Antiarrhythmic drug effect mimicked by spontaneous variability of ventricular ectopy.

Twenty patients with frequent ventricular ectopic beats had a 5 1/2 hour ECG rhythm strip recorded. Individual patients showed a marked spontaneous variability from one half-hour to the next in the total number of ectopic beats (-99% to +1100%) and the occurrence of pairs or salvos. Although no patient received antiarrhythmic drugs, some patients showed a spontaneous change in arrhythmia which mimicked either drug suppression or drug-induced worsening of arrhythmias. If an antiarrhythmic drug had been given to these patients after the first half-hour, 65% would have been termed "drug responders," using the criteria of 50% reduction in ectopic beats and elimination of pairs or salvos during any half-hour period in the subsequent three hours. Spontaneous variability in ventricular ectopic beats causes serious problems when using ECG monitoring to evaluate antiarrhythmic drug response in individual patients. The arrhythmias averaged for the entire group remained stable during the recording period. Evaluating antiarrhythmic drugs by examining group response rather than individual patient response minimizes the effect of spontaneous variability.