PHARMACOKINETICS OF EPIDURAL BUTORPHANOL IN ISOFLURANE-ANESTHETIZED DOGS

Sixteen healthy male dogs were used at random in this protocol. The do gs were anaesthetized with isoflurane in oxygen. Eight of the dogs rec eived 0.25 mg/kg of butorphanol (group B) and the others an equal volu me of isotonic saline (group S) administered by a catheter inserted in the lumbosacral epidural space. Butorphanol concentrations in plasma and cerebrospinal fluid (CSF) were measured using high-performance liq uid chromatography with electrochemical detection, Maximum concentrati on of butorphanol and time to obtain this concentration were 42.28 ng/ mL at 13.88 min in blood, and 18.03 ng/mL at 30 min in CSF. Volume of distribution, clearance, mean distribution and elimination half-lives were respectively 4.39 L/kg, 2.02 L/h.kg, 16.5 min and 189.1 min, Mean isoflurane minimal alveolar concentration values for group B obtained following hind- or forelimb stimulation decreased by 31% after epidur al butorphanol. Cutaneous analgesia (to pin-prick test) persisted for 3 h after the end of isoflurane anaesthesia in group B and was in corr elation with the plasmatic analgesic dose of butorphanol (9 ng/mL), Th ese results suggested that analgesia was predominantly obtained by act ion of butorphanol on the supraspinal structures following its vascula r systemic absorption.