Model-based administration of inhalation anaesthesia. 4. Applying the system model.

Abstract

BACKGROUND: We developed and tested a simple dosing strategy for rapid induction with isoflurane followed by maintenance under minimal-flow conditions, that is 0.5 litre min(-1) total fresh gas flow (FGF). An end-expired concentration was to be achieved within 5 min in a desired therapeutic window, that is 0.8-1.1 vol%, and to be maintained within it for at least 30 min. METHODS: With our new model we computed a three-stage regimen using one fixed vaporizer setting: 3 vol% isoflurane in a FGF of 3 and 1.5 litre min(-1), each for 3 min, and 0.5 litre min thereafter. The ratio of nitrous oxide:oxygen was, consecutively, 2:1, 2:1, and 2:3. We evaluated this scheme in 58 adult patients (body mass 74 (SD 13) kg), mostly during eye and ear, nose, and throat surgery. RESULTS: Measured oxygen (33-45 vol%) and nitrous oxide concentrations (66-50 vol%) evolved in accordance with those computed. In five patients with a median of body mass 92 kg (range 76-126 kg), inspired oxygen concentrations decreased to less than 30 vol%. End-expired isoflurane concentration entered the window after 2 min (range 1.0-5.67 min) and attained its maximum, that is 0.96 vol% (0.8-1.2 vol%), after 3.45 min (1.67-6.33 min). The mean end-expired concentration was in the desired window from 3-60 min and an average of 72% of individual measurements were within the window from 3-30 min. The scheme was adapted in six patients (excluded from analysis) because of hypotension. CONCLUSION: The regimen is easily remembered, reliable, and lends itself to alternative strategies, but must be guided by the monitoring of gas and vapour concentrations and haemodynamic variables.