Factors affecting heart rate variability and heart rate responses to tilting in infants aged 1 and 3 months

Heart rate variability (HRV) and heart rate (HR) responses following a 60 d egrees head-up tilt were measured in 60 infants at I and 3 mo of age to inv estigate the effects on these of age, sleep state, steep position, and moth er's smoking status. HRV was determined from Poincare plots of 500 sequenti al RR intervals to measure overall variability derived from the SDRR of thi s plot, and instantaneous variability derived from the SD Delta RR. HR resp onses to the tilt were measured as changes in RR interval length from rest to immediately following the tilt and again once a stable pattern was reach ed. SDRR and SD Delta RR increased 20 and 40%, respectively, with age (p < 0.0001), SDRR was higher in active sleep (AS) than quiet sleep (QS, +72%, p < 0.0001) but both measures of variability (SDRR and SD Delta RR) were low er in the prone position compared with supine (- 18%, p < 0.0001). However, several findings were dependent on the basal RR interval, thus the age eff ect disappeared once RR interval was taken into account, sleep state remain ed an important factor and the lower variability when prone now became a di fference of -3% (p = 0.034). The tilt generally provoked a reflex tachycard ia followed by a bradycardia and settling to a stable HR level below, at, o r above baseline within 30 s. The more unusual responses were no HR change, sustained tachycardia or sustained bradycardia (15% of total), These were more likely to occur in younger infants (p = 0.008) and in BS (p < 0.0001). No changes were seen in any of the cardiac indices related to maternal smo king status. The findings confirm several reports indicating that prone sle eping damps some physiologic responses. The data emphasize the need to cons ider basal heart rate, and sleep position as well as sleep state in autonom ic function testing during infant sleep.