Surviving Without Oxygen: How Low Can the Human Brain Go?

Hypoxic cerebral vasodilation is a highly conserved physiological response coupling cerebral O2 delivery (CDO2) to metabolic demand with increasingly important roles identified for the red blood cell (sensor) and nitric oxide (effector). In the current article, we reexamine previously published cerebral blood flow (CBF) and arterial blood gas data obtained in freedivers and mountaineers, extreme athletes in whom the lowest arterial partial pressures of O2 (19-23 mmHg) and greatest extremes of carbon dioxide (16-61 mmHg) were recorded during (acute) maximal static dry apnea or (chronic) exposure to terrestrial high altitude. Data highlight compensatory increases in CBF (+96% in freedivers to +209% in mountaineers relative to normoxic baseline controls) that were sufficient to sustain CDO2 (+24% in freedivers to +183% in mountaineers) even in the face of the most severe reductions in arterial O2 content (-61% in freedivers to -9% in mountaineers) reported in the literature, consistent with the conservation of mass principle. These unique findings highlight to what extent cerebral vasodilation likely contributes toward these athletes' extraordinary abilities to survive in such harsh environments characterized by physiological extremes of hypoxemia, alkalosis, and acidosis helping define the human brain's remarkable limits of tolerance.