Oxygen saturation is the measurement of how much oxygen is available in the bloodstream. As blood is pumped from the heart into the body, it first goes through the lungs where oxygen molecules bind to red blood cells in order to be carried throughout the rest of the body. The percentage of red blood cells that are fully saturated with oxygen is referred to as arterial oxygen saturation or blood oxygen level. Healthy blood oxygen saturation is between 95 and 100 percent, but patients with lung disease often have a lower percentage unless they use supplementary oxygen.
A pulse oximeter is commonly used to determine oxygen saturation. This is a small device that clips onto the patient's fingertip or ear lobe and shines two beams of light, one red and one infrared, through the skin of the patient. The light beams enable the pulse oximeter to read small changes in the color of the patient's blood caused by the pulse, which in turn provides an immediate estimate of blood oxygen saturation. Pulse oximeters are most accurate when a strong pulse is present.
For a more precise reading of arterial oxygen saturation, an arterial blood gas (ABG) test may be administered. In this test, blood is typically drawn from the radial artery in the wrist, although other arteries may be used as well. This test can be a bit more painful than other blood tests — which draw from a vein rather than from an artery — but the test is quick and usually well-tolerated with minimal risk to the patient. Results from an ABG are available in minutes.
Patients with blood oxygen levels below 90 percent are considered to have hypoxemia, and a blood oxygen level below 80 percent is known as severe hypoxemia. Shortness of breath is the primary symptom of hypoxemia. There are various causes for this condition, including congenital heart disease, low cardiac output, and interstitial lung disease.
Other lung diseases that could cause hypoxemia include pulmonary fibrosis, emphysema, chronic obstructive pulmonary disease (COPD), acute respiratory distress syndrome (ARDS), anemia, airway obstruction, collapsed lung, fluid buildup in the lungs, and sleep apnea. Patients with hypoxemia may be administered oxygen to raise blood oxygen saturation and usually are advised to not smoke, to avoid air pollutants such as secondhand smoke, and to get regular exercise if at all possible. Chronic lung or heart disease will need to be treated under the advice of a specialist in order to maintain optimal health.