Polygraphic Studies of Normal Infants and Infants at Risk for the Sudden Infant Death Syndrome: Heart Rate and Variability as a Function of State

Abstract

Summary: 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. Speculation: There are distinct developmental differences in mean heart rate, and to a lesser extent in mean heart rate variability, between groups of normal infants and infants at increased risk for sudden infant death. These differences may reflect delayed maturation or impaired functioning of the autonomic nervous system, and particularly of vagal control. However, some of these differences, for example, increased heart rate in the risk group at 3 months of age and a delayed decline in this group's heart rate values after 1 month of age, are also compatible with an interpretation of chronic hypoxia. The tendency of the differences to be most pronounced in the awake state adds support to the possibility of impaired oxygen regulation in the risk group. Thus, both chronic hypoxia and the dysfunction in autonomic control that might result from it could play a significant role in sudden infant death.