Air-oxygen mixtures in circle systems

Study Objective: To determine the effect of different air-O-2 mixtures and fresh gas flows F (FGF) on the relationship between the delivered (FDelO2) and inspired O-2 fraction (FIO2) in a circle system. Study Design: Randomized clinical study. Setting: Large teaching hospital. Patients: 160 ASA physical status I, II, and III patients undergoing a variety of cardiovascular procedures with general endotracheal anesthesia. Interventions: 160 Patients were randomly assigned to one of 20 groups (n = 8 each), depending on the combination of total FGF (0.5, 1, 2, 4, or 8 L/min) and air-O-2 mixture used (ratios of 4/1, 3/2, 2/3, or 1/4), corresponding to a FDelO2 of 0.37, 0.53, 0.68, and 0.84. For each combination of FGF and air-O-2 mixture, FIO2 after equilibration was compared with FDelO2. Measurements and Main Results: With any air-O-2 mixture with a FGF less than or equal to 2 L/min, FIO2 became lower than FDelO2. Because FIO2 decreased below 0.25 after 13 and 26 minutes in the first two Patients of the 4/1 0.5 L/min air-O-2 group, this study limb was terminated. Conclusions: When using air-O-2 mixtures in a circle system, FIO2 becomes lower than the FDelO2 with FGF less than or equal to 2 L/min. The relative proportion of 02 in the FGF has to be increased accordingly. (C) 2001 by Elsevier Science Inc.