Radial artery: course, radial pulse, snuffbox and Allen test

What it is

One of the two terminal branches of the brachial artery, running on the lateral forearm.

Location

It arises in the cubital fossa, runs under brachioradialis, crosses the anatomical snuffbox and ends by forming the deep palmar arch with the ulnar artery.

Function

It supplies the lateral forearm, thumb and index finger.

Body system

Cardiovascular · paired (left and right)

FMA ID

FMA22734 (Foundational Model of Anatomy)

Key facts

• Its pulse is taken at the wrist.
• At the wrist it lies lateral to the flexor carpi radialis tendon, where the radial pulse is taken against the radius.

Clinical relevance

It is used for pulse measurement, arterial blood gas sampling and coronary angiography access.

The radial artery starts in the lower part of the cubital fossa, at the front of the elbow, where the brachial artery divides into the radial and ulnar arteries. It looks almost like a straight continuation of the brachial artery. (Kenhub: Radial artery.)

Clinicians describe it in three portions: one in the forearm, one at the wrist and one in the palm. Its mean inner diameter is about 2.5 mm, which matters when it is used for catheters. (Tsigkas G et al. Preventing and Managing Radial Artery Occlusion following Transradial Procedures: Strategies and Considerations. J Cardiovasc Dev Dis. 2023.)

In the forearm it runs along the radial (lateral) side of the anterior compartment, under brachioradialis and lateral to the tendon of flexor carpi radialis. Its lower part lies directly on the front of the radius. It then leaves the front of the forearm by curving laterally around the wrist, crosses the floor of the anatomical snuffbox, passes round the scaphoid and trapezium, and enters the palm between the oblique and transverse heads of adductor pollicis. (Radiopaedia: Radial artery.)

In the palm it ends by forming the deep palmar arch, which usually joins the ulnar artery to make a complete loop. The deep arch crosses the palm about 1 cm proximal to the superficial palmar arch and gives three palmar metacarpal arteries. (Radiopaedia: Deep palmar arch.)

Just below its origin the radial artery crosses the tendon of biceps brachii, then passes along the medial side of the neck of the radius and runs down towards the styloid process. In the upper forearm it is overlapped by the fleshy belly of brachioradialis; in the lower forearm only skin and fascia cover it, in the groove between the tendons of brachioradialis and flexor carpi radialis. (Kenhub: Radial artery.)

At the wrist the radial artery is the most lateral of the arteries, on the thumb side. (Kenhub: Radial artery.)

Its territory is the elbow joint, the muscles of the lateral forearm, the radial nerve, the carpal bones and joints, the thumb and the lateral side of the index finger. (Radiopaedia: Radial artery.)

In the forearm the radial artery gives the radial recurrent artery, muscular branches, a palmar carpal branch and a superficial palmar branch. At the wrist it gives a dorsal carpal branch, and in the hand it gives the princeps pollicis artery and the radialis indicis artery before forming the deep palmar arch. (Radiopaedia: Radial artery.)

The radial recurrent artery arises just below the origin and runs upwards to join the radial collateral artery, supplying the elbow joint. The palmar carpal branch arises near the lower border of pronator quadratus and the dorsal carpal branch arises in the snuffbox; they help form the palmar and dorsal carpal arches. (Kenhub: Radial artery.)

The superficial palmar branch completes the lateral end of the superficial palmar arch, which is fed mainly by the ulnar artery. The princeps pollicis artery is the main blood supply of the thumb, and the radialis indicis artery supplies the lateral side of the index finger. (Kenhub: Radial artery.)

The superficial palmar arch is complete, joined by a radial branch, in only about one-third of hands; in the other two-thirds it ends without meeting the radial artery. (Radiopaedia: Superficial palmar arch.)

OpenStax notes that the pulse is most often checked at the radial artery in the wrist or at the common carotid artery in the neck. (OpenStax Anatomy and Physiology 2e, 20.2: Blood Flow, Blood Pressure, and Resistance.)

To find it, place two fingers on the front of the wrist on the thumb side: TeachMeAnatomy gives the landmark as just lateral to the tendon of flexor carpi radialis, the tendon in the middle of the wrist that stands out when you bend the wrist against resistance. Here the artery lies on the radius and can be pressed gently against it. (TeachMeAnatomy: Arterial Supply to the Upper Limb.)

