FIELD OF THE INVENTION The present invention relates generally to invasive devices. and methods for treatment of the heart, and specifically to devices and methods for pacing and electrical control of the heart muscle.

BACKGROUND OF THE INVENTION Cardiac pacemakers known in the art are designed to provide pulses continually over extended periods of time to electrodes implanted in the heart. The pulses may be applied in a bipolar mode, between pairs of electrodes connected to the myocardium, or in a unipolar mode, between a myocardial electrode and a reference electrode in contact with tissue away from the heart. Typically, the case (or"can") containing the pacemaker circuitry is implanted in the patient's thorax and serves as the distant electrode. For reasons of patient safety, the pacemaker must be designed so as to block DC coupling between the circuitry and the electrodes. In order to prevent degradation of the electrodes, it is also important that current flow be balanced between forward and reverse directions, so that there is no net charge flow through the tissue or build-up at any of the electrodes.

Fig. 1 is a schematic diagram illustrating pulse generating elements of a pacemaker 20 for pacing a heart 22 in bipolar mode, as is known in the art. Such pacemakers are described, for example, in Design of Cardiac Pacemakers, John G. Webster, ed. (IEEE Press, Piscataway, New Jersey, 1995), which is incorporated herein by reference. Pacemaker 20 comprises a battery 24 or other power source, which charges a tank capacitor 28 via a charge pump 26 (or voltage multiplier). To apply a pacing pulse to heart 22, a switch 30 is closed, transferring stored charge from capacitor 28 via a DC-blocking capacitor 34 to electrodes 36. Switch 30 is then opened, and a discharge switch 32 is preferably closed in order to remove charge built up on capacitor 34, thus generating a reverse, balancing current through the electrodes.

Fig. 2 is a timing diagram illustrating a typical pacing signal 38 generated by pacemaker 20 across electrodes 36. Switch 30 is closed for a short period in order to produce a sharp,

narrow, cathodic (negative voltage) pacing pulse 40. After pacing switch 30 is opened, and discharge switch 32 is closed, typically for about 20 ms, an anodic (positive voltage) discharge phase 42 appears across electrodes 36. The specific duration and amplitude of this phase of the pacing waveform are not significant from the point of view of pacing, since it is intended only to remove residual charge and does not provide any stimulation to the heart.

Biphasic pulses include both cathodic and anodic phases. To generate such pulses, the energy stored in tank capacitor 28 must typically be switched rapidly so that a cathodic phase of the pulse (similar to cathodic pulse 40) is followed immediately by an anodic phase of similar amplitude and duration. Some intracardiac defibrillation devices generate biphasic pulses, which are believed to reduce the pulse amplitude or charge flux needed to defibrillate the heart.

As defibrillators are intended to discharge only intermittently, in life-threatening situations, DC blocking and current balancing are not a consideration in such devices as they are in pacemakers. Although biphasic pacing pulses and bursts of pulses have been used in research, these features have not been incorporated in practical, commercially-available implantable pacemakers.

PCT patent application PCT/IL97/00012, published as WO 97/25098, to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes methods for modifying the force of contraction of at least a portion of a heart chamber by applying a non-excitatory electric field to the heart at a delay after electrical activation of the portion. The non-excitatory field is such as does not induce new activation potentials in cardiac muscle cells, but rather modifies the cells'response to the activation. In the context of the present patent application, the use of such a non-excitatory field is referred to as Excitable Tissue Control (ETC). The non-excitatory field may be applied in combination with a pacemaker or defibrillator, which applies an excitatory signal (i. e., pacing or defibrillation pulses) to the heart muscle.

PCT patent application PCT/IL97/00236, whose disclosure is also incorporated herein by reference, describes a pacemaker that gives cardiac output enhancement. This pacemaker applies both excitatory (pacing) and non-excitatory (ETC) electrical pulses to the heart. By applying non-excitatory pulses of suitable strength, appropriately timed with respect to the heart's electrical activation, the contraction of selected segments of the heart muscle can be increased or decreased, thus increasing or decreasing the stroke volume of the heart.

SUMMARY OF THE INVENTION It is an object of some aspects of the present invention to provide a flexible output stage for cardiac control devices, such as pacemakers and ETC devices.

