Cerebral normoxia in the rhesus monkey during isoflurane- or propofol-induced hypotension and hypocapnia, despite disparate blood-flow patterns - A positron emission tomography study

Background: Due to a few reports of cerebral dysfunction in connection with isofluane-induced hypotension and concomitant hypocapnia, positron emission tomography (PET) was used to measure cerebral oxygenation and blood flow during similar conditions with isoflurane or propofol. Methods: The short-lived radionuclide O-15 was used for measurement of cerebral metabolic rate of oxygen (CMRO2), cerebral blood flow (CBF) and oxygen extraction ratio (OER) regionally in rhesus monkeys during normotensive/normocapnic and hypotensive/hypocapnic conditions, mean arterial pressure 100-110 and 50-65 mmHg and PaCO2 4.4-5.4 and 3.4-4.9 kPa, respectively. Isoflurane or propofol anaesthesia was given (n=4 in both groups), supported with 70% nitrous oxide and preceded by ketamine anaesthesia (baseline). Results: PET revealed wide variations in CBF between regions during isoflurane anaesthesia, particularly In comparison with propofol anaesthesia, while rCMRO(2) decreased globally in a dose-dependent manner during both isoflurane and propofol anaesthesia. The metabolism-flow coupling was intact during propofol but not during isoflurane anaesthesia. Hypotension reduced rCBF, and rOER increased globally with both study drugs when changing from normo-to hypotension. However, this rOER increase was not significant when using PaCO2 as a covariate, and rOER was never above an arbitrary limit for hypoxia of 70%. Thus, hypocapnia, rather than hypotension, was responsible for the somewhat higher rOER measured. Conclusion: PET indicated adequate cerebral oxygenation during isoflurane and propofol anaesthesia, despite disparate blood-flow patterns. Hypotension and concomitant moderate hyperventilation reduced rCBF, but did not result in hypoxia.