Cardiac Arrest in Antelope During Capture and Chemical Immobilization

Cardiac arrest, or cardiopulmonary arrest (CPA) is characterized by an abrupt, complete failure of the respiratory and circulatory systems. The subsequent lack of oxygen transport can quickly cause systemic cellular death from oxygen depletion.1 If left untreated, cerebral hypoxia can result in death within four to six minutes of a CPA event.2 In these cases, prompt cardiopulmonary resuscitation is imperative.

Capture and/or chemical immobilization can result in CPA events in antelope, 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)¹ 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.

Antelope Chemical Immobilization

Antelope are ruminants belonging to the families Antilocapridae and Bovidae. The pronghorn antelope, Antilocapra americana, is the only member of the former.³ Antelope males and the females of some species have unbranched horns attached to the frontal bones of the skull.² It is generally accepted that there are several distinct subfamilies that fall within the general term of antelope, but this remains a matter of scientific debate. For example, some of the literature holds that the pronghorn antelope is not actually a true antelope.

Antelope are very widespread animals, comprising around 90 of the approximately 140 known species of the Bovidaefamily (which includes sheep, goats, and domesticated cattle). Antelope belong to the order Artiodactyla, which includes giraffe and pigs. The most distinctive feature of the order Artiodactyla is their even number of hooves.

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.^3,4 These variations often present an increased risk of complications during anesthetic events. It has been widely reported that until the advent of potent opiates, the pronghorn antelope was very difficult to safely capture or anesthetize.³ 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,^3,4 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.³

Responding to Cardiac Arrest in Antelope

Adopted from human emergency medicine, cardiopulmonary cerebral resuscitation in antelope involves three stages: basic life support (BLS), advanced life support (ALS), and post resuscitation care.³ The first stage involves establishing an open and clear airway, providing assisted ventilation, and performing chest compressions. If an antelope’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,¹ the caveat being that this may not be available under field conditions.

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.¹ 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

Patients 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 patients usually should have cardiovascular and ventilatory support during the period following CPA. Mild hypothermia after resuscitation from CPA decreases cerebral oxygen demand and has been shown to improve outcomes.¹