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Initial base-line electrocardiograms are used to assess the efficacy of treatment for ventricular arrhythmias. This approach assumes that in the absence of treatment the frequency of arrhythmia would remain constant. To test the validity of this assumption, we studied 26 clinically stable patients with symptomatic but not life-threatening ventricular arrhythmias, during two periods of placebo treatment separated by a mean of 17 months. As compared with the initial placebo period, there were significant reductions in ventricular premature depolarizations (50 per cent), pairs (65 per cent), and ventricular tachycardia (83 per cent) during the second period of placebo administration (P less than or equal to 0.05 for all comparisons). Over one third of the patients gave the appearance of receiving successful therapy during the second placebo period, even when the reported spontaneous variability of ventricular arrhythmia was taken into consideration. If unrecognized, these long-term spontaneous changes in the frequency of arrhythmia could result in continuation of unnecessary and potentially toxic therapy and lead to incorrect conclusions regarding the efficacy of antiarrhythmic drugs in clinical trials. We therefore recommend that the frequency of arrhythmia be reassessed annually in the absence of treatment in patients similar to those in our study. These recommendations should not be applied to patients with life-threatening ventricular arrhythmias.

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