Assessment of a carbon dioxide laser for the measurement of thermal nociceptive thresholds following intramuscular administration of analgesic drugs in pain-free female cats.

OBJECTIVE To assess the potential of a thermal carbon dioxide (CO2) laser to explore antinociception in pain-free cats. STUDY DESIGN Experimental, prospective, blinded, randomized study. ANIMALS Sixty healthy adult female cats with a (mean±standard deviation) weight of 3.3±0.6 kg. METHODS Cats were systematically allocated to one of six treatments: saline 0.2 mL per cat; morphine 0.5 mg kg(-1); buprenorphine 20 μg kg(-1); medetomidine 2 μg kg(-1); tramadol 2 mg kg(-1), and ketoprofen 2 mg kg(-1). Latency to respond to thermal stimulation was assessed at baseline and at intervals of 15-30, 30-45, 45-60, 60-75, 90-105 and 120-135 minutes. Thermal thresholds were assessed using time to respond behaviourally to stimulation with a 500 mW CO2 laser. Within-treatment differences in response latency were assessed using Friedman's test. Differences amongst treatments were assessed using independent Kruskal-Wallis tests. Where significant effects were identified, pairwise comparisons were conducted to elucidate the direction of the effect. RESULTS Cats treated with morphine (χ2=12.90, df=6, p=0.045) and tramadol (χ2=20.28, df=6, p=0.002) showed significant increases in latency to respond. However, subsequent pairwise comparisons indicated that differences in latencies at specific time-points were significant (p<0.05) only for tramadol at 60-75 and 90-105 minutes after administration (21.9 and 43.6 seconds, respectively) in comparison with baseline (11.0 seconds). No significant pairwise comparisons were found within the morphine treatment. Injections of saline, ketoprofen, medetomidine or buprenorphine showed no significant effect on latency to respond. CONCLUSIONS AND CLINICAL RELEVANCE The CO2 laser technique may have utility in the assessment of thermal nociceptive thresholds in pain-free cats after analgesic administration and may provide a simpler alternative to existing systems. Further exploration is required to examine its sensitivity and comparative utility.