22: Arterial Access: Arterial Puncture (Radial Artery Puncture)

1. To obtain arterial blood for blood gas measurements.

2. When blood is needed and venous or capillary blood samples cannot be obtained. Not preferred.

3. To obtain ammonia levels. Venous blood can be used if it is collected, transported appropriately, and done quickly.

4. To obtain lactate and pyruvate levels. Free-flowing arterial blood; stasis of blood increases lactate.

A 23–25-gauge scalp vein (butterfly) needle or a 23–25-gauge venipuncture needle (safety-engineered self-shielding), 1- or 3-mL syringe, povidone-iodine and alcohol swabs, 4 × 4 gauze pad, gloves, 1:1000 heparin or self-contained blood gas kit, high-intensity fiber optic light for transillumination, or a Doppler ultrasound (optional, can be useful to locate the artery). Smaller needles preferred (25 gauge for preterm infants, 23 gauge for term infants).

1. For a blood gas, most hospitals have kits with 1-mL syringes coated with heparin. If this is not available, draw a small amount of heparin (1:1000) into the blood gas syringe (coat the surfaces and discard excess heparin from the syringe). The small amount of heparin coating the syringe is sufficient to prevent coagulation. Excessive heparin may interfere with laboratory results (see later). If any other laboratory test is to be performed, do not use heparin.

2. The radial artery is the most frequently used site and is described in detail here. One of the advantages of the radial artery site is that the radial nerve does not lie close to the artery so there is no concern of nerve damage. Alternative sites are the posterior tibial (preferred second site) or the dorsalis pedis artery. Femoral arteries should be reserved for emergency situations. Brachial arteries should not be used (unless absolutely necessary) because there is minimal collateral circulation and a risk of median nerve damage. Temporal arteries should not be used because of the high risk of neurologic complications.

3. Check for collateral circulation and patency of the ulnar artery by means of the Modified Allen's test. Elevate the arm and simultaneously occlude the radial and ulnar arteries at the wrist; rub the palm to cause blanching. Release pressure on the ulnar artery. If normal color returns in the palm in <10 seconds, adequate collateral circulation from the ulnar artery is present. If normal color does not return for >15 seconds or does not return at all, the collateral circulation is poor, and it is best not to use the radial artery in this arm. The radial and ulnar arteries in the other arm should then be tested for collateral circulation. Because of concern on the reliability of the modified Allen test, other methods such as the modified Allen test with Doppler ultrasound evaluation of collateral flow are being used (controversial). One study found that the best way to perform the Allen test was using the laser Doppler flowmetry method. Some are combining the use of pulse oximetry with the modified Allen test.

4. Pain. Use of topical local anesthetic agents (EMLA) may diminish pain from arterial puncture. Evidence of distress responses exist even before and after a radial arterial puncture. The literature is conflicting concerning the utility of EMLA for arterial puncture, but there may be some benefit. Oral sucrose, breast milk, and/or pacifier are preferred. Other nonpharmacologic techniques are also recommended.

5. To obtain the sample. Take the patient's hand in your left hand (for a right-handed operator) and slightly extend the wrist. Hyperextension can occlude the vessel. Palpate the radial artery with the index finger of your left hand (Figure 22–1). Transillumination with a high-intensity fiber optic light and marking the puncture site with a fingernail imprint may be helpful (see Chapter 40). A Doppler ultrasound by noting characteristic sounds or real-time 2-dimensional ultrasonography can also help locate the vessel. Real-time ultrasound can result in fewer attempts and less chance of a hematoma as compared with palpation.

6. Clean the puncture site. Clean with a povidone-iodine swab and then with an alcohol swab.

7. Puncture the skin at about a 30-degree angle. Then slowly advance the needle and penetrate the artery with the bevel up until blood appears in the tubing (see Figure 22–1). If the entire artery is perforated (anterior and posterior wall) and no blood is obtained, slowly withdraw the needle until blood is obtained. Because the artery can spasm, you may need to wait for blood return. For a more superficial artery or in a premature or extremely low birthweight infant, puncture the skin at 15–25 degrees with the bevel down. With arterial blood samples, little aspiration is usually needed to fill the syringe. If there is no return of blood, withdraw the needle slowly because the artery may have been punctured through and through. (Best to limit to 2 attempts.)

8. Collect the least amount of blood needed. The volume of blood taken at one time should not exceed 3–5% of the total blood volume (the total blood volume in a neonate is ∽80 mL/kg). As an example, if 4 mL of blood is drawn from an infant weighing 1 kg, this represents 5% of the total blood volume.

9. Withdraw the needle and apply firm, but not occlusive, pressure to the site for ≥5 minutes with a 4 × 4 gauze pad to ensure adequate hemostasis. Shield and dispose the needle in appropriate container. Check fingers for adequate circulation.

10. Before submitting an arterial blood gas sample, expel air bubbles from the sample and tightly cap the syringe. Failure to do this can lead to errors in testing. (See the following discussion of inaccuracy of blood gas results.)

11. Place the syringe in ice, and take it to the laboratory immediately. Note the collection time and the patient's temperature and hemoglobin on the laboratory slip.

12. Inaccurate blood gas results. Excessive heparin in the syringe may result in a falsely low pH and Paco₂. Remove excess heparin before obtaining the blood sample. Air bubbles caused by failure to cap the syringe may falsely elevate the Pao₂ and falsely lower the Paco₂. Crying during arterial puncture can decrease Paco₂, decrease HCO₃, and oxygen saturation. Blood gases by intermittent arterial punctures may not accurately reflect the infant's respiratory status. A sudden decrease in the Paco₂ and Pao₂ can occur during the puncture. Note: Neutrophil counts are lower in samples from arterial blood than venous samples.

1. Bleeding/hematoma. To minimize hematoma risk, use the smallest gauge needle possible and hold pressure for 5 minutes immediately after withdrawing the needle. Hematomas usually resolve spontaneously.

2. Vasospasm, thrombosis, and embolism. These can cause distal ischemia and can be minimized by using the smallest gauge needle possible. With thrombosis, the vessel usually recanalizes over a period of time. Arteriospasm usually resolves spontaneously (see Chapter 79).

3. Infection. Risk is rare and can be minimized by using strict sterile technique. Infection is commonly caused by gram-positive organisms such as Staphylococcus epidermidis, which should be treated with nafcillin or vancomycin and gentamicin (see Chapter 148). Drug sensitivities at the specific hospital should be checked. Osteomyelitis has been reported.

4. Arteriovenous fistula. May occur after multiple arterial punctures and is treated surgically. Because the brachial artery and median cubital vein are anatomically very close, a single puncture can cause a fistula. Diagnosis is by Doppler flow of the brachial vessels.

5. Nerve damage. Median nerve damage has been reported after brachial artery puncture. Posterior tibial and femoral nerve damage have also been reported.

6. Rare complications. Forearm compartment syndrome with brachial artery puncture. Extensor tendon sheath injury from repeated radial artery punctures. Pseudoaneurysm of the brachial artery can occur from trauma to the wall of an artery and may require surgical treatment.