Almost four million people worldwide have pacemakers, which were first deployed experimentally 50 years ago to speed hearts that beat too slowly.

In a healthy heart, electrically conductive muscle cells fire for each contraction of the atria and ventricles. In cases of distress, a pacemaker sends an electrical impulse to select points in the heart to speed up or coordinate the firing. Historically pacemakers could deliver one pulse rate, but most modern instruments are rate-responsive--they track conduction activity as well as body motion and adjust heart rate accordingly. Doctors can now also implant special pacemakers that stimulate each ventricle directly, resynchronizing ventricles that contract out of phase. This condition often occurs in people whose cardiac muscle has been weakened by a heart attack.

This article was originally published with the title Keep the Beat.

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