During normal rhythm, the heart beats regularly, producing a single coordinated electrical wave that can be seen as a normal electrocardiogram (ECG). During arrhythmias such as ventricular tachycardia and ventricular fibrillation, this normal behavior is disrupted and the ECG records rapid rates with increased complexity.

The underlying cause of many arrhythmias is the development of a reentrant circuit of electrical activity that repetitively stimulates the heart and produces contractions at a rapid rate. During tachycardia, a single wave can rotate as a spiral wave, producing fast rates and complexity. During fibrillation, a single spiral wave can degenerate into multiple waves. Because contraction is stimulated by the pattern of electrical waves, arrhythmias can compromise the heart's ability to pump blood and sometimes may be lethal.

To learn more about sinus rhythm and several kinds of arrhythmias, click on the tabs above. Each rhythm is demonstrated using a three-dimensional interactive simulation and a sample ECG.

The contracting three-dimensional heart on the left can be rotated, zoomed, and panned by clicking and moving the mouse as indicated.