Effect of hypoglycemia on cerebral metabolism and carbon dioxide responsivity

This study examined the effects of hypoglycemia (HG) on cerebral metabolism and cerebrovascular reactivity to carbon dioxide. Cerebral blood flow (CBF) was determined using radiolabeled microspheres in pentobarbital-anesthetized dogs. Cerebral oxygen, glucose, lactate, pyruvate, acetoacetate, and β-hydroxybutyrate uptakes were calculated using the respective concentrations measured in arterial and sagittal sinus blood samples. EEG was recorded throughout each experiment. HG was induced with insulin to obtain a blood glucose <30 mg/100 ml. Hypercapnia was studied in 10 animals (3 control, 7 HG) by increasing arterial carbon dioxide tension (Pa(CO₂) from control (35 ± 4; mean ± SE) to 54 ± 2 Torr during normoglycemia (NG) and HG. Hypocapnia was studied in 11 animals (3 control, 8 HG) by decreasing Pa(CO₂) from control (39 ± 1) to 14 ± 1 Torr in NG and HG. Measurements were taken after reaching steady-state Pa(CO₂) in both groups at each control and altered Pa(CO₂) state. In the hypercapnic group, glucose decreased from 71 ± 3 to 28 ± 3 mg/100 ml. CBF increased with hypercapnia to 175% of control in both NG and HG. Cerebral metabolic rate of oxygen and electroencephalogram (EEG) did not change in the hypercapnic group. In the hypocapnic group glucose decreased from 71 ± 3 to 19 ± 2 mg/100 ml. CBF decreased with hypocapnia to 62 ± 5% of control in NG but remained at control in HG. This was not accompanied by changes in cerebral oxygen consumption; however, a flat EEG occurred in all HG hypocapnic animals. No change occurred in uptake of the other cerebral metabolites measured in any group. This study shows that the CBF hypercapnic response remains intact during HG; however, hypocapnia causes severe EEG disturbances and impairs the cerebral vasoconstriction response.