Evaluation of the PhysioFlex (TM) closed-circuit anaesthesia machine

The concentrations of nitrous oxide, sevoflurane and oxygen in the circle s ystem of a closed-circuit anaesthesia machine, the PhysioFlex(TM), were mea sured in seven patients. During anaesthesia, the settings for each gas were changed and their concentrations recorded. At the induction of anaesthesia , it took 80-510 s (median 190 s) for the end-tidal sevoflurane concentrati on to reach 2.0%, and 920-2640s (median 1500s) for the oxygen in the breath ing circuit to reach 30%. At this time, the nitrous oxide concentration was 60+/-3% (mean+/-SD). During anaesthesia, it took 90-480s (median 140s) for the end-tidal sevoflurane concentration setting to decrease from 3.0 to 1. 0%, and 90-400s (median 110s) to return from 1.0 to 3.0%. When the inspired oxygen was increased from 30 to 50%, circuit concentrations reached equili brium in 40-60s (median 40s), and when decreased from 50% back to 30%, equi librium took 310-470s (median 450s). During recovery from anaesthesia, insp iratory sevoflurane concentration took 40-70s (median 50s) to decrease to 0 .2%. The PhysioFlex(TM) provided adequate control of sevoflurane and oxygen concentrations, but not of increasing nitrous oxide concentrations.