Effects of three different types of management on the elimination kinetics of volatile anaesthetics. Implications for malignant hyperthermia treatment

The effectiveness of three types of management on the elimination kinetics of volatile anaesthetics was studied prospectively in 45 patients randomised to one of three groups. Patients were anaesthetised using isoflurane. Inspiratory and expiratory isoflurane concentrations were measured. After reaching a steady-state isoflurane concentration, the vaporizer was turned off. In group 1, only the fresh gas flow was increased from 40 to 120 ml.kg-1 x min-1. Patients in group 2, in addition to the increase in the fresh gas flow, had a charcoal filter connected in the inspiratory limb of the circuit. Patients in group 3 had the fresh gas flow increased and the anaesthetic machine and breathing system changed. There was a statistically significant difference in the isoflurane washout from the anaesthetic machines between group 1 (90% elimination time 39 10] s) and group 2 (90% elimination time 25 5] s) (p < 0.01). However, there was no significant difference in the isoflurane washout from the patients in any of the groups. Thus the use of a charcoal filter or a change of the anaesthetic machine and breathing system proved to be of no clinical advantage.