QUANTIFYING THE EFFECT OF ENFLURANE ON ATRACURIUM INFUSION REQUIREMENTS

The present study was designed to evaluate the interaction between atr acurium and enflurane in 40 adult surgical patients using closed-loop feedback control of infusions of atracurium. Anaesthesia was induced w ith thiopentone and fentanyl and intubation was facilitated with atrac urium 0.5 mg . kg(-1) lean body mass. During the first 90 min, anaesth esia was maintained with nitrous oxide in oxygen (2:1) and fentanyl. F or the following 90 min the patients were randomly assigned to receive enflurane at different end-tidal concentrations: Group I, control, fe ntanyl-nitrous oxide anaesthesia; Group II, enflurane 0.3%- nitrous ox ide; Group III, enflurane 0.6%-nitrous oxide; Group IV enflurane 0.9%- nitrous oxide. The possible interaction of atracurium with enflurane w as quantified by determining the asymptotic steady-state rate of infus ion (I-ss of atracurium necessary to produce a constant 90% neuromuscu lar block. This was accomplished by applying nonlinear curve filling t o data on the cumulative dose requirements. Every patient served as hi s/her own control and the changes in the infusion rates were determine d individually. Patient characteristics and controller performance, i. e., the ability of the controller to maintain the neuromuscular blocka de constant at the setpoint, did not differ among groups. In Group II I-ss decreased from 0.33 +/- 0.12 to 0.26 +/- 0.08 mg . kg(-1) hr(-1) (P < 0.01), in Group III from 0.32 +/- to 0.12 to 0.24 +/- 0.08 mg kg( -1) hr(-1) (P < 0.001) and in Group IV from 0.29 +/- 0.09 to 0.21 +/- 0.09 mg . kg(-1) hr(-1) (P < 0.001). In the control group atracurium r equirements remained unchanged throughout the study. Enflurane reduces atracurium requirements in a dose-dependent manner. During enflurane anaesthesia the rate of atracurium infusion should be reduced but beca use of interindividual differences the monitoring of the neuromuscular function is important to ensure the appropriate level of neuromuscula r block.