Cardiac Arrest in Nyala Antelope During Capture and Chemical Immobilization
The nyala is a large antelope that is found in the eastern part of southern Africa. Comprised of two subspecies, the lowland nyala (Tragelaphus agasi) and the mountain nyala (Tragelaphus buxtoni), these antelope live near areas of dense brush and on the fringes of forests, close to water sources. The males, females and calves have vertical white stripes along the torso and flanks. Males are larger than the females, grey in color, while the females and calves are a reddish-brown. The males also have spiral horns and a crest of longer hair down the back of the neck, down the throat and under the abdomen.1
Nyala prefer thick vegetation, although they often venture into open areas to graze on grasses and succulent, high-protein vegetation.1 It is thought that these animals are not widely spread chiefly due to their dietary preferences. Nyala do not have the explosive running ability of many other African antelope, so they depend on blending into vegetation to evade predators.2
Chemical Immobilization of Nyala
In the field and in the zoo setting, the chemical immobilization of nyala is sometimes necessary for the purposes of medical evaluation and treatment, research or relocation. Cardiac arrest, or cardiopulmonary arrest (CPA) is one of several serious complications that can arise during chemical immobilization events. Cardiac arrest is characterized by an abrupt, complete failure of the respiratory and circulatory systems. The resulting lack of oxygen transport can quickly cause systemic cellular death from oxygen depletion.3 If left untreated, cerebral hypoxia can result in death within four to six minutes of a CPA event.4 In these cases, prompt cardiopulmonary resuscitation is imperative.
Capture and/or chemical immobilization can result in CPA events in nyala, particularly under field conditions. In some instances, the stress of capture (depending upon the method of capture) can significantly increase the likelihood of cardiac arrest in these animals. While under anesthesia, common causes of CPA can include vagal stimulation, unstable cardiac arrhythmias, severe electrolyte disturbances, exacerbated cardiorespiratory disorders (e.g., congestive heart failure, hypoxia)3 or a variety of comorbidities. Signs of an impending CPA event can include dramatic changes in breathing effort, rate, or rhythm, significant hypotension, absence of a pulse, irregular or inaudible heart sounds, changes in the heart rate or rhythm; changes in mucous membrane color and fixed, dilated pupils.
Each species of antelope has its own anesthesia recommendation with intra-species variations of dosages because of the diverse individual responses to anesthetic agents.5,6 These can present an increased risk of complications like cardiac arrest during anesthetic events. It has been widely reported that until the advent of potent opiates, certain antelope species were very difficult to safely capture or anesthetize.5 Although carfentanil was reported as effective in many captures, more recently, the combination of butorphanol and azaperone have become popular in the chemical immobilization of pronghorn.
Monitoring core body temperature is essential in antelope anesthesia,5,6 and intubation has been widely recommended for any anesthetized antelope that needs to be transported or anesthetized for greater than one hour. Until the more recent use of formulated drugs (e.g., combinations of α2-agonists such as medetomidine, detomidine, xylazine and their reversal agents), opioids were the mainstay of antelope anesthesia in wildlife and captive care.5
Responding to Cardiac Arrest in Nyala
Cardiopulmonary resuscitation in nyala involves three stages: basic life support (BLS), advanced life support (ALS), and post resuscitation care.5 The first stage involves establishing an open and clear airway, providing assisted ventilation, and performing chest compressions. If the animal’s pulse becomes absent or weak, all administration of immobilizing drugs must be suspended and external cardiac massage should be initiated. Veterinary patients can usually be easily and safely ventilated with a bag-valve mask.3
Venous access can be established by using such methods as intraosseus catheter placement and venous cutdown, in which a small opening is created in a vein to allow passage of a needle or cannula.3 Epinephrine at 0.2 mg/kg (concentrated at 1/10,000) should be given IV or intracardially (IC) while cardiac massage continues. If the animal fails to respond, 0.1 ml/kg IV or IC calcium chloride may be given. If there is still no response, the epinephrine and calcium chloride may be re-administered with 10-20 mEq IV or IC sodium bicarbonate.6
Nyala that are restored to a perfusing cardiac rhythm can experience rearrest, especially if the original cause of the CPA event has not been identified. Therefore, resuscitated animals should have cardiovascular and ventilatory support during the period following a CPA event. Mild hypothermia after resuscitation from CPA decreases cerebral oxygen demand and has been shown to improve outcomes.3
4Pablo L.S. Current concepts in cardiopulmonary resuscitation. World Small Anim Vet Assoc World Congr Proc:2003.
5Ball, L. Antelope Anesthesia. Wiley Online Library, 25 July 2014, https://doi.org/10.1002/9781118792919.ch60.
6Kreeger T., Arnemo, J., Raath, J. Handbook of Wildlife Chemical Immobilization, International Edition, Wildlife Pharmaceuticals, Inc., Fort Collins, CO. (2002).
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NexGen Pharmaceuticals is an industry-leading veterinary compounding pharmacy, offering sterile and non-sterile compounding services nationwide. Unlike other veterinary compounding pharmacies, NexGen focuses on drugs that are difficult to find or are no longer available due to manufacturer discontinuance or have yet to be offered commercially for veterinary applications, but which still serve a critical need for our customers. We also specialize in wildlife pharmaceuticals, including sedatives and their antagonists, offering many unique options to serve a wide array of zoo animal and wildlife immobilization and anesthesia requirements.
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