Limitations of routine long-term electrocardiographic monitoring to assess ventricular ectopic frequency.

Variations in the frequency of ventricular premature depolarizations (VPDs) were evaluated with three consecutive 24-hour long-term electrocardiography monitor recordings from 15 clinically stable patients with various cardiac disorders. Mean hourly VPD frequencies ranged from 37--1,801 per hour. Data were subjected to 4 and 5 factor nested analyses of variance. The extent of spontaneous variation in arrhythmia frequency that occurred in individual patients from day to day was 23%, between 8-hour periods within days was 29%, and from hour to hour was 48%. In addition, the variability between repeated three-day monitoring periods over time was quantified in five patients and found to be 37%. This analysis determined that to distinguish a reduction in VPD frequency attributable to therapeutic intervention rather than biologic or spontaneous variation alone required a greater than 83% reduction in VPD frequency if only two-24-hour monitoring periods were compared, and greater than 65% reduction if two 72-hour periods were compared. The limitations of routine 24-hour electrocardiographic monitoring must be considered in diagnostic and therapeutic decision-making.