A clinical study of low flow anaesthesia by conventional strategy vis a vis by computer simulation derived strategy

Abstract

Objectives: To achieve an optimal anaesthetic condition by using low flows in short duration procedures is not easy and it needs consideration of many factors to detail. Hence, a prospective, randomized study was conducted to develop an easily memorised dosing strategy by computer simulation study and it was compared with conventional strategy of low flow anaesthesia. The main target was to achieve an end-tidal concentration of isoflurane of 0.8 -1.2 vol% within 5 minutes of the start of low fresh gas flow and to maintain it for at least 30 minutes.

Methodology: We selected sixty patients and randomly divided them into two equal groups. Ethics committee approval and informed consent from the selected patients was obtained. In Group-I patients, conventional strategy was used, in which fresh gas flow (FGF) was set at 5.5 L/min (O₂ 1.5 L plus N₂O 4 L) and isoflurane 1.5 Vol% was given for fifteen minutes followed by 2.5 L/min. (O₂ 1 L plus N₂O 1.5 L) and isoflurane 2 Vol % for next five minutes. After twenty minutes flow was reduced to a final 0.8 L (<1 L) (O₂0.4 L plus N₂O 0.4 L) and isoflurane 2.5 Vol% till the end of surgery. In Group-II, one of the three schemes designed by computer simulation studies fulfilling our objective of reaching the therapeutic window in less then five minutes was taken for clinical validation and comparison. Selected scheme had one fixed vaporizer setting, e.g. 2.5 vol% Isoflurane, with varying FGF rates of 4.5 L/min for first 3 minutes, 2.5 L/min for next 3 minutes, and 0.8 L/min thereafter. The ratio of N₂O:O₂ was 2:1, 3:2, and 1:1 consecutively.