Polygraphic studies of normal infants during the first six months of life. I. Heart rate and variability as a function of state.

This study examined spontaneous heart rate (HR) and variability as a function of age and sleep state in eight normal full term infants from birth to 6 months of age. Heart rates recorded during sleep were lower and less regular at 1 week (quiet sleep (QS) mean rate= 128, interquartile range = 6.4 beats/min; rapid eye movement (REM) = 134.5, 11.6) than at 1 month (QS = 138.6, 3.4; REM 139.6, 4.2). Rate decreased sharply from 1 to 3 months (QS = 118; REM 123.8) and decreased only slightly therafter (6-month QS = 113.5; REM 118.9). Variability decreased rapidly in REM from 2 to 4 months (from 11.4 to 9.1) and less quickly from 4 to 6 months (from 9.1 to 8.2), while QS variability decreased at 1 month (from 6.4 to 5.7) and became stable from that point (6.0 at 6 months). Waking heart rate and variability were both relatively low at 1 week (163, 11.2 beats/min) and increased from that age to 1 month (167.4, 14.3). Rate decreased rapidly in waking at 3 months (152 beats/min) and more slowly therafter (152 beats/min at 4 months, 149 beats/min at 6 months), whereas variability remained elevated until after 3 months, becoming stable at a lower level during later infancy (3 months = 14.8,6 months 11.7). Lowest values of rate and variability were found in QS and the highest values were found in waking at all ages, except at 1 week. Heart rates during REM closely approximated those in QS, whereas variability values more closely resembled those of waking.