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DEAR DR. ROACH: What's the difference between a pacemaker and a defibrillator?
-- T.S.B.
ANSWER: Both are implantable devices to treat electrical problems of the heart. A pacemaker is a much older device; the first implantable pacemaker was made in 1958.
The heart has an electrical system, which coordinates the mechanical activity of the heart so that blood is pumped out efficiently. Each cardiac muscle cell has an intrinsic rhythm, but specialized cells in the right atrium normally act as a pacemaker to the heart. They respond to nerve stimulation from the sympathetic and vagus nerves, as well as chemical stimuli from hormones like adrenaline.
Like any system in the body, the natural pacemaker can fail, and the heart rate will then slow down, as the intrinsic rhythm of other heart cells tend to be slower than the natural pacemaker. The heart doesn't generally stop because other cells take over the pacemaker's job, but they don't work as well.
The heart rate can get so slow that a person can develop symptoms such as fatigue, lightheadedness and fainting. When this happens, an electronic pacemaker can be placed. The smallest current pacemaker is described by the manufacturer as the size of a vitamin supplement, and even smaller ones have been designed. An electronic pacemaker can be programmed for several actions (when it's installed but also remotely), but in general, it'll provide an electric impulse if the heart doesn't. It can be set at any rate, but it's often set to 60 bpm.
A defibrillator (an automatic implantable cardioverter-defibrillator, or an AICD) is a more recent device; these were first approved in 1985. The primary goal of an AICD is to sense when the heart goes into a dangerous rhythm. When it does, it provides an electric shock to stop the rhythm. It's effective at stopping the rhythm about 95% of the time, so AICDs have saved many lives.
They're used in people who have had a life-threatening rhythm disturbance or for those who are at a high risk for one, such as a person with severe heart failure. Modern AICDs also have a pacemaker functionality built in.
Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu.
North America Syndicate Inc.