Abstract
The use of pigs as research animals in survival surgery has increased greatly in the last 15 years. Personnel conducting pig research have been hampered by a lack of proven long-acting analgesics for treatment of surgical pain of longer duration, and by a lack of reliable non-subjective parameters for the assessment of pain relief. The efficacy of the mixed opioid agonist-antagonist buprenorphine hydrochloride 0.10 mg/kg prn (n = 2) in the treatment of post-thoracotomy pain was compared with that of a transdermal therapeutic system (TTS) delivering 25 μg/h (n = 3) or 50 μg/h (n = 2) of the mu opioid agonist fentanyl hydrochloride. Food consumption, pain score, activity level and rate of movement were assessed under four conditions: normal pre-operative control (24 h), pre-operative with analgesic alone (24 h) and post-operative days 1, 2, 3 (72 h). Serum concentration-time curves for fentanyl in clinical cases revealed that female Yorkshire cross pigs weighing 26.2±2.1 kg achieved serum values in the recognized human therapeutic range when treated with TTS fentanyl at 50 μg/h and experienced adequate pain control. Pigs treated with 25 μg/h TTS fentanyl had serum levels below the human analgesic range, experienced less adequate analgesia, and required supplemental analgesia in some cases. Based on existing pharmacokinetic data for fentanyl in pigs, the rate of uptake of TTS fentanyl when attached on inter-scapular skin was lower than predicted. Clinical pain scores and time intervals between each major postural change were not affected by analgesics in the absence of pain, but increased in all groups after surgery regardless of treatment. Food consumption was unaffected by analgesic treatment alone but decreased in all groups after surgery regardless of treatment. Analgesic effects on postoperative activity level were variable. TTS fentanyl at appropriate doses is a cost effective means of delivering basal analgesia following major surgery in pigs.