Automatic capture using independent channels in bi-chamber stimulation

A cardiac stimulation device and method deliver independent stimulation pulses to right and left cardiac chambers, based on the capture thresholds of each chamber, and confirm capture in each chamber. A threshold test is performed in one chamber while stimulating the opposite chamber at increased pulse energy and adjusted interchamber delay.

FIELD OF THE INVENTION

The device relates to an implantable cardiac stimulation device that provides biatrial or biventricular stimulation therapy. More specifically, what is described is a method of automatically detecting and maintaining capture and performing threshold tests in each chamber individually during bi-chamber or multichamber stimulation.

BACKGROUND OF THE INVENTION

In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions.

Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac stimulation devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or anti-arrhythmia therapies to the heart at a desired energy and rate. A cardiac stimulation device is electrically coupled to the heart by one or more leads possessing one or more electrodes in contact with the heart muscle tissue (myocardium). One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.

A stimulation pulse delivered to the myocardium must be of sufficient energy to depolarize the tissue, thereby causing a contraction, a condition commonly known as “capture.” In early pacemakers, a fixed, high-energy pacing pulse was delivered to ensure capture. While this approach is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.

“Threshold” is defined as the lowest stimulation pulse energy at which capture occurs. By stimulating the heart chambers at or just above threshold, comfortable and effective cardiac stimulation is provided without unnecessary depletion of battery energy. Threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Furthermore, threshold will vary over time within a patient as, for example, fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state.

Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modern pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such “capture-verification” algorithms, a threshold test is performed by the cardiac pacing device in order to re-determine the threshold and automatically adjust the stimulating pulse energy. This approach, called “automatic capture”, improves the cardiac stimulation device performance in at least two ways: 1) by verifying that the stimulation pulse delivered to the patient's heart has been effective, and 2) greatly increasing the device's battery longevity by conserving the battery charge used to generate stimulation pulses.

Commonly implemented techniques for verifying that capture has occurred involve monitoring the internal electrocardiogram (IEGM) signals received on the implanted cardiac electrodes. When a stimulation pulse is delivered to the heart, the IEGM signals that are manifest concurrent with depolarization of the myocardium are examined. When capture occurs, an “evoked response” may be detected, which is seen as the intracardiac P-wave or R-wave on the IEGM that indicates contraction of the respective cardiac tissue. Through sampling and signal processing algorithms, the presence of an evoked response following a stimulation pulse is determined. For example, if a stimulation pulse is applied to the ventricle, an R-wave sensed by ventricular sensing circuits of the pacemaker immediately following application of the ventricular stimulation pulse evidences capture of the ventricles.

If no evoked response is detected, typically a high-energy back-up stimulation pulse is delivered to the heart within a short period of time in order to prevent asystole. An automatic threshold test is next invoked in order to re-determine the minimum pulse energy required to capture the heart. An exemplary automatic threshold determination procedure is performed by first increasing the stimulation pulse output level to a relatively high predetermined testing level at which capture is certain to occur. Thereafter, the output level is progressively decremented until capture is lost. The stimulation pulse energy is then set to a level safely above the lowest output level at which capture was attained. Thus, reliable capture verification is of utmost importance in proper determination of the threshold.

Conventional cardiac stimulation devices include single-chamber, or bi-chamber pacemakers or implantable defibrillators. A single-chamber device is used to deliver stimulation to only one heart chamber, typically the right atrium or the right ventricle. A bi-chamber stimulation device is used to stimulate both an atrial and ventricular chamber, for example the right atrium and the right ventricle. It has become apparent in clinical practice that the timing interval between atrial stimulation and ventricular stimulation, known as the AV interval or AV delay, may be important in achieving the desired benefit of bi-chamber pacing. Hence, capture verification in each chamber is important in maintaining the desired atrial-ventricular synchrony.

Mounting clinical evidence now supports the evolution of cardiac stimulating devices capable of stimulating both the left and right heart chambers, e.g., the left and right atrium or the left and right ventricle, or even three or all four heart chambers. Therapeutic applications indicated for bi-chamber (left and right heart chamber) stimulation or multi-chamber stimulation include stabilization of arrhythmias or re-synchronization of heart chamber contractions in patients suffering from congestive heart failure. The precise synchronization of the left and right heart chamber depolarizations is expected to be important in achieving the desired hemodynamic or anti-arrhythmic benefit. Thus, verifying capture in each chamber being stimulated would be essential in maintaining the desired stimulation benefit.

However, in order to achieve bi-chamber or multi-chamber stimulation in a clinical setting, conventional pacemakers have sometimes been used in conjunction with an adapter or a special bifurcated lead so that electrodes may be positioned in both the left and right heart chambers with electrical communication via only one lead connection to the same output channel of the stimulation device.

