Medical device used to stimulate tissue

Abstract: A medical device (1) comprising a pulse generator (3),for generating electrical pulses, having an electrical energy storage means (8) connectable to an output means (4), an electrode arrangement (10), for conducting the electrical pulses to tissue (21), having an input end (10') and an output end (10"), wherein the output means (4) of the pulse generator (3) is connectable to the input end (10') of the electrode arrangement (10), and the capacitance of said output means (4) is less than 1 µF and said electrode arrangement (10) has capacitor means (16) with a capacitance greater than 1 µF.

Description: Technical Field The present invention relates to inplantable medical devices used to stimulate tissue. Electrodes are used in implantable medical devices to stimulate tissue or transmit signals from tissues to a sensing device. In, for example, implantable pacemakers stimulating electrodes form part of a stimulation circuit which comprises a stimulation pulse generator connected to a coupling capacitor which is connected to a stimulation electrode fixed in or near the heart. The return path of the circuit to the pacemaker is usually through the intervening body tissues to an indifferent electrode on the casing of the pacemaker. Background art Stimulation pulse generators in implanted pacemakers contain pulse generator capacitors which are charged by the pacemaker battery to a desired stimulation voltage and discharged through the stimulation electrode when the pacemaker programme calls for a stimulation pulse to be sent. As soon as the pulse has been discharged the pulse generator capacitors are recharged to their pre-discharge voltage levels. A type of stimulation electrode known from US-A 4 336 811 has a metal tip covered by a non-metallic coating of a compound of the metal in the tip and a two-layer membrane. In one embodiment of the electrode the metal is silver and the compound is silver chloride. In order to prevent the movement of soluble ions of the tip coating migrating into body tissues during use, the tip is covered by a two-layer membrane. The inner membrane layer inhibits movement into the body tissue of soluble ions of the coating on the electrode which may be toxic to the surrounding body tissue. The outer membrane layer is composed of material which is non-toxic to body tissue. Leakage currents arise in the stimulation circuit from electrons migrating by "tunnelling" through the semiconductor materials used as switches in the pacemaker circuitry. Measurements taken at the terminals of pacemakers give leakage current values of the order of 1 nano-Ampere (1 x 10 -9 A). It is important that there is no, or as little as possible, leakage current from the electrodes to the surrounding tissue. This is because leakage currents can cause the growth of fibrous tissue around the electrodes which lead to increased stimulation thresholds, high current consumption and hence reduced battery life. A further problem with leakage currents is that under their influence some electrode materials will electrolyse when in contact with body fluids which leads to damaged electrodes and contaminated body tissues. In order to prevent leakage currents from the charged stimulation pulse capacitors an extra capacitor, known as a coupling capacitor, with a capacitance usually in the range of 1-15 µF is positioned in the circuit between the stimulation pulse capacitors and the electrode see, for example, the pacemaker known from EP-A323 56. The use of a coupling capacitor has disadvantages, for example, a coupling capacitor is an additional component which adds to the cost of the pacemaker and makes it more complicated to construct. A further disadvantage of using coupling capacitors arises when they are used in Autocapture pacemakers. In Autocapture pacemakers it is essential that the evoked response to a stimulation impulse can be sensed. Using a single stimulation impulse leads to polarisation, that is ionisation of the fluid between the electrode tip and the surrounding tissue which leaves it with an electrical potential. This electrical potential, known as the polarisation voltage, is zero prior to the emission of a stimulation impulse, rises exponentially during the impulse and falls inversely exponentially when the impulse finishes. To avoid the problem of the polarisation voltage of the tissue around the electrode tip masking the evoked response signal, it is usual to use a biphasic stimulation complex in which an impulse of specified energy is quickly followed by a second impulse of equal energy but with the opposite polarity. The net sum of the polarisation voltage is zero and hence polarisation is eliminated. The use of a coupling capacitor complicates the design and production of pacemakers using biphasic stimulation impulses and also brings about an increase in the time required to perform the polarisation cancelation procedure. Object of the invention The