Numerous studies have documented the additive or synergistic effects of combining opioids and alpha-2 adrenergic agonists. This study assessed duration of the anesthetic and/or hypoalgesic actions of 4 treatments: saline (S), butorphanol (B, 0.2 mg/kg), medetomidine (M, 5.0 ug/kg), and a combination of butorphanol (0.2 mg/kg) and medetomidine (5.0 ug/kg) (MB), administered intramuscularly to six mixed breed dogs (age; 1-3 years, weight; 11-31 kg) receiving subanesthetic doses of isoflurane (ISO, 0.9 MAC).Individual ISO MAC values were determined one week before any treatment administration. Dogs were fasted 12 hours, mask induced with ISO in oxygen, intubated, ventilated to maintain ETCO2 partial pressure between 35-45 mmHg, placed in right lateral recumbency, and instrumented with a rectal temperature probe, and ETCO2 and anesthetic agent analyzer. To determine MAC, ETISO concentration was allowed to equilibrate 30 minutes. A clamp was placed on the tail at a point with a circumference of 10 cm, closed to full ratchet, and left in place for 60 seconds. If the dog did not show gross purposeful movement, ETISO was reduced 15%, a 15 minute equilibration period was allowed, and the clamp was reapplied. When the dog responded, ETISO was increased 10% to further bracket the MAC value. Seven days following MAC determination, the first treatment was administered. Order was randomly assigned in a crossover design and all treatments were separated by at least 7 days. After induction and instrumentation, ETISO concentration was reduced to 90% of the individual’s MAC, and the tail was clamped 15 minutes later. Following a positive response, one of the 4 treatments was administered in the vastus lateralis muscle. Response to application of the tail clamp was assessed every 15 minutes until gross purposeful movement reoccurred. Response times were analyzed using mixed linear model analysis.
Duration of non-response following S, B, M, and MB was 0.0 ± 0.0, 1.5 ± 1.5, 2.63 ± 0.49, and 5.58 ± 2.28 hours (mean ± SD), respectively. Medetomidine produced an effect significantly longer than S, while MB produced an effect significantly longer than S, B, or M (P 0.05).
In conclusion, during 0.9 MAC ISO administration M, but not B provided longer and more consistent duration of non-response when compared to saline, and the combination of M and B prolonged the duration of non-response beyond that of either M or B alone.
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