Hermosa Veterinary Clinic Animal Articles

Anatomy of a Heat Stroke
by Celeste A. Clements, DVM, Diplomate ACVIM
Courtesy of VetCentric.com

   As summer temperatures continue to rise, thoughts turn to outdoor activities, and it's hard to imagine fun in the sun without including the family dog. Picnicking, boating, and trips to the beach all seem tailor-made for dog hijinks. Of course, Rover always wants to go along, even if it's for a short trip to the convenience mart. But the car itself may be Rover's biggest summer hazard.

   Summer weather conditions of high heat and humidity create the perfect environment for dogs to overheat. Confinement in a car or kennel that is poorly ventilated and lacking a water supply may very well be a death sentence. Air temperatures rise quickly in a parked car, even if it is shaded or the windows are open. Unfortunately, the dog's body temperature rises accordingly.

   Hyperthermia refers to a high core body temperature that occurs when body heat is gained faster than physiologic cooling mechanisms can alleviate the heat. When a dog's core body temperature exceeds 104.9 F, cellular damage usually ensues. Sustained body temperatures ranging from 105-109 F or greater may trigger a series of pathologic events described as heat stroke. Organs may be permanently damaged, resulting in the pet's serious impairment or death.

   When not confined, dogs and cats can use various strategies to dissipate heat and lower body temperature. They avoid hot areas, seek shelter, and adopt body postures to release heat to the environment, such as stretching out on a cooler surface or digging in the earth. Physiologic mechanisms such as dilation of superficial blood vessels of the skin and redirection of blood from the center of the body also aid in heat loss.

   Confinement limits the pets' cooling strategies, raising the risk of hyperthermia and heatstroke. Panting cycles air in the upper airways to maximize evaporation of water off the large moist surfaces of the nasal passages, mouth and throat. As the process continues, body water is lost. Humidity reduces the efficiency of the cooling process. Although dogs possess sweat glands on the feet and other parts of the body, sweating contributes little to their evaporative cooling. Alone in the car without enough air, unable to cool off in the stifling climate, dogs become seriously impaired.

Anatomy of a heat stroke

   Upon overheating, dogs will pant rapidly and display an elevated heart rate, bounding pulses, and dry red to brick-red mucous membranes. With progressive thermal injury, affected pets become depressed and "shocky," with collapse, weak pulses and pale mucous membranes. Vomiting and diarrhea are common as heat stroke progresses, and seizures and coma can occur, resulting in death. However, the full effects of heat induced tissue damage may not be immediately apparent -- so if you return to the car and your dog does not seem gravely ill, do not assume you are out of the woods. Patients that have overheated are closely monitored for several days for delayed consequences.

   Vital organs such as the kidneys, the brain, and the gastrointestinal tract are sensitive to heat and to disruptions in blood circulation that accompany heat stroke. The formation of microscopic blood clots and disruption of the normal balance of clotting in a process termed disseminated intravascular coagulopathy, or DIC, is a common consequence of heat stroke that can impact on any organ system. The prognosis for patients with kidney failure or DIC is uncertain to poor; anticipating these complications impacts on diagnostics and treatments offered by the emergency clinician.

Emergency intervention

   Prompt recognition and treatment of heat stroke is essential to a good outcome. During the warm months dogs discovered in a state of depression or collapse should be suspected of heat stroke, even if no specific risk factors are known. Documenting an elevated rectal temperature in such pets increases suspicion for the problem, and veterinary attention should be sought immediately.

   If heat stroke is strongly suspected, the pet should be removed to a cooler environment and placed under a fan. The coat should be thoroughly wetted down prior to travel to the emergency facility. While driving, leave the windows open to increase air movement over the wet coat. Overcooling is undesirable, however, so those offering first aid should limit cooling aids when the rectal temperature returns to 103 F.

   Once in the hospital, an emergency clinician must rapidly discriminate between fever, caused by some inflammatory disease process that has reset the thermostat, and hyperthermia or heat stroke. Patients that have elevated body temperature due to fever usually will not benefit from cooling, while hyperthermic patients must be cooled to reverse the process.

   Emergency veterinarians will frequently administer cooled intravenous fluids, balancing the patient's needs for rehydration and management of shock, with concerns about possible brain swelling. Ice water enemas or stomach infusions may aid in reducing the core temperature without causing vessels on the exterior to constrict. Aspirin or other antipyretics should not be given as they can be very dangerous to overheating pets; such medications should never be offered to pets without a veterinarian's recommendation.

   After the dog has been stabilized, treatment will depend on what type of damage hyperthermia has done to the body. In some situations, intravenous fluids will suffice. Other situations may require management of kidney failure, cardiac rhythm disturbance, or neurologic complications such as seizures or DIC. Monitoring for changes over four to five days will likely require laboratory testing, and possibly x-rays, as the patient's clinical status changes.

   Patients that survive heat severe stroke may require several weeks of hospitalization, but may be left with permanent organ dysfunction or neurologic impairment. Upon overheating, the brain's thermoregulatory center or internal thermostat is frequently damaged, thereby predisposing the pet to repeated bouts of overheating.

   Better safe than sorry

   Most cases of heat stroke can be prevented. Leaving a pet unattended in a vehicle is an unsafe, and potentially illegal, practice. If stops are planned on your trip to the beach, Rover should probably remain at home. Alone in the house or at a boarding facility, he may not be able to participate in a family game of Frisbee once you reach your seaside destination, but at least he will remain safe, healthy ... and alive.