There are few literatures which show the role of tidal volume on the excretion of nitrous oxide(N2O). The purpose of this study is to pharmacokinetically evaluate the effect of varying oxygen inflow rates and tidal volumes on the excretion of N2O.

Ten patients undergoing general anesthesia on supine position were selected. Administering N2O 3 L/min and oxygen 2 L/min with enflurane, ventilatory pattern was tidal volume 10 mL/kg and respiratory rate 11/min. Administration of N2O was stopped after 30 minutes, when end-tidal N2O concentration(ETN2O) were near 60% at steady state. Oxygen was given at 4 L/min with tidal volume(VT) 8 mL/kg(treatment 1), 4 L/min with VT 16 mL/kg (treatment 2), 8 L/min with VT 8 mL/kg(treatment 3) and 8 L/min with VT 16 mL/kg(treatment 4). ETN2O was measured every 15 seconds for first 3 minutes and every 1 minute thereafter. The order of 4 treatments were determined with bloc randomization. The time interval between treatments was 30 minutes. Pharmacokinetic parameters were obtained using PKCALC data program.

Area under curve and clearance in treatments 1 and 2 were significantly larger than those in treatments 3 and 4. Mean residence time and half-life in treatment 4 were significantly shorter than those in other treatments.

The oxygen inflow rate enhances the excretion of N2O more than the tidal volume does, and the tidal volume does so when the oxygen inflow rate is 8 L/min.