Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
1999-02-12
2001-10-23
Jastrzab, Jeffrey R. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
C607S014000
Reexamination Certificate
active
06308095
ABSTRACT:
FIELD OF THE INVENTION
The subject matter relates generally to implantable medical devices and more particularly to arrhythmia discrimination with an implantable medical device.
BACKGROUND
Effective, efficient ventricular pumping action depends on proper cardiac function. Proper cardiac function, in turn, relies on the synchronized contractions of the heart at regular intervals. When normal cardiac rhythm is initiated at the sinoatrial node, the heart is said to be in sinus rhythm. However, when the heart experiences irregularities in its coordinated contraction, due to electrophysiologic disturbances caused by a disease process or from an electrical disturbance, the heart is denoted to be arrhythmic. The resulting cardiac arrhythmia impairs cardiac efficiency and can be a potential life threatening event.
Cardiac arrhythmias occurring in the atrial of the heart are called supraventricular tachyarrhythmias (SVTs). Cardiac arrhythmias occurring in the ventricular region of the heart are called ventricular tachyarrhythmias (VTs). SVTs and VTs are morphologically and physiologically distinct events. VTs take many forms, including ventricular fibrillation and ventricular tachycardia. Ventricular fibrillation is a condition denoted by extremely rapid, nonsynchronous contractions of the ventricles. This condition is fatal unless the heart is returned to sinus rhythm within a few minutes. Ventricular tachycardia are conditions denoted by a rapid heart beat, 150 to 250 beats per minute, that has its origin in some abnormal location within the ventricular myocardium. The abnormal location is typically results from damage to the ventricular myocardium from a myocardial infarction. Ventricular tachycardia can quickly degenerate into ventricular fibrillation.
SVTs also take many forms, including atrial fibrillation and atrial flutter. Both conditions are characterized by rapid uncoordinated contractions of the atria. Besides being hemodynamically inefficient, the rapid contractions of the atria can also adversely effect the ventricular rate. This occurs when the aberrant contractile impulse in the atria are transmitted to the ventricles. It is then possible for the aberrant atrial signals to induce VTs, such as a ventricular tachycardia.
Implantable cardioverter/defibrillators (ICDs) have been established as an effective treatment for patients with serious ventricular tachyarrhythmias. ICDs are able to recognize and treat tachyarrhythmias with a variety of tiered therapies. These tiered therapies range from providing antitachycardia pacing or cardioversion energy for treating ventricular tachycardia to defibrillation energy for treating ventricular fibrillation. To effectively deliver these treatments, the ICD must first identify the type of tachyrhythmia that is occurring, after which appropriate therapy is provided to the heart. A problem arises, however, when the ICD delivers therapy to treat a ventricular tachycardia that is caused and sustained by an SVT.
Delivered therapy is typically ineffective in treating the ventricular tachycardia in these instances, as the pacing and/or cardioverting electrical energy has little or no effect on the true source of the ventricular tachycardia. As a result, the ICD delivers inappropriate treatment to the patient, which besides being painful is also very disconcerting to the patient. Accurate discrimination of an SVT versus a malignant ventricular tachycardia is, therefore, an important factor in ensuring the appropriate therapy is delivered to an arrhythmic heart.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for providing a reliable system of discriminating SVT induced ventricular tachycardia from malignant ventricular tachycardia which can provide effective and reliable therapy to patients experiencing malignant ventricular tachycardia.
SUMMARY OF THE INVENTION
As explained in detail below, the present invention is directed to a system for distinguishing between the occurrence of a ventricular tachycardia (VT) and a supraventricular tachycardia (SVT) during a tachycardia episode. Upon detecting a tachycardia episode, the system determines features and/or metric values from the sensed cardiac complexes and compares them to the same general features sensed during normal sinus rhythm. Using this comparison, the system is able to distinguish the underlying cause of a tachycardia episode as either being an SVT or as a VT. Once the determination has been made, the system then provides therapy to treat the underlying cause of the tachycardia episode. In turn, this provides more effective and efficient treatment to the patient.
The present system uses information sensed from normal sinus rhythm complexes to create a template against which cardiac complexes sensed during a tachycardia episode are compared in order to classify them as either VT or SVT complexes. In one embodiment, the system uses information contained in cardiac rate signals (near-field signals) and cardiac morphology signals (far-field signals) of a transvenous lead system in distinguishing VT from SVT during a tachycardia episode. To make this distinction, the system first uses cardiac complexes sensed during normal sinus rhythm to create a normal sinus rhythm template. In one embodiment, the normal sinus rhythm template is derived from the timing differences between at a first feature point on a first signal of a cardiac complex and a second feature point on a second signal of the cardiac complex. In one embodiment, the first signal is a near-field signal and the second signal is a far-field signal.
In one embodiment, the first feature point along the first cardiac signal and the second feature point along the second cardiac signal are determined from morphological features along the cardiac signals. In one embodiment, the morphological features along the cardiac signals can be any combination of maximum deflection points of the cardiac signals, the beginning or ending of cardiac signals, and/or fiducial points along the cardiac signals. Timing differences between the first and second feature points are then determined and median or average values of the timing differences are calculated.
In addition, other metric values can be derived from the feature points, such as median or average signal amplitude values, standard deviation values from the median or average signal amplitude values, slopes and/or slew rates from the cardiac complex signals can also be used in creating the normal sinus rhythm template. Feature values from the normal sinus rhythm cardiac signals can also be recorded and stored for future comparison to cardiac signal sensed during a tachycardia episode. The stored feature values can then be used alone or in conjunction with the timing difference values and/or metric values in distinguishing VT from SVT.
In one embodiment, the system determines the timing differences and/or the other metric values from a first feature point and a second feature point taken from cardiac complexes sensed during a tachycardia episode. The system then compares the timing differences and/or cardiac metric values from the cardiac complexes and the values stored for the normal sinus rhythm template. In one embodiment, if the timing differences exceed a template time difference of the normal sinus rhythm template by a predetermined amount, the cardiac complex is characterized as a VT complex. In an additional embodiment, changes in the metric values can also be used in determining whether a cardiac complex sensed during a tachycardia episode is a VT complex or an SVT complex. When a predetermined number, or percentage, of the cardiac complexes sensed during a tachycardia episode are classified as either VT or SVT complexes, the system then responds to treat the heart.
In an additional embodiment, if the timing differences are less than the template time difference of the normal sinus rhythm template by the predetermined amount, the car
Hsu William
Smith Joseph Martin
Cardiac Pacemakers Inc.
Jastrzab Jeffrey R.
Schwegman Lundberg Woessner & Kluth P.A.
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