The elk (Cervus elaphus canadensis) is the second-largest wild cervid after the moose. The North American elk (Cervus elaphus) is further split by some biologists into six subspecies (two of which are now considered extinct). Once found throughout North America, large elk populations are now found only in the western United States and Canada, and in scattered herds in the east.1 Elk living in the Rocky Mountains usually migrate to mountain meadows for the summer, and return to the lower foothills during winter.
Elk are popular as game animals; their antlers are used commercially and in traditional Eastern medicine. The word “elk” historically had a meaning like “large deer.” Elk are related to a breed of red deer in Asia, and are still called red deer in these areas. They are believed to have come to North America across the Bering Strait over 100,000 years ago, as did the caribou and other animals. Between 5,000 to 10,000 years ago, they advanced as far as what is now the southwestern U.S., and it is believed that there may have been as many as 10 million elk in North America when Europeans began to settle the continent.2
Male elk are called bulls and the females are called cows. Male elk have antlers that are made of bone; these grow about an inch a day and can grow over 20 inches in length. As they grow, a layer of velvet covers. This is shed in the spring after the antlers have finished developing. Elk rub against trees and other objects to shed their winter coats in early summer.2
Elk are considered to be the most vocal members of the deer family. Bugling bulls signal the beginning of the fall rut, or breeding season; these calls advertise the bulls’ fitness to mate. Cow elk bark alarms, and calves bleat and squeal. During the rut, cows form small groups or harems that are controlled by a dominant bull. For the rest of the year, cows, yearlings and calves form herds of 25 or more animals, while bulls form smaller bachelor herds.1 Elk are some of the most social cervids, with group sizes during the summer reaching up to 400 individuals.1,2
The chemical immobilization of elk is often necessary for the purposes of study, research and wildlife management. The drugs used for immobilizing these animals can negatively affect their cardiovascular and respiratory systems, however. In some circumstances, they can lead to complications such as respiratory depression and/or respiratory arrest.
Respiratory arrest and cardiac arrest are different complications however, if left untreated, the former inevitably leads to the latter. Interruption of pulmonary gas exchange (respiration) for more than a few minutes can irreversible vital organ damage, particularly in the brain.3 Cardiac arrest almost always follows without an intervention where respiratory function is restored.
Respiratory arrest in elk during chemical immobilization can occur due to drug overdose, but it can also come about as a spontaneous adverse reaction to immobilizing drugs. Central nervous system disorders that affect the brain stem can also cause hypoventilation leading to respiratory arrest, as can compression of the brain stem during a capture event.3
In the case of respiratory arrest brought on by chemical immobilization, the decreased respiratory effort reflects central nervous system (CNS) impairment due to the immobilizing drugs. Drugs that decrease respiratory effort include opioids and certain sedatives. Certain combinations of drugs can increase the risk for respiratory depression, although some of the newer species-specific formulations can actually lower the risk of complications, including respiratory depression and arrest. The risk for opioid-induced respiratory depression (ORID) is usually most common in the immediate postoperative recovery period but it can persist and lead to catastrophic outcomes such as severe brain damage or death.3,4
According to the literature, the various cervid species each have their own anesthesia recommendations with dosage variations due to their varied individual responses to anesthetic agents.3,4 These variations are factors in the risk of complications. Monitoring core body temperature is essential in elk sedation and anesthesia, and intubation has also been recommended for any anesthetized elk that needs to be transported or anesthetized for greater than one hour. Until the more recent use of formulated drugs, opioids were the mainstay of elk anesthesia in wildlife and captive care.3
In the area of wildlife immobilization, the most significant group of drugs that carry the potential to depress cardiopulmonary processes are opioids, which include both the natural derivatives, semisynthetic opioids and synthetic opioids.4 When respiratory arrest occurs in an immobilized elk as a result of immobilizing drugs, the probability is high that it is in reaction to opioids.
Respiratory depression, or hypoventilation, is defined as reduced and/or ineffective breathing. Respiratory arrest is the complete cessation of breathing. There are several approaches to alleviating respiratory arrest in elk as a result of chemical immobilization. Antagonists (or reversal agents) are some of the notable pharmacological developments to wildlife immobilization that are able to reverse the effects of opioid anesthetics and tranquilizers.3,5 Antagonists are typically able to completely reverse sedative and anesthetic effects and return an animal to a normal physiological state. The chief benefits of antagonists include preventing predation in the wild after immobilization events and to avoid complications. Antagonists also decrease the personnel and equipment time needed for monitoring the immobilized animal through its recovery.
When respiratory arrest occurs, the ultimate goal is to restore adequate ventilation and oxygenation without further compromising an already compromised cardiovascular situation.5 In the event of respiratory arrest in an immobilized elk, the administration of all immobilizing drugs should be ceased. Naltrexone is frequently used to fully reverse opioid-based immobilization after capture, especially if the animal needs to be released back into the field and must be fully alert. If residual analgesic or sedative effects are required, partial opioid antagonists or mixed agonists/antagonists can be used for the reversal of opioids such as diprenorphine, nalorphine or butorphanol.3,6 Atipamezole is often used as a reversal agent for medetomidine and dexmedetomidine in order to reduce their sedative and analgesic effects. It has also been used for the reversal of other α2- adrenergic agonists (e.g., xylazine, clonidine, tizanidine and brimonidine).
Doxapram and other potassium channel blockers can also be used to stimulate breathing in elk suffering from respiratory depression/arrest. Doxapram is widely used as a respiratory stimulant by veterinarians and has been shown to increase the minute ventilation in large herbivores immobilized with etorphine.Oxygen is recommended during elk immobilization whenever possible, as it can lower the risk of respiratory arrest occurring.3