Mechanisms for arrhythmias.

Two broad descriptors have been applied to the genesis of cardiac arrhythmias: abnormalities of impulse initiation and abnormalities of propagation. The first category include arrhythmias that are automatic, arising over a wide range of membrane potentials, and those that are triggered by early or delayed afterdepolarizations. Among conduction abnormalities, the mechanisms most widely referred to are reentry and reflection. Although it is relatively easy to distinguish among mechanisms at the cellular electrophysiologic level, distinction is far more difficult in the intact heart and in the clinic. The patterns of response to electrical stimulation of the heart are reviewed to demonstrate the confusion that can occur when one attempts to distinguish mechanisms. In addition, means to aid in the process of distinction are suggested.