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For persons who have metabolic or respiratory problems, an arterial blood gas test, commonly called an ABG test, can provide the physician with necessary information to make treatment decisions. There are three main points of arterial blood gas interpretation: the pathophysiology necessitating the test, the actual test procedure itself, and the results.
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Pathophysiology Requiring ABG Testing
Arterial blood gasses cover two basic types of body processes respiratory and metabolic. For metabolic conditions the ABG tests the acidity of the blood, or pH, and the amount of bicarbonate or buffer that is used to counter blood acidity.
The arterial blood gasses also measure the partial pressure of oxygen in the blood (PaO2) and the partial pressure of carbon dioxide (PaCO2). In addition the oxygen saturation or O2 sat is also measured. O2 sat measures how much of the hemoglobin in the blood red blood cells is carrying the maximal amount of oxygen. The ABG may be used to determine the effectiveness of oxygen therapy.
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Test Prep and Procedure
There is no special preparation for an ABG procedure. For patients on oxygen, a room air test may be ordered. A room air test is an ABG test performed after the oxygen has been removed for 20 minutes.
An ABG is performed by inserting a thin needle into an artery to obtain a blood specimen. Usually the radial artery on the thumb side of the wrist is used, but the femoral artery in the groin or the brachial artery in the upper arm may be used. For neonates the artery in the umbilical cord may be used to obtain the sample. A specimen may also be obtained from a central line that is in place in one of these arteries.
Before the nurse or other specially trained person performs the procedure he/she will perform an Allen test if the radial artery is going to be used. The Allen test is performed by occluding both the radial and ulnar arteries at the same time, and then releasing the ulnar artery and watching for color to return in the hand. This test is performed to prevent circulatory loss to the hand in the event the radial artery would become occluded following the procedure. If the hand remains white, another artery is selected for the test.
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Increased pH of the blood above 7.45 means that the blood has become too alkaline, the cause can come from either metabolic or respiratory disorders. Metabolic conditions that can cause disturbances in the ABG include: vomiting, aldosteronism, and disturbances in fluid and electrolytes such as dehydration or burns. Respiratory causes can be cystic fibrosis, chronic heart failure, anxiety, and pregnancy.
Metabolic conditions which result is decreased pH of the blood below 7.35 causing it to be acidotic include renal impairment, diabetes, lactic acidosis, and diarrhea. Respiratory conditions which can cause acidosis frequently come from some type of breathing difficulty such as asthma or respiratory failure.
While the most common reasons for altered ABG results have been presented, those listed are not all inclusive. Please consult your physician with questions about the implications of your ABG results.
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Pagana KD, Pagana TJ (2005). Mosby’s Manual of Diagnostic and Laboratory Tests, 7th ed. St. Louis: Mosby.