In some ways, drawing an arterial blood gas sample is easier than drawing venous blood: Unlike veins, arteries pulsate, which
makes them easier to find—and they don't roll like veins tend to do. To collect an ABG sample, you simply feel for a pulse
and sink the tip of the needle into the underlying vessel.
But because you're often called upon to perform an arterial puncture in an emergency, the task can be daunting. This review
will help you draw ABGs safely and confidently, no matter what the circumstance.
ABG analysis: When and why
Acute pulmonary edema, an exacerbation of COPD, and other medical emergencies hamper the body's ability to take in oxygen
and eliminate carbon dioxide. All too often, the patient's survival depends upon rapid intervention. But ABG results are likely
to be needed first, both to assess the extent of the problem and to guide treatment. Non-emergencies, such as weaning from
a ventilator or administering anesthesia, require ABG analysis, as well. The test measures the pH of arterial blood, partial pressure of both oxygen (PaO2) and carbon dioxide (PaCO2), oxygen saturation (SaO2), and bicarbonate (HCO3) levels. Since these findings guide interventions, accuracy is essential.
Respiratory therapy tops the list of factors that can interfere with ABG results. If your patient is given supplemental oxygen,
for example, you'll need to wait at least 15 minutes before drawing the sample to get an accurate reading. Wait at least 20
minutes after a nebulizer treatment or suctioning.
Temperature, respiratory rate, and level of anxiety can alter the results, as well. So too, can heparin, which is needed to
prevent the sample from clotting. Too much heparin in the syringe can decrease the pH. Similarly, air bubbles that haven't
been expelled from the syringe will falsely elevate the PaO2.
Select the site with care
Site selection is crucial. While you can draw an ABG sample from a radial, brachial, or femoral artery, the femoral artery
is linked to higher rates of hematoma and infection and should be used only as a last resort. What's more, the pulse in both
the radial and brachial arteries is easier to palpate and access.
For most patients, your first choice will be the radial artery of the non-dominant wrist (You'll need to perform a modified
Allen's test, described in the box below titled, "How to test collateral circulation," before proceeding with a radial stick.).
Radial sites are contraindicated, however, in a patient who has a fistula or shunt for dialysis in place or has had a portion
of the radial artery used in a coronary artery bypass graft.
Avoid brachial sites in obese patients because of the difficulty in attaining hemostasis. They're contraindicated, too, in
patients with sclerotic vessels and anyone who's had a cardiac catheterization via the brachial route.
Once you've settled on a site, you can prepare for the blood draw. First, fill a clean plastic bag, paper cup, or emesis basin
with ice. You will use it to transport the sample to the lab after it's drawn. Then gather other supplies: sterile and nonsterile
gloves, goggles, and an ABG kit containing a special pre-heparinized glass or plastic syringe.
Inform the patient that drawing blood from an artery may cause a burning sensation. Enlist his cooperation in remaining still
despite the discomfort until the procedure is completed.
Next, help the patient into a position that's comfortable for him and gives you the best access. For a blood draw from a radial
site, a semi-recumbent position with the bed elevated to working height is ideal.
Turn the patient's palm face up and mildly hyperextend his wrist, using a small rolled towel under the joint to maintain the
position. Doing so moves the radial artery closer to the skin's surface, making it easier to find and palpate the pulse.
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