Acute Oxygen Therapy
October 14, 2016 | Special HTA Reports
Hypoxemia is an abnormally low oxygen level in the blood. It is usually defined as arterial partial pressure of oxygen (PaO2) <60 mm Hg. Hypoxia refers to low oxygen tension in the tissues. In heathy individuals, PaO2 is between 75 and 100 mm Hg. This is considered normoxia and is affected by posture, age, body build, and physiologic states such as pregnancy. (1) Hypoxemia may be present in critically ill patients with ischemic heart disease, sepsis, or trauma. Hypoxemia may also be present in patients with anemia, in which the blood's oxygen-carrying capacity is compromised.
Hemoglobin contained in red blood cells is responsible for binding oxygen and carrying it through the body. (2,3) About 98.5% of blood oxygen is bound to hemoglobin, with only 1.5% dissolved in blood plasma. Oxygen saturation indicates hemoglobin's ability to bind oxygen and can be measured directly in an arterial blood sample (SO2) or using pulse oximetry (SpO2 for peripheral capillary oxygen saturation). Normal pulse oximeter readings usually range from 95% to 100%; values <90% are considered low. An SO2 <90% indicates a PaO2 <60 mm Hg and a need for supplemental oxygen for the patient. (4) Oxygen saturation is typically at or near 90% in the arteries when PaO2 is between 75 and 100 mm Hg. Oxygenated blood has a PaO2 of 100 mm Hg, but when the blood reaches the tissue capillaries, oxygen unbinds from hemoglobin and diffuses out of the blood into the tissues. The resulting venous blood has a PaO2 of 40 mm Hg, but SO2 is 75%. An increase in blood pH will decrease hemoglobin's binding capacity, and hemoglobin becomes less saturated with oxygen. An increase in the partial pressure of carbon dioxide (PaCO2) has the same effect.
Hyperoxemia is PaO2 above 120 mm Hg and may generate reactive oxygen species that will damage lung tissue. (2) Hyperoxemia and hyperoxia also damage more distant organs, primarily by inducing systemic vasoconstriction and reducing cardiac output. (5) Reduced tissue perfusion then leads to progressive cellular damage and organ injury.
In type 1 respiratory failure, the patient is hypoxemic with a PaO2 <60 mm Hg with a normal or low arterial PaCO2 (see Respiratory Failure). Type 1 is the most common and can be associated with acute diseases of the lung involving fluid filling or collapse of alveolar units. In type 2 respiratory failure, the patient is hypercapnic with a PaCO2 >50 mm Hg. Hypoxemia is also common in these patients. Patients with asthma and COPD...