A NON-REBREATHING COAXIAL ANESTHESIA SYSTEM - DEPENDENCE OF END-TIDALGAS CONCENTRATIONS ON FRESH GAS-FLOW AND TIDAL VOLUME

A non-rebreathing adaptation of the Bain coaxial anaesthesia circuit w as developed in Nepal as a simple and economical anaesthetic system fo r underdeveloped countries. It was made by inserting a coaxial (Bain) tubing between an Ambu-E valve and an Ambu self-inflating bag. The pre sent study examined the dependence of end-tidal gas concentrations on fresh gas flow and tidal volume during halothane/oxygen/air inhalation anaesthesia. Four levels of fresh gas flow with normocapnia (0.2-31.m in(-1)) and three levels of tidal volume at a constant respiratory rat e of 15 breath.min(-1) (to achieve end-tidal carbon dioxide values of 4 +/- 0.5%, 5 +/- 0.5% and 6 +/- 0.5%) were introduced in random order . Twelve ASA class 1 and 2 adult patients having intra-abdominal or pe lvic surgery were studied. With increasing fresh gas flow rates, there were proportionate increases in the end-tidal concentrations of oxyge n and halothane; with decreasing tidal volume and therefore less air d ilution, there were proportionate increases in the end-tidal concentra tions of carbon dioxide, oxygen and halothane. Both effects were stati stically and clinically significant. Thus, when this system is used as described, the end-tidal concentrations of oxygen and halothane are h ighly dependent upon both the fresh gas flow and the tidal volume.