Understanding Heat Stroke in Pets
Heat stroke is the most severe type of heat-related illness. Heat-related illnesses are broken down based on severity of elevated temperature and clinical signs. In humans, heat stroke has been categorized by two definitions: either a patient whose temperature is >40° C with central nervous system derangements or a hyperthermia case resulting in a severe inflammatory response that causes multi-organ dysfunction with predominant encephalopathy.
There are four categories of heat-related illnesses for veterinary patients that range from mild to severe.
These are:
- Heat Stress. This is the most mild form of heat-related illness. Core body temperature is normal and clinical signs are mild with discomfort or physiological strain.
- Heat Cramps. Temperature is normal, but a limp or difficulty walking is noted. This is thought to be due to water and sodium derangement.
- Heat Exhaustion. Core body temperature is normal, decreased, or mildly increased. Clinical signs are more severe than with cramps or stress, including weakness, fainting, and anxiety.
- Heat Stroke. As described above, this is the most severe form of heat illness and can be life threatening.
There are two forms of heat stroke: exertional and non-exertional (or classic). Exertional is not commonly seen in veterinary medicine, but results from strenuous physical activity leading to hyperthermia. This is typically seen in marathon runners or other elite athletes due to strenuous exercise, often in conjunction with warm environmental temperatures. It is also common in working animals, like race horses and greyhounds, and can be caused by any form of hard exercise for animals. Classic/exertional heat stroke is caused by exposure to high environmental temperatures. This is the type of heat stroke most commonly seen and treated in veterinary medicine.
Heat Compensation Mechanisms
All animals and humans are able to compensate for some level of heat exposure through built in mechanisms, but if prolonged exposure or underlying medical conditions exist, hyperthermia can occur.
The four compensation mechanisms are:
- Conduction. Where heat is transferred from the body to a colder surface through contact.
- Convection. Where heat is released into the environment through airflow. This is encouraged by fans.
- Evaporation. When a liquid becomes a vapor, such as with perspiration in humans/horses and panting with dogs (and, to a lesser extent, cats).
- Radiation. The process of releasing heat into the environment. This only works when the environment temperature is lower than body temperature.
Radiation and convection help with 70% of heat regulation in dogs and cats when environmental temperature is lower than body temperature. When ambient temperature rises, evaporation plays a larger part.
There are some predisposing conditions that can make heat stroke more likely in veterinary patients. These can be endogenous, meaning that they are outside factors, like underlying medical conditions or physical attributes, that increase the risk of developing heat stroke.
The documented endogenous and exogenous factors include:
| Endogenous Factors | Exogenous Factors |
| Obesity | Lack of acclimatization to environment |
| Abnormal airway, common in brachycephalic breeds (such as pugs, bulldogs, and boston terriers) | Water deprivation |
| Heart/lung disease | Isolated to airway with minimal ventilation or shade (such as a dog trapped in a closed car on a hot day) |
| Dense hair coat | Concurrent medication administration, mainly caused by heart medications, like diuretics |
| Muscular or neuromuscular disease |
Organ Systems Involved in Heat Stroke
The common organ systems involved include the gastrointestinal tract, kidneys, neurological system, heart, lungs, and the coagulation system. The body can suffer direct cellular injury due to high temperatures, but can also suffer secondary injuries due to the hypoxia (low oxygenation) and hypovolemia (low blood pressure and dehydration).
Clinical Signs
Almost all organ systems can be affected by heat stroke, therefore clinical signs can be variable depending on the patient. Clinical signs can also be mild, even with heat stroke, and recovery can be quick or severe and life threatening.
Clinical signs can be broken down based on what organ system is dysfunctional:
- Gastrointestinal
- Vomiting (can be bloody)
- Diarrhea (can be bloody)
- Inappetence
- Kidney Injury
- Increased thirst/urination
- Bloody urine
- Absence of urination
- Cardiac (Heart) Dysfunction
- Increased heart rate (tachycardia)
- Decreased heart rate (bradycardia)
- Weak pulses
- Arrhythmias (abnormal heart beats)
- Pulmonary (Lung) Dysfunction
- Rapid/labored breathing
- Cyanotic mucous membranes
- Hypoxia (poor oxygenation)
- Neurological Abnormalities
- Seizures
- Tremors
- Mentally dull/disoriented
- Coagulation Disorders
- Petechiae (bruising along the skin)
- Bloody vomit/diarrhea/urine
Diagnosis
Determining if a pet has heat stroke can be difficult, since observation may be hours after injury. The first step to a diagnosis is obtaining a detailed history and trying to isolate a heat injury if it occurred. A thorough physical is used to look for corresponding clinical signs and then utilizing these findings to create a treatment plan. Baseline temperature at triage may still be elevated when patients are at the veterinarian hospital, but is typically normal or decreased based on cooling measures at home or resulting from shock.
