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The Racing Siberian Husky Online
Early Fall 1998 Web Feature Edition
Published by Heritage North Press & WorkingDogWeb



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A Risk for Top-performing Sled Dogs
Heat Build-up during Intensive Exercise

PART ONE

Heat build-up occurs during exercise. Muscles are able to convert on the order of 20-30 percent of the chemical energy they use into mechanical work. [1]  Thus, 80-70 percent of the energy production of an intensively working dog ends up creating body heat.  During intensive pulling, or even high-speed "going along," there is, therefore, a large quantity of heat generated by sled dog muscles that must be dissipated.

When muscles get too warm, they function less efficiently.  When a dog's body warms too much, its non-muscular body functions not only work less efficiently, but actually may fail entirely, thus presenting a life-threatening situation to the dog. [2]

Normal rectal temperature for Canis familiaris is considered to be 38.7 degrees Centigrade plus or minus 0.6 degree.  That is a range of 100.6 to 102.7 degrees on the Fahrenheit scale. [3] Temperatures below 100F are considered to be indicative of systemic hypothermia [low body temperature]. Rectal temperatures greater than 41.4C or 106.5F may be displayed by sled dogs after intensive exercise, especially after sprint racing, but within 15-20 minutes those rectal temperatures should have returned to normal. [4]

Early signs of exercise hyperthermia for sled dogs are increased respiratory rate, excessive salivation and bright red gums.

What's called pathological exertional hyperthermia [exercise-induced heat exhaustion or heat stroke] is life-threatening.  Sustained rectal temperatures above 41.4C or 107F signal the latter condition.  It has most typically been found to occur with sled dogs as a result of over-exertion when air temperature was great than 20 degrees F. [4]

Early signs of exercise hyperthermia for sled dogs include the following: increased respiratory rate, excessive salivation, and bright red gums.  The hardest working dogs are the most at risk.  If caught early, exercise hyperthermia can be halted by merely stopping the exercise and allowing the dog to drink cold water. 

However, if the condition advances, collapse and shock will result and these situations are immediately life-threatening.  Body temperature must be lowered immediately.  Cold water immersion is the recommended way to lower body temperature.  [4]

Thus, the hardest-working dogs in a team should always be considered those most at risk for hyperthermia effects.

Sled dogs can't  sweat.  If they did, they could improve their rate of exercise-created heat dissipation through their skin surface.   Instead, it is by increasing their breathing rate and the discharge and evaporation of water from their mouth area that sled dogs can increase the amount of heat dissipated during exercise -- short of running into or through cold water, or stopping and taking cold drinks.

If the air temperature dropped 10 degrees F, this temperature change should result in at least an order of a 10 percent increase in heat dissipation PER BREATH for the dog! [5] 

It would also result in roughly a similar percentage increase in the amount of heat lost through the dog's coat.  HOWEVER, the rate of heat loss through the dog's coat is much less than the rate of heat loss that results from a three- or four-fold increase in respiration rate [number of breaths per minute] due to exercise.

-- Richard Petura, Racing Editor, WorkingDogWeb

End of Part One

FOOTNOTES:

[1]  
In the past half century, numerous approaches have been taken in trying to estimate the mechanical efficiency of muscles.  The essential conclusion is that muscles are about as efficient as the average auto engine at turning chemical energy into mechanical work.  The average auto engine, on a good-engine day, operates in the 20-30 percent range in converting chemical energy to mechanical work.

[2]  
The biochemical reactions within a sled dog's body are temperature sensitive.  In addition, cell damage starts to occur at body temperatures above 42 degrees C [108 degrees F] when oxygen delivery can no longer keep pace with the oxygen demand.  The magnitude of cellular damage depends on the degree of temperature elevation and the length of time it is experienced. 

When the oxygen demand so exceeds its delivery rate, cell function and integrity break down.  Severe hyperthermia causes multiple organ dysfunction:  kidney failure, liver failure, intravascular bleeding and clot formation, intestinal breeding and cell sloughing as well as endotoxin absorption, metabolic acidosis, heart rate dysfunction, skeletal muscle tissue death, heart failure and mental dysfunction.  In humans, at least, there is evidence for permanent derangement of the thermoregulatory function and greater risk for future heat exhaustion/stroke episodes.

In addition, with exertional heat stroke, attempts to treat hypocalcemia (calcium ion deficiency) tend to result in only transient Ca plasma elevations with the added calcium tending to end up in the DAMAGED muscle.  this may later dissolve out and be reabsorbed. , leading to severe hypercalcemia (calcium ion excess).  Exertional heat stroke is also associated with a much higher incidence of renal (kidney) failure than conventional (non exertion caused) heat stroke.

Source: 
S. J. Ettinger and E.c. Feldman, Textbook of Veterinary Internal Medicine, Diseases of the Dog and Cat, 1995, Vol. 1, 4th edition.

[3]
Body temperature of a homeotherm [warm-blooded animal] is affected by time of day, age, sex, activity, and adaptation to climate extremes.  The normal rectal temperature and range of variation noted represent diurnal [24-hour] measurements.   they also assume a normal statistical distribution with 95 percent confidence interval set at two standard deviations. 

Source: 
P.L. Altman & D.S. Dittmer, "Metabolism," Biological Handbooks, Federation of American Societies for Experimental Biology, 1968.

[4]  Source: 
Stuart Nelson, DVM, "Thermal Injuries," Vet Check, International Sled Dog Veterinary Medical Association, Vol. 2, No. 1, 1995.

[5] 
An exercising dog's lungs also act much like a heat pump.  Cool atmospheric temperature air is drawn into the nose and mouth and -- via the air passages -- ends up in the lungs.  The cool, dry air is warmed to body temperature traversing the air passages to the lungs.  The air exiting the nose and mouth from the lungs is warm, moist air close to body temperature.  If all the air inspired at each breath were to heat from 20 degrees F to 103 degrees F, rather than from 30F to 103F, a 13.6 percent increase in heat dissipation would have occurred. 

Since water produced by the body [water thus originally at body temperature] also exits from the mouth with each expiration, an estimate of 10 percent increase in heat dissipation for a 10 degree F air temperature DECREASE is quite likely to UNDERESTIMATE what would actually occur.

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