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ECG of the Month

COMMENT The first two ventricular complexes are pacemaker initiated. The next three are sinus in origin with a fixed rate pacemaker. The fifth pacemaker stimulus falls on the nadir of the T wave, the vulnerable period of the ventricle, initiating a ventricular tachyarrhythmia. The vulnerable period has been elicited by artificial pacemakers and ventricular premature complexes interrupting the T wave (Fig. 1). As a rule, the vulnerable period is located in the midportion of the T wave during a 30-millisecond period at the apex of the T wave. Asynchrony of refractoriness and conduction comprises the matrix responsible for vulnerability.