Effect of total flow rate on the concentration of degradation products generated by reaction between sevoflurane and soda lime

Abstract

We have compared concentrations of degradation products in the circle system during sevoflurane anaesthesia at different fresh gas flows. Twentyfour patients underwent sevoflurane anaesthesia with fresh gas flows of 1 litre min-1 (1L group), 3 litre min-1 (3L group), or 6 litre min-1 (6L group) (n = 8 in each group). During anaesthesia, the concentrations of degradation products were measured every hour, and the temperature of soda lime, end-tidal carbon dioxide concentration, inspired and end-tidal sevoflurane concentrations, and carbon dioxide elimination were measured. CF2 = C(CF3)-O-CH2F (compound A) was the only degradation product detected. The mean maximum concentration of compound A was 19.7 (SD 4.3) ppm in the 1L group, 8.1 (2.7) ppm in the 3L group and 2.1 (1.0) ppm in the 6L group (P < 0.05). The maximum temperature of soda lime was 44.6 (1.5) °C in the 1L group, 37.0 (4.4) °C in the 3L group and 29.1 (5.1)°C in the 6L group (P < 0.05). There were no significant differences in end-tidal sevoflurane concentration or mean carbon dioxide elimination between the groups. Only compound A was detected following anaesthesia, with higher concentrations observed at lower flow rates.