R ECENT REPORTS and communications'6 show that paired electrical stimuli repetitively applied to the mammalian myocardium can slow the effective mechanical rate of the heart and/or bring about a sustained potentiation of its contractile performance. The techniqueintroducing a highly premature extrasystole into each cycle-creates a bigeminal rhythm in which mechanically ineffective beats alternate with potentiated effective contractions. The extrasystolic depolarization is induced shortly after the termination of the absolute refractory period and produces a barely perceptible pressure event or only a slight prolongation of the downstroke of the ventricular pressure trace. In "paired pulse pacemaking,"2 "paired electrical stimulation,"5 or "paired stimulation" for short, both mechanically effective and ineffective beats are "driven," and the rate can be adjusted continuously over a wide range down to approximately 50 to 6o per c.nt of the prevailing control rate. The interval between the first stimuli of succeeding pairs determines the driven "basal rate," and the interval between the first and second stimuli of each pair sets the time in the cycle at which the extrasystole is introduced. In "R-wave coupled pacing,"3 or "coupled stimulation"1 a spontaneous depolarization is permitted to occur, and the artificial extrasystole, triggered by the R wave of the preceding normal QRS, is introduced into the cycle with an appropriate delay. This technique cuts the effective ventricular rate in half. Stimuli can be delivered to the myocardium by endocavitary catheter electrodes, intramyocardially placed wire electrodes, or epicardial surface electrodes. Either of the ventricles or auricles can be used as a
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