Implantable cardiac stimulation device including an output circuit that provides arbitrarily shaped defibrillation waveforms

An output circuit for use in an implantable cardiac defibrillation provides an output pulse having a waveform of virtually any desired shape. The device includes a sensing circuit that senses cardiac activity. An arrhythmia detector detects fibrillation responsive to the cardiac activity signal. The device further includes an output circuit that provides a stimulation output pulse when the arrhythmia detector detects a cardiac arrhythmia. The output circuit includes an H-bridge having a pair of switching devices which control the output pulse waveform with pulse-width modulation and a second pair of switching devices that control the output pulse polarity.

FIELD OF THE INVENTION

The present invention is generally directed to an implantable cardiac defibrillation device (ICD). The present invention is more particularly directed to such a device having an output circuit capable of providing a defibrillation pulse having a waveform of virtually any desired shape.

BACKGROUND OF THE INVENTION

Implantable cardiac defibrillators (ICD's) are well known in the art. These devices, encapsulated in a conductive housing or enclosure, are generally implanted in a pectoral region of a patient and electrically connected to the heart with one or more electrode carrying leads. One lead includes at least one defibrillation electrode arranged to be positioned in the right ventricle. An arrhythmia detector detects ventricular arrhythmias, such as ventricular fibrillation. When such an arrhythmia is detected, a pulse generator delivers a defibrillation shock pulse from the defibrillation electrode in the right ventricle to the conductive housing to terminate the arrhythmia. Alternatively, such arrhythmia terminating systems may further include another defibrillation electrode arranged to be positioned in the right atrium and electrically connected to the right ventricular defibrillation electrode. In this arrangement, the defibrillating shock is delivered from the parallel connected right ventricular and right atrial electrodes to the conductive housing.

For defibrillation therapy to be effective, the defibrillation pulse provided by the device must be at an output level above the defibrillation threshold. Implantable defibrillators generally provide a biphasic output pulse having a waveform which is shaped based upon a capacitive discharge. Such waveform shapes are generally characterized by a peak starting voltage followed by a capacitive decay. In a biphasic output, the polarity of the pulse is reversed during the duration of the output pulse.

Recently, output pulse waveform shapes, other than those based upon a capacitive discharge have been proposed and possess the potential to lower defibrillation thresholds. Since these devices are fully implantable and rely on battery power, a reduced defibrillation threshold, requiring less defibrillation pulse energy, would lead to extending the useful life of the implanted device. Further, reduced defibrillation energies also have the potential of being less traumatic to the patient.

SUMMARY

What is described herein is an output circuit for use in an implantable cardiac device which is capable of providing stimulation outputs having arbitrary waveforms previously unavailable with standard capacitive discharge devices. These waveforms offer decreased output energy requirements and the ability to tailor a stimulation output for a given patient.

In accordance with one embodiment, the output circuit may comprise a voltage supply circuit that provides an output voltage and a control circuit comprising an H-bridge that pulse-width modulates the output voltage to provide a stimulation output having a pulse-width modulated waveform.

The H-bridge may include a first leg and a second leg. Each leg may include a first switching device that controls the waveform shape of the stimulation output.

The output circuit may further include a pulse-width modulation circuit coupled to the first switching device of each leg of the H-bridge. Each leg of the H-bridge may further include a second switching device that controls polarity of the stimulation output. A polarity control circuit may be coupled to the second switching device of each leg of the H-bridge to control polarity of the output waveform. The output circuit may further include a comparison circuit that compares a desired output waveform to a timing waveform and provides control signals. The first switching may then be responsive to the control signals for creating the desired waveform.

The output circuit may further comprise a capacitor coupled between the legs of the H-bridge. The capacitor may be a non-polar capacitor.

The output circuit may further comprise an inductor coupled in series with the legs of the H-bridge. The inductor may provide additional filtering of the stimulation output.

In accordance with other aspects of the present invention, the H-bridge may comprise a plurality of legs, each leg including an output voltage modulating device. Each leg may further include a polarity control device.

The H-bridge may more particularly comprise first, second, and third legs. When the polarity control device of the first leg controls the polarity, the output voltage modulating devices of the second and third legs may independently modulate the output voltage.

