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Effects of five consecutive nocturnal hypoxic exposures on the cerebrovascular responses to acute hypoxia and hypercapnia in humans

¹Faculty of Kinesiology, Departments of ²Physiology and Biophysics and ³Clinical Neurosciences, Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada T2N 4N1

Submitted 10 September 2003 ; accepted in final form 7 January 2004

The effects of discontinuous hypoxia on cerebrovascular regulation in humans are unknown. We hypothesized that five nocturnal hypoxic exposures (8 h/day) at a simulated altitude of 4,300 m (inspired O₂ fraction = 13.8%) would elicit cerebrovascular responses that are similar to those that have been reported during chronic altitude exposures. Twelve male subjects (26.6 ± 4.1 yr, mean ± SD) volunteered for this study. The technique of end-tidal forcing was used to examine cerebral blood flow (CBF) and regional cerebral O₂ saturation (Sr_O2) responses to acute variations in O₂ and CO₂ twice before, immediately after, and 5 days after the overnight hypoxic exposures. Transcranial Doppler ultrasound was used to assess CBF, and near-infrared spectroscopy was used to assess Sr_O2. Throughout the nocturnal hypoxic exposures, end-tidal PCO₂ decreased (P < 0.001) whereas arterial O₂ saturation increased (P < 0.001) compared with overnight normoxic control measurements. Symptoms associated with altitude illness were significantly greater than control values on the first night (P < 0.001) and second night (P < 0.01) of nocturnal hypoxia. Immediately after the nocturnal hypoxic intervention, the sensitivity of CBF to acute variations in O₂ and CO₂ increased 116% (P < 0.01) and 33% (P < 0.05), respectively, compared with control values. Sr_O2 was highly correlated with arterial O₂ saturation (R² = 0.94 ± 0.04). These results show that discontinuous hypoxia elicits increases in the sensitivity of CBF to acute variations in O₂ and CO₂, which are similar to those observed during chronic hypoxia.

cerebral blood flow; cerebral O₂ saturation; discontinuous hypoxia

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