- Critical Care Associates
Shea, M.D., F.C.C.P.
Click here for a close look at how an ABG is performed
Arterial Blood Gases or ABGs are done to evaluate oxygenation and acid-base-balance. Numerous factors can be evaluated to determine whether tissue oxygenation is adequate and the excretion of waste products (CO2- carbon dioxide) is occurring properly from the lungs.
Arterial blood samples are usually obtained from either the radial artery (at the wrist) or the brachial artery (bend of the arm). The radial artery is generally the first site of choice to stick because it is easily accessible.
Testing for proper circulation of the arm is done using the Allens Test. This test is done by having the patient make a fist to force blood from the hand; pressure is then applied to both the ulnar and radial arteries (both sides of the inner wrist). When the hand is relaxed, the palm and fingers are blanched (white or pale). Pressure is then removed from the ulnar artery while the radial artery remains compressed. If the ulnar artery is patent, the hand should quickly become flushed within 10 to 15 seconds. If this does not occur, radial artery puncture should be avoided in that wrist. When adequate collateral circulation has been demonstrated, the patient is prepared for puncture of the radial artery.
The patients arm may be positioned using a rolled or folded towel to hyperextend the radial or brachial site. The skin surface should be cleansed thoroughly before the arterial puncture. A local anesthetic of 2% xylocaine can be injected around the artery to achieve surface and subcutaneous (just under the skin) anesthesia, provided the patient gives no history of allergy to local anesthesia. The use of a local anesthetic is not essential in many cases but may relieve apprehensiveness or be helpful in reducing arterial spasm if difficulty in obtaining an arterial sample occurs.
The needle is advanced just slightly under the skin. Once the needle is in the artery the syringe will readily fill. Arterial blood will pulsate whereas blood from a vein will not. Arterial blood is also brighter red (if oxygenation is adequate) than blood from a vein. When the needle is withdrawn, a gauze should be placed over the punctured site and pressure applied for about 5 minutes (longer if the patient has bleeding tendencies.) If any bleeding occurs at the end of this time, pressure should be maintained until no further bleeding occurs.