The chemical immobilization of wild and captive deer is often necessary for a variety of reasons, including research, physiological study 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 different complications, but 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 that restores respiratory function.
The Eld's deer (Rucervus eldii) is indigenous to Southeast Asia. It is a large deer that is considered majestic in appearance. These deer are similar in size to white-tailed deer, but differ slightly in appearance. Their legs are long and thin, and they have slender bodies with a large head and ears. Their rough coats change color with the season, appearing reddish-brown in summer and dark brown in winter. Stags often have darker coloring than hinds (females) and have a thick mane of long hair around the neck.3
Eld’s deer stags have large, bow-shaped antlers; these sweep back in a curve of about 40 inches in length, with one smaller tine growing toward the front of the head. Antlers are shed every year; these reach their largest size during the breeding season.4 Male Eld’s deer grow to approximately 71 inches in length and weigh from 276 to 386 pounds. They are taller and larger than the hinds, which stand about 60 inches tall.
Rucervus eldii are primarily nocturnal. Throughout most of the year, stags are chiefly loners, except in the spring when mating commences. Females are generally found alone or in pairs with their young. They remain close to their fawns and other female-fawn pairs. Larger groups are often formed when males join groups of females prior to the breeding season, and groups of up to 20 animals are common.3
In their native ranges, Eld’s deer inhabit suitable forest habitats, lowland valleys and plains, avoiding dense forests and coastal areas. This also includes monsoonal forests. Today, they occur in a number of protected areas throughout these areas and have been introduced to numerous countries as game animals, including the United States.3,4
Respiratory arrest in can occur in Eld’s deer during chemical immobilization 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
According to the literature, the different species of deer each have their own anesthesia recommendations with dosage variations due to their individual responses to anesthetic agents.4,5 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 Eld’s deer sedation and anesthesia in wildlife and captive care.4 In the area of wildlife immobilization, opioids happen to be the most significant group of drugs that carry the potential to depress cardiopulmonary processes; these include both the natural derivatives, semisynthetic opioids and synthetic opioids.5 When respiratory arrest occurs in an immobilized Eld’s deer as a result of immobilizing drugs, the probability is high that it is in reaction to opioids.
Respiratory depression (hypoventilation) is reduced and/or ineffective breathing, whereas respiratory arrest is a complete cessation of breathing. There are several approaches to alleviating respiratory arrest in an Eld’s deer as a result of chemical immobilization. Antagonists (reversal agents) are some of the notable pharmacological developments to wildlife immobilization; these 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 an immobilized animal through its recovery.
In cases of respiratory arrest, 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 Eld’s 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.5 Oxygen is recommended during the immobilization of Eld’s deer whenever possible, as it can significantly lower the risk of respiratory arrest occurring.1