Effect of enflurane and isoflurane on norepinephrine kinetics: a new approach to assessment of sympathetic function during anesthesia.

We investigated the effects of enflurane [1.0 and 1.5 minimum alveolar concentration (MAC)] and isoflurane (1.0, 1.5, and 2.0 MAC) on norepinephrine (NE) spillover (a measure of NE release rate) and NE clearance in 16 dogs, by using an isotope (3H-NE) dilution technique. During anesthesia with either enflurane or isoflurane, endogenous NE concentrations decreased markedly. In contrast, 3H-NE concentrations increased or remained unchanged. There was a significant (P < 0.05) reduction in NE spillover with both anesthetics at all doses studied. NE spillover decreased by 54% +/- 6% and 68% +/- 4% at 1.0 MAC enflurane and 1.5 MAC enflurane, respectively. Isoflurane decreased NE spillover by 54% +/- 8%, 59% +/- 9%, and 71% +/- 8% at 1.0 MAC, 1.5 MAC, and 2.0 MAC isoflurane, respectively. NE clearance was reduced by both anesthetics; enflurane reduced NE clearance (P < 0.05) at all concentrations studied, whereas NE clearance was reduced by 24% +/- 8% from awake values only at the highest dose of isoflurane (P < 0.05, 2.0 MAC). For enflurane, NE clearance was decreased by 17% +/- 5% at 1.0 MAC and 37% +/- 7% at 1.5 MAC enflurane. Because decreased NE clearance would otherwise have increased NE plasma concentration, our measured decreases in endogenous plasma NE concentrations probably did not reflect the actual extent of sympathetic inhibition produced by enflurane and isoflurane.(ABSTRACT TRUNCATED AT 250 WORDS)