In accordance with further aspects of the present invention, the invention provides an output circuit for use in an implantable cardiac device that provides a stimulation output having a desired waveform. The output circuit includes a power source that provides an output voltage, a pulse-width modulation circuit that generates a pulse-width modulation control signal corresponding to a desired waveform, and an H-bridge coupled to the power source and to the pulse-width modulation control circuit that modulates the output voltage to provide a stimulation output having the desired waveform.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown inFIG. 1, there is a stimulation device10in electrical communication with a patient's heart12by way of three leads,20,24and30, suitable for delivering multi-chamber stimulation and shock therapy. To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, the stimulation device10is coupled to an implantable right atrial lead20having at least an atrial tip electrode22, which typically is implanted in the patient's right atrial appendage.

To sense left atrial and ventricular cardiac signals and to provide left chamber pacing therapy, the stimulation device10is coupled to a “coronary sinus” lead24designed for placement in the “coronary sinus region” via the coronary sinus ostium for positioning a distal electrode adjacent to the left ventricle and/or additional electrode(s) adjacent to the left atrium. As used herein, the phrase “coronary sinus region” refers to the vasculature of the left ventricle, including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus.

Accordingly, an exemplary coronary sinus lead24is designed to receive atrial and ventricular cardiac signals and to deliver left ventricular pacing therapy using at least a left ventricular tip electrode26, left atrial pacing therapy using at least a left atrial ring electrode27, and shocking therapy using at least a left atrial coil electrode28.

The stimulation device10is also shown in electrical communication with the patient's heart12by way of an implantable right ventricular lead30having, in this embodiment, a right ventricular tip electrode32, a right ventricular ring electrode34, a right ventricular (RV) coil electrode36, and an SVC coil electrode38. Typically, the right ventricular lead30is transvenously inserted into the heart12so as to place the right ventricular tip electrode32in the right ventricular apex so that the RV coil electrode will be positioned in the right ventricle and the SVC coil electrode38will be positioned in the superior vena cava. Accordingly, the right ventricular lead30is capable of receiving cardiac signals, and delivering stimulation in the form of pacing and shock therapy to the right ventricle.

The housing40for the stimulation device10, shown schematically inFIG. 2, is often referred to as the “can”, “case” or “case electrode” and may be programmably selected to act as the return electrode for all “unipolar” modes. The housing40may further be used as a return electrode alone or in combination with one or more of the coil electrodes,28,36and38, for shocking purposes. The housing40further includes a connector (not shown) having a plurality of terminals,42,44,46,48,52,54,56, and58(shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the terminals). As such, to achieve right atrial sensing and pacing, the connector includes at least a right atrial tip terminal (ARTIP)42adapted for connection to the atrial tip electrode22.

To achieve left chamber sensing, pacing and shocking, the connector includes at least a left ventricular tip terminal (VLTIP)44, a left atrial ring terminal (ALRING)46, and a left atrial shocking terminal (ALCOIL)48, which are adapted for connection to the left ventricular ring electrode26, the left atrial tip electrode27, and the left atrial coil electrode28, respectively.

To support right chamber sensing, pacing and shocking, the connector further includes a right ventricular tip terminal (VRTIP)52, a right ventricular ring terminal (VRRING)54, a right ventricular shocking terminal (RVCOIL)56, and an SVC shocking terminal (SVC COIL)58, which are adapted for connection to the right ventricular tip electrode32, right ventricular ring electrode34, the RV coil electrode36, and the SVC coil electrode38, respectively.

As shown inFIG. 2, an atrial pulse generator70and a ventricular pulse generator72generate pacing stimulation pulses for delivery by the right atrial lead20, the right ventricular lead30, and/or the coronary sinus lead24via an electrode configuration switch74. It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial and ventricular pulse generators,70and72, may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. The pulse generators,70and72, are controlled by the microcontroller60via appropriate control signals,76and78, respectively, to trigger or inhibit the stimulation pulses.

The microcontroller60further includes timing control circuitry79which is used to control the timing of such stimulation pulses (e.g., pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A-A) delay, or ventricular interconduction (V-V) delay, etc.) as well as to keep track of the timing of refractory periods, blanking intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc., which is well known in the art.

The switch74includes a plurality of switches for connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, the switch74, in response to a control signal80from the microcontroller60, determines the polarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) by selectively closing the appropriate combination of switches (not shown) as is known in the art.

Atrial sensing circuits82and ventricular sensing circuits84may also be selectively coupled to the right atrial lead20, coronary sinus lead24, and the right ventricular lead30, through the switch74for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR. SENSE) sensing circuits,82and84, may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. The switch74determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity.

