Surgery: light – thermal – and electrical application – Light – thermal – and electrical application – Electrical therapeutic systems
Reexamination Certificate
2000-09-26
2003-09-09
Evanisko, George R. (Department: 3762)
Surgery: light, thermal, and electrical application
Light, thermal, and electrical application
Electrical therapeutic systems
C607S028000
Reexamination Certificate
active
06618619
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention generally relates to pacemakers or other implantable cardiac stimulation devices and in particular to techniques for reducing the effects of evoked response bleed-through on electrical polarization measurements performed in connection with calibrating an automatic capture system of an implantable cardiac stimulation device, e.g., a pacemaker, an implantable cardioverter/defibrillator (ICD), or the like.
DESCRIPTION OF RELATED ART
A pacemaker is a medical device, typically implanted within a patient, that provides electrical stimulation pulses to selected chambers of the heart, i.e., the atria and/or the ventricles. Such stimulation pulses cause the muscle tissue of the heart (myocardial tissue) to depolarize and contract, thereby causing the heart to beat at a controlled rate.
Most pacemakers can be programmed to operate in a demand mode of operation, i.e., to generate and deliver stimulation pulses to the heart only when the heart fails to beat on its own. To this end, the pacemaker senses cardiac activity, i.e., heart beats, and if the heart beats do not occur at a prescribed rate, then stimulation pulses are generated and delivered to an appropriate heart chamber, either an atrium or a ventricle, in order to force the heart to beat.
When operating in a demand mode of operation, the pacemaker defines a period of time, referred to generally as the “escape interval” (which may further be referred to as either an “atrial escape interval” or a “ventricular escape interval,” depending upon the mode of operation of the pacemaker) that is slightly longer than the period of time between normal heart beats. Upon sensing such a “natural” (non-stimulated or non-paced) heart beat within the allotted time period, the escape interval is reset, and a new escape interval is started. A stimulation (or pacing) pulse is generated at the conclusion of this new escape interval unless a natural heart beat is again sensed during the escape interval. In this way, stimulation pulses are generated “on demand,” i.e., only when needed to maintain the heart rate at a rate that never drops below the rate set by the escape interval.
The heart rate is monitored by examining the electrical signals that are manifest concurrent with the depolarization or contraction of the myocardial tissue. The contraction of atrial muscle tissue is manifest by the generation of a P-wave. The contraction of ventricular muscle tissue is manifest by the generation of an R-wave (sometimes referred to as the “QRS complex”). The sequence of electrical signals that represent P-waves followed by R-waves (or QRS complexes) can be sensed from inside of or proximate to the heart by using sensing leads implanted inside or on the heart, e.g., pacemaker leads, or by using external electrodes attached to the skin of the patient.
Most modern implantable pacemakers are programmable. That is, the basic escape interval (atrial and/or ventricular) of the pacemaker, as well as the sensitivity (threshold level) of the sensing circuits used in the pacemaker to sense P-waves and/or R-waves, and numerous other operating parameters of the pacemaker, may be programmably set at the time of implantation or thereafter to best suit the needs of a particular patient. Hence, the pacemaker can be programmed so as to yield a desired performance.
The operation of a pacemaker as described above presupposes that a stimulation pulse generated by the pacemaker effectuates capture. As used herein, the term “capture” refers to the ability of a given stimulation pulse generated by a pacemaker to cause depolarization of the myocardium, i.e., to cause the heart muscle to contract, or to cause the heart to “beat.” A stimulation pulse that does not capture the heart is thus a stimulation pulse that may just as well have not been generated, since it has not caused the heart to beat. Such a non-captured stimulation pulse not only represents wasted energy—energy drawn from a limited energy resource (e.g., a battery) of the pacemaker—but worse still may provide the pacemaker logic circuits with false information. That is, the logic circuits of the pacemaker may presuppose that each stimulation pulse generated by the pacemaker captured the heart. If the stimulation pulse does not capture the heart, then the pacemaker logic circuits control the operation of the pacemaker may be based on false information, and may thus control the pacemaker in an inappropriate manner. Thus, there is a critical need for a pacemaker to properly determine whether a given stimulation pulse has effectuated capture.
While there are many factors that influence whether a given stimulation pulse effectuates capture, a principal factor is the energy of the stimulation pulse. The energy of the stimulation pulse, in turn, is determined by the amplitude and width of the stimulation pulse generated by the pacemaker. Advantageously, in a programmable pacemaker, both the amplitude and pulse width of the stimulation pulse are parameters that may be programmably controlled or set to a desired value.
An implantable pacemaker derives its operating power, including the power to generate a stimulation pulse, from a battery. The power required to repeatedly generate a stimulation pulse dominates the total power consumed by a pacemaker. Hence, to the degree that the power associated with the stimulation pulse can be minimized, the life of the battery can be extended and/or the size and weight of the battery can be reduced. Unfortunately, however, if the power associated with a stimulation pulse is reduced too far, the stimulation pulse is not able to consistently effectuate capture, and the pacemaker is thus rendered ineffective at performing its intended function. Thus, it is desirable for a pacemaker to adjust the energy of a stimulation pulse to an appropriate level that provides sufficient energy to effectuate capture, i.e., generate an evoked response, but does not expend any significant energy beyond that required to effectuate capture.
Initially, the most common technique used to adjust the stimulation energy to an appropriate level was manual, using the programmable features of the pacemaker. That is, at the time of implant, a cardiologist or other physician conducts some preliminary stimulation tests to determine how much energy a given stimulation pulse must have to effectuate capture at a given tissue location. If the preliminary tests indicate that the capture threshold is high (compared to the average patient) then the lead will be repositioned until a “good” threshold is found. Once it has been determined that the thresholds are acceptable, the stimulation electrode is then left in place and the amplitude and/or width of the stimulation pulse is set to a level that is typically 2 to 3 times greater than the amplitude and/or width determined in the preliminary tests. The increase in energy above and beyond the energy needed to effectuate capture is considered as a “safety margin.”
During the acute phase, e.g., over a period of days or weeks after implant, the stimulation pulse energy needed to effectuate capture usually changes. This stimulation pulse energy is hereafter referred to as the “capture-determining threshold.” Hence, having a safety margin factored into the stimulation pulse energy allows the stimulation pulses generated by the pacemaker to continue to effectuate capture despite changes in the capture-determining threshold. Unfortunately, however, much of the energy associated with the safety margin represents wasted energy, and shortens the life of the battery. Furthermore, after the acute phase (when the lead is considered in the chronic phase), the capture-determining threshold is typically much lower than that determined at implant. Thus, if left unchecked, the safety margin determined necessary at implant is extremely wasteful during the chronic phase.
State of the art pacemakers now include an automatic capture system (see, for example the AUTOCAPTURE™ pacing system used by Pacesetter, Inc., which, after implant of the pacemaker, automatically
Bornzin Gene A.
Florio Joseph J.
Bradford Roderick
Evanisko George R.
Pacesetter Inc.
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