Shock in Deer During Capture and Chemical Immobilization

Deer are cervids (family Cervidae), a group of animals belonging to the order Artiodactyla. They have a four-chambered stomach, and are cud-chewing herbivores (ruminants). Most deer have 32 teeth, although they lack upper incisors, and most species also lack upper canines.¹ Cervids differ from other ruminants in that males grow antlers that are comprised of bone. These are shed and regrown annually, and increase each year in size and intricacy. Except in the case of caribou(Rangifer tarandus), female deer do not grow antlers.² Antlers serve as ornamentation for breeding purposes, as well as weapons during the mating season.

Globally, there are over 40 species of deer, most of which are found in Asia, although many species have been introduced outside of their original habitats as game animals.¹ There are seven species of deer that are native to North America, with many existing sub-species. Deer have a range that blankets the North American continent, making them popular among big game hunters and, more recently, deer farmers.

Cervids vary in size and weight from just under 100 pounds to over 1000 pounds (technically, elk and moose are members of the deer family). 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.^2-4 These animals can reach a length of approximately six feet and a height of approximately three feet at the shoulder. Adult males (bucks) can weigh up to 180 pounds, while females (does) average between 80–120 pounds.

Shock and How it Arises in Deer

Shock is a critical condition that comes about due to a sudden drop in blood flow throughout an animal’s body. It can be the result of a wide variety of conditions or circumstances, including extreme physical stress, trauma, disease, heatstroke, blood loss, allergic reactions or severe infection. When an animal is suffering from shock, its organs are not receiving an adequate amount of blood or oxygen. If untreated, this condition can lead to permanent organ damage or death.

Unfortunately, the processes surrounding capture and/or chemical immobilization can include extreme physical stress and/or trauma sufficient to induce shock in deer and other animals. The degree of risk is dependent upon factors such as species, sex, age, overall health, environmental factors, length of immobilization, the degree of stress involved in the capture/immobilization event itself, the specific chemical agents involved in immobilizing the animal and others. It should be noted that cervids are known to be susceptible to a variety of complication during chemical immobilization, including shock.

There are three main categories of shock, which include:

Circulatory Shock. This occurs when there is a significant decrease in effective circulating blood volume. This category is further divided into the three subcategories of cardiogenic, hypovolemic and distributive shock. Cardiogenic shock occurs when the circulating volume of blood decreases despite normal or increased blood volume. Hypovolemic shock occurs when blood volume is decreased through hemorrhage, third space fluid distribution, or dehydration. Distributive shock occurs when an animal’s body is unable to maintain the vasoconstriction of blood vessels.⁵

The last two categories of shock are hypoxic shock and metabolic shock. Hypoxic shock results from impaired oxygen delivery to cells, while metabolic shock involves cells that have become unable to utilize oxygen for energy production.^5,6For the purposes of this discussion, the types of shock being discussed are the subcategories of circulatory shock and hypoxic shock, which are the most likely to be brought on due to capture and/or chemical immobilization events.

The Mechanics of Shock in Deer

Even under well-controlled circumstances, chemical immobilization is a risky proposition in deer as well as humans. Almost all of the drugs that produce anesthesia compromise cardiovascular stability by producing dose-dependent impairment of cardiac function, vascular reactivity and autoregulatory responses.⁶ Hemoglobin is found within red blood cells and carries oxygen to tissues. In normal circumstances, the amount of oxygen delivered to cells is 2 to 4 times the amount required, which ensures an adequate supply.² However, if tissues are not adequately perfused with blood, the oxygen fails to get to the cells, regardless of the oxygen content in the blood.⁵

Significant changes in the mean arterial pressure (MAP) trigger changes in heart rate.^6,7 An increase in MAP also causes bradycardia and vasodilation, while a decrease produces tachycardia and vasoconstriction.^5,6 While anesthesia-related depression of cardiac function and arterial vasodilation are adverse effects that are well-recognized as contributing to anesthetic risk, far less emphasis is usually placed on effects impacting venous physiology and venous return.²

Approximately 70% of an animal’s total blood volume is represented by venous circulation, and this is a chief contributor to stroke volume and cardiac output.⁵ Vasodilation is the primary cause of hypovolemia produced by anesthetic drugs. It is often associated with increased venous compliance, decreased venous return, and reduced response to vasoactive substances.⁶ Depending on things like patient status and monitoring, a state of relative hypovolemia can remain clinically undetected for long periods of time.^5-7

Diagnosis and Treatment of Shock in Deer

The clinical signs of shock in deer can include any combination of the following symptoms:

• Unresponsiveness
• Hypothermia
• Tachycardia
• Bradycardia
• Tachypnea
• Bradypnea
• Marked hypotension
• Cyanosis
• Orthopnea

The treatment of shock in deer must primarily focus on increasing oxygen delivery to the tissues. This can be accomplished by providing supplemental oxygen, increasing effective circulating volume, increasing hemoglobin concentration and increasing cardiac output with stimulants.^5,7 An intravenous catheter should be placed for vascular access if possible. If venous access cannot be established, an intraosseous catheter can be placed. Oxygen supplementation, when available, will also provide benefits to the deer suffering from shock. This can be accomplished via flow-by oxygen, mask, nasal cannulas or an oxygen cage.⁶

Fluid Therapy for Shock in Deer

Lactated Ringer’s solution, Normosol-R, and Plasma-Lyte are the preferred choices for fluid therapy in treating shock in deer, as these have been shown to cause fewer complications as well as decrease the risk of mortality⁵ as compared to other options. Hypertonic saline is also a popular option for fluid therapy (as this increases vascular volume). Hypertonic saline increases plasma osmolarity, pulling water into the vascular space from the interstitial space, thereby expanding plasma volume. It should be noted that hypertonic saline does have unwanted side effects, such as a transient, dose-dependent increase in sodium and chloride.^6,7

Blood products are an important adjunct for the treatment of shock in deer. In normal patients, anemia can be well-tolerated with oxygen delivery being maintained. In patients with trauma and acute loss of blood volume however, the associated stressors can contribute to decreased oxygen delivery.⁷