CHAPTER 20 Midhumeral block
The humeral canal, containing the terminal nerves of the brachial plexus and the brachial artery, lies on the medial aspect of the arm. At this location it is possible to anesthetize the four major nerves of the upper limb separately. The humeral canal is bounded superiorly by the biceps muscle, inferiorly by triceps, laterally by coracobrachialis, and medially by skin and subcutaneous tissue. The needle insertion site is at the junction between the upper one-third and the lower two-thirds of the humerus, in proximity to the brachial artery. A common mistake is to choose a needle insertion site at the midpoint of the humerus. Here the radial nerve is inaccessible because it lies in the radial groove on the posterior aspect of the humerus. At the needle insertion site, the four major nerves of the upper limb have a characteristic location in relation to the brachial artery. The median nerve lies anterior to the artery, the ulnar nerve posteromedially, and the radial nerve posteriorly, adjacent to the humerus. The musculocutaneous nerve lies superior to the artery and under the biceps at this point. The medial cutaneous nerve of the arm lies medial to the artery within the canal.
The main landmarks for the midhumeral block include the junction between the upper one-third and lower two-thirds of the humerus and the brachial artery. This can be approximated as three fingers’ breadth below the anterior axillary fold (Fig. 20.1).
At the humeral canal level, the four main branches of the brachial plexus, the radial, the ulnar, the median, and the musculocutaneous nerves, are anatomically separated from each other. The relation to blood vessels is less variable than at the axillary level. These characteristics favor nerve identification and selective injection under ultrasound guidance. The nerves of the brachial plexus do not appear together on the same ultrasound screen at the level of the humeral canal. Consequently, blockade of these nerves with the classical single point of puncture is technically difficult.
Ultrasound examinations of the brachial plexus through the humeral canal show that ulnar and median nerves are located superficially under the skin. The radial nerve, located beside the humerus, is the most dorsally located nerve of the plexus. The musculocutaneous nerve is situated midway between these two nerves. Images obtained from ultrasound examinations of the brachial plexus through the humeral canal can be illustrated on a graphical synthesis and can be divided into two compartments (Fig. 20.2). A superficial and a dorsal nerve can be found within each compartment. Median and musculocutaneous nerves are located inside the cephalic compartment, whereas the musculocutaneous nerve is located dorsally. Ulnar and radial nerves are located inside the caudal compartment; the radial nerve is located dorsally.
Figure 20.2 Graphical synthesis of the brachial plexus at the level of the humeral canal under ultrasound description. RN: radial nerve; UN: ulnar nerve: MCN: musculocutaneous nerve: MN: median nerve: HA: humeral artery: BV: basilic vein: HB: humerus. The dotted line separates the caudal compartment from the cephalic compartment. The underlined numbers correspond to volunteers. The MN is between 12 and 1 o’clock in 66% of the cases. The UN is situated at the 3 o’clock position in 46% of the cases. The UN and the RN cannot be blocked from a single point of puncture located on the cephalic side of the probe (long line arrows). The 2 points of puncture inside the 2 compartments are mandatory (broken line arrows).1
Nerves in the midhumeral region have mixed echogenicity (honeycomb appearance with a mixture of hypoechoic nerve fascicles and hyperechoic connective tissues). The nerves are round or oval. Move the transducer towards the axilla and distally towards the elbow to appreciate the course of each nerve.
As for all regional anesthetic procedures, after checking that emergency equipment is complete and in working order, intravenous access, ECG, pulse oximetry, and blood pressure monitoring are established. Asepsis is observed.
The patient is placed supine and the arm abducted at 90°. At the junction between the upper and middle thirds of the arm, a line is drawn over the brachial artery. The needle insertion point is infiltrated with local anesthetic using a 25-G needle. A 50-mm 22-G insulated needle connected to a peripheral nerve stimulator is inserted almost tangentially to the skin, between the brachial artery and the palpating finger, in the direction of the axilla, in order to locate the median nerve (Fig. 20.3). The stimulating current is set at 1.0 mA, 2 Hz, and 0.1 ms. The needle is advanced slowly until the appropriate muscle response is obtained. The needle position is adjusted while decreasing the current to 0.35 mA with maintenance of the muscle response. Stimulation of the median nerve will produce contraction of the flexor carpi radialis and flexor digitorum superficialis of the fingers. Incremental injections of local anesthetic (6–8 mL) are made with repeated aspiration.