Blood gas analysis

Blood gas analysis, also called arterial blood gas (ABG) analysis, is a testthat measures the amounts of oxygen and carbon dioxide in the blood, and theacidity (pH) of the blood.

An ABG analysis evaluates how effectively the lungs are delivering oxygen tothe blood and how efficiently they are eliminating carbon dioxide from it. The test also indicates how well the lungs and kidneys are interacting to maintain normal blood pH (acid-base balance). Blood gas studies are usually done to assess respiratory disease and other conditions that may affect the lungs,and to manage patients receiving oxygen therapy. In addition, the acid-base component of the test provides information on kidney function.

Blood gas analysis is performed on blood from an artery. It measures the partial pressures of oxygen and carbon dioxide in the blood, as well as oxygen content, oxygen saturation, bicarbonate content, and blood pH.

Oxygen in the lungs is carried to the tissues through the body attached to red blood cells. Only a small amount of this oxygen can actually dissolve in arterial blood. How much oxygen dissolves depends on the pressure that the gasexerts on the walls of the arteries. This is called the partial pressure. Thepartial pressure of oxygen tells physicians how much oxygen the lungs are delivering to the blood. Carbon dioxide is released into the blood as a by-product of cell metabolism. The partial carbon dioxide pressure indicates how well the lungs are eliminating this carbon dioxide.

The remainder of oxygen that is not dissolved in the blood combines with hemoglobin, a protein--iron compound found in the red blood cells. The oxygen content measurement in an ABG analysis indicates how much oxygen is combined with the hemoglobin. A related value is the oxygen saturation, which compares the amount of oxygen actually combined with hemoglobin to the total amount of oxygen that the hemoglobin is capable of combining with.

Carbon dioxide dissolves in the blood, primarily forming bicarbonate and smaller amounts of carbonic acid. When present in normal amounts, the ratio of carbonic acid to bicarbonate creates an acid-base balance in the blood, helpingto keep the pH at a level where the body's cellular functions are most efficient. The lungs and kidneys both participate in maintaining the carbonic acid-bicarbonate balance. The lungs control the carbonic acid level and the kidneys regulate the bicarbonate. If either organ is not functioning properly, anacid-base imbalance can result. Determination of bicarbonate and pH levels, aids in diagnosing the cause of abnormal blood gas values.

To perform an ABG analysis, blood is obtained by arterial puncture (usually in the wrist, although it could be in the groin or arm) or from an arterial line already in place. A technician then collects the blood with a small sterile needle attached to a disposable syringe. After the blood is drawn, the sample must be transported to the laboratory as soon as possible for analysis.

When the blood has been drawn, the technician or the patient applies pressureto the puncture site for 10-15 minutes to stop the bleeding, and then placesa dressing over the puncture. Risks are very low when the test is done correctly, but include bleeding or bruising at the site.

Normal blood gas values are as follows:

  • Partial pressure of oxygen (PaO2): 75-100 mm Hg
  • Partial pressure of carbon dioxide (PaCO2): 35-45 mm Hg
  • Oxygen content (O2CT): 15-23%
  • Oxygen saturation (SaO2): 94-100%
  • Bicarbonate (HCO3): 22-26 mEq/liter
  • PH: 7.35-7.45.

Values that differ from those listed above may indicate respiratory, metabolic, or kidney disease. These results also may be abnormal if the patient has experienced trauma that affects breathing (especially head and neck injuries).Disorders such as anemia that affect the oxygen-carrying capacity of blood,can produce an abnormally low oxygen content value.

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