Regional dynamic cerebral autoregulation in acute poikilocapnic hypoxia

Dynamic cerebral autoregulation (dCA) of the posterior circulation has been shown to be more pressure-passive compared with the anterior circulation, possibly due to a lower basal vascular tone. In hypoxia, vascular tone and dCA are typically reduced; however, evidence using volumetric assessment is limited to the anterior circulation. We hypothesized that the posterior circulation would have an exacerbated reduction in dCA than the anterior circulation in acute hypoxia. Twenty participants (14 males, 6 females) were exposed to 120 min of normoxia and acute poikilocapnic hypoxia (12.5% fraction of inspired oxygen). dCA was assessed as the rate of regulation (RoR) of vascular conductance to thigh cuff-induced acute hypotension, in the internal carotid artery (ICA) and vertebral artery (VA) by duplex ultrasound, and the middle cerebral artery (MCA) and posterior cerebral artery (PCA) by transcranial Doppler ultrasound, representing anterior (ICA and MCA) and posterior (VA and PCA) circulations. Linear mixed model analysis revealed that ICA RoR [-0.06 (0.22) s^-1, P = 0.279] and VA RoR [-0.05 (0.21) s^-1, P = 0.343] were comparable in normoxia and hypoxia. MCA RoR (P = 0.995) and PCA RoR (P = 0.895) were also comparable between conditions. In males only, hypoxia reduced VA RoR [-0.15 (0.19) s^-1, P = 0.012], but not ICA RoR [-0.07 (0.21) s^-1, P = 0.264]. In addition, hypoxia induced vasodilation of the ICA [+0.30 (0.32) mm, P = 0.009] but not the VA [+0.08 (0.33) mm, P = 0.398] in males. In conclusion, volumetric dCA of the cerebral conduit arteries to acute hypotension in hypoxia was regionally different in males and may not be influenced by changes in vascular tone.NEW & NOTEWORTHY We demonstrate that hypoxia causes regional dynamic cerebral autoregulation (dCA) in males, where volumetric dCA was reduced in the vertebral artery but not the internal carotid artery. In addition, immediately before the dCA assessment, the vertebral artery diameter was unchanged, whereas the internal carotid artery diameter was increased. In combination, these findings challenge the prevailing view that reductions in dCA in hypoxia are due to a reduction in vascular tone.