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).