Abstract

Investigation of new chemical immobilization agents or current agents in novel combination or new species is important in improved safety and effectiveness in exotic ungulate species. The majority of investigation of immobilization utilizing alpha-2-adrenoreceptor agonists in a variety of species, including ungulates, was conducted by Harry Jalanka.¹ The goal of this study was to determine if the alpha-2-adrenoreceptor agonist medetomidine (MET) combined with ketamine HCl (KET) and antagonized with the alpha-2-adrenoreceptor antagonist atipamezole (ATI) would serve as a safe and effective anesthetic alternative to the narcotic chemical immobilization agents typically used in ungulate species, particularly bovids. The impala was selected due to its abundance, access, and importance as a sentinel species in disease studies in ungulates in the Kruger National Park, South Africa. Also, it is representative of bovid species difficult to anesthetize with currently available narcotic agents.

In April of 1996, 64 anesthetic procedures were performed on 27 individual impala. Preliminary procedures were conducted on recently captured, boma-acclimated male impala to determine an effective and safe MET/KET dose for remote-delivery anesthesia. After determining an effective dose, free-ranging impala were anesthetized; these animals were placed in bomas to permit additional anesthetic procedures for comparison within individuals. The data collected during each anesthetic procedure included: 1) induction time (time of darting to time of recumbency) recorded in minutes (min) and level of activity during this period; 2) level of sedation attained; 3) physiologic parameters of respiration, heart rate, muscle relaxation, blood pressure, blood oxygen saturation, arterial pH, pO₂, pCO₂, during immobilization; 4) response to manipulative procedures of physical examination, transport, tracheal intubation, blood collection; 5) time from injection of antagonist to standing; and 6) degree of anesthetic reversal attained after antagonism. Each anesthetic procedure was given an overall rating of poor, fair, or good assessing the general plane of anesthesia, response to manipulation or touch, and ability to collect data. The established protocol called for darting, permitting the animal to achieve voluntary recumbency (no handling before 15 min after darting), initial physiologic assessment and weighing, tracheal intubation, transport to veterinary laboratories, data collection, transport to the bomas, and reversal of anesthesia. The same size plastic dart and same needle gauge and length was employed in all animals.

There were four anesthetic groups, based upon the dose of MET and/or KET actually received (based upon known body weight), with MET ranging from 150–400 µg/kg and KET ranging from 1–8 mg/kg. Two additional anesthesia groups included animals receiving MET alone or KET alone. Variables included: field versus pen immobilization, hyaluronidase (HYL) added to the MET/KET dose to enhance absorption of the anesthetic agents, particularly in field anesthesias (HYL dose of 7500 IU consistent in all cases), sex, age, dose of MET and/or KET. In all procedures where the level of sedation was sufficient to obtain repeated samples, the following data points were collected every 5 min for eight data points, beginning 15–30 min after down time: 1) respiratory rate, 2) heart rate (auscultation and palpable pulse), 3) rectal temperature, 4) blood oxygen saturation via pulse oximetry, 5) indirect systolic and diastolic blood pressure (non-invasive cuff on front limb), 6) presence of ocular reflexive responses (palpebral, iris diameter, globe position, 7) reflex of ear to touch, 8) muscle tension in limbs (relaxed or tense/ability to flex manually), 9) pain response to needle prick to limbs, perineal area, and 10) muscle jaw tone. Simultaneous arterial and venous blood samples were taken every 10 min during data collection period (goal of four arterial and venous samples per animal) for blood gas analysis. EDTA and whole blood samples were collected at the first observation point for CBC and serum chemistry analysis.

Antagonism of MET was achieved utilizing atipamezole, administered intramuscularly in two sites, at three doses ranging from 200–1500 µg/kg; the mean time from injection to standing was 6.1 min in all animals receiving MET/KET, 2.9 min in animals receiving MET alone.

Observations

1.  KET alone, at 8 mg/kg body weight, did not result in recumbency in any animal and only minimal visible effect.

2.  MET alone in the dose range of 250–400 µg/kg body weight resulted in recumbency and a relaxed animal, but arousal could be induced with loud noise or physical manipulation.

3.  MET at 300–400 µg/kg and KET at 5–8 mg/kg produced the shortest anesthesia induction time (mean 4 min) and best relaxation. This was followed by MET at 200–300 µg/kg and KET at 3–6 mg/kg with a mean induction time of 6.5 min.

4.  The impala less stressed prior to darting, represented by the field anesthetized animals, had shorter induction times and more consistent and sustained relaxation compared to boma-anesthetized impala.

5.  7500 I.U. of hyaluronidase mixed in the dart with the anesthetic drugs resulted in shorter induction times and more consistent and sustained relaxation in almost all cases.

6.  Atipamezole produced a rapid, smooth reversal/recovery from anesthesia when given intramuscularly. Intravenous antagonism resulted in a rapid reversal with hyperexcitation. The most effective dose rate was 400–600 µg/kg body weight (ATI:MET ratio of 2.3:1).

7.  Atipamezole did not fully antagonize MET in any impala as evidenced by post-reversal sedation that was observed as mild sedation for $4 hr post reversal. The animals were easily aroused with visualization of humans or disturbance by other impala. This sedation would make the animal susceptible to predators in free-ranging conditions and susceptible to aggression from other impala.

8.  MET/KET is not a reliable field combination due to the post-reversal sedation and the need to wait 15 min post-darting to achieve adequate drug effect for physical manipulation. This combination does have applicability in captive situations.

Literature Cited

1.  Jalanka HH. 1993. New alpha-2-adrenoceptor agonists and antagonists. Artiodactylids. In: Zoo and Wildlife Animal Medicine. Current Therapy 3. Pp. 477–481.