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  • Author or Editor: J. Desmond Baggot x
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Objective—To quantitate dose- and time-related magnitudes of interactive effects of morphine (MOR) and isoflurane (ISO) in horses and to characterize pharmacokinetics of MOR in plasma and the ventilatory response to MOR during administration of ISO.

Animals—6 adult horses.

Procedure—Horses were anesthetized 3 times to determine the minimum alveolar concentration (MAC) of ISO in O2 and then to characterize the change in anesthetic requirement as defined by the alteration in ISO MAC following IV administration of saline (0.9% NaCl) solution and 2 doses of MOR (low dose, 0.25 mg/kg; high dose, 2.0 mg/kg). Arterial blood samples were obtained before and after MOR and analyzed.

Results—Mean ± SD baseline ISO MAC was 1.43 ± 0.06%. The ISO MAC did not change with time after administration of saline solution. Effects of MOR on ISO MAC varied. Maximal change in MAC ranged from –20.2 to +28.3% and –18.9 to +56.2% after low and high doses of MOR, respectively. Typical half-life of MOR in plasma was 40 to 60 minutes and related to dose. Mean PaCO2 increased from 70 mm Hg before MOR to 88 to 102 mm Hg for 30 to 240 minutes after the high dose of MOR. Recovery from anesthesia after administration of the high dose of MOR was considered undesirable and dangerous.

Conclusions and Clinical Relevance—Our results do not support routine clinical use of MOR administered IV at dosages of 0.25 or 2.0 mg/kg as an adjuvant to anesthesia in horses administered ISO. (Am J Vet Res 2003;64:166–175)

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in American Journal of Veterinary Research


Objective—To determine pharmacokinetics and selected cardiopulmonary effects of fentanyl in isoflurane-anesthetized rhesus monkeys.

Animals—6 adult male rhesus monkeys.

Procedure—Fentanyl (8 mg/kg of body weight, IV) was administered to 6 monkeys anesthetized with isoflurane. End-tidal isoflurane concentration and esophageal temperature were kept constant, and ventilation was mechanically assisted. Heart rate, rhythm, aortic blood pressure, and blood pH, gas, and fentanyl concentrations were determined before and for 8 hours after administration of fentanyl. Pharmacokinetics of fentanyl were derived by use of noncompartmental methods based on statistical moment theory.

Results—Heart rate and mean arterial pressure decreased transiently following fentanyl administration. Maximal decreases were observed 5 to 15 minutes after administration. Arterial pH, PaCO2, and PaO2 ranged from 7.46 ± 0.04 to 7.51 ± 0.05 units, 29.2 ± 3 to 34.6 ± 4.4 mm Hg, and 412.6 ± 105.3 to 482.9 ± 71.2 mm Hg, respectively. The clearance, volume of distribution area, volume of distribution steady state, mean residence time, area under the curve, elimination rate constant, and half-life were 32.5 ± 2.48 ml/kg/min, 9.04 ± 1.91 L/kg, 7.0 ± 1.2 L/kg, 218.5 ± 35.5 min, 0.247 ± 0.019 mg/ml/min, 0.004 ± 0.001/min, and 192.0 ± 33.5 min, respectively.

Conclusions and Clinical Relevance—Transient but potentially clinically important decreases in heart rate and mean arterial pressure were observed following fentanyl administration. Distribution and clearance data were similar to those reported for dogs and humans. (Am J Vet Res 2000;61:931–934)

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in American Journal of Veterinary Research