Maturational changes in heart rate and heart rate variability in low birthweight infants

To provide insight into the maturation of neural mechanisms responsible for variability in heart rate during quiet and active sleep, 6-hour continuous electrocardiographic recordings and simultaneous minute-by-minute behavior al activity state assignments were performed in 61 healthy growing low birt h weighs infants. The infants weighed 795-1600 g at birth and ranged betwee n 31-38 weeks in postconceptional age. During this age interval there was a decrease in heart rate during quiet sleep and an increase in both time dom ain and frequency domain measures of the variability in cardiac interbeat i ntervals. In quiet sleep, global variability measured as SD of R-R interval s increased in relation to age, as did higher frequency variability, measur ed as the square root of the mean of squared successive differences in R-R intervals. Developmental changes in the 0.5-2.0 Hz spectral power band of R R-interval variability, another measure of high frequency variability, para lleled the changes seen in the rime domain measure. Evaluation of patterns of changes in the magnitude and direction of successive interbeat intervals provided evidence that the incidence of sustained accelerations or deceler ations increased whereas the incidence of no change in consecutive RR-inter vals decreased as infants matured Among the various measures of heart rate variability, the incidence of sustained change and no change in successive interbeat intervals were most closely related to postconceptional age in bo th sleep states. The overall decrease in heart rate, increase in heart rate variability, and increase in the pattern of changes in interbeat interval with postconceptional age are consistent with the maturation of the autonom ic cardio-regulatory activity from 31-38 weeks age. (C) 2000 John Wiley di Sons, Inc.