Article Summary

The Natural Pacemaker Activity of the Heart

by The DiagnosisHeart.com Physician Team
Posted: October 2, 2004

Summary:


How is the rate of a healthy, normal heart controlled? The answer is simpler than you may think. There is a group of cells in the heart, called the sinoatrial node, which serves as the normal pacemaker of the heart. The sinoatrial node (SA Node for short) is located high up in the right atrium, or filling chamber, of the heart. It generates constant, regular pulses of electricity that get sent out to the ventricles, or pumping chambers. The rhythm that the SA Node generates is referred to as sinus rhythm, and it paces the heart at a rate of about 60 beats per minute at rest.

Sometimes, however, the SA Node fails. This can be a result of scarring, as can occur after a heart attack, or if the heart’s conduction system is diseased. When this happens, one of the heart’s “backup” pacemaker sites, called an ectopic focus, takes over. These ectopic foci may be located in the collecting or pumping chambers of the heart, or around the AV Junction (the junction between the atria and ventricles). The rate of these ectopic foci depends on their location.

Ectopic foci in the atria, near where the SA Node is located, pace anywhere from 60-80 beats per minute – similar to the SA Node. Ectopic foci located farther away from the SA Node pace much more slowly. Those in the AV Junction pace anywhere from 40-60 beats per minute, while those in the ventricles pace from 20-40 beats per minute.

When the SA Node fails, how does the heart decide which ectopic focus will take over? The fastest one always wins. This concept is known as overdrive suppression. The fastest focus effectively suppresses the activity of all other foci in the heart. This is a great design, when you think about it. Heart rates as slow as those generated by ectopic foci in the ventricles may exist normally in well-conditioned athletes at rest. Most of us, however, require faster heart rates to carry out our daily activities. That is why some people whose natural pacemakers have failed require an artificial pacemaker.

Occasionally, the heart may beat in a slow or uncoordinated manner but the pacemakers themselves will be fine. This is possible if part of the conduction system is blocked. Such a phenomenon is aptly called heart block. Several different kinds of heart block exist, and they are usually a result of the same diseases that lead to pacemaker failure.

Reference:
The Heart's Pacemaker, DiagnosisHeart.com

Date of Article Publication:
2003

Web Site: http://www.diagnosisheart.com