Pharmacology
The commonly used local anesthetics, such as lidocaine and bupivacaine, are amides. Ester class anesthetics, such as procaine, cocaine, tetracaine, and benzocaine, have limited clinical use as a result of the increased incidence of allergic reactions and toxicity. Lidocaine is metabolized by the liver and excreted by the kidney, and therefore needs to be adjusted in patients with impaired hepatic or renal function. It is given at a dose of 3–5 mg/kg, not to exceed a total dose of 300 mg; or 7 mg/kg when combined with epinephrine, not to exceed a total dose of 500 mg. Onset is 2–5 minutes, and action lasts 90 minutes to 3 hours. Bupivacaine is contraindicated in pregnant women or in patients with compromised respiration. The dosage of bupivacaine alone is 1–2 mg/kg, and up to 3 mg/kg with epinephrine, not to exceed a total dose of 400 mg. Its action lasts from 4 to 18 hours. The addition of epinephrine allows for a greater amount of anesthetics to be delivered locally. However, use of this adjuvant with local anesthetics should be avoided in areas supplied by end arteries.
Injection of local anesthesia for nerve blocks commonly causes discomfort at the site of injection. Techniques to decrease the discomfort include the use of a small needle, slow injection rate, warming the anesthetics to body temperature, buffering the medication in alkaline solution, and using topical anesthetics or skin infiltration with local anesthetics before nerve block. Serious toxicity, such as respiratory failure, seizure, or cardiac arrest, can occur with allergic reaction or excessive injection, especially if given intravascularly. To prevent serious complications, infiltrative local anesthetics should be administered slowly in a monitored environment, with frequent but gentle aspiration to prevent intravascular injection. A “crash cart” for cardiopulmonary resuscitation and rapid access to lipid emulsion must be readily available.
Intercostal Nerve Block
Anatomic Relationships
The intercostal nerves, branches of dorsal spinal nerves, run segmentally under the respective ribs external to the endothoracic fascia. After passing the angle of the rib, the nerve continues below the artery and vein in the costal groove between the internal and external intercostal muscles.
Procedure
Place the patient in a lateral position with the ipsilateral arm extended over the head. The midaxillary line is the safest approach to the intercostal space. Palpate the lower margin of the rib just beyond the angle. Insert a fine needle vertically until it touches the lower half of the rib ( Fig. 5.1 ). With the free hand, pull the skin with the embedded needle caudally until the needle point slips off the rib. Advance it 3 mm deeper until a fascial click is felt. Then redirect the angle of the needle upward and advance it 2–3 cm under the lower edge of the rib. Gently aspirate for detection of air or blood. Inject 5 mL of anesthetic agent, preferably bupivacaine 0.5% with epinephrine. Close monitoring is needed because systemic toxicity from rapid reuptake after multiple-level injections and pneumothorax can occur in a delayed fashion.
Penile Block
Anatomic Relationships
The two dorsal nerves of the penis arise from the pudendal nerve, pass under the symphysis pubis, and penetrate the suspensory ligament of the penis to run under the deep (Buck) fascia. The ventral aspect of the penis is partially innervated by perineal nerves.
Procedure
Palpate the symphysis pubis. Insert a short 22-gauge needle to one side of the midline at the 10-o’clock position to reach the caudal border of the symphysis ( Fig. 5.2 ). The needle is then withdrawn slightly and redirected to pass below the symphysis. The needle should be directed through the Buck fascia. Aspirate and inject 10 mL of 1% lidocaine or 5 mL of 0.5% bupivacaine, both without epinephrine. Repeat the procedure at the 2-o’clock position. Alternatively, a subcutaneous ring block at the base of the penis can be performed with 0.5% bupivacaine. For an intracorporeal block, apply a tourniquet to the base of the penis, and inject 20–25 mL of 1% lidocaine into a corpus through a butterfly scalp vein needle. Release the tourniquet after waiting 1 minute. This block should be performed in a monitored setting because systemic lidocaine absorption is pro-arrhythmic.
