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Canadian Journal of Anesthesia, Vol 27, 357-362, Copyright © 1980 by Canadian Anesthesiologists' Society

Rebreathing and Carbon Dioxide Elimination with the Bain Circuit

WOLFGANG E. SPOEREL 1

1 Department of Anaesthesia, University of Western Ontario, University Hospital, London, Canada

The carbon dioxide distribution inside the Bain breathing circuit was measured during controlled ventilation in 14 patients. A circuit with seven equi-distant sampling ports was constructed and gas samples were also taken at the gas machine end of the circuit and from a catheter ending near the tip of the tracheal tube. The carbon dioxide concentration was measured with a mass spectrometer. After 30 minutes of a stable halothane or ethrane anaesthesia, the maximum and minimum carbon dioxide concentration was determined at each sampling site; after that either tidal volume, inspiration to expiration ratio, respiratory rate (with and without change in minute ventilation) or fresh gas flow were changed, keeping all other parameters constant. The carbon dioxide measurements were repeated after an interval of at least 20 minutes. Each change was recorded in at least two patients. Only qualitative changes obtained in an individual patient were compared.

The results indicate that in conditions where the expired minute volume (Ve) was greater than the volume of fresh gas entering the circuit (VF), end-tidal carbon dioxide concentration (FETCOCO2) and the mean carbon dioxide concentration in gas eliminated from the circuit (FECOCO2) depended only on Vf and its utilization. The lowest inspired carbon dioxide concentration (FiCOCO2) could be varied independent of FetCOCO2 and FECOCO2 by changing respiratory rate (with or without changing Ve), tidal volume or the inspiratory:expiratory ratio, all of which influence the degree of rebreathing or mixing of expired and fresh gas within the circuit. The FiCOCO2 will rise toward the FECOCO2 as mixing becomes more complete. Although an increase in Vf (providing Ve > Vf) did lower FETCOCO2, FECOCO2 and FiCOCO2, the reduction of the latter did not correspond to the fall in FETCOCO2.

It is concluded that, with controlled ventilation, the distribution of carbon dioxide within the Bain circuit and the FICOCO2 have no influence on the alveolar carbon dioxide concentration or the carbon dioxide elimination from the circuit.






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Copyright © 1980 by the Canadian Anesthesiologists' Society.