W4 Arterial Cannulation and Invasive Blood Pressure Measurement
Before Procedure
Indications
• Data source for noninvasive cardiac output monitoring, pulse pressure and stroke volume variation monitoring, and transpulmonary thermodilution
• Pain-free, convenient, and repeated access to arterial blood for assessing pulmonary and cardiovascular function
Anatomy
The Radial and Ulnar Arteries
• The radial artery originates in the antecubital fossa at the level of the neck of the radius as a terminal branch of the brachial artery. The artery runs down the length of the forearm laterally. For the distal part of its course, it is covered only by fascia and skin and lies above the radius, where it is easily palpated. At the level of the wrist, the artery winds laterally around the radius and enters the posterior aspect of the hand. It terminates by dividing into the superficial and deep palmar arches, which are anastomoses with the ulnar artery. The radial artery lies near the superficial branch of the radial nerve in its distal course.
• The ulnar artery is the other terminal branch of the brachial artery, also originating in the antecubital fossa at the level of the radial neck. It is usually larger than the radial artery. The ulnar artery runs medially along the length of the forearm. As opposed to the radial artery, for most of its course the ulnar artery lies deep to the muscles of the forearm, becoming superficial only toward the wrist. The ulnar artery lies close to the ulnar nerve in its distal course.
• When compared with the ulnar artery, the radial artery is superficial for a longer part of its course, is easily palpated above the radius, and is less closely associated with neural structures. It is, however, a smaller artery. The radial artery is cannulated within a few centimeters of the anterior wrist creases, where it lies conveniently over the radius.
• Advantages of radial artery cannulation: huge experience and safety, peripheral position, double blood supply to the dependent territory (by the ulnar artery), and easy compression in the event of bleeding
• Disadvantages: technical difficulties owing to the small size of the vessel or vasoconstriction and inaccurate blood pressure measurements (when compared with the central circulation)
• The Allen test has high interobserver variability and lacks sensitivity and specificity. It is not widely used. Avoid insertion of an arterial catheter into the radial or ulnar artery when the other artery is known to be absent or occluded.
The Axillary Artery
• The axillary artery is a continuation of the subclavian artery beginning at the outer border of the first rib. The artery is surrounded by the cords of the brachial plexus. Its position relative to the other structures of the axilla varies according to the position of the arm. The artery ends at the inferior border of the teres major muscle, where it becomes the brachial artery.
• Disadvantages: arm position (see later) may be difficult for some patients. Catheter tip may be proximal to the origin of the brachiocephalic artery/left common carotid artery, creating a potential source for brain emboli (air bubbles or thrombus).
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