Arterial cannulation and invasive blood pressure measurement

Peripheral artery cannulation is one of the most commonly performed invasive procedures in the intensive care unit (ICU), and the resulting arterial line is an integral part of intensive care patient management. Arterial line usage varies according to unit type, patient age, and severity of illness, ranging from approximately 22% of medical ICU patients to over 50% of surgical ICU patients. There are three main indications for arterial line insertion: (1) to allow continuous beat-to-beat monitoring of blood pressure; (2) to provide pain-free, convenient, and repeated access to arterial blood for the assessment of pulmonary and cardiovascular function (including measures of pulse pressure variation and semi-invasive cardiac output measurements using transpulmonary thermodilution); and (3) to provide a source of blood for blood tests as required without the need for repeated venipuncture, including continuous measures of blood chemistry (such as continuous glucose measurement). A review of the relevant anatomy, equipment, and techniques for arterial line placement is provided in this chapter along with some of the more common complications.

Sites of insertion

An arterial line can be inserted into almost any palpable peripheral artery. The most common sites in clinical practice are the radial artery (employed in up to 78% of ICU patients), the femoral artery (employed in up to 45%), and the dorsalis pedis artery. Axillary and ulnar artery cannulations are performed somewhat more rarely; brachial and temporal artery cannulation is not recommended. Cannulation of the carotid arteries is absolutely contraindicated for obvious reasons. Each arterial site has advantages and disadvantages.

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 anastomosed 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

Advantages of radial artery cannulation include 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

Disadvantages include technical difficulties because of the small size of the vessel or vasoconstriction (the radial artery pulse may not be palpable when blood pressure is less than 80 mm Hg) and inaccurate blood pressure measurements (when compared with the central circulation).

The modified Allen test has been proposed as a screening tool before radial artery cannulation to ensure the presence of adequate distal collateral circulation. The Allen test has, however, been found to have high interobserver variability and to lack sensitivity and specificity. It is not widely used. Radial artery catheterization for coronary angiography has been compared in patients with normal and abnormal Allen tests with no significant adverse events occurring in patients with an abnormal Allen test. It might be prudent, however, to avoid insertion of an arterial catheter into the radial or ulnar artery when the other artery is known to be absent or occluded.

Positioning for cannulation

The forearm should be supine and the wrist slightly extended and supported ( ).

The axillary and brachial arteries

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. The brachial artery runs down the upper arm to the elbow. Initially, it is medial to the humerus, but distally it spirals anteriorly to end as the radial and ulnar arteries approximately 1 cm distal to the elbow. The brachial artery lies near the ulnar and median nerves in its proximal course and near the median nerve in its distal course.

Advantages

The axillary artery is a large artery, and pressure measurements reflect the central circulation.

Disadvantages

The arm position required for axillary artery cannulation may be contraindicated or difficult for some patients. Care should be taken if a long catheter is used, because its tip might be proximal to the origin of the brachiocephalic artery/left common carotid artery. In this case embolic material from the line (i.e., air bubbles or thrombus) could be introduced into the brain. The risk of line infection may also be higher relative to other sites.

Positioning for cannulation

For axillary artery cannulation, the arm should be bent at the elbow and raised above the head (abducted and flexed to 90 degrees). The pulse can then be palpated in the axilla.

The brachial artery is punctured where it is palpable medially on the anterior aspect of the elbow. Generally, cannulation of the brachial artery is not recommended, because it is associated with specific and potentially severe complications (see later). Despite this, large case series with low complications rates have been published.

The femoral artery

The femoral artery originates as a continuation of the external iliac artery at the level of the inguinal ligament. At the level of the inguinal ligament, it lies midway between the anterior superior iliac spine and the symphysis pubis. Distal to the inguinal ligament, the artery lies medial to the femoral nerve and lateral to the femoral vein and is superficial, being covered only by fascia, fat, and skin. The femoral artery runs down the thigh and terminates as the popliteal artery in the knee.

Advantages

The femoral artery is a large artery that is easier to locate and puncture than the radial artery. Blood pressure measurements reflect central blood pressure, and the femoral artery is palpable at a lower blood pressure than the radial artery. The femoral arterial line has a lower rate of catheter malfunction and greater longevity (compared with the radial artery).

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