March 29, 1999
Contrary to expectations, scientists at Johns Hopkins have found that a pacemaker keyed to the single area of a failing heart that is activated late due to an electrical blockage is far superior to the current thinking of pacing at a standard location or pacing both lower chambers of the heart at once.
In a study of 18 patients with dilated cardiomyopathy (DCM) -- a potentially fatal form of heart failure in which the heart's main pumping chamber becomes stretched and weakened -- researchers found that placing a pacemaker at the area of greatest delay unified and improved pumping action by nearly 25 percent and increased bloodflow to the body by as much as 40 percent.
Results of the study, published in the March 30 issue of Circulation, show that pacing at the conventional right heart location has no benefit for heart function. The results also contradict current thinking that placing a pacemaker lead at each of the heart's two lower chambers works best. In this study, doing so yielded only a 13 percent better pump.
When the heart functions normally, 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.
Likening the problem to a cluster of homes on the same cable television line, David A. Kass, M.D., lead author of the study and professor of medicine and biomedical engineering, says, "Just as cable companies want to be sure that neighboring homes receive electrical signals at the same time, lest you have to wait to hear about last-night's movie from your friends, we want to make sure that the various areas of the heart receive the signal to contract at the same time. That's what single-site pacing does."
For the study, researchers varied the site and timing of the pacemaker, a battery-operated device that sends electrical impulses to the heart to help it pump properly. They found that pacing the area of greatest signal delay, usually on the left ventricular outer wall, yielded a greater benefit than pacing at any other site or than pacing both ventricles.
In order to pace the most delay-activated region, the electrical timing between contraction of the upper and lower heart chambers has to be shortened. This may have the additional benefit of improving filling of the heart and, therefore, the amount of bloodflow sent out to the body.
Congestive heart failure is the leading cause of cardiovascular morbidity and mortality, especially in those over 60. There is no cure; treatment involves some combination of a modified diet, salt and activity restriction, and drugs such as digitalis, diuretics, angiotensin converting enzyme (ACE) inhibitors, and beta blockers. Pacemakers may be joining this list.
The study was supported in part by Guidant/Cardiac Pacemakers Inc. and by a General Clinical Research Center Grant from the National Health Service.
Other authors of the study were Chen-Huan Chen, M.D.; Cecilia Curry, M.S.E.; Maurice Talbot, R.N.; Ronald Berger, M.D.; Barry Feltics, M.S.E.; and Erez Nevo, M.D., Sc.D.
Funding for the study described in this article was provided by Cardiac Pacemakers, Inc. (CPI). Dr. Kass is a consultant to CPI. The terms of this arrangement are being managed by the Johns Hopkins University in accordance with its conflict of interest policies.
Relevant Web sites:
Johns Hopkins Cardiomyopathy and Heart Transplant Service -- http://www.med.jhu.edu/heart/
American Heart Association--http://www.amhrt.org/