The present invention relates in general to implantable, cardiac electrical stimulation devices such as pacemakers or implantable cardioverter-defibrillators. In particular, this invention pertains to a system and method for detecting ventricular capture using far-field sensing of the ventricular R-wave.
Implantable medical devices, such as pacemakers, defibrillators, cardioverters, and implantable cardioverter-defibrillators (xe2x80x9cICDsxe2x80x9d), collectively referred to herein as implantable cardiac stimulating devices, are designed to monitor and stimulate the heart of a patient who suffers from a cardiac arrhythmia. Using leads connected to a patient""s heart, these devices typically stimulate the cardiac muscle (myocardium) by delivering electrical pulses in response to measured cardiac events that are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart""s regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, shape, location, and frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program, which is responsible for the operation of the implantable device, can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Conventional programmable cardiac stimulation devices are generally of two types: (1) single-chamber, or (2) dual-chamber. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart, either the right ventricle or the right atrium. In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart, e.g., both the right atrium and the right ventricle. The left atrium and left ventricle can also be sensed and paced, provided that suitable electrical contacts are effected therewith. The recent development of multi-chamber cardiac stimulation devices allows sensing and pacing in up to all four chambers of the heart. Recent clinical evidence suggests multi-chamber stimulation may have important hemodynamic benefit in patients suffering from heart failure and may be effective in preventing arrhythmias in patients prone to sustained or frequent arrhythmias.
Cardiac stimulation devices have a great number of adjustable parameters that must be tailored to a particular patient""s therapeutic needs. One adjustable parameter of particular importance is the output stimulation energy. For the stimulation device to perform its intended function, it is critically important that the delivered electrical stimuli be of sufficient energy to depolarize the cardiac tissue, a condition known as xe2x80x9ccapturexe2x80x9d.
When a pacemaker stimulation pulse stimulates either the atrium or the ventricle during an appropriate portion of a cardiac cycle, it is desirable to have the heart properly respond to the stimulus provided. Every patient has a xe2x80x9ccapture thresholdxe2x80x9d which is generally defined as the minimum amount of stimulation energy necessary to effect capture. Capture should be achieved at the lowest possible energy setting yet provide enough of a safety margin so that, should a patient""s threshold increase, the output of an implantable stimulation device, i.e. the stimulation energy, will still be sufficient to maintain capture. Dual-chamber and multi-chamber stimulation devices may have differing atrial and ventricular stimulation energy that correspond to the capture thresholds of the targeted cardiac chamber.
The earliest pacemakers had a predetermined and unchangeable stimulation energy, which proved to be problematic because the capture threshold is not a static value and may be affected by a variety of physiological and other factors. For example, certain cardiac medications may temporarily raise or lower the threshold from its normal value. In another example, fibrous tissue that forms around stimulation electrodes within several months after implantation may raise the capture threshold.
As a result, some patients eventually suffered from loss of capture as their pacemakers were unable to adjust the pre-set stimulation energy to match the changed capture thresholds. One attempted solution was to set the level of stimulation pulses fairly high so as to avoid loss of capture due to a change in the capture threshold. However, this approach resulted in some discomfort to patients who were forced to endure unnecessarily high levels of cardiac stimulation. Furthermore, such stimulation pulses consumed extra battery resources, thus shortening the useful life of the pacemaker.
When programmable pacemakers were developed, the stimulation energy was implemented as an adjustable parameter that could be set or changed by a medical practitioner. Typically, such adjustments were effected by the medical practitioner using an external programmer capable of communication with an implanted pacemaker via a magnet applied to a patient""s chest or via telemetry. The particular setting for the pacemaker""s stimulation energy was usually derived from the results of extensive physiological tests performed by the medical practitioner to determine the patient""s capture threshold, from the patient""s medical history, and from a listing of the patient""s medications. While the adjustable stimulation energy feature proved to be superior to the previously known fixed energy, some significant problems remained unsolved. In particular, when a patient""s capture threshold changed, the patient was forced to visit the medical practitioner to adjust the stimulation energy accordingly.
To address this pressing problem, pacemaker manufacturers have developed advanced stimulation devices that are capable of determining a patient""s capture threshold and automatically adjusting the stimulation pulses to a level just above that which is needed to maintain capture. This automatic capture feature improves the patient""s comfort, reduces the necessity of unscheduled visits to the medical practitioner, and increases the pacemaker""s battery life by conserving the energy used to generate stimulation pulses.
However, many of these advanced pacemakers require additional circuitry and/or special sensors that must be dedicated to capture verification. This requirement increases the complexity of the pacemaker system and reduces the precious space available within a pacemaker""s casing, and also increases the pacemaker""s cost. As a result, pacemaker manufacturers have attempted to develop automatic capture verification techniques that may be implemented in a typical programmable pacemaker without requiring additional circuitry or special dedicated sensors.
A common technique used to determine whether capture has been effected is monitoring the patient""s cardiac activity and searching for the presence of an xe2x80x9cevoked responsexe2x80x9d following a stimulation pulse. The evoked response is the response of the heart to the application of a stimulation pulse. The patient""s heart activity is typically monitored by the stimulation device by keeping track of the stimulation pulses delivered to the heart and examining, through the leads connected to the heart, electrical signals that are manifest concurrent with depolarization or contraction of muscle tissue (myocardial tissue) of the heart. The contraction of atrial muscle tissue is evidenced by generation of a P-wave, while the contraction of ventricular muscle tissue is evidenced by generation of an R-wave (sometimes referred to as the xe2x80x9cQRSxe2x80x9d complex).