A four chamber pacing system has been proposed in which unipolar right and left atrial leads are connected via a bifurcated bipolar adapter to the atrial port of a bipolar dual chamber pacemaker. Likewise, unipolar right and left ventricular leads are connected via a bifurcated bipolar adapter to the ventricular channel. The left chamber leads are connected to the anode terminals and the right chamber leads are connected to the cathode terminals of the dual chamber device. In this way, simultaneous bi-atrial or simultaneous bi-ventricular pacing is achieved via bipolar stimulation but with several limitations.

One limitation is that simultaneous stimulation of left and right chambers, as required when two leads are coupled together by one adapter, or by internal hardwiring, is not always desirable. First, such a configuration is sub-optimal in terms of energy delivery because the right chamber lead acts as an additional load during left chamber stimulation and the left chamber lead acts as an additional load during right chamber stimulation.

Second, when inter-atrial or inter-ventricular conduction is intact, stimulation in one chamber may be conducted naturally to depolarize the second chamber. A stimulation pulse delivered in one chamber, using the minimum energy required to depolarize that chamber, often is conducted to the opposing chamber, thereby depolarizing both chambers. In this case, stimulation of both chambers would be wasteful of battery energy.

Precise control of the depolarization sequence and timing may be necessary in order to provide the anti-arrhythmic or hemodynamic support desired. Multi-chamber stimulation systems have been proposed that allow independent stimulation in each chamber, in some cases related to a coupling interval based on sensed or paced events in other chambers.

In order to ensure and maintain a desired depolarization sequence, performing capture verification in each chamber being stimulated is essential. One proposed method of performing capture verification during multisite cardiac pacing verifies capture in one area of the heart by detecting a conducted depolarization in another area of the heart that is electrically continuous with the stimulated area. The limitation of such a method is that it relies on the natural conduction of the depolarization within the cardiac tissue.

In bi-chamber or multi-chamber stimulation, the inter-chamber conduction may not be intact, or stimulation of both chambers may be preferred at a prescribed interval rather than waiting for a naturally conducted depolarization to travel from one chamber to the opposing chamber. Immediate detection of the local evoked response in the chamber being stimulated would be necessary in these situations. As a result, the proposed method would not be appropriate in all patients.

There remains an unmet need for a bi-chamber or multi-chamber cardiac stimulation device that allows independent stimulation and sensing in both right and left chambers of the heart and further provides reliable capture verification in each chamber. It would thus be desirable to provide a system and method for bi-chamber or multi-chamber stimulation with capture verification and automatic threshold determination made possible in each chamber independently.

SUMMARY OF THE INVENTION

An implantable bi-chamber or multi-chamber cardiac stimulation device is described that is capable of performing independent capture verification and threshold measurements in each chamber. A feature of the device is to verify that separate stimulation pulses delivered to the left and the right heart chambers, either atrial or ventricular, effectively capture the intended chamber in order to ensure a desired sequence of activation between the left and right chambers. Another feature of the device is to provide stimulation of each chamber according to its own capture threshold in order to conserve battery longevity.

In a preferred embodiment, the control system controls the delivery of stimulation pulses to the left and right heart chambers (atria or ventricles) at prescribed interchamber delays. Stimulation pulses are delivered by independent output channels such that each chamber receives stimulation pulses according to the capture threshold for that chamber. The capture threshold for each chamber is determined by performing independent threshold tests in each chamber periodically or in response to a loss of capture detection. Capture verification following the delivery of a stimulation pulse is verified by sensing the local evoked response. If loss of capture is detected, a threshold test is performed to allow adjustment of the stimulation pulse energy if the capture threshold has increased.

In one embodiment, a threshold test during biatrial or biventricular stimulation is performed in one chamber while stimulating the opposite chamber at an extended interchamber delay and at an increased pulse energy guaranteed to ensure capture of the opposite chamber. If all four heart chambers are stimulated, and atrioventricular conduction is intact, ventricular threshold tests are performed in one ventricle at a time while the stimulation pulse energy and atrial-ventricular delays to the opposite chamber are temporarily adjusted.

The adjusted interchamber delay is used to prevent a stimulation pulse delivered in the opposite chamber from causing interference during an evoked response detection window in the tested chamber and to prevent a conducted depolarization from the opposite chamber from interfering with evoked response detection in the tested chamber.

The increased stimulation pulse output in the opposite chamber ensures capture of the opposite chamber, which allows a depolarization to be conducted back to the tested in chamber in case a threshold test pulse does not capture the test chamber. In this way, a back-up stimulation pulse may not be needed in the tested chamber. After completing a threshold test in the first chamber, a threshold test may be performed in the opposite chamber while stimulating the first chamber at an extended interchamber delay and increased stimulation pulse output.

