An evaluation of rebreathing with the bain system during anaesthesia with spontaneous ventilation.

Data from 12 anaesthetized patients breathing spontaneously from the Bain system were used to calculate the degree of rebreathing occurring when the fresh gas flowrate (VF) was equal to 2, 1 and 0.7 times the estimated normal minute ventilation (Vtot). Measurements of the expired minute volume (VE) and end-tidal carbon dioxide tension (PE'CO2) were made to determine the effects of this rebreathing. No rebreathing occurred when VF was equal to twice Vtot. When VF was equal to Vtot rebreathing was usually small in amount and produced no changes in VE or PE'CO2. Changes attributable to rebreathing occurred in only two patients when VF was reduced to 0.7 Vtot. These results are explained by the presence of anaesthesia-induced ventilatory depression and favourable changes occurring in the respiratory wave forms in the majority of patients studied. In some patients, greater values of VE and rebreathing occurred in response to strong surgical stimulation. The net result of increased ventilation in these patients was a decrease in PE'CO2. It is concluded that during anaesthesia, when the Bain system is used with VF equal to Vtot, any increase in PE'CO2 which may result from rebreathing is likely to be small and seldom of clinical importance.