The isoflurane-sparing and clinical effects of a constant rate infusion of remifentanil in dogs

To evaluate the isoflurane-sparing and clinical effects of two constant rate infusions of remifentanil in healthy dogs undergoing orthopaedic surgery. Prospective, randomized clinical study. Forty-one American Society of Anesthesiologists I-II client-owned dogs (age, 7 months-9 years; body mass 11-59 kg). Dogs were randomly assigned to one of three groups and received either: intramuscular (IM) meperidine 2 mg kg^-1 every 2 hours throughout surgery (control group (C); n = 13); remifentanil infused intravenously (IV) at 0.1 μg kg^-1 minute^-1 (low remifentanil group (L); n = 14) or remifentanil infused at 0.25 μg kg^-1 minute^-1 IV (high remifentanil group (H); n = 14). Anaesthesia was induced with thiopental administered to effect and maintained using isoflurane in 100% oxygen. During controlled ventilation when the end-tidal CO₂ was maintained between 4.65 and 5.98 kPa [35-45 mmHg], the end-tidal isoflurane concentration (e′iso%), mean arterial blood pressure (MAP) and heart rate (HR) were measured every 5 minutes. Bradycardia (HR < 40 minute^-1 lasting >5 minutes) was corrected with 0.01 mg kg^-1 IV glycopyrrolate. Data were analysed using the Kruskal-Wallis test with a post-hoc Mann-Whitney U-test and Bonferroni correction. Statistical significance was accepted at ≤ 0.05. Data are expressed as mean ± standard deviation. The e′iso% was reduced in a dose-dependent manner by remifentanil. In C, e′iso% was 1.28 ±0.13 and was significantly different from L (0.78 ± 0.17, p < 0.001) and H (0.65 ± 0.16, p < 0.001). HR was significantly different between groups (p < 0.001). There were no significant differences in MAP between groups. Glycopyrrolate was required in two, three and six dogs in the C, L and H groups respectively. Remifentanil infusion reduced the isoflurane concentration required for surgical anaesthesia during orthopaedic surgery. Remifentanil infusions may be a useful additive to isoflurane anaesthesia in healthy dogs.