Respiratory Arrest in Deer During Capture and Chemical Immobilization

The chemical immobilization of deer is often necessary for the purposes of physiological study, research and wildlife management. The drugs used for immobilizing these animals can adversely 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 singular 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.¹ Cardiac arrest almost always follows without an intervention where respiratory function is restored.

Causes of Respiratory Arrest

Respiratory arrest in deer 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.¹

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.¹

Deer Background and Biology

Deer are hoofed mammals with an even number of toes on each foot which belong to the order Artiodactyla. These animals are also known as cervids, which differ from other ruminants in that males grow antlers that are comprised of bone. The antlers are shed and regrown annually, and increase each year in size and complexity. Except in the case of caribou, female deer do not grow antlers.² Antlers serve as ornamentation for breeding purposes, as well as weapons during the mating season.

There are over 40 species of deer globally, most of which are found in Asia.¹ There are seven species of deer that are native to North America, with many existing sub-species. Deer have a range that essentially blankets the North American continent, making them sought-after among big game hunters and deer farmers.

Deer vary in size and weight from just under 100 pounds to over 1000 pounds. The Whitetail Deer (Odocoileus virginianus) is the most widely distributed deer in North America, with a range that extends east from the Rocky Mountains into Mexico.² They can reach a length of approximately six feet and a height of approximately three feet at the shoulder. Bucks can weigh up to 180 pounds, while does average between 80–120 pounds.

Deer and Chemical Immobilization

According to the literature, the various species of deer each have their own anesthesia recommendations with dosage variations due to their varied individual responses to anesthetic agents.^2,3 These variations are factors in the risk of complications. Monitoring core body temperature is essential in deer sedation and anesthesia, and intubation has also been recommended for any anesthetized deer 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 deer 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 deer as a result of immobilizing drugs, the probability is high that it is in reaction to opioids.

Respiratory Depression Leading to Respiratory Arrest in Deer

Respiratory depression (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 deer as a result of chemical immobilization. Antagonists (reversal agents) are some of the notable pharmacological developments to wildlife immobilization that are able to reverse the effects of opioid anesthetics and tranquilizers.^2,4 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 deer, 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.^2,4 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 deer 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.³ Oxygen is recommended during deer immobilization whenever possible, as it can lower the risk of respiratory arrest occurring.¹