title: Effects of five consecutive nocturnal hypoxic exposures on the cerebrovascular responses to acute hypoxia and hypercapnia in humans.
abstract: 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 = approximately 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.
authors.affiliation: Faculty of Kinesiology, Department of Physiology & Biophysics, University of Calgary, Calgary, Alberta, Canada T2N 4N1.
authors.forename: Jon C
authors.initials: JC
authors.lastname: Kolb
authors.forename: Philip N
authors.initials: PN
authors.lastname: Ainslie
authors.forename: Kojiro
authors.initials: K
authors.lastname: Ide
authors.forename: Marc J
authors.initials: MJ
authors.lastname: Poulin
mesh_terms: D000208:Acute Disease; D000328:Adult; D001783:Blood Flow Velocity; D002245:Carbon Dioxide; D002560:Cerebrovascular Circulation; D002940:Circadian Rhythm; D006339:Heart Rate; D006801:Humans; D006935:Hypercapnia / Q000503:physiopathology*; D000860:Hypoxia / Q000503:physiopathology*; D008297:Male; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D012008:Recurrence; D012119:Respiration; D013990:Tidal Volume
chemical_list: D002245:Carbon Dioxide; D010100:Oxygen