Diagnostic measures include:
- Complete Blood Count (CBC). This evaluates the red blood cell count, since anemia can be noted in heat stroke patients due to GI loss or bleeding abnormalities. Evaluating the white blood cell count also helps, since decreased counts are common in the late stages of heat stroke.
- Chemistry Panel. This inspects for changes in liver and kidney values, which can increase with heat injury.
- Coagulation Testing. Prolongation of clotting times indicates injury to the coagulation system.
- Chest Radiographs. Radiographs should be taken if the patient is having respiratory or cardiac signs, since they evaluate the heart and lungs.
- Electrocardiogram (ECG). This test looks for arrhythmias and abnormal heart beats.
Ultimately, a diagnosis is based on a patient’s history, clinical signs, and the results of diagnostic screening.
Treatment Measures
- Initiate Cooling. If a patient is still hyperthermic on presentation, active cooling should be initiated. Cooling should be performed in a controlled fashion and temperature should be monitored to ensure that the patient isn’t becoming hypothermic. Cooling is achieved by increasing airflow on the patient with fans, leading to convection-driven heat dissipation. Cool, water-soaked towels can also be placed on the patient and replaced frequently. Using ice packs can cause hot peripheral blood to shunt towards the internal organs, so these should be avoided. Placing ice packs around large vessels, such as jugular and femoral arteries, can be considered. An IV catheter should be inserted and room-temperature IV fluids should be given. Cooled/iced IV fluids are not necessary, as room temperature is already cooler than the patient. A few sources in veterinary and human literature discuss cold-water enemas and cool-water gastric lavage to help with internal cooling, though these are not routinely used due to increased risk of pneumonia.
- Gastroprotectants. Anti-nausea medications and gastroprotectant medications should be initiated early in the treatment process. This can help reduce the amount of gastrointestinal sloughing and nausea. Nausea/vomiting can increase the risk of pneumonia, so they should be avoided.
- Broad-spectrum Antibiotics. Antibiotics should be administered, due to risk of gastrointestinal translocation of bacteria to the bloodstream. These are continued during hospitalization.
- Hydration. In addition to fluids being used in the initial stages of cooling, these are needed to maintain hydration and keep up with losses (vomiting/diarrhea/increased urination). Typically, heat stroke patients need large volumes of IV fluids to keep an equal balance.
- Plasma. Due to the damage to the coagulation system, plasma is given to patients with prolonged clotting times. Plasma restores normal clotting factors to the body and helps to stop bleeding.
- Electrolyte/Dextrose Supplementation. Based on blood work, electrolyte and dextrose supplementation may be needed. This can change during hospitalization, therefore these parameters are monitored frequently.
- Blood Pressure Medications. If a patient has high or low-blood pressure that isn’t responding to IV fluids, blood pressure medications need to be administered. They are often discontinued as the patient’s condition improves.
What Should I Do If My Pet Has Heat Stroke?
First of all, your pet should be removed from the source of high external temperature. Secondly, work towards getting them to a veterinary hospital immediately. During the car ride to the hospital, make the car as cool as possible with air conditioning or open windows to increase convection. Place cool towels on their body in the car and replace if they get warm. DO NOT submerge them in water, since this can increase the risk of water aspiration into the lungs. Treatment at home or on the way should not delay the arrival to the hospital, but rather be performed at the same time.
How Can I Avoid Heat Stroke In My Pets?
- Awareness. Be cognizant of your pet’s environment. Avoid any outside activity when the ambient temperature is warm to hot with minimal shade, or areas without space for them to go and cool off. Do not leave them in the car unattended.
- Identify Risk Factors. If your pet has a known predisposing factor, such as obesity or underlying medical conditions, extra precautions should be taken to prevent them from being exposed to high temperatures.
- Pay Attention to Humidity. Humidity can make it harder for pets to utilize evaporation and can greatly increase the heat index for the day. On high humidity days, minimize outside adventures and let them stay indoors with air conditioning or fans.
- Acclimation. If your pet is going to be participating in an exercise event, make sure they have been appropriately trained for the level of exertion and associated weather. Do not assume just because they have gone hiking or running before when temps are cool that it will be safe in hot weather or at high altitudes. Take things slow and make sure your pet is comfortable with the exercise and environmental temperature.
- Adjust Schedule. Try to plan your outdoor excursions at dusk or dawn when it is coolest to minimize the heat impact.
- Fresh Water. Always have fresh water available, because pet’s rely heavily on evaporation during hot weather and can dehydrate easily.
- Be Safe. If you are worried about it being too hot outside or an outing too strenuous for your pet, err on the side of caution and skip it. Heat stroke can be serious and life threatening, so there’s no reason to risk it.