Chapter 140: Hypoxemia and Hyperoxia

INTRODUCTION

Hypoxemia is defined as low oxygen content in the blood, with a Pao₂ of less than 60 mm Hg or Spo₂ of less than 90%. The main causes of hypoxemia include:

1. V/Q mismatch—The most common etiology for hypoxemia is V/Q mismatch. Dead space is ventilation without perfusion, as seen with pulmonary embolism. Shunt is perfusion without ventilation, as seen with pneumothorax. Hypoxic pulmonary vasoconstriction improves V/Q matching by reducing shunt as poorly oxygenated areas of the lung vasoconstrict, diverting blood to more oxygenated regions.

   Functional residual capacity (FRC) is the volume remaining in the lung after normal exhalation. Closing capacity (CC) is the lung volume at which small airways without cartilaginous support close. If CC exceeds FRC, atelectasis occurs. Atelectasis commonly occurs in the postoperative period during anesthetic recovery as a result of inadequate tidal volumes. Pneumonia and bronchospasm can also cause V/Q mismatch in the perioperative setting.

2. Hypoventilation—Hypoventilation leads to hypoxemia by reducing fresh O₂-rich gas from entering the alveolar space, resulting in the accumulation of CO₂. If hypoventilation is left uncorrected, hypoxemia rapidly develops. Use of respiratory depressants such as narcotics and benzodiazepines during anesthesia predisposes patients to hypoventilation. Residual neuromuscular blockade can decrease tidal volume and minute ventilation, and lead to airway obstruction. Intraoperatively, ventilator failure or disconnect can cause hypoventilation.

3. Low Fio₂—Alveolar oxygen content is dependent on Fio_2, which is tightly controlled perioperatively. Patients may require increased Fio₂ with V/Q mismatch or hypoventilation. Inadequate Fio₂ can occur from failure to recognize increased patient O₂ demand or equipment malfunction. If mechanical failure is suspected, an immediate change to an alternative O₂ source is indicated.

4. Right-to-left shunts—Right-to-left shunting of blood permits deoxygenated venous blood to bypass the lungs and enter systemic circulation. Intracardiac right-to-left shunt lesions include: Tetralogy of Fallot, pulmonary stenosis with atrial-septal defect, transposition of the great vessels, and Eisenmenger syndrome. Other important causes of right-to-left shunting include states of hyperdynamic circulation such as sepsis and liver failure, where transit time through the lungs is reduced.

5. Diffusion impairment—Patients with interstitial lung disease have impaired gas exchange across their pulmonary capillary beds. Increased cardiac output during exercise or times of stress worsens diffusion impairment because blood spends less time at the alveolar:pulmonary capillary interface; thereby, limiting time for gas exchange.

6. Impaired oxygen-carrying capacity—Oxygen is transported to tissues by hemoglobin. Anemia leads to decreased global oxygen carrying capacity. Functional impairment of hemoglobin such as carbon monoxide poisoning, methemoglobinemia, and hemoglobinopathies prevents normal binding and unbinding of oxygen, and can lead to tissue hypoxemia.

7. Impaired oxygen delivery—Tissue hypoxia can result from impaired delivery of oxygen. Low cardiac output and low circulating blood volumes are the most common causes. Pulmonary thromboembolism and air embolism can cause a rapid drop in venous return and cardiac output, impairing O₂ delivery to tissue.