Because the state of alertness exerts a profound influence on autonomic cardiac control, we hypothesized, that the heart rate response to a vagal stimulus, i.e., ocular compression, may differ during different states of alertness. We studied 8 healthy infants with a postconceptional age of 35-41 weeks (mean +/- SD 37.9 +/- 2.1 weeks). They underwent a standardized ocular compression test during polygraphically controlled wakefulness, rapid eye movement (REM) sleep, and non-REM (NREM) sleep. The R-R intervals were measured (1) during the 60 s preceding the ocular compression test, to determine the mean control R-R interval, and (2) during compression. Percent R-R interval was defined as the longest R-R interval in milliseconds during the test divided by the mean control R-R interval and multiplied by 100. The longest R-R interval during the test was significantly greater in REM sleep than in wakefulness (p < 0.05) and in NREM sleep (p < 0.01):939 +/- 360, 623 +/- 355, and 538 +/- 60 ms, respectively. The percent R-R interval was significantly greater in REM sleep than in NREM sleep (p < 0.01):236 +/- 91 and 129 +/- 16, respectively. The time from the longest R-R interval to return to mean control R-R interval, i.e., vagal escape, was significantly shorter in REM sleep than in NREM sleep and in wakefulness (p < 0.01): 843 +/- 168, 2,131 +/- 712, and 2,078 +/- 913 ms, respectively. This study indicates that the state of alertness should be defined when performing tests on autonomic reflexes in infants.
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