Effectiveness of Transdermal Lidocaine After Canine Ovariohysterectomy

In a recent study, researchers investigated the use of a transdermal lidocaine patch for added pain control in dogs receiving morphine and carprofen after ovariohysterectomy.

According to the results, which were published May 15 in the Journal of the American Veterinary Medical Association, the transdermal lidocaine patch produced no additional analgesic effect.

Opioids and nonsteroidal anti-inflammatory drugs (NSAIDs) are often used for postoperative pain relief in dogs, say the authors.

However, in some situations additional or alternative analgesics may be needed:

• When opioids and NSAIDs are contraindicated or cause adverse effects
• When opioids and NSAIDs do not adequately control postoperative pain
• When multimodal analgesia is desired

Studies of the efficacy of transdermal lidocaine patches in humans have had mixed results, note the authors. According to published reports, the patches appear to reduce neuropathic pain but do not necessarily affect traumatic and postoperative pain in humans.

“To the authors’ knowledge, no studies have been conducted to investigate the effects of [transdermal lidocaine patch] application on postoperative pain in veterinary patients,” they write. Pharmacokinetic studies in animals have shown that systemic absorption of lidocaine is minimal after patch application, so the risk for systemic adverse effects is low.

The investigators tested transdermal lidocaine patches in 19 dogs undergoing ovariohysterectomy at the University of Illinois Veterinary Teaching Hospital (21 dogs received placebo). Dogs included were healthy shelter dogs of various ages and breeds. The surgical procedures were performed by third-year veterinary students. All dogs received morphine and carprofen during anesthetic recovery and were given oral carprofen every 12 hours for 2 days thereafter.

For dogs in the lidocaine group, lidocaine patches were applied to each side of the surgical incision 1 cm from the incision edge and extending the length of the incision. (The researchers did not apply patches directly to the incisions because the patches were not sterile and lidocaine absorption through broken skin had not been reported.) The patches were covered with a nonadhesive wound dressing. Dogs in the placebo group received only the nonadhesive wound dressing. In all dogs, wound dressings were covered with abdominal bandages and were not disturbed for 18 hours.

Observers blinded to the dogs’ group assignments performed pain assessments before and 1, 2, 4, 6, 8, 10, and 18 hours after the surgical procedures. Blinded observers evaluated the incisions for inflammation 18, 36, and 42 hours after the operations. The investigators measured serum cortisol levels after anesthetic induction and 1, 2, 4, 6, 8, 10, and 18 hours after the procedures. Dogs remained in the hospital for 48 hours after ovariohysterectomy, at which time they were adopted or returned to the shelters to be adopted.

Statistical analysis revealed no significant differences between groups in body weight, incision length, pain score, inflammation score, or serum cortisol concentration. No dogs had signs of incision infection.

The authors mention several potentially limiting factors:

• Because all dogs received morphine and carprofen and because ovariohysterectomy is not extremely painful, overall pain levels may have been too low to reveal a significant difference between groups. The patch might be more helpful in dogs with more severe pain, they suggest.
• The statistical power of the study may have been low.
• Serum cortisol levels could have been affected by hospital stress.
• Pain scores may have been affected by interobserver variation.
• The distance between the lidocaine patches and the incision may have been too great to allow lidocaine to penetrate to the incision site.

The results, conclude the authors, show that the transdermal lidocaine patch does not provide additional pain relief to dogs given morphine and carprofen after ovariohysterectomy. “Additional studies are needed to investigate whether similar results might be achieved in dogs treated concurrently with other analgesics,” they write.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.