Because the artery is so close to the surface here, the radial pulse is used to judge pulse rate, rhythm and strength. Comparing both wrists can reveal a radio-radial delay, which may point to aortic dissection, and comparing the radial with the femoral pulse can reveal a radio-femoral delay, which may point to coarctation of the aorta. (Kenhub: Radial artery.)

The anatomical snuffbox is the hollow on the back of the wrist at the base of the thumb when the thumb is extended. Its ulnar border is the tendon of extensor pollicis longus, its radial border the tendons of extensor pollicis brevis and abductor pollicis longus, its proximal border the radial styloid process, and its floor the scaphoid and trapezium. (TeachMeAnatomy: The Anatomical Snuffbox.)

The radial artery crosses the floor of the snuffbox, and in some people its pulse can be felt with two fingers in the proximal part of the hollow. The superficial branch of the radial nerve runs in the skin and subcutaneous tissue over it, and the cephalic vein crosses it. (TeachMeAnatomy: The Anatomical Snuffbox.)

The snuffbox is also used as a puncture site for distal radial access, in which the catheter enters the radial artery between the thumb tendons. Rates of radial artery occlusion after this approach are reported between 0.4% and 5%, depending on when they are measured, and the review authors consider it superior to the standard wrist approach in this respect. (Tsigkas G et al. Preventing and Managing Radial Artery Occlusion following Transradial Procedures: Strategies and Considerations. J Cardiovasc Dev Dis. 2023.)

Because the radial and ulnar arteries are joined by the palmar arches, the hand normally has a double blood supply. Hand ischaemia after the radial artery is blocked occurs only in people whose collateral circulation is inadequate, mostly because of anatomical variation. (Tsigkas G et al. Preventing and Managing Radial Artery Occlusion following Transradial Procedures: Strategies and Considerations. J Cardiovasc Dev Dis. 2023.)

The modified Allen test checks this collateral flow at the bedside. The patient clenches the fist for 30 seconds while the examiner presses on both the radial and ulnar arteries at the wrist; the patient opens the now pale hand, the examiner releases the ulnar artery only, and the ulnar artery is judged open if the palm flushes pink again within about 5 seconds. (Di Santo P et al. Photoplethysmography using a smartphone application for assessment of ulnar artery patency: a randomized clinical trial. CMAJ. 2018.)

Kenhub lists two common reasons for the test: before taking an arterial blood gas sample from the radial artery, and before removing the radial artery for coronary artery bypass grafting. (Kenhub: Radial artery.)

The test is quick but imperfect: in a randomized trial of 438 patients it was less accurate than a smartphone plethysmography app, and the authors note that abnormal results have excluded up to 27% of patients from radial procedures. (Di Santo P et al. Photoplethysmography using a smartphone application for assessment of ulnar artery patency: a randomized clinical trial. CMAJ. 2018.)

Coronary angiography and stenting are now often done through the radial artery at the wrist rather than the femoral artery in the groin. Compared with femoral access, radial access has fewer vascular complications, less major bleeding and lower all-cause mortality. (Tsigkas G et al. Preventing and Managing Radial Artery Occlusion following Transradial Procedures: Strategies and Considerations. J Cardiovasc Dev Dis. 2023.)

The commonest complication is radial artery occlusion, a clot that blocks the artery. Its reported incidence ranges from 1% to 30% depending on when it is looked for, but it is usually silent because the ulnar artery keeps the hand supplied; symptomatic occlusion occurs in about 0.2% of patients. (Tsigkas G et al. Preventing and Managing Radial Artery Occlusion following Transradial Procedures: Strategies and Considerations. J Cardiovasc Dev Dis. 2023.)

The radial artery is also a versatile conduit for coronary artery bypass surgery, where it is removed from the forearm and used to carry blood around a blocked coronary artery. (Sophia L et al. Upper limb arterial pattern: clinical correlation and embryological perspective. J Vasc Bras. 2021.)

A high division of the brachial artery in the arm can give rise to a brachioradial artery, a radial artery that starts above the elbow. It matters to anyone passing a catheter up the arm. (Radiopaedia: Radial artery.)

The nearest named structures in the 3D model, measured from the centre of each part:

• Interosseous membrane of forearm · skeletal
• Radial vein · cardiovascular
• Radius · skeletal
• Ulnar artery · cardiovascular
• Palmaris longus · muscular
• Ulnar vein · cardiovascular
• Flexor carpi radialis · muscular
• Anterior interosseous artery · cardiovascular
• Extensor digiti minimi · muscular
• Extensor carpi ulnaris · muscular