It is a further object of some aspects of the present invention to provide a cardiac control device that can be programmed to provide signals to the heart in unipolar or bipolar mode.

It is still a further object of some aspects of the present invention to provide a cardiac control device that can be programmed to provide monophasic or biphasic signals to the heart, while blocking exposure of the heart tissue to direct current (DC) from the device and preventing charge build-up at electrodes in the heart.

In preferred embodiments of the present invention, an implantable cardiac control device comprises an energy storage component, typically a tank capacitor, which is coupled to drive one or more electrically-conductive elements to apply electrical signals to the heart of a patient. The conductive elements, which include one or more electrodes in or on the heart, are coupled to the energy storage element via respective output circuits, which comprise respective switches and DC blocking capacitors. Preferably, the energy storage component is contained in an implantable conductive case, which itself serves as one of the conductive elements and thus is coupled by its own programmable output circuit to the energy storage component. The device further comprises control circuitry, which opens and closes selected switches in the output circuits so that the device is able provide to signals to the heart with substantially any desired polarity, and including monophasic and biphasic signals, as well as pulse trains.

The term"cardiac control device,"as used in the context of the present patent application and in the claims, refers to any device that is intended to provide signals for controlling long-term cardiac function. The term"cardiac control signals"similarly refers to any and all signals applied to the heart by a cardiac control device. Such devices include both excitatory devices, such as pacemakers, and non-excitatory devices, such as the ETC devices describe hereinabove, along with multi-purpose devices that combine excitatory and non- excitatory functions. The present invention affords such devices substantially greater flexibility in adjusting the type of stimulation provide to the heart than can be achieved by devices known in the art. Although preferred embodiments are described herein with reference to control of the heart, it will be appreciated that the principles of the present invention are similarly applicable to physiological controllers of other types, as well.

There is therefore provided, in accordance with a preferred embodiment of the present invention, a device for control of excitable physiological tissue of a patient, including: an electrical energy storage component; a plurality of conductive elements, which are adapted to apply the electrical energy to the tissue ; and a plurality of output circuits, respectively coupled to the conductive elements so as to convey the electrical energy from the energy storage component through the tissue via the conductive elements, each output circuit including: one or more switches, having a first configuration in which the respective conductive element acts as a cathode in conveying the energy through the tissue, and a second configuration in which the respective conductive element acts as an anode in conveying the energy through the tissue; and a DC blocking capacitor, which is interposed between the energy storage component and the respective conductive element in at least one of the first and second configurations.

Preferably, the plurality of conductive elements include two or more electrodes, and the switches are configurable so that the electrodes apply the energy to the tissue in a bipolar configuration.

Alternatively or additionally, the plurality of conductive elements include at least one electrode and a conductive case of the device, and the switches are configurable so that the energy is applied to the tissue in a unipolar configuration between the at least one electrode and the conductive case. Preferably, the device is implanted in the body of the patient.

Preferably, the one or more switches include a first selection switch, coupled between the DC blocking capacitor and the energy storage component so as to convey the energy from the energy storage component to the respective conductive element, and a second selection switch, coupled between the respective conductive element and a ground. Most preferably, in the first configuration of the output circuit, the first selection switch is closed, and the second selection switch is opened, and in the second configuration, the first selection switch is opened, and the second selection switch is closed. Additionally or alternatively, the one or more switches include a discharge switch, coupled between the DC blocking capacitor and the ground, such that when the first selection switch is opened and the discharge switch is closed, residual charge on the DC blocking capacitor is discharged through the discharge switch.

Preferably, the device includes control circuitry, which is adapted to configure the switches in the output circuits so that the electrical energy is applied to the tissue in a monophasic pulse or, additionally or alternatively, in a biphasic pulse.

Preferably, the tissue includes the patient's cardiac tissue, wherein the energy is applied to pace the cardiac tissue or, additionally or alternatively, to perform Excitable Tissue Control of the cardiac tissue.

There is also provided, in accordance with a preferred embodiment of the present invention, a method for control of excitable physiological tissue of a patient, including: placing a plurality of conductive elements in electrical contact with the tissue; in a first configuration, selecting a first one of the conductive elements to serve as a cathode and a second one of the conductive elements to serve as an anode; in a second configuration, selecting the first one of the conductive elements to serve as the anode, and the second one of the conductive elements to serve as the cathode; and applying energy from an energy source to the tissue through the first and second conductive elements in both the first and second configurations, while blocking direct current from the energy source to the tissue.