In another embodiment, the threshold testing may be performed concurrently in both the right and left chambers. Simultaneous stimulation pulses are delivered followed by sensing for an evoked response in each chamber during a designated evoked response detection window that expires before a conducted depolarization from the opposite chamber could interfere with local evoked response detection.

The methods and devices disclosed ensure the full benefit of bi-chamber or multichamber stimulation by verifying capture in each chamber in response to independent stimulation pulses delivered at prescribed interchamber delays. Independent stimulation of each chamber further improves battery longevity of the stimulation device since leads to both left and right chambers are not loading a single output channel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description is of a best mode presently contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. The device and method are directed at providing automatic capture in an implantable cardiac stimulating device possessing pacemaking, cardioversion and defibrillation capabilities. A general cardiac stimulation device will thus be described in conjunction withFIGS. 1 and 2, in which the automatic capture feature of the device could be implemented. It is recognized, however, that numerous variations of such a device exist in which the methods could be implemented.

FIG. 1illustrates a stimulation device10in electrical communication with a patient's heart12by way of three leads20,24and30suitable 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. The right atrial lead20may also have an atrial ring electrode23to allow bipolar stimulation or sensing in combination with the atrial tip electrode22.

To sense the left atrial and ventricular cardiac signals and to provide left-chamber stimulation therapy, the stimulation device10is coupled to a “coronary sinus” lead24designed for placement in the “coronary sinus region” via the coronary sinus ostium so as to place a distal electrode adjacent to the left ventricle and 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, the 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 electrode26for unipolar configurations or in combination with left ventricular ring electrode25for bipolar configurations; 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 a superior vena cava (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 electrode36will be positioned in the right ventricle and the SVC coil electrode38will be positioned in the right atrium and/or 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 stimulation device10includes a housing40which is often referred to as “can”, “case” or “case electrode”, and which 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 electrodes28,36, or38, for defibrillation shocking purposes. The stimulation device40further includes a connector having a plurality of terminals42,43,44,45,46,48,52,54,56, and58(shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the corresponding terminals). As such, to achieve right atrial sensing and stimulation, the connector includes at least a right atrial tip terminal (ARTIP)42adapted for connection to the atrial tip electrode22. The connector may also include a right atrial ring terminal (ARRING)43for connection to the atrial ring electrode23.

To achieve left chamber sensing, pacing, and shocking, the connector includes at least a left ventricular tip terminal (VLTIP)44, a left ventricular ring terminal (VLRING)45, a left atrial ring terminal (ALRING)46, and a left atrial shocking coil terminal (ALCOIL)48, which are adapted for connection to the left ventricular tip electrode26, the left ventricular ring electrode25, the left atrial ring electrode27, and the left atrial coil electrode28, respectively.

To support right ventricular 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 coil terminal (RV COIL)56, and an SVC shocking coil 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.

At the core of the stimulation device10is a programmable microcontroller60that controls the various modes of stimulation therapy. The microcontroller60typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy, and may further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, the microcontroller60includes the ability to process or monitor input signals (data) as controlled by a program code stored in a designated block of memory. Any suitable microcontroller60may be used that carries out the functions described herein.

FIG. 2illustrates an atrial pulse generator70and a ventricular pulse generator72that generate stimulation pulses for delivery by the right atrial lead20, the right ventricular lead30, and/or the coronary sinus lead24via a switch74. It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, the atrial pulse generator70and the ventricular pulse generator72may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. In accordance with the device, independent output channels are provided for each heart chamber to undergo stimulation in order to achieve independent stimulation of the right and left atrial or ventricular chambers. The atrial pulse generator70and the ventricular pulse generator72are controlled by the microcontroller60via appropriate control signals76and78, 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 interchamber (A—A) delay, or ventricular interchamber (V—V) delay, etc.), as well as to keep track of the timing of refractory periods, PVARP intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc.

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, cross-chamber, etc.) by selectively closing the appropriate combination of switches. 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 switch74, for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial and ventricular sensing circuits82and84may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. The switch74determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches. In this way, the clinician may program the sensing polarity independent of the stimulation polarity.

