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Arterial blood gases
Author
Arthur C Theodore, MD
Section Editor
Scott Manaker, MD, PhD
Deputy Editor
Kevin C Wilson, MD
Disclosures
Last literature review version 19.3: Fri Sep 30 00:00:00 GMT 2011 | This topic last updated: Mon Jul 12 00:00:00 GMT 2010 (More)
INTRODUCTION — An arterial blood gas (ABG) is a test that measures the arterial oxygen tension (PaO2), carbon dioxide tension(PaCO2), and acidity (pH). In addition, arterial oxyhemoglobin saturation (SaO2) can be determined. Such information is vital when caring for patients with critical illness or respiratory disease. As a result, the ABG is one of the most common tests performed on patients in intensive care units (ICUs).
The sites, technique, and complications of arterial sampling are discussed here. Transport andanalysis of the arterial blood are also reviewed. Finally, pulse oximetry plus transcutaneous carbon dioxide measurement (an alternative method of obtaining similar information) is discussed.
ARTERIAL SAMPLING — Arterial blood is required for an ABG. It can be obtained by percutaneous needle puncture or from an indwelling arterial catheter.
Needle puncture — Percutaneous needle puncture refers tothe withdrawal of arterial blood via a needle stick. It needs to be repeated every time an ABG is performed, since an indwelling catheter is not inserted.
Site selection — The initial step in percutaneous needle puncture is locating a palpable artery. Common sites include the radial, femoral, brachial, dorsalis pedis, or axillary artery. There is no evidence that any site is superior to theothers. However, the radial artery is used most often because it is accessible, easily positioned, and more comfortable for the patient than the alternative sites.
The radial artery is best palpated between the distal radius and the tendon of the flexor carpi radialis when the wrist is extended (figure 1 and figure 2). To get the wrist into this position, the arm should be positioned on an armboardwith the palm facing upward and a large roll of gauze should be placed between the wrist and the armboard in a position that extends the wrist. Taping the forearm and palm to the armboard helps maintain the position.
The brachial artery is best palpated medial to the biceps tendon in the antecubital fossa, when the arm is extended and the palm is facing up (figure 3). The needle should be insertedjust above the elbow crease (figure 4).
The femoral artery is best palpated just below the midpoint of the inguinal ligament, when the lower extremity is extended (figure 5). The needle should be inserted at a 90 degree angle just below the inguinal ligament (figure 6).
The dorsalis pedis artery is best palpated lateral to the extensor hallucis longus tendon (figure 7). It receives collateralflow from the lateral plantar artery through an arch similar to that in the hand, which should be checked prior to percutaneous needle puncture. This is done by occluding the dorsalis pedis artery and compressing the nailbed of the great toe. Color should return to the nailbed rapidly when pressure on the toe is released (figure 8).
The axillary artery is best palpated in the axilla, when the armis abducted and externally rotated (figure 9). There is good collateral flow to the arm through the thyrocervical trunk and subscapular artery; thus, the risk of ischemic complications to the arm is low. The needle should be inserted as high into the axilla as possible (figure 10).
Collateral circulation — Patients undergoing radial or dorsalis pedis artery puncture should have the collateral flowto those vessels evaluated prior to puncture.
The Allen test or modified Allen test can be performed in patients undergoing radial artery puncture. These are bedside tests that demonstrate collateral flow through the superficial palmar arch [1]. Their purpose is to identify patients who have impaired collateral circulation in the hand and, therefore, may be at increased risk of an ischemic...