Thursday, April 23, 2015

What Happens in a Dog's Body with Hypothermia?

by Jennifer Coates, DVM

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, but 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.

Articles by Dr. Coates:
Kidney Disease – Say What? 
What Happens In The Dog's Body When The Kidneys Fail To Function Properly? 
Heat Stroke: What Happens In The Dog's Body?  
The Perplexities of Pancreatitis
The Other Side Of The Coin: The Cost Of Defensive Medicine
To Neuter Or Not To Neuter… That Is The Question
Don’t Forget the Physical Therapy
Common Misdiagnoses (Part 1)
Common Misdiagnoses (Part 2)
Picking the Right Dog to Breed
When Is It An Emergency?
Dog Allergies: Common, Commonly Misdiagnosed, or Both? 
Why Does The Spleen Get No Respect?
Protect Your Dog From Snake Bites 
More Creepy Crawlies
Why I Dislike Inflammatory Bowel Disease
Salmonella – A Significant Problem, Or Not? 
What’s In the Vomit?
Cortisol: What Happens In A Dog’s Body When It Goes Awry?
What Happens In The Dog's Body With Zinc Toxicity? 
What Happens In The Dog’s Body: Xylitol Poisoning 
What Happens In The Dog's Body: Insulin 
When Is Hypothyroidism not Hypothyroidism? 
What Happens in a Dog's Body with Severe Vomiting? 

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