Preferably, providing the plurality of conductive elements includes implanting the elements in the body of the patient.

The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic electrical diagram of a pacemaker pulse generator, as is known in the art; Fig. 2 is a timing diagram illustrating signals associated with the pacemaker of Fig. 1; Fig. 3 is a schematic block diagram of a cardiac control device, in accordance with a preferred embodiment of the present invention; Fig. 4 is a schematic electrical diagram illustrating details of output circuits used in the device of Fig. 3, in accordance with a preferred embodiment of the present invention; and Fig. 5 is a schematic electrical diagram illustrating further details of implementation of one of the output circuits shown in Fig. 4, in accordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Reference is now made to Fig. 3, which is a schematic block diagram illustrating a cardiac control device 50, coupled to apply control signals to heart 52 of a patient, in accordance with a preferred embodiment of the present invention. Preferably, device 50 is adapted to deliver electrical signals to the heart for the purposes of both pacing the heart and excitable tissue control (ETC). The ETC signals are typically used to increase the stroke volume of the patient's heart, or otherwise to promote recovery of the heart following surgery or other trauma, as described in the above-mentioned PCT patent applications and in corresponding U. S. patent applications 09/101,723 and 09/254,900, which are assigned to the assignee of the present patent application and whose disclosures are incorporated herein by reference. Devices and methods for pacing and ETC of the heart are further described in U. S. patent application 09/260,769, which is assigned to the assignee of the present patent application, and whose disclosure is similarly incorporated herein by reference. Alternatively, device 50 may be used only for pacing or only for ETC.

Further alternatively, device 50 may be used, mlttatis mrrtandis, for stimulation of other types of excitable tissue in the body. For example, the device may be used to induce enhancement of glandular function, such as of the pancreas; in obstetrics and gynecology, such as to stimulate the uterus; and in stimulation of portions of the gastro-intestinal tract. Other applications of device 50 or of elements thereof, as described hereinbelow, will be apparent to those skilled in the art.

Device 50 comprises a charge pump 60, which charges a tank capacitor 62, substantially in the manner described above with reference to Fig. 1. Capacitor 62 is discharged through output circuits 64,66 and 68, in accordance with signal parameters determined by a controller 70, as described in detail hereinbelow. Circuits 64 and 66 are coupled to drive electrodes 54 and 56, which are typically implanted on or in the wall of heart 52, using methods described in the above-mentioned patent applications or any other suitable method known in the art. Circuit 68 is coupled in a similar fashion to a conductive case 58, which is also typically implanted in the patient's body or is otherwise electrically coupled to the body.

A user interface 72 communicates with controller 70 in order to program the operation of device 50. Communication between the user interface and the controller preferably takes place over a wireless link, such as a radio frequency link, as is known in the art, to allow non- invasive programming after implantation of the controller in the patient's body. Programming

of the device includes selection of a mode of operation, such as pacing, ETC, or both pacing and ETC. The parameters of signals to be applied by device 50 are also programmable, so that a user may select, ij7ter alia: Unipolar or bipolar modality-In bipolar mode, the signals are applied between electrodes 54 and 56, whereas in unipolar mode, the signals are applied between one of the electrodes and case 58.

Monophasic or biphasic signals-including application of a train of monophasic or biphasic pulses, as well as selection of anodic or cathodic signal polarity.

Whether to discharge the DC blocking capacitors (shown in Fig. 4, below) after pulse application, so as to balance the net current applied to the electrodes and prevent charge buildup.

Signal duration, intensity and waveform shape.

Additionally, controller 70 is preferably programmed so as to determine automatically when and whether pacing or ETC signals are to be applied and to choose appropriate signal parameters.

