Autonomic dysfunction at different ambient temperatures in infants at risk of sudden infant death syndrome.

Autonomic function was assessed by measuring the heart rate and blood pressure responses to a change from the horizontal to upright posture at various ambient temperatures in four groups of infants aged 8-12 weeks: 30 infants who had suffered a well-defined unexplained apparent life-threatening event (ALTE); 8 infants who had suffered a cyanotic attack; 24 healthy infants with a sibling who had died from sudden infant death syndrome (SIDS); and 17 healthy infants. Autonomic dysfunction was uncommon in the control group; no infant showed an abnormal heart rate response to postural change (R to R interval maximum/minimum ratio less than 1.0) and a postural fall in blood pressure of greater than 10% occurred in only 1 infant. In contrast, in the ALTE group 9 of 26, 9 of 30, and 4 of 22 infants showed an abnormal heart rate response and 20 of 26, 14 of 30, and 10 of 22 a greater than 10% fall in blood pressure on postural change at 20 degrees C, 25 degrees C, and 30 degrees C, respectively. 1 ALTE infant died of SIDS 14 h after showing an RR max/min ratio of 0.8 and a postural fall in blood pressure of 11% and 14% at 20 degrees C and 30 degrees C. Autonomic function testing should become part of the clinical evaluation of infants at high risk of a sudden unexpected death.