Spontaneous breathing with a T-piece circuit: minimum fresh gas/minute volume ratio which prevents rebreathing

Thirty adults undergoing elective superficial surgery under enflurane-nitrous oxide anesthesia which intubated and breathing spontaneously via a modified Mapleson D (Brain) T-piece circuit were studied with their consent. Total fresh gas flows which were initially high were adjusted downward until minimal rebreathing (inspired CO2 tension of 5 to 10 mmHg) was present. At this point both fresh gas flow (VF) and minute volume (VE) were recorded, and the ratio of the two (VF/VE) was calculated. The mean VF/VE ratio was found to be 1.89 +/- 0.27 (SD). Linear regression was used to plot VF against VE breathing frequency, tidal volume, age, weight, and end-tidal CO2 tension. Significant correlation was found only with VE (r2 = 0.48, P less than 0.001) and frequency (r2 = 0.44, P less than 0.001). When the ratio VF/VE was plotted against the same variables, no significant correlations was found. This study showed a wide variability in the minimum VF/VE ratio which prevents rebreathing. The respiratory waveform, which was not studied, probably played a role in determining the VF/VE. Nevertheless, 87 per cent of our patients required a VF/VE ratio of 2.0 or less to prevent rebreathing. If one is especially concerned about rebreathing, VE should be measured in the VF adjusted to about twice the measured.