Atrial fibrillation: minimal sampling interval to estimate average rate.

The average heart rate and maximal heart rate (average R-R and shortest R-R intervals) in atrial fibrillation are frequently calculated to assess drug effect. The minimum recording sample to obtain accurate estimates of "instantaneous" average and maximum heart rate has not been established. We analyzed the R-R intervals after a 4 minute ECG recording in 40 supine patients with chronic atrial fibrillation. A "life table" analysis revealed that the average heart rate derived from a 20 second sample of atrial fibrillation approximates (+/- 5%) the true value in an estimated 92.5% of patients. On the other hand, a two minute sample is required to approximate the shortest R-R interval within 5% of the true value in 90% of patients. Studies utilizing average R-R interval and shortest R-R interval as an index of drug response should use electrocardiographic sampling intervals of at least 30 seconds and two minutes respectively.