Objective: The study of biological rhythms and the influence of environmental factors in the timing and synchronization of different rhythmic events have important implications for neonatal health. Preterm infants in the neonatal intensive care unit (NICU) are deprived of the patterned influences of maternal sleep, temperature, heart rate, and hormonal cycles. The impact of the NICU and nursing interventions on the development of the circadian system was studied in 17 stable preterm infants in the Intermediate Intensive Care Nursery at Stanford University for three consecutive days at about 35 weeks postconceptional age.
Outcome measurements: Rectal temperature, abdominal skin temperature, heart rate, and activity were simultaneously recorded at 2-minute intervals during each 3-day study by a small microcomputer (Vitalog).
Results: Very low amplitude circadian rhythms were found for rectal and skin temperatures (maximum range 36.8 to 37.0 degrees C); population mean values for heart rate (158 bpm) and activity (3.5 counts per 2-min bin) did not differ significantly as a function of time of day. Rectal temperature, averaged in 6-hour bins over the 24-hour day as a function of both postconceptional age and postnatal age, was significantly higher during the first part of the circadian cycle. In all infants, rhythmicity in each variable was dominated by ultradian periodicities that were coincident with feedings and related interventions; moreover, several physiological variables charted during feeding differed significantly from values obtained during periods in which caregiving interventions did not occur.
Conclusion: Quantitative data on the preterm infant circadian system may facilitate evaluation of factors that improve therapeutic responses, recovery, and outcome of neonatal intensive care patients.