Hypoxemia and Hypoxia

Although the terms hypoxia and hypoxemia are often used interchangeably, they are not synonymous. Hypoxemia is defined as a condition where arterial oxygen tension (Pao₂) is below normal (normal Pao₂ = 80–100mmHg). Hypoxia is defined as the failure of oxygenation at the tissue level. It is not measured directly by a laboratory value (though an increased arterial lactate level usually accompanies tissue hypoxia). Hypoxia and hypoxemia may or may not occur together. Generally, the presence of hypoxemia suggests hypoxia. However, hypoxia may not be present in patients with hypoxemia if the patient compensates for a low Pao₂ by increasing oxygen delivery. This is typically achieved by increasing cardiac output or decreasing tissue oxygen consumption. Conversely, patients who are not hypoxemic may be hypoxic if oxygen delivery to tissues is impaired or if tissues are unable to use oxygen effectively. Nevertheless, hypoxemia is by far the most common cause of tissue hypoxia.