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Figure 1 A programmed ventricular stimulation study done during the early 1970s. The selected extra and intracardiac electrograms in the four panels show the reproducible initiation of a ventricular tachycardia by single ventricular premature beats during right ventricular pacing with a basic cycle length of 700 ms. As shown, ventricular premature beats given in the interval range 500 to 410 ms result in the initiation of the ventricular tachycardia. Premature beats given later than 500 ms or earlier then 410 ms do not create the conditions required for initiation and maintenance of a reentrant ventricular tachycardia.