Chemical Immobilization: Medical Emergencies 1 - Respiratory & Cardiac Complications
Chemical immobilization is the chief method of capture for large wildlife species for the purposes of translocation, diagnostic testing or medical treatment. In zoos, farms, breeding facilities and 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
The handling of captured animals should be performed in as careful a manner as possible to ensure that minimal trauma, behavioral distress and discomfort occur. In the United States, the Food and Drug Administration (FDA) determines the immobilizing drugs that can be used on certain animals. The FDA also requires a registration certificate for those using controlled substances, which are obtained through the US Drug Enforcement Agency (DEA). Personnel or field technicians administering controlled drugs must be working under the supervision of a certificate holder, usually a licensed veterinarian.
The San Diego Zoo Wildlife Alliance (SDZWA) currently offers a variety of courses in safe capture chemical immobilization techniques for those who are interested in or who are pursuing careers in veterinary medicine, wildlife management, biology, zoology, animal control and related disciplines. Among these are in-depth modules on the pharmacology of chemical immobilization drugs and their use. Chemical Immobilization Pharmacology: Medical Emergencies 1 - Respiratory & Cardiac Complications is one of the available modules.
Respiratory Depression and Respiratory Arrest
All of the methods of capture mentioned above can cause significant stress and trauma to animals, potentially giving rise to complications. The most common problems encountered during wildlife immobilization include respiratory depression, cardiovascular disturbances, bloat, impaired thermoregulation, hypoxia and capture myopathy.1-3 In order to avoid these life-threatening events when possible, and to effectively address them when they do occur, persons engaging in the chemical immobilization of wildlife and exotic species must understand the dynamics of these complications.
In most cases, animals in the field cannot be examined with regards to their health status before an immobilization event, and they often cannot receive adequate supportive treatment during immobilization. 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.2
Potent opioids are often used for the chemical immobilization of wild species. One chief 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.3 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.2
Fortunately, there are several approaches available to alleviate opioid-induced respiratory depression in animals 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 routine use of oxygen is recommended during wildlife immobilization and can be combined with a partial opioid reversal to better alleviate hypoxia.1 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.1
Respiratory arrest during chemical immobilization can occur due to drug overdose, but in many cases, it can come about as a spontaneous adverse reaction to immobilizing drugs (e.g., the animal was inordinately stressed, comorbidities existed, etc.). 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. 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
Cardiac Arrest
Cardiac arrest, or cardiopulmonary arrest (CPA) is characterized by an abrupt, complete failure of the respiratory and circulatory systems. The subsequent lack of oxygen transport can quickly cause systemic cellular death from oxygen depletion.1 If left untreated, cerebral hypoxia can result in death within four to six minutes of a CPA event.2 In these cases, prompt cardiopulmonary resuscitation is imperative.
Capture and/or chemical immobilization can result in CPA events, particularly under field conditions. In some instances, the stress of capture (depending upon the method of capture) can significantly increase the likelihood of cardiac arrest. While under anesthesia, common causes of CPA can include vagal stimulation, unstable cardiac arrhythmias, severe electrolyte disturbances, exacerbated cardiorespiratory disorders (e.g., congestive heart failure, hypoxia)1 or a variety of comorbidities. Signs of an impending CPA event can include dramatic changes in breathing effort, rate, or rhythm, significant hypotension, absence of a pulse, irregular or inaudible heart sounds, changes in the heart rate or rhythm; changes in mucous membrane color and fixed, dilated pupils.
Cardiopulmonary cerebral resuscitation in the immobilized animal involves three stages: basic life support (BLS), advanced life support (ALS), and post resuscitation care.3 The first stage involves establishing an open and clear airway, providing assisted ventilation, and performing chest compressions. If an animal’s pulse becomes absent or weak, all administration of immobilizing drugs must be suspended and external cardiac massage should be initiated. Veterinary patients can usually be easily and safely ventilated with a bag-valve mask,1 although these are not always available under field conditions.
Venous access can be established by using such methods as intraosseus catheter placement and venous cutdown, in which a small opening is created in a vein to allow passage of a needle or cannula.1 Epinephrine at 0.2 mg/kg (concentrated at 1/10,000) should be given IV or intracardially (IC) while cardiac massage continues. If the animal fails to respond, 0.1 ml/kg IV or IC calcium chloride may be given. If there is still no response, the epinephrine and calcium chloride may be re-administered with 10-20 mEq IV or IC sodium bicarbonate.4
It is important to note that while efficacious drug combinations used for the chemical immobilization of wildlife once 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 respiratory and cardiac complications than drugs and combinations used in the past.
Interested in learning more about safe capture? The San Diego Zoo now offers courses in safe capture techniques and best practices. Learn reliable, safe, and effective techniques for the species you work with and the scenarios you encounter!
1Arnemo, J. Kreeger, T. (2018). Handbook of Wildlife Chemical Immobilization 5th Ed. Sunquest Publishing, 2007.
2Arnemo, 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.
3Bailey, P.L., et. al. (1985) The ED50 of carfentanil for elk immobilization with and without the Tranquilizer R51703. The Journal of Wildlife Management, 49(4), pp.931–934.
4Van 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.
Our pharmacists are also encouraged to develop strong working relationships with our veterinarians in order to better care for veterinary patients. Such relationships foster an ever-increasing knowledge base upon which pharmacists and veterinarians can draw, making both significantly more effective in their professional roles.
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