Hypercapnic cerebral vascular reactivity is decreased, in humans, during sleep compared with wakefulness

During wakefulness, increases in the partial pressure of arterial CO2 result in marked rises in cortical blood flow. However, during stage III-IV, non-rapid eye movement (NREM) sleep, and despite a relative state of hypercapnia, cortical blood flow is reduced compared with wakefulness. In the present study, we tested the hypothesis that, in normal subjects, hypercapnic cerebral vascular reactivity is decreased during stage III-IV NREM sleep compared with wakefulness. A 2-MHz pulsed Doppler ultrasound system was used to measure the left middle cerebral artery velocity (MCAV; cm/s) in 12 healthy individuals while awake and during stage III-IV NREM sleep. The end-tidal PCO2 ( PETCO2) was elevated during the awake and sleep states by regulating the inspired CO2 load. The cerebral vascular reactivity to CO2 was calculated from the relationship between PETCO2 and MCAV by using linear regression. From wakefulness to sleep, the PETCO2 increased by 3.4 Torr (P < 0.001) and the MCAV fell by 11.7% (P < 0.001). A marked decrease in cerebral vascular reactivity was noted in all subjects, with an average fall of 70.1% ( P = 0.001). This decrease in hypercapnic cerebral vascular reactivity may, at least in part, explain the stage III-IV NREM sleep-related reduction in cortical blood flow.