Spontaneous Breathing with a T-Piece Circuit

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Abstract
Adults (30) undergoing elective superficial surgery under enflurane-nitrous oxide anesthesia while intubated and breathing spontaneously via a modified Mapleson D (Bain) 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 5-10 mmHg) was present. Both fresh gas flow (.ovrhdot.VF) and minute volume (.ovrhdot.VE) were recorded, and the ratio of the 2 (.ovrhdot.VF/.ovrhdot.VE) was calculated. The mean .ovrhdot.VF/.ovrhdot.VE ratio 1.89 .+-. 0.27 (SD). Linear regression was used to plot .ovrhdot.VF against .ovrhdot.VE, breathing frequency, tidal volume, age, weight and end-tidal CO2 tension. Significant correlation was found only with .ovrhdot.VE (r2 = 0.48, P < 0.001) and frequency (r2 = 0.44, P < 0.001). When the ratio .ovrhdot.VF/.ovrhdot.VE was plotted against the same variables, no significant correlation was found. This study showed a wide variability in the minimum .ovrhdot.VF/.ovrhdot.VE ratio which presents rebreathing. The respiratory waveform, which was not studied, probably played a role in determining the .ovrhdot.VF/.ovrhdot.VE. Nevertheless, 87% of the patients required a .ovrhdot.VF/.ovrhdot.VE ratio of 2.0 or less to prevent rebreathing. With concern about rebreathing, .ovrhdot.VE should be measured and the .ovrhdot.VF adjusted to about twice the measured .ovrhdot.VE.

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