Expiratory valve – Normal                                                        Incompetent expiratory valve

The capnogram shows prolonged phase II as well as slanting of descending limb of inspiratory segement. The expiratory unidirectional valve was noted to be fixed in the open position and thus allowing previously exhaled gases, high in CO2, to be rebreathed with each inspiration. The CO2 waveform returned to normal soon after the valve was removed, cleaned, dried, and replaced.

Reference:
Pyles, ST, Berman LS, Modell JH. Expiratory valve dysfunction in a semiclosed circle  anesthesia circuit-verification by analysis of carbon dioxide waveform. Anesthesia Analgesia 1984;63:536-7.