Effects of extremes of respiratory and metabolic alkalosis on cerebral blood flow in man.

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Abstract
Cerebral blood flow has been measured during steady states of both respiratory and metabolic alkalosis in lightly anesthetized man. When mechanical hyperventilation decreased arterial PCO2 (PaCO2) [arterial CO2 pressure] from 37 to 19 torr [torricellian units], cerebral blood flow was lowered from 44.1 to 25.3 ml/100 g/min, a 43% diminution. Additional reduction of PaCO2 to 10 torr lowered mean cerebral blood flow an additional 17% to 21.0 ml/100 g/min. In 2 subjects minimum values of cerebral blood flow were achieved at PaCO2 levels near 19 torr, but in 4 others only at greater degrees of hypocarbia. The mean value for maximum cerebral vascular resistance during hyperventilation and respiratory alkalosis was 3.5 torr/ml/100 g/min. Metabolic alkalosis induced by sodium bicarbonate infusion, with PaCO2 controlled at 19 torr, raised arterial pH from 7.63 to 7.79; and cerebral blood flow was increased from 26.1 to 31.3 ml/100 g/min. This 17% increase in cerebral blood flow occurred despite a decrease in blood pressure, and was significant at the 0.01 level. Metabolic alkalosis can exert a slight dilating effect on cerebral vessels.

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