Effects of Prior Hypoxia Exposure, Endotoxin and Sleep State on Arousal Ability to Airway Obstruction in Piglets: Implications for Sudden Infant Death Syndrome

Abstract

Background: Respiratory tract infections may be an important component in many deaths attributed to sudden infant death syndrome (SIDS), although the mechanism of involvement remains unclear. Objectives: The hypothesis was tested that prolonged hypoxia and a thermogenic state (simulating a fever due to respiratory tract infection) would impair respiratory responsiveness to airway obstruction during sleep. Methods: Thirty nine piglets aged 5–7 days were exposed to 24 h of moderate hypoxia and/or a low dose of endotoxin derived from Salmonella abortus equi. Responsiveness to complete and subtotal upper airway obstruction was tested during non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. The end-point for airway obstruction tests was taken as the first protective response, either arousal or initiation of mouth breathing. Responsiveness was assessed as response time and response threshold (measured as respiratory effort, i.e. esophageal pressure swing). Results: All animals demonstrated a thermogenic state following endotoxin delivery (drop in ear temperature of 5.8 ± 0.2°C and a small but significant increase in rectal temperature). Response time to subtotal airway obstruction was reduced during the heat conserving phase of the fever (thermogenesis; 2.8 ± 0.5 s compared to 4.3 ± 0.7 s during pre-endotoxin tests), but markedly increased during the recovery period (20.3 ± 5.1 compared to 14.0 ± 2.5 s pre-endotoxin) in NREM sleep. Response threshold was not significantly affected by either endotoxin or hypoxia in NREM sleep. Respiratory responsiveness to subtotal obstruction was markedly reduced during REM sleep (response time 40.3 ± 10.9 s compared to 14.7 ± 2.2 s in NREM; response threshold –14.0 ± 1.3 mm Hg compared to –11.7 ± 1.0 mm Hg in NREM). Conclusions: This study has demonstrated in a neonatal animal model that respiratory responsiveness to airways obstruction is delayed during recovery from fever. The findings may have implications for the human infant recovering from a respiratory illness.