Time course variations in the mechanisms by which cerebral oxygen delivery is maintained on exposure to hypoxia/altitude.

Imray, Christopher, Colin Chan, Alison Stubbings, Hannah Rhodes, Susannah Patey, Mark H. Wilson, Damian M. Bailey, and Alex D. Wright for the Birmingham Medical Research Expeditionary Society. Time course variations in the mechanisms by which cerebral oxygen delivery is maintained on exposure to hypoxia/altitude. High Alt Med Biol. 15:21-27, 2014.Normal cerebral function is dependent upon an adequate and continuous supply of oxygen. This study calculated cerebral blood flow based on assessment of the right middle cerebral artery (MCA) velocity (MCA(Vel)) and MCA diameter (MCA(Diam)) by trans-cranial Doppler and trans-cranial Duplex in normoxia, during acute exposure to 12% normobaric hypoxia for up to 6 hours, and after 3 days exposure to the equivalent altitude, 4392m, in nine subjects. Mean (SD) MCA(Vel) increased both after 6 hours hypoxia from 76.8 (11.4) to 97.2 (17.4) cms/sec (p<0.001), and after 3 days at altitude from 68.1 (7.5) [sea level] to 76.2 (10.2) [4392 m] (p=0.015). MCA(Diam) increased from 5.07 (0.6) to 6.1 (0.6) mm (p<0.001) after 6 hours of 12% hypoxia. Calculated mean MCA blood flow increased after 6 hours of 12% hypoxia from 5.0 (0.6) mL/sec to 8.9 (1.2) mL/sec, but there was no difference between sea level and 4392m. Calculated mean cerebral oxygen delivery increased from 72.4 (14.4) to 107 (20.1) mL/sec (p<0.001) after 6 hours of 12% hypoxia and was maintained unchanged at 4392m. An increase in MCA caliber, rather than blood velocity, was a major contributor to increased oxygen delivery accompanying within the first few hours of exposure to acute hypoxia. During more long-term exposure, increases in MCA velocity and a rise in hemoglobin appeared to be the more important mechanisms in maintaining cerebral oxygen delivery. The implication of this observed change in MCA diameter questions the widely held assumption that MCA velocity is a surrogate for flow during acute hypoxic exposure.