Cardiac Arrest in Impala During Capture and Chemical Immobilization
The impala (Aepyceros melampus) is one of the most widely-distributed African antelopes, and one of the most dominant species of antelope on the African savannah. Known for their spectacular, iconic leaps when alarmed or pursued by predators, this antelope encompasses several subspecies, some of which are not universally agreed upon.1 The most common subspecies is Aepyceros melampus rendilis, which is found in East Africa. The black-faced impala, Aepyceros melampus petersi, is found further south. Impala are most often found in Kenya, Zimbabwe, Uganda, Zambia, Botswana, and Southern Angola to northern South Africa.
The impala stands 28–36 inches high and weighs 90–170 pounds, and males are approximately 20% heavier than females. They are tan in color with white underparts and a black stripe that extends from the top of the rump down the back of each thigh.2 Rams have horns which are lyre-shaped and reach a length of around 30 inches.
Impala are able to adapt to different environments of the savannas. In some areas, they are grazers and browsers in others. Impala herds typically stay within a few miles of water sources. Younger rams live in bachelor herds, while those strong enough establish or take over breeding herds.3
Impalas have a polygynous mating system; this means that each male mates with numerous females within the herd.3Most of their breeding occurs between March and May, during which pregnant females live in isolation until they give birth. After 190 to 200 days, they give birth to a single calf, at which time the calf and the female rejoin the herd.
Risks for Cardiac Arrest
One of the most serious complications associated with chemical immobilization events is cardiac arrest in the target animal. Cardiac arrest, or cardiopulmonary arrest (CPA) is 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.4 If left untreated, cerebral hypoxia can result in death within four to six minutes.5 In these cases, immediate cardiopulmonary resuscitation is imperative.
Capture and/or chemical immobilization can result in CPA events in impala, particularly under field conditions. In certain 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)4 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.
According to the available literature, each species of antelope has unique anesthesia requirements, and intra-species dosages vary because of individual responses to anesthetic agents.6,7 These variations often present an increased risk of complications during anesthetic events. It has been widely reported that until the advent of potent opiates and drug formulations, many antelope species were notoriously difficult to safely capture or anesthetize.6
Monitoring core body temperature is essential when anesthetizing impala,6,7 and intubation has been widely recommended for any anesthetized antelope that needs to be anesthetized for longer 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 chief component antelope anesthesia.6
Responding to Cardiac Arrest in Impala
Cardiopulmonary cerebral resuscitation in impala involves three stages: basic life support (BLS), advanced life support (ALS), and post resuscitation care.6 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 safely ventilated with a bag-valve mask,4 although this may not always 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.4 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.7
Impala 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.4
5Pablo L.S. Current concepts in cardiopulmonary resuscitation. World Small Anim Vet Assoc World Congr Proc:2003.
6Ball, L. Antelope Anesthesia. Wiley Online Library, 25 July 2014, https://doi.org/10.1002/9781118792919.ch60.
7Kreeger T., Arnemo, J., Raath, J. Handbook of Wildlife Chemical Immobilization, International Edition, Wildlife Pharmaceuticals, Inc., Fort Collins, CO. (2002).
About NexGen Pharmaceuticals
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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