REBREATHING CHARACTERISTICS OF THE BAIN CIRCUIT An Experimental and Theoretical Study

The Bain circuit was studied in a model lung on the assumption that, in addition to the ratio of fresh gas flow to total ventilation ($V˙$FG/$V˙$E), different time fractions of the respiratory cycle might influence rebreathing. We found that the time fraction for active expiration (FE_t) governed rebreathing for each $V˙$FG/$V˙$E value. With FE_t, as an independent variable, a theoretical formula was derived for rebreathing. Rearranging this formula made it possible to calculate the necessary increase in ventilation to keep end-tidal carbon dioxide constant for each $V˙$FG/$V˙$E. Thus, at a fresh gas flow of 70 ml kg^-1 min^-1, $V˙$I has to be increased 2.6 times. For spontaneously breathing patients inhalation anaesthetics that do not depress carbon dioxide sensitivity seem to be better suited to use in the Bain circuit. The FE_CO2 can then kept constant through increased ventilation in spite of the concomitant increase in rebreathing