Each of the atrial sensing circuit82or the ventricular sensing circuit84preferably employs one or more low power, precision amplifiers with programmable gain and automatic gain or sensitivity control, bandpass filtering, and a threshold detection circuit, to selectively sense the cardiac signal of interest. In accordance with the device, two, independent wide-band amplifiers are included in ventricular sensing circuit84to achieve independent sensing in each ventricle. One amplifier is used for sensing in the right ventricle and the other is used for sensing in the left ventricle. Likewise, if the device10is used for biatrial stimulation and sensing, atrial sensing circuit82includes two, wide-band amplifiers for independent sensing in the right atrium and in the left atrium. The automatic sensitivity control enables the stimulation 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 circuits82and84are connected to the microcontroller60for triggering or inhibiting the atrial and ventricular pulse generators70and72, respectively, in a demand fashion, in response to the absence or presence of cardiac activity, respectively, in the appropriate chambers of the heart. The atrial and ventricular sensing circuits82and84, in turn, receive control signals over signal lines86and88from the microcontroller60, for controlling the gain, threshold, polarization charge removal circuitry, and the timing of any blocking circuitry coupled to the inputs of the atrial and ventricular sensing circuits82and84.

Cardiac signals are also applied to the inputs of a data acquisition system90, which is depicted as an analog-to-digital (A/D) converter for simplicity of illustration. The data acquisition system90is configured to acquire intracardiac electrogram (IEGM) signals, convert the raw analog data into digital signals, 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 switch bank74to sample cardiac signals across any pair of desired electrodes.

Advantageously, the data acquisition system90may be coupled to the microcontroller60or another detection circuitry, for detecting an evoked response from the heart12in response to an applied stimulus, thereby aiding in the detection of “capture”. In the embodiment shown inFIG. 2, the microcontroller60includes an automatic capture detector65that searches for an evoked response signal following a stimulation pulse during an “evoked response detection window” set by timing control circuitry79within microcontroller60.

The microcontroller60enables the data acquisition system90via control signal92to sample the cardiac signal that falls in the evoked response detection window. The sampled signal is evaluated to determine if it is an evoked response signal based on its amplitude, peak slope, or another signal feature or combination of features. The detection of an evoked response during the detection window indicates that capture has occurred.

Independent sense amplifiers for the right and left heart chambers provided by the device aid in capture verification of the right and left heart chambers during bi-chamber stimulation. A signal may be sampled from the right or left heart chamber such that an evoked response signal may be detected for verification of capture in each chamber independently and, if necessary, concurrently. Capture detection may occur on a beat-by-beat basis or on a sampled basis. When loss of capture is detected, a safety, back-up pulse is delivered shortly after the primary pulse in order to prevent asystole.

Preferably, a capture threshold search is then performed in order to re-determine the threshold and appropriately adjust the stimulation pulse output. A capture threshold search may also be performed on a periodic basis, preferably once a day during at least the acute phase (e.g., the first 30 days) and less frequently thereafter. A capture threshold search would begin at a desired starting point (either a high output level or the level at which capture is currently occurring) and continue by decreasing the output level until capture is lost. The output level is then increased again until capture is regained. The lowest output level at which sustained capture is regained is known as the capture threshold. Thereafter, the stimulation output is adjusted to a level equal to the capture threshold plus a working margin. The method for performing a threshold test in each chamber during bi-chamber stimulation will be described in detail in conjunction withFIGS. 5 and 6.

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, stimulation pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each stimulation pulse to be delivered to the patient's heart12within each respective tier of therapy.

Advantageously, the operating parameters of the stimulation 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 microcontroller60by a control signal106. The telemetry circuit100advantageously allows intracardiac electrograms and status information relating to the operation of the stimulation device10(as contained in the microcontroller60or memory94) to be sent to the external device102through the established communication link104.

The stimulation device10may further include a physiologic sensor108, commonly referred to as a “rate-responsive” sensor because it is typically used to adjust 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 stimulation parameters (such as rate, AV Delay, interventricular or interatrial delay, etc.) at which the atrial and ventricular pulse generators70and72generate stimulation pulses.

The stimulation device10additionally includes a power source such as a battery110that provides operating power to all the circuits shown in FIG.2. For the stimulation device10, which employs shocking therapy, the battery110must be capable of operating at low current drains for long periods of time, preferably less than 10 μA, and also be capable of providing high-current pulses when the patient requires a shock pulse, preferably, in excess of 2 A, at voltages above 2 V, for periods of 10 seconds or more. The battery110preferably has a predictable discharge characteristic so that elective replacement time can be detected.

As further illustrated inFIG. 2, the stimulation device10is shown to include an impedance measuring circuit112which is enabled by the microcontroller60by means of a control signal114. The known uses for an impedance measuring circuit112include, but are not limited to, lead impedance surveillance during the acute and chronic phases for proper lead positioning or dislodgment; detecting operable electrodes and automatically switching to an operable pair if dislodgment occurs; measuring respiration or minute ventilation; measuring thoracic impedance for determining shock thresholds; detecting when the device has been implanted; measuring stroke volume; and detecting the opening of heart valves, etc. The impedance measuring circuit112is advantageously coupled to the switch74so that any desired electrode may be used.

