4.3 Ectopic arrhythmias caused by increased excitability of the myocardium

Normally, myocardial cells do not discharge spontaneously, and the possibility of spontaneous discharge of the His bundle and Purkinje system is low because the normal pacemaker discharge of the SA node is more rapid than their rate of spontaneous discharge. However, in abnormal conditions, the His-Purkinje fibers or the myocardial fibers may discharge spontaneously. In these conditions, increased automaticity of the heart is said to be present.

If an irritable ectopic focus discharges once, the result is a beat that occurs before the expected next normal beat and transiently interrupts the cardiac rhythm (atrial, junctional or ventricular extrasystole or premature beat).

If the focus discharges repetitively at a rate higher than that of the SA node, it produces rapid, regular tachycardia (atrial, ventricular, or nodal paroxysmal tachycardia or atrial flutter). Traditionally, atrial and junctional premature contractions refer to supraventricular.