Objectives: To determine the effects of body position (supine vs prone) on dynamic changes in cardiac inter-beat interval during quiet (QS) and active sleep (AS) in growing low birth weight (LBW) infants.

Methods: Two 6-hour continuous electrocardiographic recordings along with simultaneous minute by minute behavioral activity state assignment were performed in 61 healthy, growing LBW infants (795-1600g), studied at 30-38 weeks postconceptional age. Infants were randomly assigned to supine or prone position for the first 3-hours of each study; the position was reversed for the second 3-hours. In each of the 122 studies minute to minute measurerments of heart rate (HR), heart period variability (RR-SD), square root of the mean of squared successive differences in RR-interval (rMSSD), and incidence of successive R-R interval changes, i.e. sustained (+/+, -/-), alternating (+/-, -/+) and no change (0/0) were computed. Effects of position within each state were tested using paired t-test.

Results: Table

Table 1 No caption available.
Full size table

Conclusions: Significant differences in multiple measures of cardiac rate and rhythm suggest that inherent autonomic control of the heart is altered by body position. Among the various measures positional differences in sustained inter-beat changes appear to be most robust.