Knowledge of the arterial anatomy is a key factor in the success of arterial puncture and cannulation. It is important to recognize that nerves and veins are located in close proximity to the desired arteries in order to avoid complications. The anatomy and positioning for radial, brachial, femoral, and dorsalis pedis artery access is described below.
The radial artery is the preferred site for arterial puncture and cannulation. One reason is the comparative ease of identifying the anatomical location of this artery. A second reason is the collateral nature of the arterial blood supply to the hand provided by the radial and ulnar arteries. The ulnar artery is not often used due to its smaller size. Terminal branches of these two arteries meet in the palm of the hand to form the deep and superficial palmar arterial arches (Figure 57-1).
Anatomical location of the radial and ulnar arteries. Collateral circulation is provided by the superficial and deep palmar arches.
The radial artery can be found just medial and proximal to the radial styloid process on the ventrolateral wrist (Figure 57-1). Dorsiflexing the wrist approximately 60° can aid in palpating the arterial pulse. Another notable landmark is the flexor carpi radialis tendon that runs immediately medial to the radial artery. The recommended point of needle or catheter insertion is at the proximal flexor crease of the wrist and directly above the radial artery pulse.
An Allen test should be performed to assess the adequacy of the collateral circulation to the hand prior to radial artery puncture or cannulation (Figure 57-2).1,2 Ask the patient to repeatedly close their hand tightly into a fist and open it, to force blood out of the fingers, while manually occluding the radial and ulnar arteries (Figure 57-2A). Continue this process for 1 minute. Ask the patient to open their hand. The fingers should be blanched and pale due to the occlusion of the arterial inflow. Release the finger occluding the ulnar artery. Measure the time it takes for blushing of the palm to occur. It is considered normal if it is <7 seconds, equivocal at 8 to 14 seconds, and abnormal if >14 seconds.1 Repeat the test, but this time release the radial artery compression to confirm arterial flow into the hand (Figure 57-2B). The purpose is to confirm arterial inflow from both the radial and ulnar arteries.
The modified Allen test. A. The distal radial and ulnar arteries are occluded. B. The ulnar artery remains occluded while determining if the radial artery can supply adequate blood flow to the hand.
An alternative method of evaluating the collateral circulation involves the use of a pulse oximeter with a visual pulse waveform display.3 This is useful in the young, unconscious, or uncooperative patient. Place the pulse oximeter sensor on the patient's thumb. Observe the visual display to confirm a waveform is present. Occlude the radial artery and examine the waveform on the monitor. Either an ulnar dominant system or a radial dominant system with adequate collateral circulation is likely if the waveform remains unchanged during radial artery occlusion.3
The performance of an Allen test to confirm adequate collateral circulation to the hand is generally advocated before radial artery puncture or cannulation. There is concern that radial artery occlusion from an intraluminal clot or an external hematoma can result in hand ischemia if the ulnar artery cannot provide adequate collateral blood flow. Some authors have questioned the utility of performing an Allen test.1,2,4,38 The Allen test is subjective, often improperly performed, and has poor sensitivity and specificity to predict complications.39 The relative safety of radial artery cannulation without the Allen test has been demonstrated in a large case series of patients without major peripheral vascular disease.4 Although an abnormal Allen test may not preclude radial artery puncture or cannulation, it may indicate a greater need for caution and alert the Emergency Physician to potential problems after the procedure is performed.2 This can include arterial thrombosis, hand ischemia, and hand necrosis. Thus it is recommended, but not required, to cannulate another site if the Allen test is abnormal.
The brachial artery courses along the medial side of the antecubital fossa just lateral to the median nerve (Figure 57-3). The brachial artery divides at approximately the level of the neck of the radius to become the ulnar and radial arteries. In the antecubital fossa, the brachial artery is located lateral to the medial epicondyle of the humerus and medial to the biceps brachii muscle. The brachial artery is more easily identified when the elbow is fully extended. In order to locate the artery, start by palpating the medial epicondyle of the humerus. Move laterally until the medial edge of the biceps muscle is palpated. The brachial artery pulse should be palpable just medial to the biceps muscle. The arterial pulsation is most easily identified at the level of the proximal flexor crease of the antecubital fossa. The preferred location for puncture or cannulation of the brachial artery is in, or just proximal to, the antecubital fossa and directly above the brachial artery pulse. If the artery is to be cannulated, the arm should remain in extension while the cannula is in place.
Anatomical location of the brachial artery. Note the median nerve running just medial to the artery and the biceps brachii muscle just lateral to the artery.
The bony anatomic landmarks used to identify the femoral artery are the anterior superior iliac spine and the tubercle of the pubic symphysis. The artery lies approximately midway between these two points after it courses under the inguinal ligament to enter the thigh (Figure 57-4). The femoral nerve and vein are found running in parallel and adjacent to the artery. The vein lies just medial to the artery and the nerve just lateral. The femoral artery is larger than other arteries commonly cannulated and lies significantly deeper than the radial or brachial arteries. This makes it necessary to use a longer cannula and, depending on the patient's body habitus, may require a longer needle for a simple arterial puncture to be successful. Extension and slight abduction of the hip maximizes access to the femoral triangle, improves the ability to palpate the artery, and provides a maximal work area for the procedure.
Anatomical location of the femoral artery. Note the proximity to the femoral nerve and vein.
The dorsalis pedis artery is a continuation of the anterior tibial artery on the dorsal surface of the foot. Puncture and cannulation of the dorsalis pedis artery is a good second choice if the radial artery is unsuccessfully cannulated or unavailable. The pulse is often easily palpable between the first and second metatarsal due to its superficial location. Its distal location does not interfere with other resuscitative efforts and is convenient for the patient. The risk of foot ischemia is minimal due to the abundant collateral circulation to the foot and ankle. There are no significant structures to injure adjacent to the dorsalis pedis artery. This artery can be absent in some people or have significant variability in its anatomic location. It may be difficult to identify the pulse in the hypotensive patient.
Alternative Arterial Sites
There are several alternative sites that are occasionally used for arterial puncture and cannulation. These include the superficial temporal artery, the axillary artery, and the ulnar artery. The superficial temporal artery is often used in neonates and young infants in the intensive care unit. Cannulation of this artery is rarely performed in the Emergency Department because of its location and the ability to maintain access.
The axillary artery is a continuation of the subclavian artery after the first rib. The axillary artery crosses the teres major tendon to enter the arm as the brachial artery. The axillary artery can be deep to the skin surface. The advantages of this artery include the ease of locating a the palpable pulse, even in the hypotensive patient. An accurate blood pressure, devoid of the effects of vasoconstriction, can be obtained. Unfortunately, the disadvantages of cannulating the axillary artery are significant. The axilla is poorly accessible. The patient's arm must be abducted, externally rotated, and immobilized during and after the procedure. The course of the artery changes with arm position. The artery is contained within the axillary sheath along with the axillary vein and brachial plexus. There is a significant risk of nerve injury if the needle penetrates the brachial plexus. The risk of an arterial embolus is higher in central arteries when compared to peripheral arteries.
The ulnar artery, along with the radial artery, is one of the terminal branches of the brachial artery. It is superficial at the wrist and the pulse is easily palpable. The ulnar artery is significantly smaller in most people when compared to the radial artery, making the procedure more technically difficult. An ulnar artery may be absent or extremely small in some patients. The ulnar artery lies adjacent to the ulnar nerve. This increases the possibility of nerve injury if the needle penetrates the ulnar nerve. It can be difficult to identify the artery in the patient with anasarca, obesity, peripheral edema, or peripheral vascular disease.