Determination and evaluation of an optimal dosage of carfentanil and xylazine for the immobilization of white-tailed deer (Odocoileus virginianus)

Using an iteration method, optimal hand-injected immobilization dosages of carfentanil/xylazine (CAR/XYL) were determined for 13 adult white-tailed deer (Odocoileus virginianus). Deer were temporarily restrained in a squeeze chute and were repeatedly immobilized one to four times at 2-5-wk intervals from December 2002 to March 2003. A fixed ratio of 1mg CAR:10 mg XYL intramuscularly was used, increasing or decreasing the dosage until the optimal dosage (defined by an induction time < 3 min and PaCO2 < 60 mmHg) was reached for each animal. Inductions were videorecorded and reviewed by observers blinded to drugs and dosages, who rated qualitative aspects of each induction. There were significant (P < 0.05) dosage-dependent decreases in induction time, time to first effect, PaO2, SaO(2), and arterial pH, and significant dosage-dependent increases in PaCO2 and quality ratings. The median optimal dosage (mOD) was 0.03 (range, 0.015-0.06) mg/kg CAR+0.3 (range, 0.15-0.6) mg/kg XYL. Induction times using the mOD were rapid (median 3.0 min [range, 1.8-10.0]), but quality ratings were considered undesirable for nine of 13 deer. Increased rectal body temperatures of 40.6 +/- 0.5 C (mean +/- SD) were noted in all deer and hyperthermia (T > 41 C) was noted in three. There was a positive correlation between body temperature and induction time (r=0.44). Heart rates significantly decreased from 5 to 15 min postinduction and remained decreased at the 20-min reading; there was occasional bradycardia. There was a significant increase in pH from 10 to 20 min postinduction, but metabolic acidemia (pH < 7.3) persisted throughout the immobilization periods for all deer. Possible hypoxemia (SaO(2) and SPO2 < 90 mmHg but PaO2 > 60 mmHg),vas present after induction, while hypercapnea (PaCO2 > 60 mmHg) did not occur. Reversal times with naltrexone and yohimbine were rapid (mean 3.7 +/- 1.5 min) and uneventful, with no evidence of renarcotization. Although the median optimal dosage produced rapid inductions, no respiratory depression, complete reversal after antagonist administration, and no renarcotization, negative attributes included elevated body temperatures, acidemia, and undesirable induction qualities.