Respiratory Arrest in Impala During Chemical Immobilization
The impala (Aepyceros melampus) is probably the most well-known of the African antelopes. Residing in the savannas of eastern and southern Africa and the bushveld regions of South Africa, the impala is the most widespread and common antelope in these areas. As with other antelopes, it is a member of the family Bovidae, and the only member of the subfamily (or tribe) Aepycerotini.1 As African antelopes go, impalas are medium-sized, with slender but muscular legs and a long neck.
Both impala males and females have a tan coat, a reddish-brown saddle and white markings at the eyes, the inside of the ears, throat, the underside of the torso and tail. They also have black markings at the ears, tail and back feet. The darker, black-faced impala (Aepyceros melampus petersi) is a less common subspecies and occupies areas of southwest Africa.1 Impala males have wide, lyre-shaped horns that are 18–36 inches long.2
Impala both graze and browse for food. When grazing, they consume grass; when browsing, they typically feed upon a wide variety of vegetation, including fruits, seedpods, shoots as well as leaves of trees and bushes.3
Impala are both diurnal and nocturnal. They are usually most active immediately after sunrise and just before sunset. Female impalas and their offspring gather into herds which number from 15 to 100 individuals. A herd’s home range covers a territory that varies from 80 to 180 hectares.3 During the wet season, females become highly territorial and defend the home ranges. Young males form non-territorial bachelor herds of up to 30 individuals. During the dry season, male and female herds mix together.
Impala have a polygynous mating system, with each male mating with a number of females.2 Breeding activity spans from March through May. During this time, pregnant females live in isolation to give birth. After gestating from 190 to 200 days, a single calf is born, following which the mother and calf rejoin the herd.3
Chemical Immobilization and Respiratory Arrest
The chemical immobilization of impala is often necessary for physiological study, research, and for the purposes of wildlife management. Unfortunately, immobilizing drugs can adversely affect the cardiovascular and respiratory systems of these animals and, in certain circumstances, can lead to complications such as respiratory depression and/or respiratory arrest.
While respiratory arrest and cardiac arrest are different complications, if left untreated, the former inevitably leads to the latter. Interruption of pulmonary gas exchange (respiration) for more than five minutes can irreversible vital organ damage, particularly in the brain.4 Cardiac arrest almost always follows without an intervention where respiratory function is effectively restored.
Causes of Respiratory Arrest
Respiratory arrest during chemical immobilization events can occur due to an inadvertent drug overdose, but it usually comes about as a spontaneous adverse reaction to immobilizing drugs (e.g., the animal was inordinately stressed, comorbidities existed, etc.).4 When respiratory arrest is 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 is usually most common in the immediate postoperative recovery period but it can persist and lead to catastrophic outcomes (e.g., severe brain damage, death).4,5
Impala Immobilization Dynamics
Each species of antelope have their own anesthesia recommendations with dosage variations due to their diverse individual responses to anesthetic agents.5,6 These variations are factors in the risk of complications. Monitoring core body temperature is widely recommended in all antelope anesthesia, and intubation has also been recommended for any anesthetized antelope 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 antelope anesthesia in wildlife and captive care, and many species were known to be very difficult to safely anesthetize.6
In wildlife immobilization, the most significant group of drugs that carry the potential to depress ventilation are opioids, which include both the natural derivatives, semisynthetic opioids and synthetic opioids.7 When respiratory arrest occurs in an immobilized antelope as a result of immobilizing drugs, the probability is high that this is in reaction to opioids.
Respiratory Arrest in Impala
While respiratory depression (hypoventilation) is characterized by reduced and/or ineffective breathing, respiratory arrest is the cessation of breathing. There are several approaches available to alleviate respiratory arrest in impala as a result of chemical immobilization. Reversal agents (antagonists) are some of the notable pharmacological developments to wildlife immobilization that are able to reverse the effects of opioid anesthetics and tranquilizers.5,7 These drugs are able to completely reverse anesthetic effects and return an animal to a normal physiological state. The chief benefits of antagonists include preventing predation in the wild after anesthetic events and to avoid or overcome complications. Antagonists also decrease the personnel and equipment time needed for monitoring the immobilized animal through its recovery.
If respiratory arrest occurs in an impala, the ultimate goal is to restore adequate ventilation and oxygenation without further compromising an already taxed cardiovascular situation.8 In the event of respiratory arrest in an immobilized impala, 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.5,7 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.
To stimulate breathing in an antelope suffering from respiratory depression/arrest, potassium channel blockers such as doxapram can also be used. 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.6 Oxygen is recommended during antelope immobilization whenever possible, as it can lower the risk of respiratory arrest occurring. Oxygen can also be combined with partial opioid reversal agents to better alleviate hypoxia.4
4Izrailtyan I, et. al. Risk factors for cardiopulmonary and respiratory arrest in medical and surgical hospital patients on opioid analgesics and sedatives. PLoS One Mar 22;13(3):e019455, 2018.
5Arnemo, J. Kreeger, T. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed. Sunquest Publishing, 2007.
6Ball, L. Antelope Anesthesia. Wiley Online Library, 25 July 2014, https://doi.org/10.1002/9781118792919.ch60.
7Arnemo, J., et. al. Field Emergencies and Complications. In: G. West, D. Heard, & N. Caulkett, eds. Zoo Animal and Wildlife Immobilization and Anaesthesia. Oxford: Wiley Blackwell, pp. 139–147.
8Van der Schier, R., et. al. (2014) Opioid-induced respiratory depression: reversal by non-opioid drugs. F1000 Prime Reports, 6, pp.1–8.
About NexGen Pharmaceuticals
NexGen Pharmaceuticals is an industry-leading veterinary compounding pharmacy, offering sterile and non-sterile compounding services nationwide. Unlike other veterinary compounding pharmacies, NexGen focuses on drugs that are difficult to find or are no longer available due to manufacturer discontinuance or have yet to be offered commercially for veterinary applications, but which still serve a critical need for our customers. We also specialize in wildlife pharmaceuticals, including sedatives and their antagonists, offering many unique options to serve a wide array of zoo animal and wildlife immobilization and anesthesia requirements.
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