Effect of 0.1, 0.2, 0.4, and 0.8 mg kg−1 of intravenous butorphanol on thermal antinociception in cats

Many cats do not receive analgesics for treatment of perioperative pain, but when they do, the opioid agonist–antagonist butorphanol (B) is widely used. B is reported to provide long‐lasting visceral analgesia, but its somatic actions are not well documented. This study aimed to assess B by using a thermal threshold (TT) model. Six cats (four spayed females, two castrated males, 4.4–6.9 kg) participated in the study. The day before each study, the lateral thorax of each of the cats was shaved and a cephalic catheter was placed. TT was measured using a thermal threshold testing device specifically developed for cats. A heater element and temperature sensor housed in a small probe was held against the cat's thorax with an elastic band and pressure bladder to assure consistent contact. Skin temperature was recorded before each test, then the heater was activated. When the cat responded by flinching, turning, or jumping, the stimulus was terminated and the threshold temperature recorded. A cut‐out temperature was set at 55.5 °C. Three baseline measurements were recorded before IV injections of 0.1, 0.2, 0.4, or 0.8 mg kg^?1 of B. Each cat received all doses in a randomized order at least 1 week apart, and the investigator was blinded to the treatments. TT was measured at every 15 minutes for 6 hours. Data were analyzed using a three‐factor anova , and the critical mean difference was calculated. Pre‐treatment threshold was 40.8 ± 2.25 °C in all cats. There was a significant increase in threshold in all groups from 15 to 90 minutes, but no dose‐related differences were observed. Peak threshold achieved was 48.35 °C, 60 minutes after 0.4 mg kg^?1 of B was injected. Mydriasis was present in all cats after treatment, and many exhibited dysphoric behavior. In this model, B had a short duration of action and no dose–response relationship. Compared to other opioids tested under similar conditions, the intensity of the effect of B was small and of shorter duration.