Respiratory Depression in Elk During Capture and Chemical Immobilization

The elk (Cervus elaphus canadensis) is a member of the deer family (Cervidae, order Artiodactyla). Also called wapiti in North America, elk were once found throughout much of the Northern Hemisphere, from Europe through northern Africa, Asia and North America. 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. Elk are exceeded in size only by the moose, the largest of the cervids.

The management of both free-ranging and captive elk frequently requires the live capture (restraint) of individual animals. The reasons for capture typically include routine medical care, research, translocating nuisance animals, or for the treatment of injured or sick animals.

For research and wildlife management purposes, many techniques have been used to capture elk, including live traps, drop nets, drive nets and rocket nets. Since these methods tend to limit the ability to select specific animals for capture, chemical immobilization using a remotely-delivered anesthetic agent has become the preferred capture technique. This is because this method is not only selective, but it reduces the stress of manual capture on elk.

With increased scientific and physiological knowledge and the refinement of immobilizing drugs, numerous chemical agents and drug formulations are now available. Still, chemical immobilization does cause physiological stress to elk. Anesthetized animals will be at risk of complications such as cardiovascular or respiratory depression and disruption of the thermoregulatory system. These effects can require supportive treatment by the attending veterinarian or support staff or the initiation of anesthetic reversal prior to completion of the procedure.^2,3

Remote drug delivery systems are typically used for the purpose of chemical immobilization, usually via a dart gun or blowpipe. Drugs are injected by means of a dart syringe which is fired from the dart gun at a distance. Since dart volume can be a limiting factor, immobilizing drugs must be highly potent and concentrated. They must also have a high therapeutic index and wide safety margin since animals cannot be examined and weighed prior to immobilization.¹ The ideal drugs will also be fast-acting to limit stress and the likelihood of escape following darting. They should also be reversible, since elk are often released back into the wild immediately after the capture event.

In zoos, farms, breeding facilities and even in free-ranging situations, chemical immobilization is usually carried out from the ground. In some circumstances however, wild animals often have to be located and darted from a helicopter.¹ All of the above methods of capture can cause significant stress and trauma to these animals, potentially giving rise to complications.

Respiratory Depression Risks in Elk

It is inevitable that the chemical immobilization of elk and other wildlife is associated with risks. In many cases, animals cannot be examined with regards to their health status beforehand and they often cannot receive adequate supportive treatment during immobilization in the field. Additionally, they are often highly-stressed and sometimes run long distances before they are immobilized. Most drugs used for immobilization have side effects; they not only cause sedation by influencing the central nervous system, but also influence cardiovascular, respiratory and thermoregulatory functions.¹The most common problems encountered during wildlife immobilization include respiratory depression, cardiovascular disturbances, bloat, impaired thermoregulation, hypoxia and capture myopathy.^2-4

Potent opioids are often used for the chemical immobilization of elk and other wild herbivores. A disadvantage of using these drugs is that they can cause clinically significant respiratory depression which is due to their potent effect on mu-opioid receptors.⁴ Activation of mu-opioid receptors in the respiratory centers of animals depresses neurons that generate the normal respiratory rhythm. At the same time, activation of these receptors activate other receptors in the brain stem, on the aortic arch and carotid bodies, which depresses normal respiratory function.

These processes in turn lead to a reduction of the respiratory frequency and tidal volume, as well as pulmonary vasoconstriction which decreases pulmonary perfusion.³ Alpha-2 agonists such as guanabenz, clonidine, medetomidine,and dexmedetomidine cause reflex bradycardia and hypotension, which can lead to hypoxemia and tissue hypoxia. Hypoxia can cause capture myopathy, which can ultimately lead to cardiac arrest and death.²

Treating Respiratory Depression in Elk

There are several approaches available to reduce the risk of opioid-induced respiratory depression in elk undergoing chemical immobilization. Assisted ventilation and oxygen insufflation can combat hypoxia,¹ while agents such as opioid antagonists or partial antagonists can be used. Unfortunately, the latter also reduce desirable effects, such as the degree of immobilization, sedation and analgesia. Respiration can also be improved during chemical immobilization events via respiratory stimulants which act on non-opioid receptor systems such as potassium channel blockers, ampakines and serotonin receptor agonists.⁴

The use of oxygen is recommended during the chemical immobilization of elk; this can be combined with a partial opioid reversal to better alleviate hypoxia.² 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 are used for the reversal of opioids such as diprenorphine, nalorphine or butorphanol.^2,3 Signs of recovery after naltrexone administration typically consist of increased respiratory depth, followed by ear twitching, eye movement and lifting of the head.²

Partial mu-receptor antagonists (e.g., butorphanol) can be used to reduce respiratory depression caused by strong mu-agonistic immobilization drugs.^1,4 Some of these partial antagonists, however, also reduce the immobilization effects of opioids. Potassium channel blockers such as doxapram can also be used to stimulate breathing. Doxapram is widely used as a respiratory stimulant by veterinarians. It has been shown to increase the minute ventilation in large herbivores immobilized with etorphine.² It should be noted that the respiratory effects of doxapram are usually short lived.

While safe and effective drug combinations used for darting were not always commercially available as pre-mixed solutions, many of these can now be purchased as highly-concentrated drug formulations for this purpose from compounding pharmacies. Such formulations are often species-specific, reliable and are less likely to bring about complications such as respiratory depression in elk than drugs and combinations used in the past.