Lidocaine Patches for Cats?

Lidocaine patches are used in human medicine to provide topical analgesia for pain caused by herpes zoster (shingles), for postsurgical incision pain, and for controlling the pain of osteoarthritis. Two studies have shown the patches to be safe in dogs; however, little is known about the safety in cats. In fact, cats are traditionally used in models of lidocaine toxicity. In this study, 8 cats were assigned to 1 of 2 treatment groups in a randomized, crossover study. Cats received either lidocaine 2 mg/kg IV or one 700-mg lidocaine patch placed on the lateral thorax for 72 hours. Plasma was collected at regular intervals. Tissue biopsy from the patch site was obtained and processed for lidocaine concentrations and one of its metabolites. Data analysis revealed a constant rate of absorption from 12 to 72 hours; however, detectable levels were found within 3 hours of application. After patch removal, the terminal elimination phase was longer with the patch (3.7 H) than with IV lidocaine (2.3 H). Lidocaine and its metabolite were recovered from the contralateral skin, suggesting systemic absorption. Plasma lidocaine concentrations stayed below systemically toxic concentrations. The skin at the site of patch application was normal. The overall bioavailability was 6%, indicating that about 44 mg of the total dose in the patch was absorbed. The authors conclude that the high local concentrations in the skin and low systemic absorption of lidocaine indicate that patches can be safely used in cats.

COMMENTARY: This is an important study because of the sensitivity of cats to lidocaine and its potential for toxicity in this species. The study found use of the patches for 72 hours under occlusion (as would be done in most practice situations) to be safe. However, the fact that the patches were used under occlusion may have enhanced absorption of the lidocaine. The authors propose these patches as a possible alternative to fentanyl patches, and one practical advantage is that lidocaine is not a controlled substance. Lidocaine patches are not designed to be rate-limiting-the drug is not in a reservoir but rather resides in the adhesive of the patch and is evenly distributed across the surface. It is not intended to deliver a certain amount of drug per hour but rather diffuses via passive diffusion from an area of high concentration to an area of low concentration. These patches offer options for local anesthesia for certain types of surgery. Additional studies may be needed to determine whether skin preparation affects absorption and toxicity. The skin of cats is much thinner than that of dogs, and use of surgical scrubs and alcohol may affect absorption. Furthermore, cats in this study were adults and some of the target procedures (spay and declaw) are performed on younger, smaller animals.Pharmacokinetics of lidocaine following application of 5% lidocaine patches to cats. Ko JCH, Maxwell LK, Abbo LA, Weil AB. J VET PHARMACOL THER 31:359-367, 2008.