Reoxygenating a patient post failed intubation with BVM is subject to the problem of pulse oximeter lag. This is the phenomenon that it takes at least 30 seconds (much longer in very sick patients) for the pulse oximeter to accurately reflect the central oxygenation. As a result even with perfect ventilation the BVM operator may not be able to confirm they are reoxygenating for some time which can lead to resus team stress, excessive ventilation or unnecessary progression to other strategies.
Most airway practitioners are used to end tidal CO2 to confirm the effectively ventilation associated with ETT placement. However Scott Weingart recommends connecting the end tidal C02 monitor to your BVM set up to overcome the problem of pulse oximeter lag. This will give the BVM operator the ability to know straight away if they are achieving affecting ventilation when reoxygenating as well as provide early warning if such adequate ventilation is interrupted. Scott describes this further in the Emcrit BVM podcast where he runs through Rueben Strayer’s video on BVM but with extra comments at the end.
Note using end tidal CO2 is not limited to just BVM ventilation but can be equally applied to LMA ventilation with the bag.