Does sympathetic activity contribute to growth of preterm infants?

Background: Emerging evidence on faster growth in preterm infants with higher heart rate as opposed to the classical approach calls for further research. Aims: to test whether (1) high or low heart rate and (2) heart rate during the first days of life predict greater weight gain in preterm infants. Methods: A retrospective study analyzing two daily measures of heart rate obtained during restful sleep, total daily calorie intake and daily weight gain measured always in the morning before meal were collected from the medical files. Results: Analysis of 90 healthy preterm infants born at 32-36 weeks of gestation revealed that increased mean heart rate during hospitalization predicted greater weight gain even when controlling for calorie intake, birth weight, gestational age, appropriateness of birth weight for gestational age, and length of hospitalization. Mean heart rate during the first three days of life yielded the same pattern of results. Post-hoc analysis of variance between infants with mean daily heart rate ≤ 139 bpm vs. ≥ 140 bpm showed that infants with higher heart rate achieved a significantly higher weight gain. Conclusions: It is suggested that, contrary to adults, in neonates an anabolic activity is represented by increased sympathetic functioning within the normal range. The implications of a slower growth rate for additional developmental care and individual considerations of appropriate stimulation in preterm infants are discussed.