Total cerebral ischemia: effect of alterations in arterial PCO2 on cerebral microcirculation.

Radiolabeled 15-microns microspheres were used to examine alterations in regional CBF and cerebrovascular resistance in response to changes in arterial PCO2. Flow measurements were obtained before and 1-3 and 24 h after 12 min of total cerebral ischemia. Striking sensitivity of blood flow in all areas of the central nervous system was shown to changes in arterial PCO2 between 24 and 50 mm Hg during the control nonischemic period. Following 12 min of total cerebral ischemia, cerebrovascular resistance increased, producing a decrease in regional blood flow when the important controlling variables for CBF were held constant. One to 3 h after total cerebral ischemia, the effect of variations in arterial PCO2 on cerebral blood flow was almost completely abolished. Within 24 h after total cerebral ischemia, the sensitivity of CBF to changes in PCO2 was almost completely restored, whereas the secondary severe neurologic deficit remained. Therapeutic interventions following global cerebral ischemia, designed to ameliorate the "no-reflow" phenomenon and minimize residual ischemic neurologic damage, must take into account this marked early post-ischemic reduction in sensitivity to normally potent cerebrovasodilatory influences.