Study objectives: There is uncertainty concerning the relative contribution of arousal, chemoreceptor stimulation, and their potentially interactive effects, to the acute cardiovascular changes observed during sleep in patients with sleep-disordered breathing. The purpose of this study was to compare cardiovascular responses (heart rate, skin blood flow, and pulse transit time, a non-invasive measure of arterial wall stiffness) to auditory induced arousal from stage 2 sleep under conditions of normoxia and overnight mild hypoxia.
Design: Randomised crossover.
Setting: Sleep Disorders Unit in a 270-bed teaching hospital.
Participants: Eleven healthy male subjects.
Interventions: Subjects slept wearing a facemask and breathed room air (one night; SaO2 approximately 98%) or an hypoxic gas mixture (two nights; SaO2 approximately 92%). Once in stage 2 sleep, subjects were administered one of 10 auditory tones (500 Hz, range 54-90 dB, 5-sec duration) via earphones or a sham tone (recording with no tone).
Measurements and results: Cardiovascular responses were examined beat-by-beat for 20 seconds before and 30 seconds after auditory tones associated with arousals (3-10 second EEG changes) and after sham tones. Sleep efficiency and the percentage of sleep spent in each stage were not different between hypoxia and normoxia nights. Baseline heart rate was elevated on hypoxia nights compared with normoxia nights (59.5+/-1.7 vs. 54.4+/-1.6 b x min(-1), p=0.007). Heart rate, pulse transit time, and skin blood flow showed significant changes after arousal consistent with rapid parasympathetic withdrawal and sympathetic nervous system activation. No changes were observed after sham tones. There were no differences in time course or magnitude of cardiovascular responses between hypoxia and normoxia nights.
Conclusions: We conclude that while mild hypoxia stimulates autonomic activity it does not augment the cardiovascular response to arousal from stage 2 sleep in normal subjects.