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| 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. The semi-recumbent position works well for the brachial approach, too, but you'll need to hyperextend the patient's arm and place a small pillow under the elbow to maintain the position. If you're using a femoral artery, place the patient in a supine position, keeping the leg to be accessed as straight as possible. Now you're ready to do the arterial stick Don nonsterile gloves and goggles and begin cleansing the site. Using a circular motion, start in the center and spiral outward for 30 seconds. Allow the area to dry completely. To reduce pain, you can anesthetize the site, as ordered: Using a 25-gauge needle and a 1 ml syringe, draw up 0.5 ml of 1% lidocaine and inject about 0.2 – 0.3 ml intradermally. Next, take off the nonsterile gloves, wash your hands, and don sterile gloves. With your non-dominant hand, feel for the pulse. Take a few seconds to note the precise point where the pulse feels the strongest and center your index finger over that point. Lightly stabilize the artery with your index and middle fingers while continuing to concentrate on the pulse point: The tip of the needle should enter the skin right in front of your index finger. Hold the ABG syringe with the needle bevel up at about a 30 – 60 degree angle for a radial or brachial puncture or a 60 – 90 degree angle for a femoral stick. Then, in one smooth motion, slowly enter the skin and arterial wall. Once the needle has entered the artery, you should see a flashback of blood pulsate into the syringe. (Blood gas syringes fill by themselves, stopping at about 2 ml.) If you don't see blood pulsing into the syringe, you may have missed the artery—or gone through it. In that case, you'll need to slowly withdraw the needle until blood starts filling the syringe. If you still don't get a blood return, withdraw the needle to skin level, re-angle the syringe toward the artery, and try again. If there's still no return, withdraw the needle completely and start over at a new site. Excessive probing could injure both the artery and any nearby nerves. Once the syringe has filled, hold it steady to prevent the aspiration of air and withdraw the needle. Immediately place a gauze pad over the site and firmly apply pressure for at least five minutes—or for 30 minutes or more if you used a femoral artery. With your other hand, hold the sample upright and check the syringe for air bubbles. Slowly eject any that you find onto a gauze pad. Then, immediately seal the needle or tip of the syringe with a rubber stopper to prevent the influx of air. Gently roll the syringe between your fingers to mix the blood with the heparin. Be sure that the sample has the correct name, date, time, and puncture site on the label and the requisition. Indicate, too, whether your patient was breathing room air or on supplemental oxygen. If he's receiving respiratory therapy, jot down the type, including the amount of oxygen being given. Plunge the syringe into the ice and send it off to the lab without delay. For best results, ABG samples should be analyzed within 10 minutes of collection. Apply a pressure dressing to the site and leave it in place for 30 minutes, or until hemostasis is established. Don't ask a patient on anticoagulant therapy to take over this task, since inadequate pressure could lead to hemorrhage and hematoma formation. If you need help, ask a colleague to assist as you prepare the specimen for transfer to the lab. It's not over yet: A look at after-care Following an arterial blood draw, you'll need to monitor the site and extremity for any sign of circulatory problems or nerve damage. Check frequently for swelling and monitor vital signs, including the assessment of distal pulses. Note the color and temperature of the extremity distal to the puncture site. Ask the patient if he's experiencing any pain, numbness or tingling. Immediately report any problems to the physician. And document everything, including the results of the Allen's test, if applicable. Obtaining a blood sample for ABG analysis by percutaneous arterial puncture is a relatively safe procedure. But it's not without risk. Your ability to perform an arterial blood draw safely, confidently, and rapidly, when necessary, could be a true lifesaver. SOURCES 1. Bucher, L. (2005). Arterial puncture (Procedure). In D. J. Lynn-McHale Wiegand, & K. K. Carlson (Eds.). AACN procedure manual for critical care (5th ed.), (pp. 630 – 637). Philadelphia: Elsevier Saunders. 2. Breuninger, C., Follin, S., et al. (Eds.). (2001). Handbook of nursing procedures (pp. 33 – 38). Springhouse, PA: Springhouse.
How to test collateral circulation Before drawing blood from a radial artery, it's necessary to assess the ulnar artery's ability to maintain circulation of the hand during and after the procedure. This assessment is known as a modified Allen's test. To perform this test, have your patient extend his hand to you, resting his arm on the bedside table with a rolled towel supporting his wrist, if necessary. Have him open and close his fist a few times. Then, while his fist is clenched, simultaneously occlude both the radial and ulnar arteries. While the arteries are occluded, ask the patient to slowly unclench his fist and relax his hand. The palm should be pale from lack of arterial blood flow. Then, release the pressure over the ulnar artery. The palm should turn pink, indicating that arterial circulation has resumed. If that doesn't happen, you'll need to select another site. |  
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