Incomplete baroreceptor responses in newborn infants.

Previous studies of the baroreceptor response to a 45 degree head-up tilt in preterm and mature infants during the first 3 days of life failed to demonstrate a significant tachycardia while eliciting a 25% reduction in limb blood flow. In order to determine if maturity or respiratory distress affected the presence of tachycardia after tilting, 34 infants between 28 and 40 weeks gestation, including 15 infants with RDS, were studied serially during their hospitalization in a neonatal intensive care unit. In the first 5 seconds after a rapid (1-second) tilt to 45 degrees, individual infants responded with changes in heart rate (HR) ranging from a fall of 8 beats/min to an increase of 13 beats/min. Individual responses were not predictable on the basis of maturity at birth, age after birth, or presence of clinical respiratory distress. The group as a whole demonstrated a statistically significant increase of 2 +/- 4 beats/min in the first 5 seconds immediately after tilting. This may be compared to a coefficient of variation of +/- 4-5% in the resting HR of the same group of infants. Because of the unpredictability of individual responses and the small magnitude of the response for the group as a whole, it can be concluded that the HR component of the baroreceptor response is poorly developed during the neonatal period.