Polygraphic studies of normal infants and infants at risk for the sudden infant death syndrome: heart rate and variability as a function of state.

Spontaneous heart rate and variability were examined as a function of age and state from birth to 6 months of age in 10 normal infants and 10 infants at risk for sudden infant death syndrome (SIDS). The risk group showed a higher heart rate at 3 months of age, particularly in the waking state. The risk infants' heart rate also increased more markedly during the first month after birth in both quiet and active sleep. Heart rate in both groups declined after 2 months in every state; however, the risk infants lagged in the 2- to 3-month decline seen with the normal infants. In the awake state, heart rate variability in the risk group did not follow the increase seen with the normals during the 1-week to 2-month age period.