Each sensing circuit,82and84, preferably employs one or more low power, precision amplifiers with programmable gain and/or automatic gain control, bandpass filtering, and a threshold detection circuit, as known in the art, to selectively sense the cardiac signal of interest. The automatic gain control enables the device10to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation. The outputs of the atrial and ventricular sensing circuits,82and84, are connected to the microcontroller60which, in turn, are able to trigger or inhibit the atrial and ventricular pulse generators,70and72, respectively, in a demand fashion in response to the absence or presence of cardiac activity in the appropriate chambers of the heart.

Cardiac signals are also applied to the inputs of an analog-to-digital (A/D) data acquisition system90. The data acquisition system90is configured to acquire intracardiac electrogram signals, convert the raw analog data into a digital signal, and store the digital signals for later processing and/or telemetric transmission to an external device102. The data acquisition system90is coupled to the right atrial lead20, the coronary sinus lead24, and the right ventricular lead30through the switch74to sample cardiac signals across any pair of desired electrodes.

The microcontroller60is further coupled to a memory94by a suitable data/address bus96, wherein the programmable operating parameters used by the microcontroller60are stored and modified, as required, in order to customize the operation of the stimulation device10to suit the needs of a particular patient. Such operating parameters define, for example, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each shocking pulse to be delivered to the patient's heart12within each respective tier of therapy.

Advantageously, the operating parameters of the implantable device10may be non-invasively programmed into the memory94through a telemetry circuit100in telemetric communication with the external device102, such as a programmer, transtelephonic transceiver, or a diagnostic system analyzer. The telemetry circuit100is activated by the microcontroller by a control signal106. The telemetry circuit100advantageously allows intracardiac electrograms and status information relating to the operation of the device10(as contained in the microcontroller60or memory94) to be sent to the external device102through an established communication link104.

In the preferred embodiment, the stimulation device10further includes a physiologic sensor108, commonly referred to as a “rate-responsive” sensor because it is typically used to adjust pacing stimulation rate according to the exercise state of the patient. However, the physiological sensor108may further be used to detect changes in cardiac output, changes in the physiological condition of the heart, or diurnal changes in activity (e.g., detecting sleep and wake states). Accordingly, the microcontroller60responds by adjusting the various pacing parameters (such as rate, AV Delay, V-V Delay, etc.) at which the atrial and ventricular pulse generators,70and72, generate stimulation pulses.

The stimulation device additionally includes a battery110which provides operating power to all of the circuits shown inFIG. 2. For the stimulation device10, which employs shocking therapy, the battery110must be capable of operating at low current drains for long periods of time, and then be capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse. The battery110must also have a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, the device10may employ lithium/silver vanadium oxide batteries.

As further shown inFIG. 2, the device10is shown as having an impedance measuring circuit112which is enabled by the microcontroller60via a control signal114. The impedance measuring circuit112is not critical to the present invention and is shown for only completeness.

In the case where the stimulation device10is intended to operate as an implantable cardioverter/defibrillator (ICD) device, it must detect the occurrence of an arrhythmia, and automatically apply an appropriate electrical shock therapy to the heart aimed at terminating the detected arrhythmia. To this end, the microcontroller60further controls a stimulation output circuit (shocking circuit)116by way of a control signal118. The shocking circuit116will be more particularly described with respect toFIG. 3. It generates shocking pulses of low (up to 0.5 joules), moderate (0.5-10 joules), or high energy (11 to 40 joules) and of virtually any desired waveform. Such shocking pulses are applied to the patient's heart12through at least two shocking electrodes, and as shown in this embodiment, selected from the left atrial coil electrode28, the RV coil electrode36, and/or the SVC coil electrode38. As noted above, the housing40may act as an active electrode in combination with the RV electrode36, or as part of a split electrical vector using the SVC coil electrode38or the left atrial coil electrode28(i.e., using the RV electrode as a common electrode).

Referring now toFIG. 3, it is a block diagram of the stimulation output circuit116which embodies the present invention and which may be used to advantage in the implantable cardiac device10ofFIG. 2. As will be seen subsequently, the output circuit116is capable of providing a stimulation output having essentially any desired waveform.

The output circuit116generally includes a charging circuit117and an output control circuit120. The output circuit116further includes a high voltage capacitor119.

The charging circuit117may be of a type well known in the art for charging the capacitor119to a desired level. Once the capacitor is charged, under control of control signal118, an output voltage (HVP) will reside across the capacitor119. It is this stored voltage that is modulated by the output control120to produce the stimulation output having the desired waveform.