invention seeks to provide a means for preventing leakage currents in the output circuit of a medical device for stimulating tissue. Disclosure of the invention In accordance with the invention, this object is accomplished by manufacturing an medical device which has a low or negligable output capacitance which can be coupled to an electrode lead which has a capacitance sufficiently large so that it can act as a coupling capacitor. This makes the use of a coupling capacitor in the output part of the medical device unnecessary which leads to a medical device of simplified design and construction and which uses the energy stored in it in a more efficient manner. Brief Description of the Drawings Figure 1 shows a diagram representing one embodiment of a pacemaker and electrode constructed in accordance with the invention. Figure 2 shows a cross-section through the electrode shown in figure 1 constructed in accordance with the invention. Figure 3 shows a circuit diagram representing a third embodiment of the invention. Description of the Invention Figure 1 shows an example of one embodiment of the invention wherein the the medical device is an Autocapture pacemaker. Figure 1 represents an Autocapture pacemaker 1 comprising a battery 2, a pulse generator 3 connected to an output circuit 4, controlling means 5 and indifferent electrode 6 mounted on casing 7. An electrode arrangement 10 has an input end 10' connectable to output circuit 4, and an output end 10" implantable in body tissue. Output circuit 4 provides a connection between the pulse generator 3 and the electrode arrangement 10 and has a negligable capacitance, less than 1 µF, and a negligable resistance. In the embodiment shown electrode arrangement 10 comprises an electrode lead 11 containing insulated metal conductors 12, 13 leading to output end 10" with tip electrode 14 and ring electrode 15, for connecting output circuit 4 to heart 20 and for conducting stimulation pulses to heart 20 via heart tissue 21 and heart fluids 22. The controlling means 5 has well-known means for determining when a stimulation pulse is required and for determining the impedance of the pulse circuit so that the correct stimulation energy reaches the heart. These means will not be described further as they do not form part of the present invention. Stimulation pulse energy is stored in a reservoir capacitor means 8 in the pulse generator 3 and capacitor means 8 is connected by semiconductor switching means 9 via output circuit 4 to conductors 12,13. After stimulation pulse discharges, the capacitor means 8 is charged to a predetermined voltage using the energy stored in battery 2. Tip electrode 14 and ring electrode 15 in this embodiment are made of titanium covered with a contact surface made of a thin layer of non-metallic titanium oxide 16. Titanium oxide is a very good electrical insulator, that is, it has a high dielectric constant, and is hard, stable and a very good biocompatable material. Furthermore a titanium oxide layer grown in a titanium to liquid interface does not degrade for moderate reversed voltages.The titanium oxide layer 16 can be provided on the electrodes before implanting or can be grown on the electrodes after implantation for example as described in the following. The electrode titanium surfaces can easily react with hydroxy ions in a liquid such as the heart fluids 22 surrounding the implanted electrodes 14, 15 to form a layer of titanium oxide 16 on the surface of the metal. This process is enhanced by passing an electrical current at a voltage which is less than the stimulation pulse voltage across the liquid to metal interface, with titanium as the anode. The insulating effect of the titanium oxide can be used to automatically limit the thickness of the titanium oxide layer 16 when it is being formed by a current flow. As the thickness and hence the resistance of the titanium oxide layer increases, the current will decrease and eventually stop when the titanium oxide layer is so thick that its resistance is sufficiently high to prevent current flow at the low voltage across it. The resulting titanium oxide layer thickness is proportional to the voltage that was used to produces it. A voltage of 9 V for example gives a layer of the order of 1 µm thick. For low to moderate voltages the titanium oxide layer will be very thin, possibly only one molecule thick. Such thin layers of dielectric materials have a high capacitance and by chosing an appropriate voltage it is possible to control the capacitance of the resulting capacitor. By forming a capacitor with a capacitance between 1-15 µF it is possible to use this layer as a coupling capacitor and thereby avoid the need to design and produce a coupling capacitor in the pulse generator. A lower voltage than that used to form a titanium oxide layer when applied across an intact titanium oxide layer 16 will not result in any current flow. If, however, an titanium oxide layer 16 is damaged or scratched