Fig. 4 is a schematic circuit diagram illustrating details of output circuits 64,66 and 68, which enable the selection of signal modality and parameters, in accordance with a preferred embodiment of the present invention. Each of circuits 64,66 and 68 comprises a cathode selection switch 80, a discharge switch 82 and an anode selection switch 86, all operated by controller 70. A DC blocking capacitor 84 is interposed between switch 80 and the respective electrode 54,56 or case 58 in each of the circuits. This capacitor attenuates DC and low- frequency voltage components reaching the output circuit, thus sharpening the high-frequency signals to be applied to the electrodes. DC leakage currents from charge pump 60, which may occur in fault conditions, charge up capacitor 84 and are blocked from reaching heart 52.

Optionally, passive elements (e. g., 10 kQ resistors, not shown in the figures) are coupled in parallel to switches 82 to enable blocking capacitors 84 to discharge current passively to ground. A supply voltage of the circuits, VDD, serves as the ground.

Several examples will elucidate the operation and function of circuits 64,66 and 68: A. To discharge tank capacitor 62 in bipolar mode through electrode 54 to electrode 56, switch 80 in output circuit 64 and switch 86 in output circuit 66 are actuated by controller 70 to close. A pulse of current passes from the tank capacitor via respective DC blocking capacitor 84 in circuit 64 to electrode 54, and returns to ground through electrode 56. This

configuration gives a monophasic pulse. It is preferably followed by opening of switch 80 in circuit 64 and of switch 86 in circuit 66 and closing of switch 82 in circuit 64 so as to discharge blocking capacitor 84 and balance the current between electrodes 54 and 56, thus producing the waveform shown generally in Fig. 2.

B. To discharge tank capacitor 62 in unipolar mode, through electrode 54 to case 58, the procedure is substantially similar to that described above with respect to example A, except that switch 86 in output circuit 68 of case 58 is closed instead of the corresponding switch in output circuit 66. In this example, the case serves as the anode, instead of electrode 56.

C. Alternatively, switch 80 in output circuit 68 and switch 86 in output circuit 64 are closed during the discharge phase, in order to allow current to be conveyed from tank capacitor 62 via the respective DC blocking capacitor 84 to case 58. In this way, a monophasic anodic pulse, in unipolar mode, is generated between case 58 and electrode 54.

D. Returning to example A, a biphasic, bipolar signal will be generated if after tank capacitor 62 has been partially discharged while switch 80 of circuit 64 and switch 86 of circuit 66 are closed, the switch configuration is reversed, so that switch 86 of circuit 64 and switch 80 of circuit 66 are closed. This reversal of the switches, whether in unipolar or bipolar mode, can be repeated a number of times in succession to generate a biphasic pulse train.

E. On the other hand, to generate a monophasic pulse train, switch 80 of circuit 64 is closed and opened multiple times in succession, while switch 86 in circuit 66 or 68 remains closed.

The examples listed above can be applied both to pacing and to other methods for controlling the heart, such as ETC, with adjustments of signal timing and amplitude (typically by varying the charging voltage of tank capacitor 62) as appropriate. Those skilled in the art will be able to vary these configurations in order to generate signals of other types and will also be able to devise other combinations and configurations of switches and capacitors that can realize some or all of the functionality of device 50. Moreover, although device 50 as described herein includes only three output circuits 64,66 and 68, it will be appreciated that additional electrodes and corresponding output circuits may be added to the device, in order to enable application of control signals to heart 52 at multiple different sites thereon. All such combinations, variations and alternative configurations are considered to be within the scope of the present invention.

Fig. 5 is a schematic electrical circuit diagram that shows details of implementation of output circuit 64, in accordance with a preferred embodiment of the present invention. The implementations of circuits 66 and 68 are substantially similar to that of circuit 64. Switch 80 in Fig. 4 is actually implemented by a group of components, as illustrated in Fig. 5, in order to be able to control a relatively high voltage (typically several times the battery voltage) using low-voltage iogic. To close switch 80, controller 70 applies a control signal to an inverter 90, thereby causing gates 92 to close. Current flows from tank capacitor 62 through gates 92 to DC blocking capacitor 84, and from there to electrode 54. Switch 86 is implemented using two gates 94 in order to provide a low-impedance path with high saturation current (typically >60 mA). The gates used in circuit 64 are chosen so as to provide low on-state impedance (preferably <1OQ), low off-state leakage (preferably <5nA) and low charge injection (preferably <5nA). Other aspects of the design of circuit 64, as well as possible variations on this design, will be apparent to those skilled in the art.

It will be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.