title: The influence of fresh gas flow and inspiratory/expiratory ratio on tidal volume and arterial CO2 tension in mechanically ventilated surgical patients.
abstract: The 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.
authors.affiliation: Department of Anesthesiology, University of California, San Diego 92103.
authors.forename: M S
authors.initials: MS
authors.lastname: Scheller
authors.forename: B R
authors.initials: BR
authors.lastname: Jones
authors.forename: J L
authors.initials: JL
authors.lastname: Benumof
mesh_terms: D000328:Adult; D000769:Anesthesia, Inhalation / Q000295:instrumentation / Q000379:methods; D002245:Carbon Dioxide / Q000097:blood*; D006801:Humans; D007320:Inspiratory Capacity / Q000502:physiology; D008449:Maximal Expiratory Flow-Volume Curves / Q000502:physiology; D010366:Peak Expiratory Flow Rate / Q000502:physiology; D012123:Pulmonary Ventilation / Q000502:physiology*; D012121:Respiration, Artificial / Q000295:instrumentation; D013990:Tidal Volume / Q000502:physiology*
chemical_list: D002245:Carbon Dioxide