. 6. Thus an electrode for electrically and mechanically stimulating and detecting evoked response is provided. One skilled in the art will appreciate that the present invention can be practised by other than the described embodiments, which are presented for purposes of illustration and of limitation, and the present invention is limited only by the claims which follow.

Claims: Claims 1. An implantable electrode for an implantable stimulation device comprising a biocompatible piezoelectric electrode (40,50) adapted to be in direct electrical contact with tissue, the entire piezoelectric surface of the electrode being adapted to electrically and mechanically transfer stimulation energy to the tissue. 2. An implantable electrode for an implantable stimulation device comprising a biocompatible piezoelectric electrode (40,50) adapted to be in direct electrical contact with tissue, the entire piezoelectric surface of the piezoelectric electrode being adapted to electrically and mechanically receive electrical and mechanical evoked or intrinsic response of tissue to which stimulation energy can be transferred. 3. An implantable electrode for an implantable stimulation device comprising a biocompatible piezoelectric electrode (40,50 ) adapted to be in direct electrical contact with tissue, the entire piezoelectric surface of the electrode being adapted to electrically and mechanically transfer stimulation energy to the tissue and electrically and mechanically receiving electrical and mechanical evoked or intrinsic response of the tissue to which stimulation energy can be transferred. 4. An implantable electrode as claimed in any one of claims 1 to 3, characterised in that the piezoelectric electrode (40,50 ) comprises a conductive core 40 covered by a piezoelectric material (50). 5. An implantable electrode as claimed in claim 4, characterised in that the capacitance (Cp) of the piezoelectric material (50) is 10 to 100 nF. 6. An implantable electrode as claimed in any one of claims 1 to 5, characterised in that the piezoelectric electrode (40,50) is a tip electrode. 7. An implantable electrode as claimed in claim 6, characterised in that the piezoelectric electrode (40,50) is hemispherical. 8. An implantable electrode as claimed in claim 6, characterised in that the distal end of the piezoelectric electrode (40,50) is planar. 9. An implantable electrode as claimed in any one of claims 1 to 5, characterised in that the piezoelectric electrode (40,50) is a ring electrode. 10. An implantable electrode as claimed in claim 9, characterised in that the piezoelectric electrode (40,50) is coaxial. 11. An implantable electrode as claimed in any one of claims 1 to 11, characterised in that the stimulating pulse generator (120) generates a chopped stimulation pulse. 12. An implantable electrode as claimed in claim 11, characterised in that the stimulation pulse is chopped with a frequency between 10 to 100 kHz. 13. An implantable electrode as claimed in any one of claims 1 to 12, characterised by further comprising conductor means (20,40) for conducting the stimulation energy to the piezoelectric electrode (50) and for conducting the electrical signals coming from the piezoelectric electrode (50) and corresponding to the electrical and mechanical evoked o intrinsic response of the tissue. 14. An electrode lead connectable to an implantable stimulation device, wherein said elektrode lead comprises an implantable electrode (10) as claimed in any one of claims 1 to 13 and further comprises a conductor (13) for delivering stimulation energy to the piezoelectric electrode (40,50) and/or for conducting signals from the piezoelectric electrode (40,50) corresponding to the electrical and mechanical evoked or intrinsic response of the tissue. 15. An implantable stimulation device for stimulating tissue comprising an implantable electrode (10) as claimed in any one of claims 1 to 13, a stimulation pulse generator (120) for delivering stimulation energy to said implantable electrode (10) and detector means (110) for receiving signals from said implantable electrode (10) corresponding to the electrical and mechanical evoked or intrinsic response of the tissue. 16. An implantable stimulation device as claimed in claim 15, characterised by further comprising control unit (130) for processing the signals received by the detector means (110). 17. An implantable stimulation device as claimed in any one of claims 15 or 16, characterised in that the control unit (130) comprises detecting means for detecting fusion beats, said detecting means determining if the electrical signal (B) corresponding to the mechanical evoked or intrinsic response registered by the electrode (10), arrives before a predetermined time window (C). 18. An implantable stimulation device as claimed in any one of claims 15 or 16, characterised in that the control unit (130) comprises analysing means for analysing the heart contractility, said analysing means determining the morphology of the electrical signal (B) corresponding to the mechanical evoked or intrinsic response registered by the electrode (10). 19. An implantable stimulation device as claimed in any one of claims 15 or 16, characterised by comprising pacing means for dual-chamber pacing having an A-V timer and in that the control unit (130) comprises means for providing a stable heart function, said providing means starting the A-V timer after the detection of the electrical signal (B) corresponding the mechanical evoked or intrinsic response. 20. An implantable stimulation device as claimed in any one of claims 15 or 16, characterised in that the control unit (130) comprises detecting means for detecting electromechanical dissociation by determining whether the electrode (10) registers both an electrical and a mechanical evoked response of the tissue after a stimulation or not.

Inventors: HEDBERG SVEN-ERIK, HOLMSTROEM NILS, NILSSON KENTH, HEDBERG, SVEN-ERIK, HOLMSTROEM, NILS, NILSSON, KENTH
Applicants: HEDBERG SVEN ERIK, HOLMSTROEM NILS, NILSSON KENTH, PACESETTER AB, HEDBERG, SVEN-ERIK, HOLMSTROEM, NILS, NILSSON, KENTH
Application year: 1999
Main classification: A61N1
IPC codes: A61N1