A New Breed of Pacemakers.
The physiologic premise for biventricular pacemakers is that they resynchronize left ventricular depolarization. Ventricular contractility in the setting of bundle branch block is inefficient and is thought to contribute to ventricular dysfunction in heart failure. Ventricular pacemakers placed in the left and right ventricles in patients with bundle branch block and a QRS interval greater than 140 milliseconds have been shown to reorganize that ventricular contractility.
Resynchronization also can decrease mitral regurgitation, which is often associated with ventricular dilatation in systolic heart failure. Resynchronization therapy with placement of electrodes in the left and right ventricles provides an opportunity to also place defibrillator leads in the ventricle as separate or composite leads. The combination of these devices-biventricular pacing and defibrillator-is an attractive therapeutic package in patients with heart failure. The procedure is applicable to perhaps as many as 25% of heart failure patients who have prolonged QRS intervals.
Nonrandomized clinical studies in Europe suggested that clinical improvement could be achieved. These studies were followed by a number of small studies in the United States: the Multisite Stimulation in Cardiomyopathy (MUSTIC) trial and the Multicenter InSync Randomized Clinical Evaluation (MIRACLE) reported variable degrees of improvement in the 6-minute walk test and measurements of the quality of life in patients with heart failure. Patients enrolled in these trials have in general had advanced heart failure and were in New York Heart Association classes III and IV.
As yet, no mortality trial has been carried out. One trial, the Comparison of Medical Therapy, Pacing and Defibrillation in Chronic Heart Failure (COMPANION), is recruiting patients and comparing synchronization therapy with and without an automatic implantable defibrillator to standard medical therapy Trials are also underway to examine the benefit of routine automatic defibrillator implantation in patients with heart failure.
The results of the early trials have been encouraging and have provided important stimulus for more rigorous examination of the benefits of these devices.
In some corners of the world, resynchronization therapy has already become routine in spite of the paucity of clinical observations. Much of this has been driven by the apocryphal stories of dramatic improvements associated with the implantation of these devices. A number of questions need to be answered, however, before resynchronization therapy becomes a mainstream treatment for heart failure. The very nature of device implantation lends this therapy to extreme preference bias among the patients receiving the device.
The mortality and morbidity benefits associated with this therapy are not yet defined. In addition, patient selection needs to be further clarified. Is the device applicable for both left and right bundle branch block? What is the QRS duration for which the device is best suited? Do all patients benefit or are there certain classes of heart failure, ischemic or nonischemic, that may benefit? What is the role of preimplant mitral regurgitation and what is the most appropriate PR interval to use? How long can we expect these devices to exhibit their benefit? And most importantly, where is the most appropriate place in the patient's ventricle to position the pacemaker lead?
Manufacturers are moving quickly to develop ever more sophisticated devices that will sense and more efficiently deliver electrical impulses for pacing and defibrillation. This fact alone should give one pause. Rather than jumping to incorporate these new technologies, we should be moving slowly to achieve the maximum technological benefit with the minimal trauma for our patients. On the surface, these devices have great potential, but we need to know more about implantation strategies before incorporating them into standard therapy for heart failure.
DR. SIDNEY GOLDSTEIN is head, emeritus, of the division of cardiovascular medicine at Henry Ford Hospital in Detroit. To respond to this column, write to Dr. Goldstein at our editorial offices.
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|Publication:||Internal Medicine News|
|Article Type:||Brief Article|
|Date:||Oct 15, 2001|
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