Postoperative Pain Management Using Fentanyl Patches in Dogs

To the ability of the fentanyl patch to control pain in a postoperative canine model, we provided two male beagles with 25-μg/h patches and two with 50-μg/h patches 24 h prior to surgery. Each animal underwent a major abdominal surgical procedure to place three separate catheters with associated vascular access ports. Serum plasma levels of fentanyl were analyzed at multiple time points throughout the study period. Animals were subjectively assessed for postoperative pain by using a Simple Descriptive Scale at regular intervals postoperatively. Other parameters observed and recorded included heart and respiration rates, rectal temperature, appetite, and activity. The fentanyl patch appeared to adequately control postoperative pain in our canine abdominal surgical model. Three animals demonstrated mild pain 1 to 2 h postoperatively. Two animals, one from each dose group, showed mild pain 8 h postoperatively. Mild pain is commensurate with USDA category C, which encompasses procedures that do not result in more than momentary or slight pain or distress and do not require intervention. At no other time points were any of the animals considered to be in pain. Our study also suggested that increased subcutaneous fat delayed the rate of absorption of fentanyl. The lower body-weight beagles, which had the 25-μg/h patches, reached reported human serum analgesic levels within 8 h after placement, whereas the heavier beagles with the 50-μg/h patches reached human serum analgesic levels 12 h after placement. Fentanyl concentrations remained at the reported human analgesic levels in all animals between 2 to 4 h after the patches were removed. Regardless of the dose, decreases in heart rate, respiration rate, and temperature were observed in all four animals 12 h after placing the patches. Issues regarding the regulatory requirements to prevent drug abuse, the side effects and potency of fentanyl, and the prolonged duration of action as a transdermal system should be addressed by the veterinarian when considering usage of this analgesic method.