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Respiratory Research Group, Departments of 1 Physiology and 2 Zoology, University of Toronto, Toronto, Ontario, Canada M5S 1A1
Mechanisms underlying the circadian rhythm in lung ventilation were investigated. Ten healthy male subjects were studied for 36 h using a constant routine protocol to minimize potentially confounding variables. Laboratory light, humidity, and temperature remained constant, subjects did not sleep, and their meals and activities were held to a strict schedule. Respiratory chemoreflex responses were measured every 3 h using an iso-oxic rebreathing technique incorporating prior hyperventilation. Subjects exhibited circadian rhythms in oral temperature and respiratory chemoreflex responses, but not in metabolic rate. Basal ventilation [i.e., at subthreshold end-tidal carbon dioxide partial pressure (PETCO2)] did not vary with time of day, but the ventilatory response to suprathreshold PETCO2 exhibited a rhythm amplitude of ~25%, mediated mainly by circadian variations in the CO2 threshold for tidal volume. We conclude that the circadian rhythm in lung ventilation is not a simple consequence of circadian variations in arousal state and metabolic rate. By raising the chemoreflex threshold, the circadian timing system may increase the propensity for respiratory instability at night.
body temperature; metabolic rate; constant routine; sleep apnea syndrome; respiratory instability
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