Arterial Blood Gas Sampling Periprocedural Care

Healthcare personnel should explain the arterial blood gas (ABG) sampling procedure to the patient, with particular attention to the associated risks and benefits. However, this may not be possible in certain clinical scenarios, such as a critically ill patient with rapid decompensation or a patient with an altered level of consciousness (eg, from encephalopathy or advanced dementia). Written consent is not necessary.

Patients should be asked not to manipulate the area involved in the procedure and should be instructed to alert healthcare personnel if new symptoms develop, such as skin color changes, persistent or worsening pain, active bleeding, impaired movement, or sensation of the limb.

Planning for the procedure focuses on the choice of a puncture site and accurate delineation of the vascular anatomy. If radial artery sampling is to be performed, the adequacy of the ulnar collateral circulation must be confirmed.

Selection of puncture site

Puncture of the radial artery is usually preferred because of the accessibility of the vessel, the presence of collateral circulation, and the artery's superficial course proximal to the wrist, which makes it easier for the clinician to identify the vascular structure and hold local pressure after the procedure is finished.

If radial artery sampling is not feasible, femoral artery puncture is a possible alternative. When femoral artery puncture is being considered, the potential risk of infection at the entry site and the artery's proximity to the femoral vein and nerve must be taken into account. The deeper the vascular structure, the higher the risk of damage to adjacent structures.

Femoral artery puncture necessitates prolonged monitoring and therefore is recommended only in the inpatient setting. Some physicians recommend that femoral artery puncture be avoided whenever possible. This consideration may play a bigger role for patients who will be undergoing an intervention that involves femoral access (eg, cardiac catheterization or intra-aortic balloon pump placement) in the near future.

The brachial artery runs deeper in the arm than the radial artery does. Consequently, its structures are typically harder to identify, and achieving hemostasis when necessary is more difficult. Furthermore, the brachial artery is a relatively small-caliber vessel and does not have extensive collateral circulation. For these reasons, the brachial artery is the least preferred site for puncture.

Repeated arterial blood sampling at the same site increases the risk of hematoma, laceration of the artery, and scarring. It also increases the chances of inadvertent venous blood sampling. If recurrent sampling is required, healthcare personnel should alternate puncture sites. If more frequent sampling is necessary, the healthcare provider should consider placing an indwelling arterial catheter through arterial cannulation.

Confirmation of vascular anatomy

ABG sampling can be difficult in patients with feeble pulses or distorted anatomic landmarks or in situations where sampling of a deep vascular structure (eg, the femoral or brachial artery) is required. In these scenarios, the use of ultrasonography (US) to guide ABG sampling should be considered, ^[7] especially when sampling by the standard approach has been unsuccessful or is not feasible.

US is a noninvasive technique with an excellent safety profile. It is an important option in cases where vascular sampling proves difficult. US enables more accurate recognition, delineation, and targeting of the chosen vascular structure. It minimizes the risk of vascular laceration and damage to surrounding structures.

Although US guidance is a safe and effective tool for patients with weak pulses, a better option for patients in profound shock or in the midst of cardiopulmonary resuscitation is to obtain arterial blood from the femoral artery using bony landmarks alone. When the tip of the fifth finger is placed on the symphysis pubis and the tip of the thumb on the anterior superior iliac spine, the femoral artery always lies beneath the middle finger.

Assessment of collateral circulation (modified Allen test)

If puncture of the radial artery is planned, a modified Allen test should be performed beforehand when feasible to assess the collateral circulation. Although the anatomy of the radial artery in the forearm and the hand is variable, most patients have adequate collateral flow should radial artery thrombosis occur. ^[1] The modified Allen test is performed as follows. ^[1]

Firm occlusive pressure is held on both the radial artery and the ulnar artery (see the first image below). The patient is asked to clench the fist several times until the palmar skin is blanched (see the second image below), then to unclench the fist. Overextension of the hand or wide spreading of the fingers should be avoided, because it may cause false-normal results. ^[8] The pressure on the ulnar artery is released while occlusion of the radial artery is maintained (see the third image below). The time required for palmar capillary refill is noted.

Modified Allen test: digital occlusion of radial and ulnar artery.

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Modified Allen test: clenching of hand.

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Modified Allen test: ulnar artery occlusion released.

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The test is then repeated, but this time the radial artery is released while the ulnar artery remains compressed (inverse modified Allen test; see the image below).

Modified Allen test: radial artery occlusion released.

