Local Anesthetics: Overview & Updates

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According to AAHA pain management guidelines, local anesthetics (LAs) are "the only class of drug that renders complete analgesia" and should be considered for every surgical procedure.¹ To effectively follow this recommendation, a firm understanding of LA action and use is necessary.

Mechanism of Action and Pharmacology

Nerve conduction depends upon an influx of sodium entering the nerve, depolarizing the membrane, and setting off an action potential.² LAs stabilize the nerve cell membrane by reversibly binding to sodium channels and preventing depolarization of the nerve.³

LAs can be grouped into two categories based on their chemical composition: esters and amides. Esters—such as procaine, cocaine (the first discovered LA), and benzocaine—have short half-lives and are metabolized by plasma cholinesterase.² Amides—such as lidocaine, bupivacaine, and ropivacaine—are very stable and are metabolized by the liver.² 

Although LAs have varying potencies, durations of action, and speed of onset based on lipid solubility, acid dissociation constant (pK_,), and degree of protein binding,² varying pH levels of each LA formulation may reduce the effectiveness of each LA in the mixture.^2,3 Evidence does not support benefits to mixing LAs to take advantage of each LA's strength (eg, a quick onset combined with long duration of action). ^2,3 However, developments in sustained-release technology have made a long-acting LA possible. This novel technology utilizes aqueous bupivacaine encased in multivesicular liposomes; the bupivacaine is slowly released into the tissues as the liposomes gradually degrade, providing up to 72 hours of analgesia.⁴

Safety

When used appropriately—both in technique and dose—LAs have relatively few adverse effects.^1,2 High blood levels of LAs (eg, from very high doses or inadvertent IV administration) can cause severe CNS and cardiovascular reactions.^2,3 Methemoglobinemia in cats and allergic reactions occur rarely and are more often associated with ester LAs.^2,3 Concerns that LAs might slow wound healing or cause seroma formation and infection are unsupported^4-7; rather, LAs have been shown to have antimicrobial activity and to slow the growth of several bacteria and fungi.^1,2,4,7

Delivery

LAs are versatile and can be administered in several ways.³ Infiltration anesthesia, which involves injecting the LA into the tissue surrounding the target region, is the most common method. Topical application, available in formulations such as sprays, creams, and gels, is also commonly used. Although neuraxial blockade (ie, epidural and intrathecal anesthesia) is an effective mode of analgesia, it is technically difficult to administer. Lastly, systemic administration can be done only with lidocaine and only in dogs. Systemically, lidocaine is thought to affect the NMDA receptors in the dorsal horn of the spine.³

Conclusion

LAs are an integral part of balanced anesthetic protocols. Veterinarians can make the most of this important class of analgesic by solidifying their knowledge base and staying abreast of current research and new label indications.

Important Safety Information: NOCITA<sup®sup> (bupivacaine liposome injectable suspension) is for use in dogs and cats only. Do not use in dogs or cats younger than 5 months of age, that are pregnant, lactating or intended for breeding. Do not administer by intravenous or intra‐arterial injection. Adverse reactions in dogs may include discharge from incision, incisional inflammation and vomiting. Adverse reactions in cats may include elevated body temperature, infection or chewing/licking at the surgical site. Avoid concurrent use with bupivacaine HCl, lidocaine or other amide local anesthetics. Please see the full Prescribing Information for more detail.

DOG INDICATION: For single-dose infiltration into the surgical site to provide local postoperative analgesia for cranial cruciate ligament surgery in dogs.

CAT INDICATION: For use as a peripheral nerve block to provide regional postoperative analgesia following onychectomy in cats.