Chemical Immobilization-Related Cardiac Arrest in Antelope

“Antelope” is a common name that has been ascribed to various members of the family Antilocapridae and Bovidae. The pronghorn antelope, Antilocapra americana, is the only member in the Antilocapridae family.1 Antelope males and the females of some species have unbranched horns attached to the frontal bones of the skull.2 Until the advent of potent opiates, the pronghorn was very difficult to safely capture or anesthetize.1

According to the available literature, each species of antelope has its own anesthesia recommendation with intra-species variations of dosages because of diverse individual responses to anesthetic agents.^1,2 These variations can present risks of various surgical complications in these species and must be taken into account when planning chemical immobilization, as well as other factors (e.g., stress, venue, individual animal and field conditions).

The veterinary care of non-domestic hoofstock has become commonplace in recent years due to the integration of veterinary medicine in wildlife management programs, zoological collections, exotic animal ranching expansions and the private collection of wildlife species.² Thus, research and wildlife veterinarians are required to amass the requisite knowledge associated with safely anesthetizing and handling these animals.

Chemical Immobilization of Antelope

Research, conservation and management programs focusing on antelope can require the capture and manipulation of animals, despite the ongoing development of non- or minimally-invasive procedures that allow for obtaining information without the need to handle them. Certain information on individual animals can only be collected via capture.

Effective wildlife management must include efficient, safe methods for short-term restraint to reduce stress on animals while facilitating actions such as sample collection, restraint, handling for the purpose of relocation, or release from snares or entanglements.^3,4 Chemical immobilization has become prevalent in large mammal capture because of the lower mortality and injury rate than other techniques. Additionally, drug induced side effects have been dramatically reduced in recent years due to the refinement of immobilizing agents and novel drug formulations. Depending on the specific drugs used, these factors have also significantly reduced the psychological stress caused to animals while immobilized and the stress caused by their displacement, which is necessary to perform manipulation and procedures.

The sedation and anesthesia of antelope requires the knowledge of the pharmacology of the drugs being used, as well as the wide variation in dose response among sub-species of these animals. A major challenge which impacts potential complications is correlating the available pharmaceutical tools with the environment and conditions, as well as the procedures and events preceding, during, and following the anesthetic event.¹ For example, a proven anesthesia protocol for a captive environment may not be appropriate in the field or in a large pasture enclosure.

Today, the quality of sedation, anesthesia and analgesia achievable in antelopes and other wildlife species has been made possible through the availability of new, receptor-specific and highly potent agonist-reversible pharmaceuticals and the improved knowledge of CNS receptors.

Planning for Procedures Mitigates Risk

Ideally, an anesthetic plan should be created, evaluated, and approved weeks to months ahead of a field-capture event.⁵An anesthetic protocol should account for the specific species, type of restraint, and the intended procedure, as well as the environmental conditions at the intended capture site.

Any involved personnel should have basic training in anesthetic emergency management. At a minimum, the anesthesia and surgical personnel should be trained in the causes of and treatments for apnea, bradycardia, tachycardia, hypothermia, and cardiopulmonary arrest.⁵ They should also be prepared to address hypoxia, hypotension, and hypoventilation if reliable monitoring of oxygen saturation, carbon dioxide excretion and blood pressure equipment is available. While it is impossible to be prepared for every eventuality, it is both prudent and ethical to be prepared for the most-common anesthetic and surgical emergencies.

The anesthetist or party administering drugs via remote delivery must be able to safely deliver the chosen drug combination in the field situation.⁵ In many situations, remote delivery of drugs to an unrestrained animal is faster and less stressful to the animal than physical restraint in a trap.⁴ Extended chasing or long restraint in a trap can increase the risk of self-injury and capture myopathy.

Cardiac Arrest in Antelope

Even when all procedures have been diligently followed, all safety measures taken and the drugs have been prepared and administered properly, medical emergencies can occur when chemically immobilizing antelope. Cardiac arrest is defined as the cessation of circulation brought on by a loss of effective cardiac function. This can be brought on by a number of causes, including the immobilizing drugs, hypoxia, acidosis, alkalosis or hypothermia. If an animal’s pulse is absent or weak, all administration of immobilizing drugs must be suspended and external cardiac massage should be initiated.⁴

Circulation should be promoted by compressing the chest externally. The following are essential when performing chest compressions:

• The animal is in lateral recumbency.
• Elbows should be locked, with one hand on top of the other and with shoulders directly above the hands.
• Compressions should be performed over the widest part of the thorax in animals with a thoracic conformation that is equally wide and tall.
• Compressions may be performed directly over the heart at the fourth and fifth intercostal space.
• The compression rate should be 100–120 compressions/minute regardless of the size of the animal.
• Each compression should be delivered quickly, compressing 1/3 to 1/2 of the width of the thoracic wall and allowing full recoil between compressions.
• Thoracic compressions should be done for a total of 2 minutes without interruption.⁵

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.⁴

It is important to understand that no procedure involving anesthesia is risk-free. These include factors such as remote location, working conditions, lack of knowledge of the health and physical condition of the animal and the fact that the animal may be severely stressed.