Referring now toFIG. 4, it is a block diagram of an output control circuit which embodies the present invention and which may be used to advantage in the shocking circuit116ofFIG. 3. As will be seen subsequently, the output control circuit120is capable of providing a stimulation output having a waveform of virtually any desired shape.

The control circuit120generally includes a waveform generator122, a chopping waveform generator124, a comparison circuit126, a pulse-width modulation circuit128, and a polarity control circuit130. The control circuit120further includes an H-bridge140, an output capacitor132and series switch133, and a feedback circuit134.

The waveform generator122and chopping waveform generator124are coupled to the comparison circuit126. The output of the comparison circuit126is coupled to the pulse modulation circuit128. A connection131may be made between the pulse-width modulation circuit128and the polarity control circuit130to assure that polarity is not changed when the pulse-width modulation circuit is active.

The outputs of the pulse-width modulation circuit128and the polarity control circuit130are coupled to the H-bridge140. The H-bridge140includes a first leg or half145and a second half147. The first half145includes a pair of switching devices142and144. The second half147of the H-bridge140includes switching devices146and148. The switching devices may be, for example, IGBT devices. Switching devices142and146control the polarity of the stimulation output and devices144and148control the waveform of the stimulation output.

The output capacitor132is coupled across the H-bridge144. It in turn is coupled in parallel to the patient. The capacitor132is preferably a bipolar capacitor.

The feedback circuit134is coupled across the capacitor132for sensing the output pulse voltage. The output of the feedback circuit134is coupled to the comparison circuit126.

Switch133is in series with the capacitor132. The capacitor132is across the circuit output. The switch133may be employed for generation of a standard truncated exponential output. It also prevents charging of capacitor132during external defibrillation. In operation, the waveform generator122provides an electrical signal representing the desired waveform for the stimulation output, such as a defibrillation pulse. The chopping waveform generator124provides a high frequency (for example 500 kilohertz) triangle or saw-tooth waveform. The output of the comparison circuit126provides a control signal in response to the signals from the waveform generator122and the chopping waveform generator124. The pulse-width modulation circuit128then provides a modulated output comprising a pulse train of pulses having varying duty cycles. The pulse-width modulation circuit further conditions the pulse train to adjust duty cycle, polarity, etc. The pulse-width modulated pulse train is coupled to switching devices144and148to control the waveform of the stimulation output. The control circuitry136provides a polarity control signal which is in turn provided to the polarity control circuit130. The outputs of the polarity control circuit130are coupled to switching devices142and146for controlling the polarity of the defibrillation output pulse.

If the polarity is as indicated inFIG. 4, the voltage to the patient will be considered positive when switching device142is turned on, switching devices144and146are turned off, and when switching device148modulates the output voltage (HVP) to control the amplitude of the output voltage. Conversely, the voltage to the patient will be considered negative when switching device146is turned on, switching devices142and148are turned off, and switching device144modulates the output voltage to control the amplitude of the output voltage. The output amplitude is fed back to the comparison circuit126to adjust the pulse widths of the output based upon the desired amplification at the output. The feedback circuit134may, for example, be a differential amplifier.

The pulse-width modulation circuit provides further conditioning of the modulation signal. For example, it may limit duty cycle or pulse train polarity based on output polarity.

One advantage of the output circuit in accordance with the present invention is that the amplitude control devices are switched at a sufficiently high frequency such that the patient, in conjunction with capacitor132, will act as a low pass filter. As a result, high efficiency pulse-width modulation amplification is obtained without the use of a low pass inductor. This is particularly important in as much as space and in an implanted device is at a premium and inductors are generally large in size. It may also be noted that only switching devices144and148are switched to control the waveform of the output pulse. This may serve to lower the amount of energy required to operate the H-bridge and thus save battery power.

Referring now toFIGS. 5 and 6, they illustrate the current flow and resulting output voltage when the control circuit120is in a first phase of the pulse-width modulation. The first phase is when the polarity switch142is closed along with the modulating switch148. When both switches142and148are closed, current flows from the high voltage storage capacitor112into the patient and filter capacitor132. As may be best seen inFIG. 6, the voltage across the patient rises at a rate determined by the capacitance value of the filter capacitor132and the impedance of the patient.

As may be seen inFIG. 7, when switch148is switched open, current ceases to flow from the high voltage capacitor119and instead is sourced by the filter capacitor132. Hence, as will be noted inFIG. 8, the voltage across the patient decreases at a rate determined by the capacitance value of capacitor132and the impedance of the patient.

