Cardiac Arrest in Elk During Capture and Chemical Immobilization

The elk (Cervus elaphus canadensis) is the largest subspecies of red deer (Cervus elaphus) found in North America and in the high mountains of Central Asia. Also called wapiti, elk are members of the deer family, (Cervidae, order Artiodactyla).

The North American elk (Cervus elaphus) is further split into six subspecies by some biologists, two of which are now considered extinct:

• Rocky Mountain (Rocky Mountain West) – these elk have the largest antlers of all subspecies
• Roosevelt’s (Coastal Pacific Northwest) – these are the largest in body size of all subspecies
• Tule (Central California) – these have the smallest body size of all subspecies
• Manitoban (northern Great Plains)
• Merriam’s (Southwest and Mexico) – Extinct
• Eastern (east of the Mississippi) – Extinct^1,2

Elk were once found throughout much of the Northern Hemisphere, from Europe through northern Africa, Asia and North America. It is believed that extensive hunting and habitat destruction have limited them to a portion of their former range. Today, large populations in North America are found only in the western United States from Canada through the Eastern Rockies to New Mexico, and in a small region of the northern lower peninsula of Michigan.

Exceeded in size only by the largest of the cervids, the moose, adult male elk average around 840 pounds. The body mass of elk populations varies considerably within and between populations and increases from south to north. The largest bulls can exceed 1,100 pounds in weight. Compared to other cervids, female elk are more similar to bulls in external appearance and body mass. During winter, both have well-developed, dark neck manes. Elk range in color from dark brown in winter to tan in summer and have a characteristic lighter-colored rump. Elk have a long head with large ears and widely branching antlers as long as 3 to 5 feet from tip to tip; these are only found on the bucks.

Elk are browsers who feed on grasses, sedges, and forbs in summer, and woody growth during the winter months. They appear to be especially fond of dandelions, aster, hawkweed, violets, clover, and the occasional mushroom. Since elk are ruminants, they regurgitate their food and remasticate to aid in digestion.^1,2

Both males and females are sexually mature at sixteen months, although young males do not usually mate until they are a few year old and can compete with more mature males. Gestation generally lasts between 240 and 262 days and results in a single birth. Shortly before the fall rut (late September and early October), male elk lose the velvet on their antlers and begin to compete for access to females. Fights between dominant males and intruders can be intense and result in injury, exhaustion, or death.

Elk are social animals, often living in herds with as many as 400 individuals. The herds are matriarchal and are typically dominated by a single cow. These herds will migrate elevationally, occupying higher elevations during the summer and lower elevations during winter. As the fall mating season approachs, bulls form harems. In the spring, the sexes separate with females leaving to give birth, while bulls form their own separate summer herds.

Chemical Immobilization and Risks for Cardiac Arrest

The management of and research involving wild or captive elk routinely call for the use of chemical immobilization(sedation and/or anesthesia). Unfortunately, this can increase the risk for a number of complications, cardiac arrest being among these.

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. If left untreated, cerebral hypoxia can result in death within four to six minutes of a CPA event.³ In these cases, prompt cardiopulmonary resuscitation is imperative.

Signs of an impending CPA event in an elk 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.

In the field, remote drug delivery via dart is the modality of choice for wild elk, 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 elk. These drugs facilitate transport of elk and help to decrease stress in captured animals. Chemical immobilization can be also induced with opioids, alpha-2 agonists combined with dissociative drugs⁴ or custom compounded preparations. In zoos and farms, elk may also be handled in drop-floor or hydraulic squeeze chutes. Free-ranging elk are sometimes captured with net guns, drive nets, or clover traps.⁴

Capture can result in CPA events in elk regardless of the capture method, particularly under field conditions. In some instances, the stress 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.

Addressing Cardiac Arrest in Immobilized Elk

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

• Basic life support (BLS)
• Advanced life support (ALS)
• Post resuscitation care³

The first stage involves establishing an open and clear airway, providing assisted ventilation, and performing chest compressions. If an elk’s pulse becomes absent or weak, all administration of immobilizing drugs must be suspended and external cardiac massage should be initiated. Elk can usually be easily and safely ventilated with a bag-valve mask,^3,4although these are not always 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.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 elk 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

An elk that is 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 CPA. Mild hypothermia after resuscitation from CPA decreases cerebral oxygen demand and has been shown to improve outcomes.^3-6