Mechanisms of Arrhythmias
Disturbances in the orderly formation and conduction of the cardiac impulse may result in heart rates that are either too fast (tachycardia) or too slow (bradycardia). In general, bradyarrhythmias result from the failure of impulse generation within the sinoatrial node or failure of the excitatory wavefront to conduct from the atrium to the ventricle through the atrioventricular node. In general, bradyarrhythmias are not amenable to long-term pharmacological therapy and may require perma-nent cardiac pacing. Tachyarrhythmias, conversely, frequently may be palliated with long-term medical therapy. The mechanisms supporting tachycardias may be classified broadly into three groups: (1) abnormal au-tomaticity, (2) triggered activity, or (3) reentry.