If it is a function of the stimulation device10to operate as an implantable cardioverter/defibrillator (ICD) device, it must detect the occurrence of an arrhythmia, and automatically apply an appropriate electrical stimulation or shock therapy to the heart aimed at terminating the detected arrhythmia. To this end, the microcontroller60further controls a shocking circuit116by way of a control signal118. The shocking circuit116generates shocking pulses of low (up to0.5Joules), moderate (0.5-10 Joules), or high (11 to 40 Joules) energy, as controlled by the microcontroller60. Such shocking pulses are applied to the patient's heart through at least two shocking electrodes, and as shown in this embodiment, selected from the coronary sinus coil electrode28, the RV coil electrode36, and/or the SVC coil electrode38(FIG.1). 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 coronary sinus coil electrode28.

In the embodiment ofFIG. 1, and as further illustrated inFIG. 3, the stimulation device10includes at least three connection ports200,220,230. A coronary sinus connection port (CS connection port)200accommodates the coronary sinus lead24with terminals44,45,46, and48that are associated with the left ventricular tip electrode (LVTE)26, the left ventricular ring electrode (LVRE)25, the left atrial ring electrode (LARE)27, and the left atrial coil electrode (LACE)28, respectively.

A right ventricular connection port (RV connection port)220accommodates the right ventricular lead30with terminals52,54,56,58that are associated with the right ventricular tip electrode (RVTE)32, the right ventricular ring electrode (RVRE)34, the right ventricular coil electrode (RVCE)36, and the SVC coil electrode (SVCCE)38, respectively.

A right atrial connection port (RA connection port)230accommodates the right atrial lead20with terminals42,43that are associated with the right atrial tip electrode (RATE)22and the right atrial ring electrode (RARE)23, respectively.

It is recognized that numerous variations exist in which combinations of unipolar, bipolar and/or multipolar leads may be positioned at desired locations within the heart in order to provide bi-chamber or multichamber stimulation. The illustrated embodiments of the device provide for the flexibility of independent stimulation and/or sensing in multiple heart chambers by providing a cardiac stimulation device that includes multiple connection ports with unique terminals for the electrode(s) associated with each heart chamber such that the electrodes may be selectively connected to independent sensing and output circuitry using switch74. As such, stimulation and sensing sites are not obligatorily coupled together by adapters or hardwiring within the stimulation device that would otherwise preclude independent sensing and stimulation of each chamber during either bi-chamber or multichamber stimulation.

InFIG. 4, a process240is shown to illustrate an overview of the operation implemented in one embodiment of the device10, for performing independent biatrial or biventricular stimulation and automatic capture verification. In this flow chart, and the other flow charts described herein, the various algorithmic steps are summarized in individual “blocks”. Such blocks describe specific actions or decisions that must be made or carried out as the algorithm proceeds. Where a microcontroller (or equivalent) is employed, the flow charts presented herein provide the basis for a “control program” that may be used by such a microcontroller (or equivalent) to effectuate the desired control of the stimulation device. Those skilled in the art may readily write such a control program based on the flow charts and other descriptions presented herein.

At step250of process240(FIG.4), a stimulation pulse is delivered to a heart chamber, either left or right, from the appropriate output channel of device10. At step252, an automatic capture detector65(FIG. 2) searches for an evoked response signal during an evoked response detection window.

This evoked response detection window follows the stimulation pulse delivered to the first heart chamber and is advantageously within the interchamber delay, prior to stimulating in the opposite heart chamber, such that the local evoked response in the first heart chamber can be detected without interference of the stimulation pulse in the opposite heart chamber. The evoked response signal may be sensed using the same electrodes used for stimulation, or different electrodes located near the stimulation electrodes.

After the interchamber delay expires at step255, a stimulation pulse is delivered to the opposite heart chamber at step260. At step262, the automatic capture detector65searches for an evoked response signal in the opposite chamber.

In the case of biatrial stimulation, a stimulation pulse may be delivered first to the right atrium, followed by an interatrial delay and a stimulation pulse to the left atrium using independent atrial output channels included in atrial pulse generator70. The evoked response signals are sensed using independent amplifiers included in atrial sensing circuit82.

Likewise, a stimulation pulse delivered first to a ventricle would be followed by an interventricular delay and a stimulation pulse to the opposite ventricle using independent ventricular output channels included in ventricular pulse generator72. The evoked response signals in each ventricle would be sensed using independent sense amplifiers included in ventricular sensing circuit84. The interchamber delays are preferably programmable settings, typically on the order of 30 ms.

