Effects of oxymorphone and hydromorphone on isoflurane minimum alveolar concentration in dogs

Abstract

The purpose of this study was to assess if the isoflurane (ISO) minimum alveolar concentration (MAC) in dogs is reduced when combined with either oxymorphone (OX) or hydromorphone (HY). We used six healthy, mixed-breed dogs (1–3 year, 17.5–31.0 kg) on three occasions in a randomized crossover study (at least 1 week between treatments). Following mask induction with ISO in 33% oxygen (+N2O), anesthesia was maintained with ISO in 100% O2 (F-circuit) using mechanical ventilation to achieve eucapnia (PaCO2 35–45 mm Hg). The dogs were assigned to receive: OX at 0.05 mg kg−1, HY at 0.1 mg kg−1 or 1 mL of saline (control). The test drug was administered IV and a minimum of 15 minutes was allowed before the first MAC assessment. The dog was equilibrated at each end-tidal ISO for at least 15 minutes. An electrical stimulus (50 V at 50 cycles second−1 for 10 ms) was applied for up to 1 minute to the toe web with a Grass stimulator to determine if purposeful response could be elicited. Adjustment of ISO vaporizer setting was made according to the response. The end-tidal ISO concentration midway between that allowing movement and that preventing it was the MAC for each treatment. If the first MAC was achieved before 1.5 hour after drug administration, it was confirmed to be the same at 1.5 hours (MAC-1.5 hour). Each dog remained anesthetized and MAC determination was repeated starting 3 hours after test drug injection. Final ISOMAC was confirmed at 4.5 hours if achieved earlier (MAC-4.5 hour). Two individuals, unaware of the treatment given, performed and agreed upon all assessments. Analysis of variance (mixed model) showed a significant drug–time interaction (p < 0.0001). A Tukey analog compared the drugs at each time and between times (significance at p < 0.05* from control is reported). MAC-1.5 hour was 1.2% (control), 0.68%* (OX), 0.62%* (HY). The MAC-4.5 hour was 1.1% (control), 0.96% (OX), 0.75%* (HY). It was concluded that OX and HY significantly reduce ISOMAC in dogs at 1.5 hours. After 4.5 hours of anesthesia HY has a significant ISOMAC sparing effect, whereas OX does not.