Capnographic detection of anaesthesia circle valve malfunctions

To determine whether capnographic waveforms can characterize valve malfunction of the anaesthesia circle, which would enable such problems to be identified and rectified immediately, we monitored capnographic respiratory waveforms during anaesthesia with simulated circle valve malfunctions. Ten mongrel dogs were anaesthetized with pentobarbitone, 25 mg.kg-1 IV, and halothane, 0.5 to 1 per cent. Respiratory gas was sampled from the elbow of the circle system for capnographic monitoring. At fresh gas flow rates of 2.5 or 5 L.min-1 during consecutive periods of controlled and spontaneous ventilation, the inspiratory valve, the expiratory valve, or both valves of the circle system were opened for 15 min. Inspired CO2 concentration increased significantly every time a valve was opened, except during spontaneous breathing at 5 L.min-1. At 2.5 L.min-1, inspired CO2 increased from baseline to 0.41 +/- 0.28 per cent with the inspiratory valve opened and to 2.22 +/- 1.72 per cent with the expiratory valve opened during controlled ventilation and to 0.43 +/- 0.20 per cent and 2.02 +/- 1.28 per cent, respectively, during spontaneous ventilation. Inspired CO2 increased to almost 1 per cent when the inspiratory valve was open and to greater than or equal to 1.89 per cent when the expiratory valve was open. The effects with the expiratory valve open and with both valves open were similar. Capnograms were affected in characteristic ways by the valve malfunctions.