At decision step265, the automatic capture detector65determines if capture was verified in the first chamber stimulated by the detection of an evoked response at step252. If not, a threshold test is performed in that chamber at step270. One method for performing a threshold test during bi-chamber stimulation will be fully described in conjunction with FIG.5.

After re-determining the capture threshold, the stimulation pulse energy used for stimulating the first chamber may be adjusted at step275. Typically the pulse energy is set equal to the capture threshold plus a working margin on the order of 0.25 V.

At decision step280, the automatic capture detector65determines if capture was verified in the opposite chamber by the detection of an evoked response at step262. If so, bi-chamber stimulation may continue at the programmed settings by returning to step250. If capture is not verified in the opposite heart chamber, a threshold test is performed in that chamber at step285.

The stimulation pulse energy used for stimulation in the opposite chamber is adjusted according to the capture threshold found in that chamber at step290. Bi-chamber stimulation then continues, by returning to step250, using right and left chamber stimulation pulse energy settings that have been automatically programmed according to the capture thresholds of the respective heart chambers.

Thus, independent stimulation of right and left heart chambers is performed according to the capture threshold of each chamber. Sensing of a local evoked response verifies capture of each of the right and left heart chambers. If loss of capture is detected in either chamber, a threshold test is performed in that chamber according to the bi-chamber threshold test to be described in conjunction with FIG.5.

InFIG. 5, a process300illustrates an overview of the operation implemented in a preferred embodiment of the device10, for performing threshold tests during bi-chamber stimulation. A threshold test may be performed in response to a loss of capture as described in connection with process240ofFIG. 4, or it may be performed on a periodic basis.

At step305ofFIG. 5, the atrial or ventricular chamber to undergo a threshold test is identified. That chamber may be either the left or right chamber, selected in either order, during a periodic threshold test, or is the chamber in which loss of capture has been detected during bi-chamber stimulation.

At step310, the stimulation pulse energy to the opposite chamber (not being tested) is increased, preferably to approximately 1.5 times the currently programmed pulse energy for that chamber. For example, if a threshold test is performed in the right chamber, the left chamber stimulation pulse energy is increased at step310. This step310of increasing pulse energy in the opposite chamber ensures continued, regular capture of the left chamber during threshold testing in the right chamber, or vice versa.

At step315, the interchamber delay is adjusted such that the opposite chamber is not stimulated during the evoked response detection window that will be applied following delivery of a test stimulation pulse in the first chamber. If threshold testing is being performed during biatrial stimulation, the interatrial delay is adjusted. If threshold testing during biventricular stimulation is being performed, the interventricular delay is adjusted.

The delay is adjusted such that the chamber to be tested is stimulated first, preferably followed by a lengthened delay period after which stimulation to the opposing chamber is delivered. The lengthened delay will not only prevent stimulation of the opposing chamber during evoked response sensing in the first chamber but also prevents a depolarization in the opposing chamber from propagating to the test chamber during the evoked response detection window, which would likely produce erroneous threshold test results.

At step320, the stimulation pulse energy in the test chamber is decreased by predetermined steps, typically 0.25 or 0.5 Volts, until capture is lost as determined at decision step325. The stimulation pulse energy starts at a level certain to achieve capture: either the currently programmed setting in the case of a periodic threshold test or, in the case of a detected loss of capture, a predetermined maximum setting.

Once loss of capture is detected at decision step325, the capture threshold is identified at step330as the previous, lowest pulse energy setting at which capture was maintained. The stimulation pulse energy for the tested chamber is adjusted at step335to be equal to the newly determined capture threshold plus a working margin, typically 0.25 Volts.

At step340, the stimulation pulse energy for the opposite chamber is reset to the programmed value, and the interchamber delay is reset to the programmed setting. The threshold test is thus terminated at step345.

Alternatively, the threshold test may be performed by beginning at a sub-threshold stimulation level and then incrementally increasing the energy level until the chamber being tested is captured. The stimulation pulse energy is then preferably set to be equal to the capture threshold plus a working margin, e.g. 0.25 Volts.

The output of device10during the implementation of method300ofFIG. 5is illustrated in FIG.6. In this example, biventricular stimulation is being performed, but similar operations may be performed in the atria. The outputs from the independent right and left ventricular output channels are shown on one trace for the sake of convenience.

The top trace depicts a normal bi-ventricular stimulation mode370, wherein the right ventricular stimulation pulse350and the left ventricular stimulation pulse352are separated by an interventricular delay362. In this example, the right ventricular stimulation pulse and the left ventricular stimulation pulse are shown having equal pulse width and pulse amplitude. However, the pulse width or pulse amplitude may be adjusted in the independent output channels to provide stimulation pulses according to the capture threshold of each chamber.

