CONTEXT: Sleep is disturbed at high altitudes. Low PO2 levels at high altitude cause hyperventilation, which results in secondary hypocapnia (low PaCO2 levels). Thus, although sleep disruption at high altitudes is generally assumed to be caused by hypoxia, it may instead be the result of hypocapnia. OBJECTIVE: To determine whether hypocapnia disrupts sleep. METHODS: Four cats were studied for a total of 345 hours of sleep recordings. Two methods were used to test this idea. First we studied their sleep when the cats breathed oxygen concentrations (15% and 10%) equivalent to those at approximately 12,000 feet and 21,000 feet. Then we studied their sleep again in response to the same hypoxic stimuli but with CO2 added to the inspirate to maintain normal CO2 levels. Second, we used mechanical hyperventilation to vary the levels of CO2 while maintaining normal O2 levels. RESULTS: Hypoxia (10% O2) decreased the amount of rapid eye movement sleep to about 20% of normal, and adding back CO2 restored rapid eye movement sleep to approximately 70% of normal. Periodic breathing and apneas were not observed during hypoxia in sleep. When mechanical hyperventilation lowered the CO2 to 85%, 75%, and 65% of normal, rapid eye movement sleep decreased progressively from a control level of 17% of total recording time to 12%, 7%, and 4%, respectively. CONCLUSION: We conclude that hypocapnia rather than hypoxia may account for most of the sleep disturbance at high altitudes.
CONTEXT: Sleep is disturbed at high altitudes. Low PO2 levels at high altitude cause hyperventilation, which results in secondary hypocapnia (low PaCO2 levels). Thus, although sleep disruption at high altitudes is generally assumed to be caused by hypoxia, it may instead be the result of hypocapnia. OBJECTIVE: To determine whether hypocapnia disrupts sleep. METHODS: Four cats were studied for a total of 345 hours of sleep recordings. Two methods were used to test this idea. First we studied their sleep when the cats breathed oxygen concentrations (15% and 10%) equivalent to those at approximately 12,000 feet and 21,000 feet. Then we studied their sleep again in response to the same hypoxic stimuli but with CO2 added to the inspirate to maintain normal CO2 levels. Second, we used mechanical hyperventilation to vary the levels of CO2 while maintaining normal O2 levels. RESULTS:Hypoxia (10% O2) decreased the amount of rapid eye movement sleep to about 20% of normal, and adding back CO2 restored rapid eye movement sleep to approximately 70% of normal. Periodic breathing and apneas were not observed during hypoxia in sleep. When mechanical hyperventilation lowered the CO2 to 85%, 75%, and 65% of normal, rapid eye movement sleep decreased progressively from a control level of 17% of total recording time to 12%, 7%, and 4%, respectively. CONCLUSION: We conclude that hypocapnia rather than hypoxia may account for most of the sleep disturbance at high altitudes.