Pacemakers

Pacemakers - What You Should Know

What is a pacemaker?

A pacemaker is a sophisticated electronic device that does two things: 1) It analyzes the function of the heart's own electrical system. 2) When necessary, it sends tiny, precisely-timed electrical signals to the heart, to correct certain abnormalities in the heart's electrical system.

The cardiac electrical system regulates the frequency of the heart beat (i.e., it sets the heart rate), and it coordinates the contraction of the heart muscle, so that the heart beats efficiently. (Click here for a quick and easy review of the normal cardiac electrical system.)

Abnormalities of the cardiac electrical system are manifested by cardiac arrhythmias. Abnormally slow arrhythmias are called the bradycardias, and abnormally rapid arrhythmias are called tachycardias.

(Click here for a quick and easy review of cardiac arrhythmias .)

Most pacemakers are designed to correct the bradycardias (the slow arrhythmias.) Abnormally slow heart rhythms can cause weakness, fatigue, lightheadedness, dizziness, loss of consciousness, or even death. Pacemakers, properly used, effectively eliminate all of these symptoms.

How do pacemakers work?

Pacemakers consist of two major parts: the generator and the leads.

The generator is essentially a tiny, hermetically sealed computer – along with a battery to run it – housed in a titanium container. Most modern pacemaker generators are roughly the size of a 50-cent piece, and approximately three times as thick. The battery life of most pacemaker generators today is 5 – 8 years.

The lead is a flexible insulated electrical wire. One end is attached to the generator and the other end is passed through a vein into the heart. Most pacemakers today use two leads – one placed in the right atrium and the other in the right ventricle. (Click here for a quick and easy review of the cardiac chambers.)

How it works. The pacemaker leads detect the heart's own electrical activity (in the right atrium and right ventricle,) and transmit that information to the pacemaker generator. The generator – which, again, is a computer – analyzes the heart's electrical signals, and uses that information to decide whether, when, and where to pace. If the heart rate becomes too slow, the generator transmits a tiny electrical signal to the heart, thus stimulating the heart muscle to contract. (This is called pacing.)

Pacemakers that have two leads not only keep the heart rate from dropping too low, they can also maintain the optimal coordination between the atria and the ventricles (by pacing the atrium and the ventricle in sequence.)

Thus, pacemakers do not take over the work of the heart – the heart still does its own beating – but instead, pacemakers merely help to regulate the timing of the heart beat.

What are "rate-responsive" pacemakers?

In the early days, pacemakers were set to work at a certain heart rate. That is, if the heart's own intrinsic rate dropped below that pre-set number (say, 70 beats per minute) the pacemaker would begin to pace at that rate. Today, most pacemakers have a means of determining what the heart rate should be from moment to moment, and of setting the rate of pacing to that optimal heart rate. These pacemakers are said to be rate-responsive.

Rate-responsive pacemakers can use several technologies to determine the optimal heart rate, but two in particular have proven quite useful. One of these technologies is the activity sensor, sensors that detect body movement. The more the patient's body is moving (so the reasoning goes) the faster the heart rate should be. The other is the breathing sensor, which measures the patient's rate of breathing. The faster the breathing, the faster the heart rate should be. Either of these technologies allow rate-responsive pacemakers to mimic the moment-to-moment changes in heart rate seen in patients with normal cardiac electrical systems.