The capnogram shows prolonged phase II as well as slanting of descending limb of inspiratory segement. The expirato

ry 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.