Effects of Blood Gases on the Pressure-Flow Relationships in Canine Cerebral Circulation

The effects of the arterial oxygen saturation and carbon dioxide pressure on the pressure-flow relationships in cerebral circulation were studied in 22 dogs. The cerebral blood flow was observed with stepwise lowering of the systemic arterial blood pressure by controlled bleeding in normoxic normocapnic, normoxic hypocapnic, normoxic hypercapnic, hypoxic hypocapnic and hypoxic normocapnic animals. The autoregulation of cerebral blood flow occurred in the animals in which the arterial oxygen saturation and carbon dioxide pressure were above 90%, and between 20 and 46 mm Hg, respectively. When the arterial oxygen saturation and carbon dioxide pressure were maintained between 17% and 40%, and 34 and 47.5 mm Ng, or above 92%, and between 65 and 82 mm Hg, respectively, almost complete loss of autoregulation was observed. However, autoregulation revived in breathing air at about 30 minutes after autoregulation had been lost during severe hypoxia or hypercapnia, which was induced for about 30 minutes. This suggests that a hypoxic or hypercapnic situation for about 30 minutes does not irreversibly damage the autoregulatory mechanism.