Effects of oxymorphone and hydromorphone on the minimum alveolar concentration of isoflurane in dogs. 
To quantify the change in the minimum alveolar concentration (MAC) of isoflurane (ISO) associated with oxymorphone (OXY) or hydromorphone (HYDRO) in dogs.  Randomized crossover study with at least 1 week between assessments.  Six young, healthy, mixed-breed dogs (1-3 years old), weighing 24.7 +/- 4.70 kg.  Following mask induction, anesthesia was maintained with ISO in 100% O(2) using mechanical ventilation.  The dogs received 0.05 mg kg(-1) OXY, 0.1 mg kg(-1) HYDRO, or 1 mL saline (control) IV.  Following equilibration (15 minutes) at each percentage ISO tested, a supramaximal electrical stimulus was applied to the toe web and the response was assessed.  Two separate MAC determinations were carried out during 4.5 hours of anesthesia, with completion of the evaluations at 1.5-2 and 4-4.5 hours after drug administration.  A two-factor anova was used to determine whether there was a time or treatment effect on MAC and a Tukey test compared the drug effects at each time.  Significance is reported at p < 0.05.  The mean MAC values (+/-SD) were 1.2 +/- 0.18 and 1.2 +/- 0.16% for control, 0.7 +/-0.15 and 1.0 +/- 0.15% for OXY, and 0.6 +/- 0.14 and 0.8 +/- 0.17% for HYDRO.  The initial MAC with OXY and the MAC determined at both times with HYDRO were significantly different from the control MAC values.  Both OXY and HYDRO significantly reduced the MAC of ISO in dogs at 2 hours.  At approximately 4.5 hours, HYDRO had a significant MAC-sparing effect, whereas OXY did not.  Although both OXY and HYDRO resulted in a significant reduction in the MAC of ISO at approximately 2 hours, HYDRO may be preferred for procedures of long duration and rarely needs repeated dosing before 4.5 hours.