Blood gases

Alternative names
Arterial blood gas analysis; ABG

Definition
Blood gases measure the pH (acidity), oxygen content, and carbon dioxide content of the blood. Usually, blood gases are used to analyze the arterial blood. In rarer cases, venous blood may be used.

How the test is performed

The test is performed by collecting a sample of blood from an artery. Using a small needle, the sample may be collected from the radial artery in the wrist, the femoral artery in the groin, or the brachial artery in the arm.

Before blood is drawn, the circulation to the hand may be tested (if the wrist is the site). After the blood is drawn, pressure must be applied to the puncture site for at least 5 minutes to completely stop the bleeding.

The test must be sent to the laboratory for analysis immediately, or the accuracy of the results cannot be guaranteed.

How to prepare for the test
There is no special preparation. If the person receiving the test is on oxygen, the oxygen concentration must remain constant for 20 minutes preceding the test. If the test is to be taken without oxygen, the oxygen must be turned off for 20 minutes before the sample is taken to ensure accurate test results.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child’s age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child’s age:

     
  • Infant test or procedure preparation (birth to 1 year)  
  • Toddler test or procedure preparation (1 to 3 years)  
  • Preschooler test or procedure preparation (3 to 6 years)  
  • Schoolage test or procedure preparation (6 to 12 years)  
  • Adolescent test or procedure preparation (12 to 18 years)

How the test will feel

A needle is inserted through the skin into the artery. The site can be anesthetized if you desire. There may be brief cramping or throbbing at the puncture site. The needle is withdrawn after the sample is collected.

Pressure will be applied over the site for 5 to 10 minutes to prevent bleeding, and then a bandage will be applied over the puncture site. The site will be observed for signs of bleeding or impairment of the circulation.

Why the test is performed
The test is used to evaluate respiratory diseases and conditions that affect the lungs. It is used to determine the effectiveness of oxygen therapy. The acid-base component of the test also gives information on how well the kidneys are functioning.

Normal Values
Values at sea level:

     
  • Partial pressure of oxygen (PaO2) - 75 to 100 mm Hg  
  • Partial pressure of carbon dioxide (PaCO2) - 35 to 45 mm Hg  
  • A pH of 7.35 to 7.45  
  • Oxygen saturation (SaO2) - 94% to 100%  
  • Bicarbonate - (HCO3) - 22 to 26 mEq/liter

Note: mEq/liter = milliequivalents per liter; mm Hg = millimeters of mercury

At altitudes of 3,000 feet and above, the values for oxygen are lower.

What abnormal results mean
Abnormal results may indicate respiratory, metabolic, or renal diseases. The results may also be abnormal in trauma, particularly with head or neck injuries that may affect breathing.

What the risks are
In general, there is a very low risk when the procedure is done correctly. There may be bleeding or bruising at the puncture site, or delayed bleeding from the site. Circulatory impairment in the area of the puncture can occur, although it is rare.

Special considerations
Notify the health care provider if there is bleeding, bruising, numbness, tingling, or discoloration at the puncture site. Notify your practitioner if you are taking any anticoagulant (blood-thinning medications) or aspirin.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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