When capture occurs, the evoked response is an intracardiac P-wave or R-wave that indicates contraction of the respective cardiac tissue in response to the applied stimulation pulse. For example, using such an evoked response technique, if a stimulation pulse is applied to the ventricle, a response sensed by ventricular sensing circuits of the stimulation device immediately following the application of the stimulation pulse is presumed to be an evoked response that evidences capture of the ventricle.
However, it is for several reasons very difficult to detect a true evoked response. First, because the ventricular evoked response is a relatively small signal, it may be obscured by a high-energy stimulation pulse and therefore difficult to detect and identify. Second, the signal sensed by the stimulation device""s sensing circuitry immediately following the application of a stimulation pulse may be not an evoked response but noise, such as electrical noise caused, for example, by electromagnetic interference, or myocardial noise caused by random myocardial or other muscle contraction.
Another signal that interferes with the detection of an evoked response, and potentially the most difficult for which to compensate because it is usually present in varying degrees, is lead polarization. A lead/tissue interface is that point at which an electrode of the stimulation lead contacts the cardiac tissue. Lead polarization is commonly caused by electrochemical reactions that occur at the lead/tissue interface due to application of an electrical stimulation pulse, such as a stimulation pulse, across the interface.
Because the evoked response is sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an xe2x80x9cafterpotentialxe2x80x9d, formed at the electrode can corrupt the evoked response that is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response. Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time.
In each of the above cases, the result may be a false positive detection of an evoked response. Such an error leads to a false capture indication, which in turn, leads to missed heartbeats, a highly undesirable and potentially life-threatening situation. In dual chamber and multichamber stimulation, successful capture all stimulated chambers is critical to maintaining proper synchrony of heart chamber contractions. Loss of optimal atrial-ventricular synchrony or inter-ventricular synchrony may have deleterious hemodynamic effects.
Another problem results from a failure by the stimulation device to detect an evoked response that has actually occurred. In that case, a loss of capture is indicated when capture is in fact present, also an undesirable situation that will cause the device to unnecessarily deliver a high-energy back-up stimulation pulse and invoke the threshold search function in a chamber of the heart.
Automatic threshold testing is invoked by the stimulation device when loss of ventricular capture is detected or on a predetermined periodic basis. An exemplary threshold test is performed as follows. When loss of capture is detected, the device increases the stimulation pulse energy to a relatively high predetermined testing level at which capture is certain to occur, and thereafter decrements the output energy until capture is lost. The stimulation energy is then set to a level slightly above the lowest output energy at which capture was still detected. Thus, capture verification is of utmost importance in proper determination of the stimulation energy.
When a ventricular stimulation pulse is properly captured in the ventricle, a subsequent ventricular contraction results in an R-wave which may be sensed through an atrial lead, in patients with intact atrioventricular (xe2x80x9cAVxe2x80x9d) conduction, as a xe2x80x9cfar-fieldxe2x80x9d signal, also referred to herein as xe2x80x9cfar-field R-wavexe2x80x9d or xe2x80x9cfar-field evoked responsexe2x80x9d. The far-field R-wave confirms successful ventricular capture because the ventricular contraction only occurs after a properly captured ventricular stimulation pulse.
However, previously known dual-chamber and multi-chamber pacemakers do not sense ventricular activity through the atrial lead for a particular interval of time (i.e., the xe2x80x9cpost-ventricular atrial refractory period,xe2x80x9d commonly known as PVARP) subsequent to the delivery of the ventricular stimulation pulse. This refractory period on the atrial channel following ventricular stimulation prevents the atrial channel from mistaking a far-field R-wave for an atrial P-wave. However, detection of the far-field R-wave can be advantageous in verifying that ventricular capture has occurred.
It would thus be desirable to provide a system and method for automating the detection of capture on one or both ventricular channels of an implantable multi-chamber stimulation device, with increased accuracy. It would also be desirable to provide a system and method for reducing the negative effect of polarization and noise on capture verification. It would further be desirable to enable the stimulation device to perform ventricular capture verification without requiring dedicated circuitry and/or special sensors.
The present invention addresses these needs by providing a system and method for automatically detecting capture of a ventricular chamber in a multi-chamber cardiac stimulation device. Ventricular capture is detected by sensing the far-field R-wave that follows a ventricular stimulation pulse that has successfully captured the ventricle.
In one illustrative embodiment of the invention, the device delivers a ventricular stimulation pulse to both the right and left ventricles and then samples a far-field R-wave, resulting from the biventricular evoked response, on the atrial channel during a predetermined far-field interval window.
In another illustrative embodiment, the device delivers a ventricular stimulation pulse to one ventricle and then samples a far-field evoked response on a pair of atrial channels during a predetermined far-field interval window.
In certain embodiments, capture may be verified by comparing one or more signal characteristics of the far-field R-wave sample, such as peak amplitude, integral, or slope, to the same characteristic(s) of an expected far-field R-wave following an evoked response in the designated ventricle.