Surgery – Diagnostic testing – Cardiovascular
Reexamination Certificate
2000-12-28
2002-08-20
Getzow, Scott M. (Department: 3762)
Surgery
Diagnostic testing
Cardiovascular
Reexamination Certificate
active
06438408
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to implantable medical devices and more specifically to implantable monitors for monitoring signs of acute or chronic cardiac mechanical dysfunction such as congestive heart failure (CHF) or cardiogenic shock to enable a physician to diagnose the condition of the heart and prescribe appropriate therapies.
BACKGROUND OF THE INVENTION
Patients suffering from chronic CHF manifest an elevation of left ventricular end-diastolic pressure, according to the well-known heterometric autoregulation principles espoused by Frank and Starling. This may occur while left ventricular end-diastolic volume remains normal due to a decrease in left ventricular compliance concomitant with increased ventricular wall stiffness. CHF due to chronic hypertension, ischemia, infarct or idiopathic cardiomyopathy is associated with compromised systolic and diastolic function involving decreased atrial and ventricular muscle compliance. These may be conditions associated with chronic disease processes or complications from cardiac surgery with or without specific disease processes. Most heart failure patients do not normally suffer from a defect in the conduction system leading to ventricular bradycardia, but rather suffer from symptoms which may include a general weakening of the contractile function of the cardiac muscle, attendant enlargement thereof, impaired myocardial relaxation and depressed ventricular filling characteristics in the diastolic phase following contraction. Pulmonary edema, shortness of breath, and disruption in systemic blood pressure are associated with acute exacerbations of heart failure. All these disease processes lead to insufficient cardiac output to sustain mild or moderate levels of exercise and proper function of other body organs, and progressive worsening eventually results in cardiogenic shock, arrhythmias, electromechanical dissociation, and death.
Such patients are normally treated with drug therapies, including digitalis, which may lead to toxicity and loss of effectiveness. Many inotropic drugs have recently become available, targeted at various receptors in the myocyte and designed for the purpose of directly stimulating cardiac tissue in order to increase contractility. However, there exist many possible undesirable side effects, in addition to the fact that these drugs do not always work for their intended purpose. This is especially characteristic of the patient suffering from end-stage heart failure.
In the early days of implantable cardiac pacing, it was observed that paired pacing (two or more closely spaced pacing pulses delivered at the time-out of an escape interval) and triggered or coupled pacing (one or more pacing pulses delivered following the detection of a P-wave or R-wave terminating an escape interval) with relatively short interpulse intervals (150 to 250 milliseconds in dogs and about 300 milliseconds in human subjects) beneficially slowed heart rate and increased cardiac output. The result of the second pulse, applied within the relative refractory period of the first paced or spontaneous depolarization, is to prolong the refractory period and effect a slowing of the heart rate from its spontaneous rhythm without an attendant mechanical myocardial contraction. This slowing effect has been employed since that time in many applications, including the treatment of atrial and ventricular tachycardias, where a single pulse or a burst of pulses are coupled to a spontaneous tachycardia event with a coupling interval that is shorter than and can be set as a fraction of the tachycardia interval as taught, for example, in U.S. Pat. Nos. 3,857,399 and 3,939,844. The slowing of the heart rate by coupled pacing is accompanied by the ability to increase or decrease the rate with subsequent coupled pacing within wide limits.
Paired and coupled stimulation of a heart chamber also cause a potentiation of contractile force effect through a phenomenon known as post-extrasystolic potentiation (PESP) described in detail in commonly assigned U.S. Pat. No. 5,213,098. The force of contraction of the heart is increased during the heart cycle that the paired or coupled stimulation is applied, and the increase persists but gradually diminishes over a number of succeeding heart cycles. Other measurable PESP effects that also persist but gradually decline over a number of heart cycles include changes in the peak systolic blood pressure, the rate of contraction of the ventricular muscle with a resulting increase of the rate of rise of intraventricular pressure (dP/dt), an increase in coronary blood flow, and an increase in the oxygen uptake of the heart per beat. Investigators observed that PESP was accompanied by an increase in the myocardial oxygen consumption of 35% to 70% as compared with single pulse stimulation at the same rate and was associated with a significant improvement in ejection fraction. The addition of a third stimulus increased the myocardial oxygen uptake even further without any attendant observed increase in cardiac contractile force. The alterations in coronary flow roughly parallel the oxygen consumption of the heart as observed in such studies.
The marked potentiation effect produced by paired stimulation led certain investigators-to speculate that-PESP stimulation would be beneficial in treating heart failure in humans and conducted studies using the technique in the treatment of acute heart failure induced in dogs. Improvements in left ventricular performance and cardiac output produced by such paired pacing in these dogs was observed by several investigators. In other studies conducted on relatively normal dogs' hearts, it was confirmed that paired pacing offered no increase in cardiac output, most likely due to reflex compensation. Early investigators conducted a large number of animal and human studies employing paired and coupled stimulation of the atrial and ventricular chambers, and medical devices were made available by Medtronic, Inc. and other companies in an effort to employ the PESP effect. However, it was realized that the application of closely timed paired and coupled pacing pulses, particularly the high energy pacing pulses that were employed at that time in implantable pacemakers, could trigger a tachyarrhythmia in patient's hearts that were susceptible. The efforts to capitalize on the PESP effects were largely abandoned. A history of the investigations and studies conducted is set forth in the above-referenced '098 patent.
Since dual chamber pacing was developed, conventional, atrioventricular (AV) synchronous pacing systems, including DDD and DDDR pacing systems, marketed by Medtronic, Inc. and other companies, have also been prescribed for treatment of CHF as well as a variety of bradycardia conditions. Certain patient groups suffering heart failure symptoms with or without bradycardia tend to do much better hemodynamically with AV synchronous pacing due to the added contribution of atrial contraction to ventricular filling and subsequent contraction. However, fixed or physiologic sensor driven rate responsive pacing in such patients does not always lead to improvement in cardiac output and alleviation of the symptoms attendant to such disease processes because it is difficult to assess the degree of compromise of cardiac output caused by CHF and to determine the pacing parameters that are optimal for maximizing cardiac output. The magnitude of the AV delay is one factor that requires obtaining pressure data involving an extensive patient work-up as set forth in commonly assigned U.S. Pat. No. 5,626,623.
The above-referenced '098 patent discloses PESP cardiac pacing energy stimulator for applying paired and/or triggered pacing stimulation pulses to the right atrium and/or ventricle incorporating one or more sensors and signal processing circuitry for controlling the frequency of or number of heart cycles between periodic delivery of triggered or paired pacing to induce and optimize the PESP effect for the treatment of CHF or other cardiac dysfuncti
Bennett Tom D.
Deno D. Curtis
Hill Michael R. S.
Igel David A.
Mulligan Lawrence J.
Getzow Scott M.
Girma Wolde-Michael
Medtronic Inc.
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