Initiation of Ventricular Tachycardia

The importance of a premature ventricular complex (PVC) initiating ventricular tachycardia (VT) or ventricular fibrillation (VF) when the PVC occurs during the vulnerable period of the antecedent T wave has been stressed in the literature.¹The vulnerable period represents an interval of 20 to 40 msec located near the apex of the T wave during which the heart, when stimulated, is more prone to developing VT or VF. The stimulus may be from an intrinsic source such as PVC (Fig 1) or from an extrinsic source such as a pacemaker (Fig 2).²During the interval of the vulnerable period, maximal electrical nonuniformity in the ventricular muscle is present; that is, ventricular muscle fibers are at varying stages of recovery of excitability. Some fibers have completely repolarized, others only partially repolarized and still others may be completely refractory. Therefore, stimulation during this stage establishes nonuniform conduction with some areas