at any point then a current will start to flow at this point if the resistance at this point is low enough to allow a current to flow. If the current is caused by a voltage source with the titanium as the anode then the current will cause an titanium oxide layer 16 to form (or become thicker) at this low resistance point thereby causing the insulating layer to self-heal and the current flow to stop when the titanium oxide layer 16 is thick enough. Thus by ensuring that the titanium is always the anode with respect to the surrounding fluid it is possible to repair damage to the titanium oxide layer. As the stimulation pulse voltages are higher than the voltage used to build the titanium oxide layer then the stimulation pulses are still able to pass through the titanium oxide layer 16 and stimulate the heart 20. In a second embodiment aluminium is used for the electrodes 14, 15 instead of titanium and an aluminium oxide layer is grown on the surface of the electrode. Of course any other suitable substance which, when subject to an electrical current, can form an insulating surface layer can be used instead of aluminium and titanium. In a third embodiment of the invention, as shown in the circuit diagram in figure 3, resistors 17, 18 which each have a resistance which is high compared to the resistance between the electrical path between the output of the pacemaker to the indifferent electrode 6 via the electrodes 14, 15, titanium oxide layers 16, heart 20 and body tissues, are connected in the output stage 4 between each electrode 12, 13 and the positive supply. For pacemakers using pulse generators which generate 3 volt pulses then resistors with a resistance of 10 MΩ would give a current of 30 nano-Ampere (30 x10 -9 A). These resistors 17,18 bias the semiconductor switches 9', 9" such that in the event of a leakage current which is normally of the order of only 1 nano-Ampere (1 x 10 -9 A) i.e. only one thirtieth of the bias current, the titanium electrodes 12, 13 are always made positive with respect to the liquid 22 surrounding them. In this way leakage currents give rise to a bias current which works to cause an increase in the thickness of the titanium oxide layer which leads to increased resistance and eventually the elimination of the leakage current. In a fourth embodiment control means 5 measures the impedance of the stimulation circuit in order to determine if the oxide layer has been damaged, thereby reducing the capacitance of the layer and hence reducing the resistance against leakage currents. If the capacitance of the oxide layer is too low then control means 5 allows a low voltage bias current to flow across the electrode. This causes the damaged oxide layer to regrow and restores the capacitance of the layer. When the capacitance has reached a sufficiently high value to reduce leakage currents then the low voltage bias current is stopped. In a fifth embodiment of the present invention the oxide layers are protected by a protective membrane of semipermeable material, for example naphion, termeon or carboxylic acid resin. Naturally the present invention is not limited to the embodiments show but covers all changes and modifications covered by the scope of the claims.

Claims: A medical device (1) comprising a pulse generator (3),for generating electrical pulses, having an electrical energy storage means (8) connectable to an output means (4) of the pulse generator (3), an electrode arrangement (10), for conducting the electrical pulses to tissue (21), having an input end (10') and an output end (10"), wherein the output means (4) of the pulse generator (3) is connectable to the input end (10') of the electrode arrangement (10), characterised in that the capacitance of said output means (4) is less than 1 µF and said electrode arrangement (10) has capacitor means (16) with a capacitance greater than 1 µF. A medical device (1) according to claim 1 characterised in that the electrode arrangement (10) comprises at least one conductor (12,13) made of a metal and having an electrode (14,15) at said output end (10"), said capacitor means (16) being formed of a layer of an electrically insulating compound of said metal formed on said electrode (14,15). A medical device according to claim 2 characterised in that said metal is titanium. A medical device according to claim 2 characterised in that said metal is aluminium. A medical device according to any of claims 2 to 4 characterised in that the compound is an oxide of said metal. A medical device according to any of the preceding claims characterised that high resistance biasing resistors (17, 18,19) are arranged in said device (1) such that in the event of a leakage current from said pulse generator (3) said electrode (14, 15) is electrically positive with respect to said tissue (21).

Inventors: EKWALL CHRISTER, EKWALL, CHRISTER
Applicants: ST JUDE MEDICAL, ST. JUDE MEDICAL AB
Application year: 1997
Main classification: A61N1
IPC codes: A61N1