[Physiological beat-to-beat variation of heart rates in normal unrestrained rabbits for 24 hours]

In 18 freely moving rabbits, R-R interval histograms, each of which included average value of 336,000 heart beats per day, were analyzed by means of long-term continuous recording of ECG. The mean heart rate of the day was 233 +/- 4 (SE) cpm and the coefficient of variation was 12.0 +/- 0.8 (SE)%. When the mean heart rate of the day increased, its coefficient of variation decreased proportionally (p less than 0.01). Not all the histograms resembled the normal distribution in contour as some of them apparently skewed to the side of tachycardia or bradycardia. The averaged heart rate of 18 rabbits showed such a circadian rhythm as it became tachycardia in the day while bradycardia at midnight. These results suggested the daily physiological rhythm in heart rates was controlled with the variation around the time varying mean heart rate, rather then by simple shortening or elongation of the cardiac cycles (i.e., "chronotrope").