Skip to Main Content




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 Fio2, 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.

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.


About MyAccess

If your institution subscribes to this resource, and you don't have a MyAccess profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.

Subscription Options

AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more

$995 USD
Buy Now

Pay Per View: Timed Access to all of AccessAnesthesiology

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.