Following each ventricular stimulation pulse350and352, is an evoked response detection window368. The automatic capture detector65searches for a local evoked response in the stimulated chamber during the evoked response detection window. An evoked response detection window may follow every stimulation pulse as illustrated inFIG. 6, to allow capture verification on a beat-by-beat basis. Alternatively, capture verification may be performed less frequently, on a periodic or sampled basis.

In the second trace, the outputs of the left and right ventricular output channels are shown during a right ventricular threshold test380. The left ventricular stimulation pulse356is increased, in this example by increasing the pulse amplitude to 1.5 times the pulse amplitude during normal stimulation370. The interventricular delay364is lengthened such that the left ventricle is stimulated at an extended delay after the right ventricle.

The evoked response detection window368is set during the interventricular delay364such that an evoked response may be detected in the right chamber prior to delivering a stimulation pulse to the left ventricle, thus avoiding interference from the left ventricular stimulation pulse or depolarization. The right ventricular stimulation pulse354can be seen to undergo a progressive decrease in pulse amplitude beginning from a predetermined maximum pulse amplitude. During an actual threshold test, the right ventricular pulse amplitude would be decreased until loss of capture is detected, i.e., no evoked response detected during the evoked response detection window.

One advantage of this bi-chamber threshold test method is that when loss of capture occurs in the right ventricle, no safety back-up stimulation pulse is needed to depolarize the right ventricle. This is because the stimulation pulse delivered to the left ventricle is set to guarantee capture of the left ventricle which will produce a depolarization wave that will be conducted to the right ventricle and cause depolarization of the previously non-captured right ventricle. Therefore, in this embodiment, left ventricular stimulation at an increased pulse energy is committed to during the right ventricular threshold test and vice versa. Alternatively, the device10may be programmed to deliver the back up pulse in the right ventricle to maintain interventricular synchronization for the hemodynamic benefit of the patient.

In an alternative embodiment, stimulation to the opposite chamber could be inhibited upon sensing an intrinsic depolarization in the opposite chamber. If the conduction time between the right and left chambers is shorter than the lengthened interchamber delay, a test pulse that captures the tested chamber will produce a depolarization that is conducted to the opposite chamber causing depolarization of the opposite chamber as well.

If this depolarization is sensed by the device10, the stimulation pulse scheduled to be delivered to the opposite chamber at the end of the interchamber delay may be inhibited. If no depolarization is sensed, the opposite chamber is stimulated at an increased pulse energy expected to guarantee capture that will further be conducted to the tested chamber if the tested chamber was not captured by the test pulse.

In the third trace, a left ventricular threshold test390is performed. In this test, the inter-ventricular delay366is adjusted such that the right ventricular stimulation pulse360is delivered at an extended delay after the left ventricular stimulation pulse358. The right ventricular stimulation pulse360is set at 1.5 times its programmed pulse amplitude. The left ventricular stimulation pulse358begins at a high level and is seen to be progressively decreased. During an actual threshold test, the left ventricular pulse amplitude would be decreased until loss of capture is detected. When loss of capture occurs in the left ventricle, a depolarization from the right ventricle will propagate to the left ventricle and cause depolarization precluding the need for a safety back-up pulse in the left ventricle. The backup pulse in the left ventricle may be programmed for delivery in the event of left-ventricular loss-of-capture.

InFIG. 7, a process500is shown describing an overview of the operations implemented in one embodiment of the device10for performing ventricular threshold tests during multichamber stimulation. At step405, microcontroller60identifies the ventricle, left or right, in which a threshold test will be performed. At step410, the stimulation pulse energy applied in the opposing ventricle (not being tested) is set to a temporary high level, preferably to 1.5 times the currently programmed setting. This increase in pulse energy ensures capture of the opposing ventricle while testing is being performed in the first ventricle.

The AV and PV delays associated with the opposing ventricle are also temporarily adjusted at step410. At step415, the AV and PV delays associated with the ventricle undergoing a threshold test are temporarily shortened, preferably to 25 ms and 50 ms respectively. For example, if a right ventricular threshold test is to be performed, the left AV delay and the left PV delay are temporarily lengthened to predetermined intervals; the right AV delay and right PV delay are shortened to 25 ms and 50 ms, respectively. The adjustment of the AV and PV delays to the opposing ventricle and the shortened AV and PV delays in the test ventricle prevent a conducted depolarization from the opposing ventricle from interfering with evoked response detection in the tested ventricle. The shortened AV and PV delays in the test ventricle also ensure stimulation of the test ventricle prior to an intrinsic depolarization due to conduction of the atrial depolarization to the ventricle. Furthermore, the shortened AV and PV delays in the test chamber avoid fusion, which would confound the threshold test results. Fusion occurs when a stimulation pulse is delivered approximately at the same time as an intrinsic depolarization, causing a distorted depolarization signal.

