Occasionally, an ectopic ventricular focus may discharge and propagate in a retrograde direction to the AV node, but not excite the ventricular myocardium. This phenomenon, which results in a concealed ventricular beat, is referred to as “exit block” and is analogous to the block that occurs between the sinus node and atrial myocardium and is the cause of sino-atrial block. On the left of the figure shown here, an ectopic ventricular focus stimulates the ventricular myocardium and travels retrograde to enter the AV node. On the right, the focus discharges and propagates retrograde to the AV node but does not stimulate the ventricular myocardium because of exit block. This illustrates the concept of a concealed ventricular premature beat.