Esthesiometric evaluations of corneal anesthesia and prolonged analgesia in rabbits

Esthesiometry was used to evaluate prolonged corneal analgesia, induced by repeated intramuscular injections in New Zealand White rabbits of 2.3 to 4.9 kg body weight of two analgesics: meperidine (10 mg/kg) or buprenorphine (200 micrograms/kg) following short term anesthesia induced by ketamine (35 mg/kg) and xylazine (5 mg/kg). The surface pressure was calculated, using data supplied by the manufacturer, for the fiber length just sufficient to elicit the corneal reflex. Thus the corneal reflex was used as a proxy for pain. For the 12 injections of meperidine, at 2 hr intervals, the surface pressure peaked one hour after each injection of meperidine, and subsequently declined at the next hour when the injection was repeated resulting in a saw-tooth pattern of responsiveness. For the 5 injections of buprenorphine, at 5 hr intervals, the surface pressure to elicit the corneal reflex rose after a second injection to overlap the pressures generated by meperidine. The consistent analgesia continued until after 24 hours. Accordingly long-term analgesia may be conveniently measured using a quantitative technique, esthesiometry. Comparison of the two regimens suggests buprenorphine is preferred over meperidine in experiments that require sustained analgesic coverage for periods of 24 hours.