Abstract

Potent opioids such as carfentanil and etorphine are commonly used for hoofstock anesthesia because they provide a rapid induction.^2,3 However, they can have serious side effects that include profound hypoxemia, cardiopulmonary depression, bradycardia, and decreased gastrointestinal motility.⁴ The purpose of this study was to investigate the use of naloxone to reverse or partially reverse these effects while maintaining adequate anesthesia.¹ Eight healthy female Rocky Mountain wapiti (Cervus elaphus nelsoni, mean age ± SD, 10.1±2.6 years; mean weight ± SD, 265±26 kg) were used in a crossover study to compare the respiratory and cardiovascular effects of IM carfentanil-xylazine-placebo (CXP) and IM carfentanil-xylazine supplemented with IV naloxone (CXN). The study was performed during April and May 1999 in Fort Collins, CO (elevation 1495 m and P_atm = 640 mm Hg).

Anesthesia was induced and maintained with IM carfentanil (10 µg/kg; Wildnil, Wildlife Pharmaceuticals, Fort Collins, CO, USA) and xylazine (0.1 mg/kg; Cervizine, Wildlife Pharmaceuticals, Fort Collins, CO, USA), dosages previously reported to be effective in wapiti.^2,3 Mean induction time was 3.7±0.9 minutes. After an IV catheter and monitoring equipment were placed, animals were administered an IV bolus of naloxone (2 µg/µg carfentanil; Naloxone HCl injection, Elkins-Sinn, Cherry Hill, NJ, USA) or an equivalent volume of placebo (normal saline). Arterial blood was obtained directly from an arterial catheter and a portable blood gas analyzer was used to measure arterial blood oxygen (P_aO₂) and carbon dioxide (P_aCO₂) partial pressures.

At the time of instrumentation, animals were profoundly hypoxemic (P_aO₂ was 23.0±4.1 mm Hg; mean ± SD). Oxygenation values increased to 50.2±7.3 mm Hg (mean ± SD) at t=10 minutes. after naloxone administration; mean P_aO₂ of placebo-treated animals did not change significantly over time. The difference in P_aO₂ between treatment groups was significant for all measurements from t=10 to t=30 minutes, however, the P_aO₂ in the CXN group gradually returned toward baseline levels over time (P_aO₂ = 34.61±4.7 at t=30 minutes). Three animals in the placebo group exhibited electrocardiogram (ECG) changes suggestive of myocardial hypoxia. Hypoxemia is likely to have been due to multiple factors including hemodynamic alterations and lateral recumbency, in addition to the respiratory depression induced by the opioids.

Anesthesia with carfentanil-xylazine also produced mild hypercapnia (54.3±4.6 mm Hg), mild acidosis (pH=7.32±0.04), and mild tachycardia (63.9±16.7 bpm). There were significant differences (p<0.05) in P_aO₂, P_aCO₂, heart rate, and respiratory rate between treatment and placebo groups. Significant differences between groups were not detected for mean blood pressure, hematocrit, or serum electrolytes.

After naloxone treatment, one animal suddenly stood and walked; all other animals remained anesthetized after naloxone administration. Reversal of anesthesia was achieved with naltrexone (100 mg/mg carfentanil; 25% IV and 75% SC; Trexonil, Wildlife Pharmaceuticals, Fort Collins, CO, USA) and yohimbine (0.1 mg/kg; Antagonil, Wildlife Pharmaceuticals, Fort Collins, CO, USA). Reversal time was s 4 minutes for both groups.

This investigation documents the profound hypoxemia that is induced with carfentanil-xylazine anesthesia in wapiti. It also demonstrates the efficacy of a bolus of naloxone as a practical method of improving oxygenation. In the wapiti of this study, an IV bolus of naloxone, at a dose of 2 µg naloxone per µg carfentanil, was effective in ameliorating carfentanil-induced hypoxemia. However, the arousal of one animal suggests that this naloxone dose may be the high-end dose for captive wapiti, that it may be more appropriate to start with a lower dose, and that caution should be used when administering this treatment.

Acknowledgments

We acknowledge Wildlife Pharmaceuticals for providing drugs and Kendall, Sherwood-Davis, and Geck for supplying equipment used in this investigation. We thank Drs. W.R. Lance and C. Pichet for valuable discussions. We also thank faculty and staff of the Anesthesia Section at Colorado State University and K.T. Castle for technical assistance.

Literature Cited

1.  Drummond GB, Davie IT, Scott DB. Naloxone: Dose-dependent antagonism of respiratory depression by fentanyl in anaesthetized patients. Br J Anaesth. 1977;49:151–154.

2.  Kreeger TJ. Handbook of Wildlife Chemical Immobilization. International Wildlife Veterinary Services. Laramie, Wyoming. 1996.

3.  Miller MW, Wild MA, Lance WR. Efficacy and safety of naltrexone hydrochloride for antagonizing carfentanil citrate immobilization in captive Rocky Mountain elk (Cervus elaphus nelsoni). J Wildl Dis. 1996;32:234–239.

4.  Wagner AE, Muir WW, Grospitch BJ. Cardiopulmonary effects of position in conscious cattle. Am J Vet Res. 1990;51:7–10.