Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-04-05
2002-12-10
Layno, Carl (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C600S504000
Reexamination Certificate
active
06491639
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to cardiac devices, and more particularly to the assessment of hemodynamic status by a cardiac device.
II. Description of the Related Art
A critical function of implantable cardioverter defibrillators (ICDs) is to identify and terminate hemodynamically unstable arrhythmias such as ventricular fibrillation (VF) and ventricular tachycardia (VT). A technical challenge is to achieve high sensitivity, so that the occurrence of such arrhythmias does not go undetected, while maximizing specificity, so that relatively benign rhythms such as atrial fibrillation (AF) and sinus tachycardia (ST) are not treated.
Conventional implantable cardioverter defibrillators (ICDs) perform discrimination through analysis of the intracardiac electrogram, where features such as rate, interval regularity, and QRS morphology are analyzed in order to identify the underlying rhythm. This approach is convenient because electrical sensing can be easily performed using the same leads that the device requires to deliver therapy. Furthermore, electrogram features provide reasonable accuracy in rhythm identification. However, the accuracy of this approach is ultimately limited because there are inherent ambiguities in cardiac electrical activity that prevent perfect identification. For example, VT, which typically generates a wide QRS complex morphology and regular intervals, may exhibit narrow complexes, particularly if bipolar sensing is used, and irregular intervals. On the other hand, AF, which typically generates narrow QRS complexes and irregular intervals, may exhibit a wide QRS complex, due to a fixed- or rate-related bundle branch block or interventricular conduction defect. In addition AF may produce extended sequences of regular intervals. Thus, there is a theoretical limit to the accuracy with which electrogram analysis can identify cardiac rhythms. The problem is compounded by the fact that practical considerations, such as power consumption and manufacturing costs, impose compromises in the algorithms and sensing configurations that are implemented.
The conventional approach to the detection of pathological arrhythmias using electrogram analysis rests on the premise that the type of therapy is appropriately determined by the underlying cardiac rhythm. In fact, from a clinical perspective, a more important criterion for determining therapy than the identification of the underlying rhythm is whether the rhythm is hemodynamically stable, that is, whether the heart is pumping blood sufficiently to ensure adequate perfusion of vital organs. For example, there are forms of VT that are hemodynamically stable which do not require immediate cardioversion. For these rhythms, low energy methods of termination such as anti-tachycardia pacing can be attempted without sacrificing safety. Such an approach improves device longevity, and avoids subjecting the conscious patient to painful and distressing electrical shocks. Conversely, there are cases of AF and supraventricular tachycardias that are hemodynamically unstable because of high rate or poor ventricular function, and therefore require rapid termination. Thus, the most relevant question clinically is not the identification of the underlying cardiac rhythm but rather whether the rhythm is hemodynamically stable. This point is well illustrated by the Universal Algorithm for emergency cardiac care, advocated by the American Heart Association, and described in the book Advanced Cardiac Life Support, R. O. Cummins, Ed., 1997. The major branch point, which occurs early in the algorithm, is the test for the presence of an arterial pulse, a rapid and robust method of assessing hemodynamic status. Subsequent evaluation and treatment along the two branches of the algorithm varies significantly depending on the outcome of this test. Only later in the algorithm is identification of the origin, ventricular vs. supraventricular, of the cardiac rhythm attempted.
Because of the inherent limitation in the accuracy that can be achieved in identifying cardiac rhythms using electrogram analysis, and more importantly, because what is of ultimate clinical relevance is the hemodynamic status of the patient, what is needed is a method and apparatus for rapidly assessing hemodynamic status that can be used by implantable cardiac devices.
Another application of rapid hemodynamic status assessment is in capture verification, a technique that verifies that a pacemaker-delivered stimulus has electrically captured the myocardium and initiated the propagation of a depolarization. The technique is useful because it allows continuous or periodic adjustment of the pacing energy to accommodate a changing threshold. It allows the pacemaker to deliver the minimum energy necessary to consistently capture the heart, which both maximizes the pacemaker longevity and enhances patient safety. A method of capture verification that is known in the art analyzes the electrical evoked response that is generated after a pace stimulus is delivered by the pacemaker. While this approach, disclosed in U.S. Pat. Nos. 5,165,404 and 5,165,405, has proven to be commercially viable, it requires sophisticated circuitry and low-polarization leads, and is potentially susceptible to electrical noise. It would be advantageous to have a capture verification technique based on hemodynamic status assessment.
One of the challenges of electrically detecting cardiac depolarizations is ensuring that the system is sufficiently sensitive so that myocardial depolarizations are recognized by the device, while at the same time not excessively sensitive so that repolarization waves or noise, such as that induced by diaphragmatic myopotentials, is incorrectly interpreted as a depolarization. Beat-to-beat hemodynamic sensing would allow oversensing of electrical noise to be recognized and distinguished from ventricular fibrillation (VF) or tachycardia. In oversensing of electrical noise, regular and robust mechanical cardiac contractions would occur along with higher-rate ventricular sensed events. In this case the sensitivity could be decreased until the ventricular sensed events occurred in concert with cardiac contractions. On the other hand, the detection of robust mechanical cardiac contractions during the absence of ventricular sensed events would indicate that the sense amplifier sensitivity is set too low and should be increased. Finally, the presence of rapid ventricular sensed events without detected cardiac contractions would indicate the presence of VF, hemodynamically unstable VT, or another hemodynamically unstable rhythm. It would thus be advantageous to provide an improved system for verification of sensed events and optimization of sensing thresholds.
Yet another application of hemodynamic sensing is in pace-parameter optimization, in which any of a number of parameters that define pacing characteristics is optimized. Pace-parameter optimization is particularly applicable to multi-site pacing. For example, in dual-chamber (atrial and ventricular) pacemakers the atrio-ventricular (AV) delay is optimized, so that the ventricular contraction is timed such that the contribution of the atrial contraction is maximally exploited. Another example is biventricular pacing for heart failure, in which ventricular synchrony is optimized by adjusting the timing that pace pulses are delivered to various sites. Currently, these parameters are set to default nominal values, or labor-intensive methods are used to assess hemodynamics in order to optimize the parameters at time of device implant. Examples of these methods include ultrasound to measure ejection fraction and left heart catheterization to measure the rate of change of left ventricular pressure during systole, which is a measure of contractility and mechanical efficiency. In addition to the substantial time and effort these approaches impose, the invasive techniques increase the perioperative risk to the patient. Furthermore, these approaches are possible only during device implant or f
Layno Carl
Mitchell Steven M.
Pacesetter Inc.
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