Local and regional anesthetic techniques are used extensively in large animals for a variety of minor and major surgical procedures. Local anesthetics are used in small animals much less frequently and primarily to facilitate suturing of minor lacerations. Because of the relative ease and safety of inducing general anesthesia in small animals, local and regional anesthetic techniques are often overlooked. Nevertheless, these techniques provide an excellent alternative to general anesthesia for select cases and are being used increasingly in conjunction with general anesthesia to improve postoperative analgesia. Local anesthetics used before surgery may decrease the requirement for potent injectable and/or inhalant general anesthetics.
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, 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 on this page.
Lidocaine transdermal patches have been developed for the treatment of neuropathic pain in people. 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 hr. However, further clinical efficacy studies are warranted. Patches must be applied close to the site of pain, and toxicity is of concern if the patch is orally ingested.
EMLA cream, a eutectic mixture of 2.5% lidocaine and 2.5% prilocaine, has been used to decrease venipuncture pain in children and has been evaluated for use in dogs, cats, rabbits, horses, and pigs. Significant results are not achieved until the cream has been in place for 60–90 min beneath an occlusive dressing. Nonclinical methemoglobinemia, lasting as long as 24 hr, 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 hr). Bupivacaine (0.25–0.75%), with or without epinephrine, is administered in a dosage not to exceed 3 mg/kg (dogs) for line or ring blocks of an incision, ring blocks before declaw in cats (epinephrine-containing solutions should not be used for distal extremity ring blocks), intercostal nerve blocks and intrapleural local anesthesia (diluted to twice the volume) after a thoracotomy or the management of pain associated with pancreatitis, proximal nerve infiltration during limb amputations, regional anesthesia in which a nerve proximal to the surgical site is targeted, tissue infiltration for lateral ear resections, and local blocks of facial nerves (maxillary, infraorbital, mental, and mandibular). Bupivacaine is frequently administered into the epidural space at the lumbosacral space for pelvic limb and perianal procedures. Bupivacaine is cardiotoxic if administered IV.
Ropivacaine is a long-lasting amide-type local anesthetic. Its duration is similar to that of bupivacaine, but it has lower potential to induce cardiovascular and CNS toxicity.
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 people. Capsaicin binds to the TRPV1 receptor on nonmyelinated primary afferent pain fibers, causing increased firing through 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 species; however, one study using intrathecal resinferatoxin, a potent analogue of capsaicin, reported encouraging results for palliative treatment of pain in a canine osteosarcoma model. 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 on this page for dose by weight.