Evaluation of analgesic and physiologic effects of epidural morphine administered at a thoracic or lumbar level in dogs undergoing thoracotomy

Objective To evaluate the analgesic and physiological effects of epidural morphine administered at the sixth and seventh lumbar or the fifth and sixth thoracic vertebrae in dogs undergoing thoracotomy. Study design Prospective, randomized, blinded trial. Animals Fourteen mixed-breed dogs, weighing 8.6 +/- 1.4kg. Methods The animals received acepromazine (0.1mgkg(-1)) IM and anesthesia was induced with propofol (4mgkg(-1)) IV. The lumbosacral space was punctured and an epidural catheter was inserted up to the region between the sixth and seventh lumbar vertebrae (L, n=6) or up to the fifth or sixth intercostal space (T, n=8). The dogs were allowed to recover and after radiographic confirmation of correct catheter position, anesthesia was reinduced with propofol IV and maintained with 1.7% isoflurane. Following stabilization of monitored parameters, animals received morphine (0.1mgkg(-1)) diluted in 0.9% NaCl to a final volume of 0.25mLkg(-1) via the epidural catheter, and after 40minutes, thoracotomy was initiated. Heart rate and rhythm, systolic, mean and diastolic arterial pressures, respiratory rate, arterial hemoglobin oxygen saturation, partial pressure of expired CO2 and body temperature were measured immediately before the epidural administration of morphine (0minute) and every 10minutes during the anesthetic period. The Melbourne pain scale and the visual analog scale were used to assess post-operative pain. The evaluation began 3hours after the epidural administration of morphine and occurred each hour until rescue analgesia. Results There were no important variations in the physiological parameters during the anesthetic period. The post-operative analgesic period differed between the groups, being longer in T (9.9 +/- 1.6hours) compared with L (5.8 +/- 0.8hours). Conclusions The use of morphine, at a volume of 0.25mLkg(-1), administered epidurally over the thoracic vertebrae provided longer lasting analgesia than when deposited over the lumbar vertebrae. Clinical relevance The deposition of epidural morphine provided longer lasting analgesia when administered near to the innervation of the injured tissue without increasing side effects.