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

Abstract

Due to a few reports of cerebral dysfunction in connection with isoflurane-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. The short-lived radionuclide 15O 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 3.4-4.4 kPa, respectively. Isoflurane or propofol anaesthesia was given (n = 4 in both groups), supported with 70% nitrous oxide and preceded by ketamine anaesthesia (baseline). PET revealed wide variations in CBF between regions during isoflurane anaesthesia, particularly in comparison with propofol anaesthesia, while rCMRO2 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. 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.