Local and regional analgesic techniques are valued for their ability to decrease both intraoperative nociception and postoperative pain. They are recommended for use in the majority of surgical procedures and traumatic injuries.
Conduction blockade of nerve fibers by local anesthetics is related to the size of the nerve, amount of myelination, and frequency of activity. Small sensory and autonomic fibers tend to be anesthetized before larger motor and proprioceptive fibers. Nerves that are repetitively stimulated are more sensitive to local anesthetics than resting nerves. The most commonly used agents are lidocaine, mepivacaine, bupivacaine, and ropivacaine.
Lidocaine, the prototype aminoamide local anesthetic, systemically used (by continuous-rate infusion), decreases minimum alveolar concentration of isoflurane in a variety of species. It also has the potential to decrease ileus in horses after general anesthesia and is used commonly as an adjunct in colic surgery. This has become the theoretical basis for its systemic use as an analgesic agent.
Constant-rate infusion of lidocaine has been advocated for pain management in a number of species; notably, a number of combination analgesic protocols (eg, morphine, lidocaine, ketamine [known as MLK]) have been developed for dogs. The use of constant-rate infusion lidocaine during anesthesia is not recommended for cats because of their increased sensitivity and its negative cardiovascular effects. Also, see the Canine Morphine/Lidocaine/Ketamine (MLK) CRI Dose calculator.
Lidocaine transdermal patches have been developed for the treatment of neuropathic pain in humans. Systemic absorption of lidocaine in dogs and cats has been reported to be minimal, while local tissue concentration is reported to be as much as 100 times greater than plasma concentration. The low systemic absorption rate coupled with high local lidocaine concentrations on the skin support the safe use of lidocaine patches in dogs and cats. These patches appear to result in a differential blockade, preserving sensory function of the skin and motor function of regional muscles while inducing analgesia at the site for as long as 72 hours. However, further clinical efficacy studies are warranted. Patches must be applied close to the site of pain, and toxicity is a concern if the patch is orally ingested.
A eutectic 2.5% lidocaine and 2.5% prilocaine cream has been used to decrease venipuncture pain in children and evaluated for use in dogs, cats, rabbits, horses, and pigs. In order to be effective, the cream must be in place for 30 minutes beneath an occlusive dressing. Nonclinical methemoglobinemia, lasting as long as 24 hours, has been reported in children. Repeated dosing of neonatal or small animals should be done cautiously.
Bupivacaine is the preferred drug for postoperative analgesia because of its relatively long duration of action (~3–8 hours). Bupivacaine liposomal-encapsulated injectable suspension is approved for single-dose infiltration into the surgical site (but not intra-articularly). The liposomes release bupivacaine locally due to their relative large molecule size.
Ropivacaine is structurally similar to bupivacaine; but has less cardiotoxic side effects and causes less motor dysfunction.
Topical capsaicin, the active component in chili peppers, has been used extensively to treat pain associated with diabetic neuropathy, post-herpetic neuralgia, and osteoarthritis in humans. Capsaicin binds to the transient receptor potential vanilloid 1 (TRPV1) receptor on nonmyelinated primary afferent pain fibers, causing increased firing via the release of substance P. With repeat application, desensitization occurs due to degeneration of these pain fibers with resultant hypoalgesia. Reinnervation occurs when use is discontinued.
No controlled studies have been published in veterinary patients; however, one study using intrathecal resiniferatoxin, a potent analogue of capsaicin, reported encouraging results for palliative treatment of pain in a canine osteosarcoma model.(1,2) The benefits of these TRPV1 ligands include specific loss of pain sensation without concomitant loss of motor and nonpain sensation.
Also see the anesthetic drug calculator for dose by weight.
References
Iadarola MJ, Sapio MR, Raithel SJ, Mannes AJ, Brown DC. Long-term pain relief in canine osteoarthritis by a single intra-articular injection of resiniferatoxin, a potent TRPV1 agonist. Pain. 2018;159(10):2105-2114. doi: 10.1097/j.pain.0000000000001314. PMID: 30015705; PMCID: PMC8121156.
Brown DC, Agnello K, Iadarola MJ. Intrathecal resiniferatoxin in a dog model: efficacy in bone cancer pain. Pain. 2015;156(6):1018-1024. doi: 10.1097/j.pain.0000000000000115. PMID: 25659068; PMCID: PMC4431903.