FIGS. 9-14provide an illustrative example of how a stimulation output having a desired waveform may be provided by the control circuit120. When the stimulation output is to be provided, control signal150is provided by the control circuitry to close the series switch133. The series switch133remains closed throughout the provision of the stimulation output.

FIG. 10shows control signal152which causes polarity switch142to close for providing a positive going waveform portion of the stimulation output. While polarity switch142is closed, a pulse-width modulation pulse train156as illustrated inFIG. 12is provided to the modulation switch148. The result is a positive going portion160of the stimulation output170illustrated inFIG. 14. When the positive going portion of the stimulation output is to terminate, the polarity switch142opens and a control signal154shown inFIG. 11causes polarity switch146to close. With polarity switch146closed, the post width modulation circuitry provides another pulse train158illustrated inFIG. 13to control the modulation switch144. This results in a negative going portion162of the stimulation output waveform170.

The foregoing illustrates the provision of a stimulation output having a 50% duty cycle. As will be appreciated by those skilled in the art, this is but a simplified example of the many different kinds of waveforms which may be generated in accordance with the present invention.

As will be noted inFIG. 14, the output waveform170exhibits a slight ripple164due to the finite capacitance of capacitor132and finite impedance of the patient. InFIG. 15, an H-bridge180is illustrated which provides for greater filtering of the output waveform to greatly eliminate the aforementioned ripple in the output waveform. To this end, the H-bridge180includes a series inductor182which is coupled in series with the legs or bridge halves184and186of the H-bridge180. As will be noted, leg184includes a polarity control switching device188and a pulse-width modulating switching device190. Similarly, leg186includes a polarity switching device192and a pulse-width modulating device194. Coupled between the legs184and186is the filter capacitor132. In use, the H-bridge is coupled to the patient such that the filter capacitor132is in parallel connection with the patient as in the previous embodiment ofFIG. 4.

The H-bridge180further includes blocking diodes200and202. Blocking diode200is switched in circuit by a switch204which closes when polarity switch188closes. Similarly, blocking diode202is switched in circuit by a switch206when polarity switch192is closed.

The operation of the H-bridge180is identical to the operation of the H-bridge140ofFIG. 4. The added inductor182serve to provide greater filtering of the output waveform for eliminating the ripple164as may be seen inFIG. 14.

Referring now toFIG. 16, it illustrates another H-bridge210embodying the present invention. The H-bridge210includes bridge halves, or legs,212,214, and216. Leg212includes polarity control switch220and modulating switch222. Leg214includes polarity switch230and modulating switch232. Similarly, leg216includes polarity switch240and modulating switch242. Coupled across adjacent legs is a filter capacitor. Hence, filter capacitor250is coupled between legs212and214, filter capacitor260is coupled between leg214and216, and filter capacitor270is coupled between leg216and212. Across the filter capacitors are outputs280,290, and300. The outputs may be connected to the electronic configuration switch74(FIG. 2) such that the outputs may be coupled to different stimulation vectors as may be known in the art.

The H-bridge210ofFIG. 16has the advantage that, for a positive polarity as indicated in the figure, when the polarity control switch of one of the legs is closed, the output voltage modulating switches of the other two legs are configured to independently modulate the output voltage. Hence, for example, if switch220is closed, switches232and242may be operating independent of one another for controlling the waveform of the stimulation output. This provides for a further degree of flexibility in providing a desired waveform. Similarly, if polarity switch230is closed, modulating switches222and242may be operating independently to modulate the stimulation output and if polarity switch240is closed, modulating switches222and232may be operated independently to modulate the output.

Still further, if it is desired to modulate the output in this manner for a negative going output waveform portion, any one of the pulse-width modulation switches may be closed and the polarity switches of the other two legs may be operated independently to control the output waveform. As a result, the H-bridge210ofFIG. 16provides improved flexibility towards providing a stimulation output having a desired waveform.

While the invention has been described by means of specific embodiments and applications thereof, it is understood that numerous modifications and variations may be made thereto by those skilled in the art without departing from the spirit and scope of the invention. For example, the switching devices may be field effect transistors (FETs). In addition, the chopping waveform may be eliminated and the feedback could be compared directly with the desired output waveform. It is therefore to be understood that within the scope of the claims, the invention may be practiced otherwise than as specifically described herein.