Nerve Block Anesthesia

Etiology

Peripheral nerve blocks have gained widespread acceptance as technological advancements have improved accuracy, efficacy, and safety. Nonetheless, in-depth knowledge of peripheral nerve anatomy remains an essential component of a successful and safe peripheral nerve blockade. Peripheral nerve blocks were originally performed by eliciting paresthesias to localize peripheral nerves, followed years later by nerve stimulation techniques. More recently, ultrasound guidance has improved anesthesiologists’ and emergency physicians' abilities to visualize nerve structures and other surrounding structures (e.g., vasculature and pleura).[4][5]

Epidemiology

Peripheral nerve blocks offer many advantages to traditional anesthetic and analgesic techniques. Patients who would otherwise have excessive risks with general anesthesia can safely undergo surgery painlessly with regional anesthesia. Additionally, perioperative opioid analgesia's adverse effects can be minimized or avoided entirely while still providing superior pain control. If a prolonged blockade is a goal, peripheral nerve catheters can remain for several days to provide more extended analgesia than a single-shot block alone.

Pathophysiology

Although peripheral nerve blocks are overall safe when performed correctly, there are rare but serious risks associated with them. Risks include block failure, bleeding, infection, damage to surrounding structures, permanent nerve injury, and intravascular uptake of local anesthetic resulting in systemic toxicity. Contraindications include an inability to cooperate with placement (sometimes seen in pediatric patients, combative patients, or those with severe dementia), pre-existent neurologic deficit, systemic or active local infection at the site of the block, body habitus, or obesity obscuring optimal ultrasonographic visualization. Relative contraindications include the presence of coagulopathy.

Toxicokinetics

Local anesthetics have a significant risk of systemic toxicity when administered intravascularly. Symptoms usually manifest in the central nervous system first (metallic taste, auditory changes, circumoral numbness, blurred vision, agitation, seizures), followed by cardiovascular effects (hypotension, decreased cardiac contractility, dysrhythmias, complete heart block, cardiovascular collapse). Bupivacaine is particularly cardiotoxic, and reports of cardiovascular collapse in the absence of antecedent neurologic symptoms exist. Neurolytic blocks with alcohol and phenol have utility in chronic pain but are usually the last resort in cancer pain. They cause semi-permanent destruction of the nerve, which blocks impulse transmission and improves pain. Risks with neurolytic destruction of a nerve include the development of central or deafferentation pain syndromes, which are very difficult to treat.[6][7][8]

Contraindications include an allergy to aminoesters (procaine, cocaine, tetracaine).

Amides-lidocaine/bupivacaine/mepivacaine/ropivacaine (usually preservative).

History and Physical

Before performing a peripheral nerve block, a complete history and physical exam are essential. The emphasis in the patient's history should be on the existence of underlying severe cardiac or pulmonary disease, opioid intolerance, pre-existing neurologic deficit, easy bleeding or bruising, and anticoagulation medications. The physical exam should direct its focus to the sensory and motor function of the extremity undergoing blockade, evidence of coagulopathy (e.g., widespread bruising), evidence of systemic infection (fever or elevated white blood cell count), and infection at the site of the block (redness, swelling, warmth).

Evaluation

A thorough risk/benefit analysis and informed consent are necessary when evaluating a patient for peripheral nerve blockade. If block administration is in the emergency department, discussion with the anesthesiology can help direct preference toward a catheter if a longer block duration is preferable. Each patient’s maximum local anesthetic dose must be calculated before performing the block and never exceeded.

Treatment / Management

Emergency airway equipment, resuscitation drugs (including lipid emulsion 20%), supplemental oxygen, and hemodynamic monitoring must be immediately available where blocks are performed. Blocks should be administered with a strict sterile technique. Anxiolytic medications may be used to facilitate patient comfort. Awake patients reduce the risk of nerve injury by reporting paresthesias so the needle can be redirected before injection of local anesthetic. Additionally, an awake patient can report early symptoms of local anesthetic systemic toxicity (i.e., tinnitus, circumoral numbness, and tachycardia with epinephrine) to avoid more serious complications that can occur a large volume local anesthetic injection (seizures, coma, and cardiac collapse). Recommended maximum dosages for local anesthetics are widely available, but practitioners should always use the lowest dose necessary to achieve the desired result due to the significant risk of systemic toxicity.

Point of care, bedside ultrasound, can be used to locate and visualize the nerve.

Differential Diagnosis

• Anaphylaxis

• Anxiety disorders

• Cocaine toxicity

• Conversion disorders

Complications

Potentially serious complications resulting from peripheral nerve blocks include nerve injury, infections at the catheter site, bleeding, and local anesthetic systemic toxicity (LAST). Other effects can include elevations of blood sugar, itching, rash, and injection site soreness.[9]