This study evaluated the effect of preoperative epidurally administered morphine (M; n = 9), lidocaine (L; n = 9), their combination (ML; n = 10), and sterile water (W; n = 11) on inhalational anesthetic requirements for dogs undergoing hindlimb orthopedic procedures. All dogs were ASA status I or II, 30.5 ± 11.8 kg, and 2.5 ± 2.2 years. Dogs were anesthetized with thiopental 10 - 15 mg/kg IV, maintained with isoflurane, and mechanically ventilated (ETCO2 30 - 40 mmHg). In a blinded, randomized manner, W [X ml, X = 0.1 x distance (cm) from occiput to lumbosacral space (LS)]; M (0.1 mg/kg) in X ml sterile water; L (2%) X ml; or ML (M, 0.1 mg/kg in X ml L); was injected into the LS epidural space. A period of 30 - 60 minutes was allowed from start of anesthesia to surgical incision to achieve steady state end-tidal isoflurane concentration (ETISO). Reductions in ETISO occurred every 10 minutes throughout the surgical procedure to determine the lowest ETISO at which response to surgical stimulation was seen. The ETISO at surgical incision (SI, 30 - 60 minutes after epidural injection) and lowest ETISO between 61 - 90 and 91 - 120 minutes after epidural injection were analyzed with ANOVA for repeated measures with significance at P < 0.05. Data are expressed as mean ± SEM. At SI: ETISO was 1.74 ± 0.07% (M), 1.65 ± 0.06% (L), 1.59 ± 0.05% (ML), 1.84 ± 0.07% (W). At 61 - 90 minutes: ETISO was 1.06 ± 0.11% (M), 0.85 ± 0.09% (L), 0.78 ± 0.07% (ML), 1.13 ± 0.10% (W). At 91 - 120 mins: ETISO was 0.85 ± 0.15% (M), 1.11 ± 0.13% (L), 0.81 ± 0.10% (ML), 1.17 ± 0.14% (W). Significant differences at SI were M and L vs W; at 61 - 90 minutes, M vs ML and L and ML vs W; and at 91 - 120 minutes, M and ML vs W. Reduction in body temperature occurred in all groups throughout the study but no significant differences between groups were noted. We conclude that lidocaine significantly reduces ETISO but only up to 90 minutes after epidural injection. Morphine also significantly reduces ETISO but onset of effect was slower as compared to lidocaine or the combination with significant differences not seen until 90 minutes after epidural injection. The morphine/lidocaine combination is superior in significantly reducing ETISO with more rapid onset of effect (< 60 minutes) compared to morphine or sterile water and a longer duration of effect (> 91 minutes) compared to lidocaine or sterile water.
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