At step420, the stimulation pulse energy is progressively decreased from a level known to evoke capture, either the currently programmed setting in the case of a periodic threshold test or, in the case of loss of capture, a predetermined maximum setting, until loss of capture is detected at decision step425. Once capture is lost, the capture threshold is identified as the lowest pulse energy at which capture was maintained at step430. Hence, at step435, the stimulation pulse energy for the tested ventricle is adjusted to the newly determined threshold plus a working margin, typically 0.25 Volts. At step440the AV and PV delays associated with both ventricles are reset to the programmed settings, and the pulse energy in the opposing chamber is reset to its programmed setting. At step445, the threshold test is terminated.

The output of device10during independent stimulation of all four heart chambers and during the ventricular threshold testing operations summarized in the flow chart ofFIG. 7is illustrated by the diagrams of FIG.8. The normal four-chamber stimulation mode450is represented by the top two traces depicting atrial output452and ventricular output456. The atrial output452from two independent atrial channels, stimulating the right and left atria, is shown on one trace for the sake of convenience. The right atrial and left atrial stimulation pulses462and464, respectively, are shown separated in time by an interatrial delay454.

The ventricular output456from two independent ventricular channels is also shown on one trace, for the sake of convenience. The right ventricular stimulation pulse466can be seen to occur after the right atrial pulse462following a right AV delay458. The left ventricular stimulation pulse468is shown to occur after the left atrial pulse464following a left AV delay460.

Thus, it can be seen that stimulation of each of the four chambers may be delivered at prescribed interchamber delays for achieving a desired sequence of heart chamber contractions. Furthermore, each stimulation pulse may be followed by an evoked response detection window492, during which no stimulation pulse is delivered to any other heart chamber, to allow beat-by-beat capture verification of each chamber.

The operation of device10during a ventricular threshold test470is illustrated by the two lower traces of FIG.8. In the example shown, a threshold test is to be performed in the left ventricle. The atrial output480from the two independent atrial channels remains the same as during normal stimulation mode450.

The stimulation pulse output to the right ventricle476is temporarily increased to an amplitude 1.5 times greater than the amplitude during the normal stimulation mode450. The right AV delay478is increased to a predetermined setting. The left AV delay490is shortened, preferably to 25 ms, to avoid fusion.

Thus, the left ventricular stimulation pulse474is seen to be delivered much earlier than the right ventricular stimulation pulse476so that a stimulation pulse or a conducted depolarization from the right ventricle will not interfere with evoked response detection in the left ventricle during the evoked response detection window492.

The left ventricular stimulation pulse474is seen to start at an initially high amplitude and progressively decrease in amplitude during the threshold test. Four decreasing pulses are shown for the sake of illustration, however, once loss of capture is detected, the left ventricular capture threshold would be identified, and the threshold test would be complete. Similar procedures may be performed in the right ventricle for determining a right ventricular capture threshold. Likewise, right and left atrial capture thresholds may be determined according to the operations described previously in conjunction with FIG.5.

Thus, the device and method provide for automatic and separate adjustment of the stimulation pulse energy for each heart chamber based on an automatically determined capture threshold for each chamber being stimulated.

In an alternative embodiment, stimulation pulses may be delivered concurrently to the left and right chambers, each followed by an appropriately timed evoked response detection window during which the local evoked response is sensed in each chamber individually using independent sense amplifiers. The evoked response detection windows in this embodiment are limited to the time immediately after the stimulation pulse during which a local evoked response is expected. The evoked response detection windows would be kept shorter than the interchamber conduction time such that if one chamber is captured and one isn't, a conducted depolarization is not detected in the non-captured chamber and mistaken for a capture detection.

Thus, a system and method for providing independent stimulation and sensing in each heart chamber during bi-chamber or multichamber stimulation therapy has been described in which each chamber is stimulated according to its own capture threshold, thus conserving battery energy. Methods provided herein for performing threshold tests in each chamber independently allow for a determination of capture threshold for each chamber while minimizing the need for safety back-up pulses by ensuring capture of the opposing chamber during a threshold test.

Furthermore, independent capture verification of each chamber is made possible through independent sensing channels, thus ensuring that precisely timed stimulation pulses effectively produce a desired synchronization of heart chambers. While detailed descriptions of specific embodiments of the device and method have been provided, it would be apparent to those reasonably skilled in the art that numerous variations of the methods described herein are possible in which the concepts of the device and method may readily be applied. The descriptions provided herein are for the sake of illustration and are not intended to be exclusive.