Developmental patterns of heart rate and heart rate variability during sleep and waking in normal infants and infants at risk for the sudden infant death syndrome.

The developmental sequence of heart rate and heart rate variability was examined during sleep and waking states in 22 normal infants, and 22 siblings of sudden infant death syndrome (SIDS) victims, using 12-h polygraphic recordings at 1 week and at 1, 2, 3, 4, and 6 months of age. Heart rate was higher in siblings of SIDS victims than in normal infants during quiet sleep over the first 6 months of life and was higher in the waking state at 3 months of age. The sibling group also had lower variability at 1 week during quiet sleep. Gender contributed no significant differences to heart rate, but females at risk for SIDS had lower waking cardiac variability than males.