Compound A does not accumulate during closed circuit sevoflurane anaesthesia with the Physioflex

Abstract

We have investigated inspiratory and end-tidal gas composition during sevoflurane anaesthesia in a closed circle system with continuous gas flow (70 litre min-1, Physioflex) to determine possible accumulation of sevoflurane degradation products. During five abdominal operations in adults lasting more than 2 h, anaesthesia was maintained with an end-tidal concentration of 2% sevoflurane in 40% oxygen-air. The circle included an absorbing canister filled with 1 litre of fresh soda lime. Samples were obtained at the end of an expiration from the tracheal tube and from the inspiratory limb before, and at selected times after, addition of sevoflurane. The temperature of soda lime was 24.7 +/- 0.7 degrees C at the beginning and reached a maximum of 31.2 +/- 1.0 degrees C after 20-30 min, followed by a plateau. Inspiratory compound A (CH2F-O-C(= CF2)(CF3)) 3-8 ppm was detected after 10 min, but did not accumulate in the circle over 2 h without flushing. Expired concentrations were consistently lower with 1.5-3 ppm signalling absorption by patients. Calculated total amounts absorbed over 2 h varied between 2.0 and 7.2 ppm h. Other degradation products such as compound B or methanol were not detected. In summary, we did not detect sevoflurane metabolites with soda lime in significant amounts during closed circle anaesthesia with the Physioflex. The observed concentrations of compound A were below the threshold of nephrotoxicity in rats by a factor of more than 20.