Significance of the number of stimuli to initiate ouabain-induced arrhythmias in the intact heart. Ouabain-induced arrhythmias are a well-known model used to study triggered activity resulting from delayed afterdepolarizations. In the intact heart, initiation of these arrhythmias is promoted by pacing, especially at fast rates. However, the relevance of the number of stimuli is unknown. In conscious dogs with formalin-induced atrioventricular block, we investigated the effect of variations in pacing mode on 1) the behavior of nonsustained triggered rhythms at progressive levels of ouabain intoxication, and 2) the induction of sustained ventricular tachycardia (VT). Twenty experiments were analyzed. Ouabain was administered as a bolus of 40 micrograms/kg followed by continuous infusion. Every 15 minutes the pacing protocol was repeated, with a maximum of 10, until completion or induction of VT. When VT could not be initiated, the experiment was repeated at least 1 week later, adding 5-10 micrograms/kg ouabain to the bolus and increasing the infusion rate correspondingly. This was repeated until VT could be induced. Four interstimulus intervals (200, 400, 600, and 800 msec) and seven numbers of stimuli (5, 10, 20, 35, 50, 100, and 150) were given in two pacing protocols. The effect of these protocols on 1) the number of induced beats per stimulation train, 2) their first postpacing interval, and 3) induction of VT were studied. Initiation of VT occurred after 75 +/- 42 minutes. The bolus of ouabain needed to induce VT was inversely related to the body weight of the animals. Progression of ouabain intoxication resulted in 1) a significant increase in the number of induced beats per stimulation train and 2) a significant shortening of the first postpacing interval. Stimulation at a faster rate and/or more stimuli resulted in 1) a significantly pronounced increase in the number of induced beats at the higher levels and 2) a significantly shorter first postpacing interval at successive levels of ouabain intoxication. 