Dose-related thermal antinociceptive effects of intravenous hydromorphone in cats

OBJECTIVE: To describe the dose-related thermal antinociceptive effects of intravenous (i.v.) hydromorphone in cats. STUDY DESIGN: Randomized, blinded, crossover design. ANIMALS: Seven adult cats (3.5-7.4 kg), two spayed females, and five neutered males. METHODS: Hydromorphone (0.025, 0.05, or 0.1 mg kg(-1)) was administered i.v.. Skin temperature and thermal threshold were measured before and at selected time points to 720 minutes post-administration. Statistical analysis of mean thermal threshold and skin temperatures over time for each dose and between doses was by way of a split-plot model and post hoc Bonferroni t-tests. p < 0.05 was considered significant. RESULTS: A significant difference from baseline for mean thermal threshold was identified for the 0.05 mg kg(-1) dose (5-80 minutes, peak thermal threshold 46.9 +/- 6.2 degrees C) and 0.1 mg kg(-1) dose (5-200 minutes, peak thermal threshold 54.9 +/-0.2 degrees C). The thermal threshold was significantly greater after the 0.1 mg kg(-1) dose from 5 to 200 minutes compared to the 0.025 mg kg(-1) and 0.5 mg kg(-1) doses. The thermal threshold was significantly greater from 35 to 80 minutes for the 0.05 mg kg(-1) dose when compared with the 0.025 mg kg(-1) dose. Skin temperature was significantly increased from 35 to 140 minutes following the 0.1 mg kg(-1) dose. CONCLUSIONS: A dose-related antinociceptive effect was demonstrated for i.v. hydromorphone in cats. CLINICAL RELEVANCE: Hydromorphone at doses less than 0.1 mg kg(-1) has a modest antinociceptive effect and a short duration of action. At a dose of 0.1 mg kg(-1) i.v., onset of analgesia is rapid with a clinically useful duration of effect, but is associated with a rise in skin temperature.