Incompetent inspiratory valve versus incompetent expiratory valve
Incompetent inspiratory valve versus incompetent expiratory valve
Capnography can help distinguish an incompetent inspiratory valve from an incompetent expiratory valve. (A) Normally, the inspiratory and expiratory valves prevent rebreathing. (B) An incompetent inspiratory valve causes some rebreathing, because exhaled gas flows into the inspiratory limb, which is small. On inspiration, end-tidal gas is re-inhaled, followed by fresh gas. The result is a long expiratory capnogram, a normal inspired CO2, and a slow rise in end-tidal CO2. (C) An incompetent expiratory valve causes more rebreathing, because the expiratory limb contains the reservoir bag or ventilator bellows. On inspiration, fresh gas from the inspiratory hose, and end-tidal gas from the expiratory hose is re-inhaled. The result is a normal length expiratory capnogram, elevated inspired CO2, and a more rapid rise in end-tidal CO2. Increasing the fresh gas flow does not decrease the inspired CO2 (which it does when the CO2 absorbent is exhausted).
CO2: carbon dioxide.
Graphic 114952 Version 1.0
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