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A Comprehensive Educational Website (established 1998)

Designed, Produced, and maintained (Edition 12, 2022) by 

Bhavani Shankar Kodali MD

Updated September 2022

expiratory valves.

Expiratory Unidirectional valve


 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.


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.