THE RESPONSIVENESS OF CEREBRAL BLOOD-FLOW TO CHANGES IN ARTERIAL CARBON-DIOXIDE IS MAINTAINED DURING PROPOFOL NITROUS-OXIDE ANESTHESIA IN HUMANS

Because it is common to manipulate Pa(CO2) during neurosurgery, it is essential to characterize the relationship between cerebral blood flow (CBF) and changes in Pa(CO2). The purpose of this study was to investigate the effects of propofol-N2O anesthesia on the CBF response to changes in Pa(CO2) in healthy subjects. In seven patients, anesthesia was induced with propofol 2.0-2.5 mg/kg and then maintained with a propofol infusion of 12 mg.kg-1.h-1 for 10 min and then 9 mg.kg-1.h-1 for 10 min and then was reduced to 3-6 mg.kg-1.h-1 for the remainder of the study. The subjects' lungs were ventilated with N2O in O2 (FI(O2) 0.3) to the end-tidal CO2 present before anesthesia, and then CBF was measured using intravenous Xe-133 and ten scintillation counters, five over each cerebral hemisphere. ET(CO2) then was increased to 50 mmHg and CBF measurement repeated, ET(CO2) then was reduced to 30 mmHg and CBF measurement repeated. Concurrent with each CBF measurement, arterial blood was sampled for Pa(CO2) and hemoglobin measurement. CBF at normocapnia (Pa(CO2) 42 +/- 2 mmHg) was 33 +/- 7 ml.100 g-1.min-1, which increased to 58 +/- 10 ml.100 g-1.min-1 and decreased to 19 +/- 4 ml.100 g-1.min-1 on increasing Pa(CO2) (53 +/- 4 mmHg) and decreasing Pa(CO2) (31 +/- 2 mmHg), respectively. Both the Pa(CO2) and CBF values were statistically different from those measured at any other time (CBF P < 0.002, Pa(CO2) P < 0.001). The slope of CBF versus Pa(CO2) was 1.56 ml.100 g-1.min-1.mmHg. Blood pressure, temperature, and hemoglobin were unchanged throughout the study. We conclude that the cerebral vasculature in healthy subjects remains responsive to changes in Pa(CO2) during propofol-N2O anesthesia and that the slope of the CBF-CO2 response is similar to that found in awake individuals and during anesthesia with other intravenous anesthetic agents.