In vitro analysis of compound A formation in a computer-controlled closed-circuit anesthetic apparatus - Comparison with a classical valve circuit

Background: Few data exist on compound A during sevoflurane anesthesia when using closed-circuit conditions and sodalime with modern computer-controlled liquid injection.

Methods: A PhysioFlex apparatus (Drager, Lubeck, Germany) was connected to an artificial test lung (Inflow approximate to 160 ml/min carbon dioxide, outflow approximate to 200 ml/min, simulating oxygen consumption). Ventilation was set to obtain an end-tidal carbon dioxide partial pressure (PETCO2) approximate to 40 mmHg. Canister inflow (T-in(o)) and outflow (T-out(o)) temperatures were measured. Fresh sodalime and charcoal were used. After baseline analysis, sevoflurane concentration was set at 2.1% end-tidal for 120 min. At baseline and at regular intervals thereafter, PETCO2, end-dial sevoflurane, T-in(o) and T-out(o) were measured. For inspiratory and expiratory compound A determination, samples of 2-ml gas were taken. These data were compared with those of a classical valve-containing closed-circuit machine. Ten runs were performed in each set-up.

Results: Inspired compound A concentrations increased from undetectable to peak at 6.0 (SD 1.3) and 14.3 (SD 2.5) ppm (P <0.05), and maximal temperature in the upper outflow part of the absorbent canister was 24.3 degrees C (SD 3.6) and 39.8 degrees C (SD 1.2) (P <0.05) in the PhysioFlex and valve circuit machines, respectively. Differences between the two machines in compound A concentrations and absorbent canister temperature at the inflow and out-flow regions were significantly different (P <0.05) at all times after 5 min.

Conclusion: Compound A concentrations in the high-flow (70 1/min), closed-circuit PhysioFlex machine were significantly lower than in conventional, valve-based machines during closed-circuit conditions. Lower absorbent temperatures, resulting from the high flow, appear to account for the lower compound A formation.