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Hypoxemia (SpO2 , 95%, severe SpO2 , 90%) is uncommon when a patient is intubated and breathing 100% oxygen.44 Observation of mucous  nembrane color is not a sensitive indicator of hypoxemia as cyanosis will likely not occur until hypoxemia is profound.45 Continuous assessment of oxygenation is best accomplished with pulse oximetry. With low SpO2, the anesthetist may be tempted to troubleshoot the pulse oximeter by repositioning the probe, moistening the mucous membranes, or trying a different monitor. These measures may work if the issue is indeed the probe, but prior to troubleshooting the probe, verify that the patient is properly intubated and connected to the oxygen source and that the supply of oxygen is adequate.

Hypoventilation can cause hypoxemia, so adequate ventilation should be ensured, as previously described. Insertion of the ETT past the thoracic inlet can cause one-lung intubation with decreased pulmonary surface area for gas exchange. If one-lung intubation is likely, the ETT can be pulled out slightly, with the goal to move the tip of the ETT into the trachea.

Hypoxemia can be secondary to atelectasis, in patients with abdominal distention or obesity positioned in dorsal recumbency, or to primary pulmonary (e.g., pneumonia) or pleural (e.g., pleural effusion) disease. If this is expected, manual or mechanical ventilation should be instituted and a positive-end expiratory pressure (PEEP) valve (2.5–5 cm H2O) can be added to the expiratory limb of the circuit to open collapsed airways.

Decreased oxygen delivery to the tissues from perfusion issues (rather than respiratory issues) can also cause decreased SpO2 readings. Treat indicators of poor perfusion such as slow capillary refill time, brady- or tachycardia, hypotension, and weak pulses. If no improvement occurs with these treatments, the patient should be positioned in sternal recumbency as soon as possible and recovered from anesthesia with continued oxygen support. 

These guidelines are supported by generous educational grants from IDEXX Laboratories, Inc., Midmark, and Zoetis Petcare.