The chemical immobilization of Chinese water deer is sometimes 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 discrete complications, but if left untreated, the former inevitably leads to the latter. Interruption of respiration (pulmonary gas exchange) 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.2
The Chinese water deer (Hydropotes inermis) is a small deer whose native range includes the lower Yangtze Basin of east-central China and in Korea. The species was also introduced in England and France in the late 1800s and subsequently became wild.3 Since then, it has been introduced into many countries, including the United States.
Chinese water deer—sometimes simply called “water deer”—can usually be found among tall reeds, along rivers, and in tall grass on mountains and cultivated fields.4 They also inhabit swampy regions and occasionally, open grasslands. They are adept at hiding, strong swimmers, and need very little cover to provide them with shelter.5
Chinese water deer are small in size, ranging in length from 25 to 30 inches. They have a short tail, and their hair is thick and coarse.5 The top of the face is grayish and reddish brown, the chin and upper throat are whitish, and the back and sides are usually a uniform yellowish brown, finely striped with black.3 Both sexes lack antlers, but the upper canine teeth of the males are enlarged, forming fairly long, slightly curved tusks. These upper canines are the most conspicuous feature of the bucks.6
Respiratory arrest in Chinese water deer during chemical immobilization can occur due to drug overdose, but it can also come about as a spontaneous adverse reaction to immobilizing drugs. Certain central nervous system disorders can also cause hypoventilation leading to respiratory arrest, as can compression of the brain stem during a capture event.1
When respiratory arrest due to chemical immobilization occurs, 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. There are also combinations of drugs that can increase the risk for respiratory depression, although many 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 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, or hypoventilation, is defined as reduced and/or ineffective breathing. Respiratory arrest is a total 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.2,6 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.7 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.2,6Atipamezole 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.7 Oxygen is recommended during the immobilization of water deer whenever possible, as it can lower the risk of respiratory arrest occurring.1