Normal body temperature for dogs is around 100.5-102.5°F (38-39.2°C).
Hypothermia sets in when the body loses heat at a faster rate than it can generate heat. In dogs, I consider a body temperature of under 99°F (37°C) to be evidence of early/mild hypothermia.
We normally think of exposure to cold environmental temperatures, particularly combined with windy and/or wet conditions, as the cause of most cases of hypothermia. However, in the veterinary setting, anesthesia and surgery are most typically to blame.
Some anesthetic agents disrupt the physiologic processes that regulate temperature and cause blood vessels to dilate thereby increasing heat loss.
Surgery often involves exposing internal organs to colder than normal temperatures, blood loss, and the administration of cool intravenous fluids. Certain diseases (e.g., shock, poisonings) can also lead to hypothermia in dogs.
Whatever the cause of hypothermia, it will cause considerable damage to a dog’s body if it is not reversed.
Let’s take a look at exactly why hypothermia is so dangerous.
In the initial stages of hypothermia, the body tries to conserve what heat is present by shunting blood towards the body core and away from the skin and extremities.
This is accomplished by narrowing the blood vessels (vasoconstriction) on the body’s surface. While helpful in a big-picture sort of way (e.g., survival), it also predisposes the dog to frostbite if environmental temperatures are low enough.
At the same time, dogs will start to shiver and their muscles become tense.
All this muscular activity is the body’s way of generating extra heat. But it can also get in the way of the dog’s ability to move in a normal manner.
As hypothermia progresses, shivering becomes more violent and dogs will become sluggish and confused.
And then… the shivering stops.
This is an indication that severe hypothermia has set in.
Essentially, the body has used up all the energy it has available to warm itself and a downward spiral is underway.
Metabolic and physiologic processes that rely on heat slow down leading to changes in body chemistry. The heart rate decreases. Arrhythmias can develop. Breathing slows, and the brain no longer gets the oxygen and energy it needs.
Stupor leading to coma and eventually death is the result.
Severe hypothermia is not without a silver lining, however.
At these low temperatures, cells within the body and especially the brain need far less oxygen and energy than do warm, metabolically active cells.
Therefore, circulation can stop for a relatively long period of time and a veterinarian may still be able to revive the dog using methods that warm them from the inside out (e.g., warm intravenous fluids, CPR using warm/humidified air, and infusing warm fluids into body cavities).
Paradoxically, warming a severely hypothermic dog from the outside in (e.g., using hot water bottles) can actually worsen their chances of survival.
To paraphrase my colleagues in human medicine, no hypothermic dog should be considered dead until the patient is “warm and dead.”
Get your dog veterinary care as quickly as possible if they are suffering from hypothermia, even if you can’t detect a heartbeat and breathing.
With hypothermia, miracles really can happen.
Low Body Temperature in Dogs