• M S Scheller, B R Jones, and J L Benumof.
• Department of Anesthesiology, University of California, San Diego 92103.
• J Cardiothorac Anesth. 1989 Oct 1;3(5):564-7.

AbstractThe relative importance of fresh gas flow and inspiratory/expiratory ratio in determining delivered tidal volume and PaCO2 was studied in anesthetized adult patients ventilated with a fixed ventilator bellows volume. The fresh gas flows studied were 2, 6, and 10 L/min, and inspiratory/expiratory ratio was either 1:2 or 1:4.5. Bellows volume and respiratory rate were held constant throughout the study. At the lowest fresh gas flow and smallest inspiratory/expiratory ratio, PaCO2 was 43 +/- 2 mm Hg. The PaCO2 decreased progressively and significantly with each increase in fresh gas flow during ventilation with either inspiratory/expiratory ratio setting. PaCO2 averaged 30 +/- 3 during ventilation with the highest fresh gas flow and largest inspiratory/expiratory ratio. As fresh gas flow increased, PaCO2 and tidal volume changed to a significantly greater degree in response to changes in inspiratory/expiratory ratio. These data demonstrate that altering either fresh gas flow or inspiratory/expiratory ratio can produce clinically significant perturbations in PaCO2 and tidal volume during anesthesia. These perturbations occur even if bellows volume is held constant. Furthermore, changes in inspiratory/expiratory ratio will affect these parameters to a greater degree as fresh gas flow is increased.

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