Arterial oxygen saturation, or SaO2, is a measurement of the oxygen carried in your blood. It quantifies the percentage of hemoglobin—the protein within red blood cells—that is bound with oxygen molecules. Hemoglobin is the primary vehicle for transporting oxygen from the lungs to the body’s tissues. Each hemoglobin protein can bind with up to four oxygen molecules, and the SaO2 value represents how much of this capacity is being used.
How SaO2 is Measured
The most accurate way to measure arterial oxygen saturation is through an arterial blood gas (ABG) test. This test requires drawing blood directly from an artery, most commonly the radial artery in the wrist. The collected sample is then analyzed by a specialized machine that directly measures the amount of oxygen bound to hemoglobin, providing a precise SaO2 value.
SaO2 should be distinguished from SpO2, the value provided by a pulse oximeter—the small clip-on device often placed on a finger or earlobe. SpO2, or peripheral oxygen saturation, is an estimate of arterial oxygen saturation. Pulse oximeters work by shining light through the skin and measuring the different light absorption patterns of oxygenated versus deoxygenated hemoglobin to approximate the saturation level.
While pulse oximetry is a convenient and non-invasive method for continuous monitoring, it is not as precise as an ABG test. The SpO2 reading can be influenced by various factors, leading to a potential discrepancy of 2% to 3% when compared to the SaO2 value. Therefore, in clinical situations where an exact measurement is needed, an ABG test is preferred.
Interpreting SaO2 Levels
For a healthy adult at sea level, a normal SaO2 value is typically between 95% and 100%. This range indicates that nearly all hemoglobin in the arterial blood is successfully saturated with oxygen. Levels can be slightly lower at higher altitudes; for instance, at an altitude of about one mile, a saturation above 92% is still considered normal.
A state of low oxygen in the arterial blood is referred to as hypoxemia. An SaO2 level below 90% is generally considered low and a cause for concern. When saturation drops to this level, it may signal that the body’s organs, particularly the brain and heart, are not receiving enough oxygen to function properly.
The “normal” range can be adjusted based on an individual’s specific health status. For example, a person with a chronic respiratory condition might have a lower baseline SaO2 that is considered acceptable for them. Medical professionals interpret these values in the context of the patient’s overall health.
Conditions That Lower SaO2
Various medical conditions can impair the body’s ability to maintain adequate oxygen saturation. Respiratory issues are a primary cause, as they affect the lungs’ function of transferring oxygen into the blood. Conditions like chronic obstructive pulmonary disease (COPD), a severe asthma attack, or pneumonia can obstruct airflow or damage lung tissue, reducing the efficiency of gas exchange. A pulmonary embolism, a blood clot in the lungs, can also block blood flow and prevent oxygenation.
Cardiovascular problems can also lead to low SaO2 levels. If the heart is not pumping effectively, as is the case in heart failure, it cannot circulate oxygenated blood throughout the body efficiently. Certain congenital heart defects that allow oxygen-poor blood to mix with oxygen-rich blood can also lower the overall saturation level.
Other conditions can impact SaO2 as well. Anemia, a condition with a lack of healthy red blood cells or insufficient hemoglobin, reduces the blood’s total oxygen-carrying capacity. Even with 100% saturation, if there isn’t enough hemoglobin to begin with, the body’s tissues may still not receive adequate oxygen. Sleep apnea, a disorder causing repeated pauses in breathing during sleep, also leads to intermittent drops in oxygen saturation overnight.
Recognizing Symptoms of Low Oxygen
When SaO2 levels fall, the body may exhibit several signs indicating a state of hypoxemia. One of the most common symptoms is shortness of breath, where a person feels as though they cannot get enough air. This is often accompanied by rapid breathing and a fast heart rate as the body attempts to compensate for the lack of oxygen.
As hypoxemia becomes more pronounced, it can affect brain function, leading to symptoms such as confusion, disorientation, or headaches. A person might feel restless or have difficulty concentrating. These neurological signs occur because the brain is highly sensitive to oxygen deprivation.
A severe, late-stage sign of low oxygen is cyanosis. This is a visible bluish discoloration of the skin, lips, and nail beds, which occurs when the amount of deoxygenated hemoglobin in the blood near the skin’s surface increases. The appearance of cyanosis indicates a substantial drop in oxygen saturation and should be treated as a medical emergency.