As a trainee, it’s helpful to rely on structured approaches.

One very practical method is to trace oxygen from source to endpoint: trace O₂ delivery from the O₂ outlet in the wall, to the machine, to the circuit, to the endotracheal tube, to the alveoli, to the bloodstream, and finally to the pulse oximeter. This “mechanical walkthrough” ensures nothing is overlooked.

In the ICU or on the wards, you may have learned a more physiologic framework: the five causes of hypoxemia:

1. V/Q mismatch
   
2. right-to-left shunt
   
3. low FiO₂
   
4. hypoventilation
   
5. diffusion impairment

This is a useful approach when dealing with a lung parenchymal process that is common on the wards or in the ICU: pneumonia, pleural effusion, etc. But intraoperatively, the issue is almost always mechanical: airway issue, poor ventilation, or the result of a procedural intervention. These fit into the above physiologic categories but require more than just diagnosis; quick action is needed.

1. Turn FiO₂ to 100% and call your attending.
   
2. Check for obstructive or mechanical issues:
   
   
   
   • Mainstem intubation? Withdraw the ETT 1–2 cm.
     
   • Mucus plug? Suction the tube and circuit thoroughly.
     
   • Circuit issue? Check for disconnections, leaks, or kinks.
   
   
3. Perform an alveolar recruitment maneuver:
   
   
   
   • An “official” recruitment maneuver for a mechanically ventilated patient in the ICU consists of holding 30–40 cm H₂O pressure for 30–40 seconds. In most anesthetized, supine patients without significant co-existing lung pathology, you can often get away with less. The 20–30 cm H₂O for 10–20 seconds is a good place to start.
     
   • In thoracic or resection cases, recruitment maneuvers should be very limited and first discussed with the surgeon.
     
   • After a successful recruitment maneuver, the saturation will typically drop transiently, as pulmonary blood flow redistributes to poorly aerated lung regions that were previously collapsed, and because some sections of the lungs can become overdistended during the maneuver. Then, the saturation will quickly rebound and improve.
     
   • If the recruitment maneuver was successful in improving the saturation, the patient has proven that they are at risk of atelectasis, and thus, a higher PEEP should be used throughout the case.