Hypoxemia occurs when oxygen delivery to the tissues is insufficient to meet metabolic demands.

1. Intraoperative etiologies

   1. Inadequate oxygen supply

      1. Loss of the main pipeline supply with an empty reserve oxygen tank.

      2. An oxygen flowmeter that is not turned to a sufficient flow.

      3. Breathing system disconnection.

      4. Large leaks in the anesthesia machine, ventilator, carbon dioxide absorber, breathing circuit, or around the endotracheal tube or laryngeal mask airway. This condition may be managed acutely by increasing oxygen flow or using a self-inflating (Ambu) bag to deliver oxygen to the patient.

      5. Obstructed endotracheal tube (eg, kinked or mucous plug).

      6. Malpositioned endotracheal tubes (eg, esophageal or mainstem bronchial intubation).

      7. Upper airway obstruction.

      8. Laryngospasm in the unintubated patient (see Section X).

   2. Hypoventilation (see Section V).

   3. Ventilation–perfusion inequalities or shunting

      1. Pulmonary shunt. As seen with atelectasis, pneumonia, pulmonary edema, aspiration, pneumothorax, bronchospasm, mucous plugging, and other parenchymal pathologic states as well as with single lung ventilation. In some cases, these inequalities may be corrected by increasing mean airway pressure or applying PEEP.

      2. Cardiac shunt. Right-to-left cardiac shunt, as in ventricular septal defect (VSD), atrial septal defect (ASD), and tetralogy of Fallot.

   4. Reduction in oxygen-carrying capacity. The oxygen-carrying capacity is reduced with anemia, carbon monoxide poisoning, and hemoglobinopathies, despite a normal oxygen saturation as measured by pulse oximetry. Methemoglobin reduces the oxygen-carrying capacity and at high levels will reduce oxygen saturation reading via pulse oximetry.

   5. A leftward shift of the hemoglobin-oxygen dissociation curve results from hypothermia, decreased 2,3-diphosphoglycerate concentration, alkalosis, hypocarbia, and carbon monoxide poisoning.

2. Treatment of hypoxemia

   1. If the patient is being mechanically ventilated, begin manual ventilation with 100% oxygen to assess pulmonary compliance. Evaluate breath sounds, check the surgical field for mechanical interference with ventilation, examine the endotracheal tube for obstruction or dislodgement, and confirm adequate movement of the chest wall or diaphragm. Elevated peak airway pressures may indicate bronchospasm,

      pneumothorax, an obstructed endotracheal tube, or endobronchial intubation.

   2. The breathing circuit, ventilator, and anesthesia machine should be checked for leaks. If a leak is present, ventilation should be started with high-flow 100% oxygen via an alternative source such as a self-inflating bag until the problem is rectified.

   3. Adequate oxygen delivery to the patient should be confirmed with an inline oxygen analyzer.

   4. Bronchoscopy may be valuable to help rule out obstructive causes, verify endotracheal tube placement, and remove mucus plugging.

   5. Ultrasound or bedside chest x-ray may help evaluate for pneumothorax.

   6. Further treatment is outlined in Chapter 13: Airway Evaluation and Management.