EFFECT OF LOW FRESH GAS-FLOW RATES ON INSPIRED GAS-COMPOSITION IN A CIRCLE ABSORBER SYSTEM

Study Objective: To determine the effects of fresh gas flow on inspired gas composition during low-flow anesthesia. Design: Randomized trial with 2-hour observation periods in patients assigned to one of three groups. Setting: Inpatient surgery clinic at a medical center. Patients: Thirty-six patients undergoing abdominal surgery with low-flow anesthesia. Interventions: Fresh gas flow was given at a starting rate of 5 L/min for 6 minutes. Thereafter, the fresh gas flow setting was nitrous oxide (N2O) 1 L/min and oxygen (O2) 0.6 L/min (Group 1), N2O 0.5 L/min and 0, 0.5 L/min (Group 2), and with a moderate surplus of N2O and 02 with respect to the patient's 02 Consumption (Group 3). Measurements and Main Results: The inspired 02 concentration (FIO2) was measured using a paramagnetic technique, and N2O levels were measured with infrared sensors; the inspired nitrogen concentration (FIN2) was calculated by the following formula: FIN2 = 1 - FIO2 - FIN2O, where FIN2O is the inspired N2O concentration. After 1 hour of anesthesia, FIO2 was significantly lower in Group 1 than in Groups 2 and 3 (p < 0.01), and FIN2 was significantly higher in Groups 2 and 3 than in Group 1 (p < 0.01). After 2 hours of anesthesia, FIN2 returned to normal in Group 2 but continued to increase in Group 3. FIN2O was close to 0. 7 % only in Group 1. Conclusions: The same initial period of denitrogenation is not adequate to denitrogenate the circle system in all cases. The lower the fresh gas flow, the longer the initial period of denitrogenation should be. Various levels of fresh gas flow for low-flow anesthesia have been suggested, but none guarantees adequate control of inspired gas composition unless flowmeters are continuously adjusted.