Peripheral nerve blocks involve the injection of local anesthetic near a nerve – or bundle of nerves – to numb a specific part of the body.1 Commonly used for surgery on the hands, arms, feet, legs, and face, peripheral nerve blocks provide pain relief during and after surgery by blocking the transmission of pain signals from the surgical site.2 Compared to general anesthesia, peripheral nerve blocks offer numerous advantages, including faster recovery time, fewer side effects, no need for an airway device, and reduced postoperative pain.1 Prior to surgery, an anesthesiologist uses ultrasound or electrical stimulation to locate the nerves and determine the optimal injection site.1 The local anesthetic is then injected without irritating the nerves themselves. Depending on the local anesthetic and amount injected, the effects can last a few hours to days.1
Local anesthetics used for peripheral nerve block include lidocaine, ropivacaine, bupivacaine, and mepivacaine.3 These drugs are often combined with additives to reduce onset time and prolong the duration of analgesia.4 Common additives include epinephrine, clonidine, dexmedetomidine, buprenorphine, dexamethasone, tramadol, sodium bicarbonate, and midazolam, with epinephrine being the most popular.4 An α1-adrenoceptor agonist, epinephrine works synergistically with local anesthetics by causing vasoconstriction and decreasing blood flow.4 Mechanistically, epinephrine decreases the diffusion of local anesthetic away from the nerve, which ultimately lengthens the duration of analgesia.4
The mechanism of peripheral nerve blocks is explained in terms of electrophysiology. Local anesthetics act on the voltage-gated sodium channels that initiate and propagate action potentials in neurons.5 Electron microscopic studies reveal that sodium channels are comprised of a bell-shaped transmembrane glycoprotein with four domains and a central pore through which sodium ions flow.5 Previously, it was thought that local anesthetics physically occlude the central pore.6 However, it is now understood that local anesthetics block neurotransmission by binding to specific sites at the pore and creating an electrostatic field that repels positively charged sodium ions.7
After injection at the surgical site, two factors determine the amount of local anesthetic that actually reaches the nerve: the relative mass of the nerve and diffusion across the perineurium. Once injected, the local anesthetic reaches an equilibrium with all tissues, including muscle, bone, connective tissue, etc.5 The mass of the nerve relative to the mass of the surrounding tissue determines the volume of local anesthetic taken up by the nerve.5 Because the mass of the surrounding tissue is typically 5-10 times more than that of the nerve, only a small portion of the injected local anesthetic actually acts at sodium channels.5 In addition to relative mass, diffusion rates affect the amount of local anesthetic that reaches the target site. The perineurium is a sheath of connective tissue that surrounds a bundle of nerve fibers.5 The thicker the perineurium, the lower the rate of diffusion, and the lower the volume of local anesthetic that reaches the site of action at nerves.5
Though generally safe, peripheral nerve blocks occasionally manifest with local anesthetic toxicity in the central nervous system and heart. Causes of local anesthetic toxicity include an allergy, excessive dosage, or intravascular injection.8 Early signs include numbness and tingling in the mouth, metallic taste, and ringing in the ears.8 Complications include seizures, arrhythmias, cardiac arrest, and nerve injuries (the last of which occurs in only 0.029-0.2% of cases).8 Causes of nerve injury include ischemia, compression, direct neurotoxicity, laceration, and inflammation.8 Some evidence suggests that the use of ultrasound lowers the risk of nerve injury to 0.0037%.9
With the widespread acceptance and adoption of ultrasound-guided techniques, peripheral nerve blocks have become much more commonly used by anesthesiologists.5 As researchers continue to develop novel drugs with lower toxicity and longer analgesia, peripheral nerve blocks will become safer and more effective.5
References
1) “Regional Anesthesia (Nerve Blocks).” UC San Diego Health. 2019.
2) Healthwise Staff. “Peripheral Nerve Blocks for Anesthesia.” Alberta Health. December 13, 2018.
3) “Common Regional Nerve Blocks (PDF).” UWHC Acute Pain Service. January 2011.
4) Brummett, Chad M, and Brian A Williams. “Additives to local anesthetics for peripheral nerve blockade.” International anesthesiology clinics. Vol. 49, 4 (2011): 104-16.
5) Vadhanan, Prasanna et al. “Physiological and pharmacologic aspects of peripheral nerve blocks.” Journal of anaesthesiology, clinical pharmacology. Vol. 31, 3 (2015): 384-93.
6) McNulty, Megan M et al. “Charge at the lidocaine binding site residue Phe-1759 affects permeation in human cardiac voltage-gated sodium channels.” The Journal of physiology. Vol. 581, Pt 2 (2007): 741-55.
7) GLadsden, Jeff. “Clinical Pharmacology of Local Anesthetics.” New York School of Regional Anesthesia. 2019.
8) El-Boghdadly, Kariem et al. “Local anesthetic systemic toxicity: current perspectives.” Local and regional anesthesia. Vol. 11, 35-44.
9) David Hardman. “Nerve Injury After Peripheral Nerve Block: Best Practices and Medical-Legal Protection Strategies.” Anesthesiology News. July 30, 2015.
