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Dear Dr. Donohue: I am 88 and had a successful triple-bypass in 1985. One year ago, a state-of-the-art pacemaker was implanted. Please explain the relationship between the pacemaker's mechanical beat and the heartbeat -- how the two work together and what the result is. Does the pacemaker's mechanical beat add to the heart's strength or longevity? -- E.B., Ph.D.

Dear E.B., Ph.D.: More than half a million North Americans walk around each day with a pacemaker in their chests. Their pacemakers, like yours, are far cries from the first pacemakers installed in the late '50s.

In most cases, but definitely not all, doctors install pacemakers when the heart's own pacemaker -- the sinus node -- falters. The heart's inborn pacemaker generates a blip of electricity 60 to 100 times a minute and even faster when the heart is called upon to support physical activity. That electrical blip travels down the heart to its pumping chambers, the ventricles. When it arrives at the ventricles, they contract and pump blood into the lungs and into the body.

When the natural pacemaker produces blips erratically, or when it fails to transmit them to the ventricles, the artificial pacemaker takes over and keeps the heart beating.

The natural pacemaker and the artificial pacemaker are not in competition with each other. Today's pacemakers are so ingeniously engineered that they can sense a normally generated heartbeat. The normal beat keeps the artificial pacemaker quiet. If the artificial pacemaker does not sense a beat, it steps in and generates an electrical blip to keep the heart beating.

The artificial pacemaker adds to the heart's longevity and the patient's longevity. Pacemakers do not add much to the strength of a heartbeat.

E.B., readers, is a retired English professor from a well-known Florida university.


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