Use of the oculocardiac reflex to assess vagal reactivity during quiet sleep in neonates

Arousal from sleep can be a protective response to life-threatening stimuli. Hence, faults within state-switching processes may lead to fatal events. To investigate the role of the nervous system during cardiac failure triggered by phasic, vagally mediated stimulation, we analysed autonomic and behavioural reactions in 50 premature neonates during quiet sleep (QS) -- a sleep state characterized by a preponderance of tonic, parasympathetic activity. Bradycardia was induced with a standardized ocular compression test. Neither awakening nor behavioural escape reactions were observed during or after an episode of bradycardia. Eighty-six per cent of the provoked bradycardic episodes induced central apnoea. During QS, the neonates' respiratory response and arousability were found to be time-dependent: when the test was performed early in the QS episode, apnoea was more frequent (94%), and no sleep state change occurred. When ocular compression was performed in the later part of the QS episode, a transition towards active sleep was observed, together with significantly fewer episodes of apnoea (64%). These results indicate that a progressive decrease in the respiratory system's responsiveness to phasic, parasympathetic stimulation occurs during QS, whereas arousability increases. Our study suggests that newborns could be more vulnerable to potentially fatal events during the initial portion of a QS episode.