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)