Understanding Oxygen Saturation - SaO2 and SpO2

What are O2 Sats and Why are They Important?

pulse monitor on finger
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Definition: Oxygen Saturation

Oxygen saturation refers to the extent to which hemoglobin is saturated with oxygen. Hemoglobin is an element in the blood that binds with oxygen to carry it through the bloodstream to the organs, tissues and cells of the body.  Normal oxygen saturation is usually between 96% and 98%.  You may hear this term referred to as O sats, or simply, "sats."

Oxygen Saturation in Relation to Hemoglobin in Red Blood Cells

Each of our red blood cells contains 4 molecules of hemoglobin.

  Iron, which is present in hemoglobin, is what oxygen binds to after diffusing from the alveoli in the lungs and into the capillaries of the lungs.  Most of the time the hemoglobin is fully saturated.


Measuring Oxygen Saturation

Oxygen saturation is most commonly measured by either:

  • Arterial blood gasses - The value obtained from arterial blood gasses or ABG's(SaO2) - describes the oxygen saturation of arterial blood, and is obtained by drawing blood from an artery such as the radial artery in the wrist or the femoral artery in the groin.
  • Pulse oximetry - The value of oxygen concentration - SpO2 - with pulse oximetry refers to the oxygen saturation levels in peripheral capillary blood, and often closely reflects the levels that would be found in arterial blood.  Pulse oximetry has the advantage of being a non-invasive test which can be measured with a probe attached to a finger or earlobe or other regions of the body, which reads the wavelengths of light reflected from the blood.  Not only are pulse oximeters a standard now for monitoring people in the hospital, but people can even follow their own saturation via the use of wearables.

    Oxygen Saturation Variables

    Oxygen saturation is dependent upon:

    • 1. Oxygen availability.
    • 2. Gas exchange in the lungs - The ability of oxygen to reach the alveoli, and once in the alveoli, to diffuse through the walls of the alveoli and capillary to reach the red blood cells.
    • 3. The concentration of hemoglobin in red blood cells.
    • 4. The affinity of hemoglobin for oxygen, in other, words how strongly hemoglobin attracts oxygen.

    Decreased Oxygen Saturation

    A drop in oxygen saturation levels is referred to desaturation, or hypoxemia, and can be caused by any changes or damage in the variables above.

    • A change in oxygen availability can be caused by a decreased concentration of oxygen in inspired air such as at higher altitudes and when flying in an airplane.
    • Problems with gas exchange causing a decrease in oxygen saturation can include anything which reduces the ability of oxygen to travel from the outside air down into the alveoli of our bodies, or during the process of oxygen being transferred from the alveoli into the capillaries of the blood.
    • Decreased oxygen saturation may result from a lower concentration of hemoglobin, such as in iron deficiency anemia.
    • A decreased affinity of hemoglobin for oxygen may occur when there is something else present which binds more strongly to hemoglobin than does oxygen, such as in carbon monoxide poisoning in which the compound carboxyhemoglobin is formed.

      Hypoxemia - Decreased Oxygen Concentration in the Blood

      The terms hypoxemia and hypoxia are often used interchangeably but mean quite different things.  Hypoxemia is the term which defines a decreased oxygen concentration in the blood.  Hypoxia, in turn, describes the consequences - tissue damage - which occurs due to hypoxemia (a decreased oxygen concentration in the blood.)

      Hypoxia - Consequences of Hypoxemia

      Hypoxia, again, refers to a deficiency in the amount of oxygen which reaches the tissues.

      Hypoxia is often caused by hypoxemia, but may also occur when:

      • There is anemia because there are too few red blood cells so that fully oxygenated blood doesn't bring enough oxygen to tissues.
      • There is inadequate blood flow so that fully oxygenated blood does not reach the tissues.
      • The tissues are unable to use the fully oxygenated blood that is delivered.
      • The tissues require even more oxygenated blood than can be delivered, such as in severe infections.

      When cells do not get enough oxygen, they may adapt if the deficiency if small, otherwise this results in cell damage followed by cell death.

      Treatment of Hypoxemia and Hypoxia Due to Hypoxemia

      When oxygen saturation drops below a certain level, supplemental oxygen therapy is usually needed, and sometimes emergently.

      It is also important to then determine the cause of a low oxygen saturation.  Most of the time the problem causing hypoxemia (a low O2 sat) occurs in gas exchange in the lungs (number 2 above in the list of what oxygen saturation depends upon.)  Treating the underlying cause is then the primary goal of treatment.

      Also Known As: O2 saturation, oxygen sats, saturation levels, O2 sats, sats


      Collins, J., Rudenski, A., Gibson, J., Howard, L., and R. O’Driscoll. Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve. Breathe. 2005. DOI: 10.1183/20734735.001415.

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