Cardiac Arrest in Axis Deer During Capture and Chemical Immobilization

The Axis deer (Axis axis) is easily identified from many other deer species by the white spots splashed across its coat. Although the axis deer is native to the Indian subcontinent (where it is called a chital), it has been introduced to nations across the globe. It is even considered an invasive species in some of these areas because it is a competitor for resources.¹

Axis deer have been introduced into parts of Hawaii, Texas, California, Mississipi, Alabama, and countries outside of the United States. Though they were originally intended to remain captive, many escaped captivity and established wild populations. Axis deer can reproduce very quickly, giving them an advantage over many local species of deer.

While some invasive species have been introduced to certain areas as game animals, in their new environments, Axis deer have fewer predators and more resources in their non-native environments, and can easily outcompete local species. For example, in the Hawaiian islands, the axis deer population is reportedly growing at 20% to 30% a year.²

Deer belong to the order Artiodactyla, and to the family Cervidae. Cervids differ from other ruminants in that males grow antlers that are comprised of bone. Male Axis deer grow antlers on a yearly basis, and they use these to compete with other males for access to females. Female Axis deer can reproduce multiple times each year.

Mating with multiple mates (polygamy) the norm for most species of the Cervidae family. Specifically, most deer species practice polygyny. In this form of mating, the largest, most aggressive males control mating access to many different females.¹ This strategy has the benefit of the superior male genetics being the only ones that get passed on. Unfortunately, many males never get the chance to mate, with some having to wait until they are very old to mate. This limits genetic variability within the population and can lead to the development of peculiar traits.³

Unlike many deer species, the Axis deer leans more toward monogamy. Instead of a male trying to create a harem or defend a lek, male Axis deer tend to pick one female to mate with.^2,3 This increases the diversity of the population by allowing more males to breed in every season. With more and younger males breeding, the genetic variability in the population is greatly increased over the practice of polygamy.¹

The Chemical Immobilization of Axis Deer

The management of and research involving wild or captive Axis deer often call for the use of chemical immobilization (sedation and/or anesthesia). In the field, remote drug delivery via dart is the modality of choice, with darts being delivered from the ground, a ground-based vehicle or from a helicopter. In a captive situation (e.g., a zoo or farm), drugs may be delivered via pole syringe or dart.

Neuroleptic drugs should be used with caution, but can be useful in the management of wild and semi-domesticated deer. These drugs facilitate transport of deer and help to decrease stress in acutely captured deer. Chemical immobilization can be also induced with opioids, alpha-2 agonists combined with dissociative drugs⁴ or custom compounded preparations. In zoos and farms, deer may also be handled in drop-floor or hydraulic squeeze chutes. Free-ranging deer are sometimes captured with net guns, drive nets, or clover traps.⁴

Cardiac arrest, or cardiopulmonary arrest (CPA) is characterized by an abrupt, complete failure of the respiratory and circulatory systems. This is a complication that can be brought on by the stress of capture, or as an undesired effect of immobilizing drugs. In CPA, the subsequent lack of oxygen transport can quickly cause systemic cellular death from oxygen depletion. If left untreated, cerebral hypoxia can result in death within four to six minutes of a CPA event.In these cases, prompt cardiopulmonary resuscitation is imperative.

Signs of an impending CPA event in Axis deer 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.

Responding to Cardiac Arrest in Axis Deer

As indicated above, capture and/or chemical immobilization can result in CPA events in Axis deer, particularly under field conditions. The stress of capture (depending upon the method of capture) can significantly increase the likelihood of cardiac arrest in these animals. While under sedation or 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.

Techniques for cardiopulmonary cerebral resuscitation in deer have been adopted from human emergency medicine, and involve three stages:

• Basic life support (BLS)
• Advanced life support (ALS)
• Post resuscitation care⁵

The first stage in managing a CPA event involves establishing an open and clear airway, providing assisted ventilation, and performing chest compressions. If a deer’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,⁴ although this may not be available under some 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 deer 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

An Axis deer that is restored to a perfusing cardiac rhythm may 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 CPA. Mild hypothermia after resuscitation from CPA decreases cerebral oxygen demand and has been shown to improve outcomes.⁴