Veterinary Clinical Medicine-I (General and Systemic)

HYPERTHERMIA

• Hyperthermia is an elevated body temperature in mammals. The effects of hyperthermia, or heat stroke, if left untreated, can cause permanent internal organ damage or even death.
• Hyperthermia is an elevated body temperature due to failed thermoregulation.
• Hyperthermia occurs when the body produces or absorbs more heat than it can dissipate.
• When the elevated body temperatures are sufficiently high, hyperthermia is a medical emergency and requires immediate treatment to prevent disability and death.
• The most common causes are heat stroke and adverse reactions to drugs.
• Heat stroke is an acute condition of hyperthermia that is caused by prolonged exposure to excessive heat and/or humidity.
• The heat-regulating mechanisms of the body eventually become overwhelmed and unable to effectively deal with the heat, causing the body temperature to climb uncontrollably.
• Hyperthermia is a relatively rare side effect of many drugs, particularly those that affect the central nervous system.
• Malignant hyperthermia is a rare complication of some types of general anesthesia.
• Hyperthermia can be created artificially by drugs or medical devices.
• Hyperthermia therapy may be used to treat some kinds of cancer and other conditions, most commonly in conjunction with radiotherapy.
• Hyperthermia differs from fever in the mechanism that causes the elevated body temperatures: a fever is caused by a change in the body's temperature set-point.

Signs and Symptoms

• Hot, dry skin is a typical sign of hyperthermia.An inability to cool the body through perspiration causes the skin to feel dry.
• Other signs and symptoms vary depending on the cause. The dehydration associated with heat stroke can produce vomiting, and low blood pressure. This can lead to fainting or dizziness, especially if the person stands suddenly.
• In the case of severe heat stroke, the animal or person may become confused or hostile, and may seem intoxicated.
• Heart rate and respiration rate will increase (tachycardia and tachypnea) as blood pressure drops and the heart attempts to supply enough oxygen to the body.
• The decrease in blood pressure can then cause blood vessels to contract, resulting in a pale or bluish skin color in advanced cases of heat stroke.
• Some, especially young animals, may have seizures. Eventually, as body organs begin to fail, unconsciousness and coma will result.

Heat Stroke

• Heat stroke is due to an environmental exposure to heat, resulting in an abnormally high body temperature.
• In severe cases, temperatures can exceed 40 °C (104 °F).
• Heat stroke may be exertional or non-exertional, depending on whether the animal or person has been exercising in the heat.
• Significant physical exertion on a very hot day can generate heat beyond a healthy body's ability to cool itself, because the heat and humidity of the environment reduces the efficiency of the body's normal cooling mechanisms.
• Other factors, such as drinking too little water, can exacerbate the condition.
• Non-exertional heat stroke is typically precipitated by medications that reduce vasodilation, sweating, and other heat-loss mechanisms, such as anticholingeric drugs, antihistamines, and diuretics.
• In this situation, the body's tolerance for the excessive environmental temperatures can be too limited to cope with the heat, even while resting.

Diagnostic Approach

• Hyperthermia is generally diagnosed in the presence of an unexpectedly high body temperature and a history that suggests hyperthermia instead of a fever.
• Most commonly this means that the elevated temperature has appeared in a animal that was working in a hot, humid environment (eg. equine heat stroke) or that was taking a drug for which hyperthermia is a known side effect (drug-induced hyperthermia).
• If fever-reducing drugs lower the body temperature, even if the temperature does not return entirely to normal, then hyperthermia is excluded.

Prevention & Mitigation

• In cases where heat stress is caused by physical exertion, hot environments or wearing protective equipment it can be prevented or mitigated by taking frequent rest breaks, staying hydrated and carefully monitoring body temperature. However, in situations demanding prolonged exposure to a hot environment or wearing protective equipment, a personal cooling system is required as a matter of health and safety.
• A variety of active or passive technologies personal cooling systems exist which can be categorized by their power sources and whether they are animal or vehicle-mounted.

Treatment

• Treatment for hyperthermia depends on its cause, as the underlying cause must be corrected.
• Mild hyperthemia caused by exertion on a hot day might be adequately treated through measures, such as allowing to drink more water and resting in a cool place.
• Hyperthermia that results from drug exposures is frequently treated by cessation of that drug, and occasionally by other drugs to counteract them. Fever-reducing drugs such as paracetamol and aspirin have no value in treating hyperthermia.
• When the body temperature is significantly elevated, mechanical methods of cooling are used to remove heat from the body and to restore the body's ability to regulate its own temperatures.
• Passive cooling techniques, such as resting in a cool, shady area and removing clothing can be applied immediately.
• Active cooling methods, such as sponging the head, neck, and trunk with cool water, remove heat from the body and thereby speed the body's return to normal temperatures.
• Making the animal to drink water and turning a fan or dehumidifying air conditioning unit on the affected animal may improve the effectiveness of the body's evaporative cooling mechanisms (sweating).
• Placing the animal in a bigger bathtub or pool of tepid or cool water (immersion method) can remove a significant amount of heat in a relatively short period of time. However, immersion in very cold water is counterproductive, as it causes vasoconstriction in the skin and thereby prevents heat from escaping the body core.
• In exertional heat stroke, studies have shown that although there are practical limitations, cool water immersion is the most effective cooling technique and the biggest predictor of outcome is degree and duration of hyperthermia.No superior cooling method found for nonexertional heat stroke.
• When the body temperature reaches about 40 C, or if the affected animal is unconscious or showing signs of confusion, hyperthermia is considered a medical emergency that requires treatment in a proper veterinary medical facility.
• In a veterinary hospital, more aggressive cooling measures are available, including intravenous hydration, gastric lavage with iced saline, and even hemodialysis to cool the blood.