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The modified Allen test has been the method most frequently used for clinical assessment of the adequacy of ulnar artery collateral circulation and the patency of the palmar arches of the hand. However, there is some controversial evidence suggesting that it can predict ischemic complications in the setting of radial artery occlusion. ^[1]

Given the low positive predictive value of the modified Allen test, the examiner should consider further testing to assess patency of circulation, such as finger pulse plethysmography, Doppler flow measurements, and measurement of the arterial systolic pressure of the thumb. ^[8]

Whether the modified Allen test is clinically reliable as a screening test for adequate collateral circulation of the hand is controversial. A wide range of values for hand reperfusion have been noted, and normal values are not consistent (ranging from 3 to 15 seconds) ^{[1, 8, 9]} ; furthermore, there has been conflicting evidence regarding the validity of the modified Allen test as a standard of care. ^{[9, 10]}

The materials needed for ABG sampling (see the first image below) include the following:

• Gloves - Nonsterile gloves may be used, but care must be taken not to touch the puncture site after cleaning the area
• Syringe for sampling - A standard syringe with a 25-gauge needle and a 3-mL capacity (a higher-capacity syringe may be difficult to maneuver, and smaller needle sizes may increase the risk of traumatic hemolysis, decreasing the accuracy of hemoglobin and potassium measurements); a 23-gauge needle may work as well ^[11]
• Lithium heparin - 1-2 mL lithium heparin (1000 U/mL) should be aspirated into the syringe through the needle and then pushed out; the plunger should be left depressed to allow the arterial blood flow to fill up the syringe
• ABG syringe (alternative) - Some ABG kits contain a prefilled heparinized syringe along with a protective needle sleeve and a syringe cap (see the second image below); the sleeve, while still attached to the syringe, locks the needle within itself to prevent direct contact between operator and needle; some syringe models have a vented plunger that allows the operator to preset a specific amount of blood to be withdrawn, and with these models, the plunger is placed midway through the syringe and is not pulled back while the puncture is performed; before the procedure, the prefilled heparin is expelled, and the vented plunger is then repositioned at the 2-mL mark
• Antiseptic skin solution - Chlorhexidine and povidone-iodine are solutions commonly used
• Syringe cap - Usually included in the ABG syringe kit
• Sterile gauze, 2 × 2 in. (5 × 5 cm)
• Adhesive bandage
• Bag with ice
• Sharp object container
• Lidocaine HCl 1% without epinephrine (optional)
• 25-gauge needle with syringe for local anesthetic (optional)

Arterial blood gas sampling equipment.

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Arterial blood gas syringe kit.

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Anesthesia

Local anesthesia with a subcutaneous injection of lidocaine HCl 1% without epinephrine may be used. Local anesthesia is not frequently employed, however, because the administration of the anesthetic is as painful as the procedure itself. ^[12]

If local anesthesia is employed, 0.5-1 mL of the anesthetic is injected so as to create a small dermal papule at the site of puncture; using larger amounts or injecting the anesthetic into deeper planes may distort the anatomy and hinder identification of the vessel. After the skin is punctured but just before the anesthetic is injected, the clinician should pull back the plunger to confirm that the needle is not inside a blood vessel; intravascular placement will be signaled by blood filling up the anesthetic syringe.

A randomized controlled trial was conducted to compare the effectiveness of three anesthetic methods (anesthetic cream, cryoanalgesia, and subcutaneous mepivacaine) with standard practice (no anesthesia) in reducing pain caused by radial artery puncture in patients requiring an ABG test in the emergency department (ED). ^[13] When the control group (no anesthesia) was compared with the three intervention groups, mepivacaine and cryoanalgesia were associated with significantly lower pain scores; anesthetic cream did not produce a statistically significant median difference in pain.

A randomized controlled study by Gur et al found that topical application of 10% lidocaine provided effective analgesia for radial artery blood gas sampling. ^[14]

Positioning

For radial artery blood sampling, the patient should be in the supine position, with the arm lying at his or her on a hard surface. The forearm should be supinated and the wrist dorsiflexed at 40º. A gauze roll may be placed under the wrist to make the patient more comfortable and to bring the radial artery to a more superficial plane. Overextension of the wrist is discouraged, because interposition of flexor tendons may make the pulse difficult to detect.

For femoral artery blood sampling, the patient is supine on a stretcher, and the patient's leg is placed in neutral anatomic position.

For brachial artery blood sampling, the arm is placed on a firm surface with the shoulder slightly abducted. The elbow is extended, with the forearm in full supination.

After the blood sampling procedure, healthcare personnel should monitor the patient for early and late signs and symptoms of potential complications. Active profuse bleeding at the puncture site might raise suspicion of vessel laceration. Femoral artery bleeding carries an increased risk of circulatory compromise because of the large caliber and deep location of the vessel, which allow larger amounts of blood to accumulate without initially giving rise to clinical findings.

A rapidly expanding hematoma may compromise regional circulation and increase the risk of compartment syndrome, especially in the forearm. This manifests as pain, paresthesias, pallor, and absence of pulses. Paresis and persistent pain may indicate a nerve lesion. Limb skin color changes, absent pulses, and distal coldness may be seen in ischemic injury from artery occlusion caused by thrombus formation or vasospasm. Infection at the puncture site should be considered in the presence of regional erythema and fever.