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A non‐rebreathing coaxial anaesthesia system: dependence of end‐tidal gas concentrations on fresh gas flow and tidal volume
Author(s) -
Tweed W. A.
Publication year - 1997
Publication title -
anaesthesia
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.839
H-Index - 117
eISSN - 1365-2044
pISSN - 0003-2409
DOI - 10.1111/j.1365-2044.1997.033-az0059.x
Subject(s) - normocapnia , tidal volume , halothane , medicine , anesthesia , fresh gas flow , volume (thermodynamics) , dilution , respiratory system , hypercapnia , sevoflurane , thermodynamics , physics , acidosis
A non‐rebreathing adaptation of the Bain coaxial anaesthesia circuit was developed in Nepal as a simple and economical anaesthetic system for underdeveloped countries. It was made by inserting a coaxial (Bain) tubing between an Ambu‐E valve and an Ambu self‐inflating bag. The present 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 (0.2–3 l.min −1 ) and three levels of tidal volume at a constant respiratory rate 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 pelvic surgery were studied. With increasing fresh gas flow rates, there were proportionate increases in the end‐tidal concentrations of oxygen and halothane; with decreasing tidal volume and therefore less air dilution, there were proportionate increases in the end‐tidal concentrations of carbon dioxide, oxygen and halothane. Both effects were statistically and clinically significant. Thus, when this system is used as described, the end‐tidal concentrations of oxygen and halothane are highly dependent upon both the fresh gas flow and the tidal volume.