Introduction Arousal from Sleep Serves as a Vital Protective Mechanism against Cardiorespiratory

AROUSAL FROM SLEEP SERVES AS A VITAL PROTECTIVE MECHANISM AGAINST CARDIORESPIRATORY FAILURE.1,2 In infants, failure to arouse (FTA) from sleep has been postulated to be involved in the sequence of events leading to the sudden infant death syndrome (SIDS).3 In order to understand factors affecting arousability in infants, studies have been performed at postnatal ages relevant to SIDS. To avoid ethical and technical concerns associated with the administration of cardiorespiratory stimuli, arousal responses of sleeping infants have been assessed using somatosensory stimuli and body tilting.4-14 These studies have consistently found that infants are more arousable in active sleep (AS) than in quiet sleep (QS).7-14 It has been proposed that mild hypoxia is not an effective stimulus for arousal in sleeping infants because the majority of normal infants fail to arouse in response to hypoxic challenges.15 However, most previous studies in infants have been conducted only in QS,15-22 with only 1 investigation being performed in both AS and QS.23 Consequently, little information is available on hypoxic arousal responses in AS, which, based on studies using somatosensory stimuli, is a state of increased arousability. In addition, arousal responses of infants to hypoxia have not been compared against the background of spontaneous arousals occurring during sleep, which occur frequently in young infants, particularly in AS. Previous studies have usually defined arousal as vigorous body movements, eyes opening, awakening, crying, or a combination thereof. However, it has now been demonstrated that full cortical arousal from sleep, in both human adults and infants, is preceded by a stereotypical sequence of subcortical events.24-29 In sleeping infants, these subcortical processes may serve as protective mechanisms that restore airway patency, allow the infant to reposition his or her head while maintaining sleep efficiency, or both.12 Because more knowledge is needed regarding arousability of infants in both sleep states in response to respiratory stimuli, the aims of this study were to compare arousal responses to mild hypoxia in both AS and QS and to contrast these induced responses with those of spontaneously occurring arousal, using indexes of subcortical arousal. In addition, we aimed to determine the effects of postnatal age on arousal responses. It was hypothesized (a) that hypoxia-induced arousal responses would be depressed in QS compared to AS and would occur more frequently and with shorter onset latencies than spontaneously occurring arousal (under normoxic conditions) in both sleep states, and (b) that these effects would be most marked at 2 to 3 months of age when the incidence of SIDS is highest.