In the presence of carbon dioxide absorbents, sevoflurane is degraded to CF2=C(CF3)OCH2F, an olefin compound A. There remains some concern of the hepatic and renal toxicity that compound A poses when using closed circuit anaesthetic techniques. We investigated the effects of the fresh gas flow rate during wash-in on the degradation of sevoflurane in the closed circuit anesthesia technique in surgical patients.

       Methods:   Fiftyseven patients, ASA 1 or 2, undergoing elective neurosurgery with planned duration more than 2 hours were studied. physical examination or laboratory tests showing evidence of abnormal hepatic or renal function. Patients were selected randomly to received sevoflurane anesthesia at a fresh gas-flow rate of 5-6(5.52 ±0.93 )L•min-1 (Group A; n=20) or 0.2-0.3(0.25±0.02) L•min-1 (Group B; n=18) during wash-in. For each study, both absorbent canisters were filled with fresh absorbent (Sofnolime; NK). Anesthesia was maintained with sevoflurane (4% in group A and 8% in group B). 1.4 MAC (1.41 ± 0.04) meaned that the duration of wash-in was finished in group B but in group A that is (1.52±0.25). Thus, in group A, the output concentration of sevoflurane was adjusted from 4% to 8% and the fresh gas-flow rate was decreased to 0.2-0.3 L•min-1 (0.25±0.02). During wash-in in group B, 2 mg•kg-1•h-1 propofol was administered in order to ensure the operation smoothly because the duration of wash-in in group B was longer than that in group A.. Gas samples were obtained from the inspiratory and expiratory limbs of the anesthetic circuit near the one-way valves using gas-tight glass syringes for compound A analysis. Inspiratory limb gas samples were obtained every 2 min after intubation. In- and expiratory limb gas samples were obtained every 15 min after the duration of wash-in,every 30 min after 180 min,at the start of wash-out and the end of anesthesia by using two gas-tight locking syringes. The temperature of the CO2 adsorbent was measured. Gas samples were taken and compound A was quantitated by gas chromatography. Standard compound A (Baxter PPI, USA )was used for calibration. Intergroup comparisons of data were performed using unpaired student t-test. Intragroup datas were compared using one-way analysis of variance (ANOVA ). P < 0.05 was considered statistically significant.

        Results:   There were no significant differences in age, height, body weight, anesthesia time or total closed circuit time in both groups (P > 0.05 ). In group A, the concentration of compound A (inspiratory limb) was 64.44±36.32 ppm (maximum), 43.17±18.89 ppm (mean). The temperature of the canister was 40.32±2.44°. In group B, the concentration of compound A (inspiratory limb) was 51.07±29.88 ppm (maximum), 36.61±20.09 ppm (mean). The temperature of the canister was 41.09±2.63°.It is no renal or hepatic injury in the two groups.