Research Status of Different Adjuvants on Nerve Block's Effect.

Background: Acute postoperative pain is one of the most common challenges faced by patients who undergo surgery. Multimodal analgesia has been recommended in recent years to effectively control this condition. Nerve blocks are an important part of multimodal analgesia; a single peripheral nerve block is widely used in clinical practice. To prolong the analgesic duration of a single nerve block, adjuvants with different mechanisms, dosages, or administration routes are added to local anesthetics; however, it is not clear which adjuvant or combination is better.

Objectives: This study aimed to provide a comprehensive review of the current utilization of diverse adjuvants in single peripheral nerve block analgesia and to recommend optimal adjuvants for single peripheral nerve blocks based on current literature.

Study design: A narrative review.

Methods: PubMed was searched using the terms "postoperative analgesia," "nerve block," "adjuvant," "epinephrine," "clonidine," "dexmedetomidine," "dexamethasone," "buprenorphine," "morphine," "magnesium sulfate," and "ketamine." The mechanisms of action of different adjuvants were investigated and clinical trials of different adjuvants for postoperative analgesia were determined and reviewed.

Results: According to current literature, there are 4 main types of adjuvants added to local anesthetics to prolong analgesic effects: adrenergic receptor agonists, anti-inflammatory agents, opioids, and N-methyl-D-aspartic acid (NMDA) receptor antagonists. As a single adjuvant, adrenergic agonists, dexmedetomidine, and anti-inflammatory agents are more effective than opioids and NMDA receptor antagonists. When added to local anesthetics, intravenous dexamethasone (10 mg) had an effect similar to that of perineural dexamethasone (8 mg). However, considering the side effects of perineural dexamethasone, intravenous injection of dexamethasone is preferable.Magnesium sulfate is a suitable NMDA receptor antagonist for peripheral nerve blocks. The combination of adjuvants with different mechanisms can further prolong local anesthetic duration.When more than one adjuvant was used, the combination of dexmedetomidine and dexamethasone was determined to be excellent.

Limitations: Additional compatibility tests with different adjuvants are required to completely determine the curative effect and optimal dosage parameters.

Conclusion: Adjuvants with diverse mechanisms of action can variably extend the duration of local anesthetic effects. When utilizing adjuvants in conjunction with local anesthetics, perineural dexmedetomidine (1 mu-g/kg) or intravenous dexamethasone (10 mg) may be preferable, considering their efficacy and side effects. Current research suggests that the combination of perineural dexmedetomidine (1 mu-g/kg) and intravenous dexamethasone (10 mg) is more effective than either dexmedetomidine or dexamethasone alone.