Narcotic Adjuvants to Local Anesthetics

The use of local anesthetics is limited by their duration of action and the dose dependent adverse effects on the cardiac and central nervous system. Hence a multimodal approach to pain management is recommended whenever possible using a combination of two or more drugs that act by different mechanisms to provide safe analgesia with minimal adverse effects. Anesthesiologists now prefer to add adjunctive drugs to local anesthetics to improve the quality of regional blocks and also ensure good residual analgesia post operatively for better patient comfort. Opioids are the most frequently used local anesthetic adjuvant. A wide range of opioids ranging from morphine, fentanyl, sufentanyl, hydromorphone, buprenorphine and tramadol have been used with varying success. The opioids potentiate anti-nociception of local anesthetics by G protein coupled receptor mechanisms, causing hyperpolarisation of the afferent sensory neurons. Their efficacy is determined by their dose, site of injection, lipophilicity and also the acid-base status at the site of drug deposition. Opioid use is limited by adverse effects like respiratory depression, nausea, vomiting and pruritus, especially with its neuraxial use.