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
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 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.
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: a large artery that is easy to locate and puncture. Blood pressure measurements reflect central blood pressure. 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.
• Disadvantages: in obese subjects, adipose tissue and skin folds may create difficulties in the approach to the groin. The skin over the puncture site also can be compromised by chronic inflammatory changes or fungal infections, and the artery itself may be very deep and difficult to locate. The insertion site also may be difficult to keep clean and well dressed. The risk of hemorrhage into the retroperitoneal space (which may initially be undetectable clinically) is unique to this site. The femoral artery is a common site for vascular surgery in the leg, and this represents a strong relative contraindication to arterial cannulation.
Dorsalis Pedis
• The dorsalis pedis artery begins anterior to the ankle as a branch of the anterior tibial artery. The artery runs distally in the foot between the tendons of the extensor digitorum longus and extensor hallucis longus. It terminates as it turns in to the foot toward the sole between the first two metatarsal bones. During its course over the foot, the artery is covered only by fascia and skin and is easily palpable.
Procedure
• Zeroing: for electronic equilibration of the monitor and pressure transduction system, and to compensate for height differential between pressure transducer location and level of measurement
• Damping: the interaction between the arterial waveform and the physical properties of the pressure transduction system (tubing etc.)
• Underdamping leads to overshoot and spuriously high blood pressure; caused by excessive length of pressure tubing, marked vasoconstriction, central pressure measurement.
After Procedure
Outcomes and Evidence
• There is vast experience with it, and the procedure is safe, although complications do occur (see earlier).
• Recent investigations have focused mainly on the infectious risk of arterial catheterization.
• Arterial lines were thought to be of lower infectious risk than central venous pressure (CVP) lines.
• This is probably not true, as a meta-analysis of infectious risk of intravascular catheters showed that arterial catheters had an infectious risk of 1.7 bloodstream infections per 1000 catheter days versus 1.6 per 1000 catheter days for chlorhexidine-impregnated CVPs. Arterial catheters should therefore be treated with the same level of suspicion as CVPs as a possible source of sepsis, particularly if they are in place more than 14 days.
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