July 14, 2000
The pacemaker has taken on an increasingly important role in recent years. Originally used to fix electrical abnormalities in people with irregular heart rhythms, it is now in favor for heart failure patients as a way to "resynchronize" a weak and struggling heart.
In a study of 22 patients, Johns Hopkins researchers have found that those whose hearts have the largest amount of timing discord, and whose hearts are often weakest, are the ones that seem to benefit most from a pacemaker. Pacing therapy improved the heart's ability to contract and pump out blood by an average of 35 percent.
Study results were published in a recent issue of Circulation, an American Heart Association journal.
"The problem we set out to solve was determining which heart failure patients would respond best to pacemakers," says David A. Kass, M.D., lead author of the study and professor of medicine and biomedical engineering at Hopkins. "The devices are expensive and permanent, and the patients are so sick we can't afford to waste time."
In normal hearts, cells in the right upper chamber, or atrium, emit an electrical impulse that travels through the heart on a specified route through a small group of cells called the A-V node and along a track that divides into the right and left lower chambers, or ventricles. As the impulse travels down the right and left branches at the same speed, the muscle contracts, or beats, in a coordinated (nearly simultaneous) manner.
In some heart failure patients, however, there is a block in one branch, causing marked delay in contraction to a portion of the heart as the electrical impulse must now reach this region by a slow detour through the heart muscle itself. As a result, the heart "wobbles" and must struggle to send blood out to the body.
Kass' team likens the failing heart to an automobile engine. If the engine pistons aren't timed correctly, the car will still move but will have worse fuel economy. Likewise, the heart that's out of "sync" will still pump blood, but not efficiently. The idea behind treatment is to use the pacemaker to stimulate the part that gets the signals later to improve the heart's overall performance.
Using nuclear magnetic resonance (NMR), an imaging technique that tracks specific parts of the heart muscle during contraction, Kass and his group studied the effects of pacing in 22 patients with a potentially fatal form of heart failure known as dilated cardiomyopathy, in which the lower left chamber of the heart becomes severely stretched and weakened.
The amount of dissynchrony between sides of the heart among heart failure patients with a blocked branch was found to be 10 times that of normal patients, proving a strong predictor for benefit from pacemaker resynchronization.
Further study will determine if the improved response to pacing therapy holds out over time, Kass says. Hospitals that don't have such high technology tools may use electrocardiograms to evaluate heart failure patients, he says.
Nearly five million people have heart failure, with 400,000 new cases being diagnosed each year, according to the American Heart Association.
The study was supported by the National Institutes of Health and by Guidant/Cardiac Pacemakers Inc. Kass is a consultant for Guidant; the terms of this arrangement are being managed by The Johns Hopkins University in accordance with its conflict of interest policies.
Other authors were Gregory S. Nelson, Ph.D.; Cecilia W. Curry, M.S.E.; Bradley T. Wyman, Ph.D.; Jerome Declerck, Ph.D.; Maurice Talbot, R.N.; Ronald D. Berger, M.D., Ph.D.; and Elliot R. McVeigh, Ph.D.
Related Web Sites:
Johns Hopkins Cardiomyopathy and Heart Transplant Service:
American Heart Association: