Postural differences in cardiac dynamics during quiet and active sleep in low birthweight infants

To study the effects of body position (supine versus prone) on changes in c ardiac inter-beat interval during quiet and active sleep, 6-h continuous el ectrocardiographic recordings and simultaneous minute-by-minute behavioural activity state assignments were made in 61 healthy, growing, low birthweig ht infants. The infants weighed 795-16008 at birth and ranged between 30-38 wk in postconceptual age. Infants were randomly assigned to the supine or prone position for the first 3 h of each study; the position was reversed f or the second 3 h. Higher heart rates and lower time and frequency domain m easures of inter-beat interval variability were observed in the prone posit ion as compared to the supine position, during both quiet and active sleep. In addition, an analysis of consecutive increases and decreases in the ins tantaneous heart rate revealed a lower incidence of sustained accelerations or decelerations in the prone position. Although consistent findings conce rning inter-beat interval variability and sleeping position were obtained f rom all analytic techniques, the differences derived from analysis of conse cutive inter-beat changes were the most robust. These differences in multip le measures of cardiac rate and rhythm between prone and supine positions s uggest that autonomic control of the heart is altered by body position, the net effect on heart rate being increased sympathetic dominance.