Abstract 744
Poster Session IV, Tuesday, 5/4 (poster 2)
This study focuses on developmental changes in autonomic responses to a challenge of head-up tilting as infants enter the period of maximum risk for Sudden Infant Death Syndrome. In a prior study we found that, unlike newborns, two and four month old infants demonstrated no significant increase in heart rate when tilted from the flat to a 30° head-up position ( Dev Psychobiol 32:147, 1998). That conclusion was based on analyses in which heart rate changes from baseline were assessed 30 seconds after infants had been in the tilted position. It is possible that older infants have responses but that they are more proximal to the time of tilting. Thus, we analyzed heart rate responses in 10-second blocks throughout the tilting procedure. The first 3 blocks were baseline data before tilting, the next 3 blocks covered the 30 second period during which infants were slowly tilted to the head-up position, and during the last 6 blocks, infants were in the head-up position. Data were included only if the infant was judged to be in quiet sleep throughout the testing period and had no significant movements either during the baseline period or while in the tilted position (newborns n=29; two months n=25; four months n=25). In newborns, heart rate was ∼2-4 bpm above baseline (p<.05) in each 10-second block after initiation of the tilt. In contrast, there were no significant increases in heart rate in either group of older infants at any time although there was a trend for heart rate to increase in two-month-old infants during the second 10-second block after initiating the tilt. In both groups of older infants, heart rates were actually lower than baseline values during the first 10 seconds of the head-up position. Three alternative hypotheses are proposed that could account for these age-specific response patterns. At two to four months: 1) baroreceptors are less sensitive to decreases in blood pressure (BP) than during the newborn period, 2) baroreceptors sense the BP change but, at this age, infants have diminished capacity to respond, 3) baroreceptor mediated mechanisms are highly responsive and, as BP starts to fall during the head-up tilt, there is an immediate compensatory increase in vascular sympathetic activity that eliminates the stimulus for a change in cardiac rate in response to tilt. We are currently testing these hypotheses by measuring beat-to-beat blood pressure changes during the tilting procedure. Contrary to hypothesis 3, preliminary evidence at six months of age suggests there are substantial decreases in BP during head-up tilting which indicates inadequate compensation in these older infants. (Supported by NICHD: HD32774)
