Relation of beat-to-beat variability to heart rate in normal sleeping infants.

The relationship of beat-to-beat heart rate variability (delta RR) and instantaneous heart rate was studied in eight normal infants while asleep during the first four months of life. The sleep state (REM or quiet) was determined using neurophysiologic and behavioral criteria. The results of regression analyses indicated that the delta RR values were positively correlated with the instantaneous heart rate (RR intervals). The correlation coefficient range was 0.49 to 0.92 in quiet sleep and 0.50 to 0.93 in REM sleep. Regression analyses supported a linear approximation of the delta RR to RR relation over the RR range investigated (400 to 520 msec). The median slope was 0.124 in REM and 0.117 in quiet sleep. The slopes of these linear functions were similar in both sleep states and at all ages. If beat-to-beat variability is to be used as an index of the integrity of the autonomic nervous system, these results suggest that delta RR be corrected for RR. A model is presented which relates the demonstrated positive correlation of delta RR to RR with the physiology of cardiac output control.