Brain strain: Blood flow and metabolism in environmental extremes

This narrative review compares and contrasts the most commonly encountered environmental stressors on human cerebrovascular functioning. From high altitude and space, extreme apnoea, heat and cold stress, the impact of these stressors on the regulation of cerebral blood flow (CBF) and oxygen metabolism ( $\mathrm{CM}{R}_{O_2}$ ) is discussed. As long as consciousness remains, $\mathrm{CM}{R}_{O_2}$ and oxygen delivery ( $C{D}_{O_2}$ ) remain stable during acute and chronic exposure to poikilocapnic hypoxia. Such a response is possible with alterations in CBF to maintain a relatively stable $C{D}_{O_2}$ . Likewise, the elevations in CBF during exercise in conditions of acute and chronic hypoxia seem to be appropriate to maintain stable $\mathrm{CM}{R}_{O_2}$ . In freedivers, prolonged periods of apnoea during breath-hold reflect marked hypoxaemia and acidosis. At these extremes in elite human freedivers, although elevations in CBF seem to maintain $C{D}_{O_2}$ , there is evidence of reductions in $\mathrm{CM}{R}_{O_2}$ . In contrast to hypoxia, heat- or cold-induced hyperventilation and related hypocapnic-induced vasoconstriction, marked reductions in CBF and $C{D}_{O_2}$ can occur. In the cold, however, the reductions in CBF seem to be partly compensated by elevations in blood pressure and haemoconcentration. In the heat, Q₁₀-mediated elevations in $\mathrm{CM}{R}_{O_2}$ are challenged by cerebral vasoconstriction and limited $C{D}_{O_2}$ , especially when hyperventilation is pronounced. Furthermore, intracranial velocity seems stable during spaceflight despite elevations in $P_{\mathrm{aC}{O}_2}$ . The implications of these changes in CBF and metabolism during environmental stressors are considered in the context of neuropsychological functioning. Finally, the limited research on cross-exposures on cerebrovascular function is reviewed, and future research directions are proposed.