Using the anesthetic requirement for halothane (MAC) to follow central nervous system depression in dogs, acute hypoxia reduced MAC starting at Pao2 38 mm Hg. The decrease in MAC related to both arterial oxygen content (Cao2) and arterial pH (group 1). Therefore, acute hypoxia to Pao2 30 mm Hg was induced in two groups of dogs during hypocapnia (group 2) and normocapnia (group 3). MAC decreased far more rapidly during normocapnic hypoxia than during hypocapnic hypoxia. Respiratory alkalosis maintained pH at 7.39 in group 2, and Cao2 remained at 10 vol per cent In group 3, arterial pH fell progressively, reducing Cao2 to 7.4 vol per cent at a constant Pao2 of 30 mm Hg. Significant regression lines correlated arterial and mixed venous oxygen contents, arterial oxygen transport, and arterial pH with the decreases in MAC in groups 2 and 3. Serial CSF pH determinations in both groups correlated poorly with the reductions in MAC.
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Cullen et al. (1970) studied this question.