Where is the thermoregulatory centre located?
In the anterior hypothalamus :
Senses deviations from the temperature set-point and sends signals to initiate heat dissipation measures
What are the 4 principle mechanisms for heat loss or heat gain in dogs and cats?
Convection: heat transfer between skin and moving air or water (loss or gain)
Radiation: heat loss or gain to the environment along a temperature gradient, without direct contact, heat can also be gained from solar energy (loss or gain)
Evaporation: moisture evaporites into the air, pulling heat away from the body (heat loss only)
Conduction: heat transfer between skin and objects in contact with the skin (loss or gain)
True fever
- Pyrexia
- normal body response to invasion or injury
- one of the several components of the “acute phase response”
- increased neutrophil counts
- increased lymphocyte activity
- increased fibroblast activity
- fatigue
What causes pyrexia?
Exogenous pathogens: bacteria, viruses, fungi, and some pharmological agents
-This leads to production of endogenous pyrogens
What are pyrogens
- proteins that are produced by leukocytes or tumour cells, and trigger a febrile response
- Also known as cytokines
- travels to the anterior hypothalamus stimulating the release of prostaglandins = cases increases in temperature
Why do you not treat mild to moderate fevers?
- fever plays a role in interfering with the growth and reproduction of the causative organism
- active cooling can be detrimental as body will work harder to maintain new set temperature and if active cooling suddenly stopped, a patient can rebound to an even higher temperature
What do you use to treat a fever?
NSAIDs
- inhibits prostaglandin synthesis
- increase mortality in rabbits with Pasterurella infections
At what temperature do you consider a fever extreme and how do you treat it?
- 41.6 degrees celcius (106 F)
* active cooling
Examples of heat dissipation in a dog
Panting and hypersalivation, peripheral vasodilation
Dogs and cats will actively look for a cooler environment by finding shaded area, and lying on cool substrates
At a low humidity ambient temperature of 32C (89.6F), what mechanisms are used to maintain body temperature?
Convection, conduction and radiation
*Passive processes and do not require additional energy expenditure to dissipate heat
What initially happens if a dog needs to actively dissipate heat?
- Pant to increase evaporative heat loss
- Peripheral vasodilation increases to increase convection and radiation of heat from the body
- Panting uses large muscle groups + vasodilation increases cardiac output to maintain MAP= generates heat
- If heat load exceeds heat dissipation = heatstroke
Definition of heatstroke
A body temperature greater than 41C (105.8F) PLUS CNS dysfunction
What causes heat stroke?
*Extreme environmental heat often in combination with high humidity
*Humidity hinders heat dissipation methods by reducing the evaporation of water
= animal has to expend more energy to dissipate heat, increasing metabolic components of heat load and not able to dissipate heat.
- Other predisposing factors: obesity, laryngeal paralysis, tracheal collapse, brachycephalic syndrome
- Exertional heatstroke : when dogs are worked in extreme heat without sufficient cooling opportunities
What is the upper lethal temperature in dogs?
43C (109.4F)
But irreversible brain damage has been identified at 41C (105.8F)
How would a heatstroke patient usually present?
Recumbent, dry or tacky, hyperemic MM, a very fast or very slow CRT and will be heavily panting
Neurological signs may be present: ranging from loss of consciousness to blindness, seizures, and coma.
Tx for suspected cerebral Edelman must be implemented as soon as possible to prevent brain herniation
Active cooling in the hospital setting
- Administer cool IVF
- wetting the patient with tepid water
- Using a fan to evaporate water
- Placing on a stainless steel table with no blankets
- Ice packs on areas of the body with large vessels such as around the inguinal area and neck
What substance is contraindicated in active cooling of a heatstroke patient?
The use of alcohol as an evaporative medium as appreciable amount of alcohol can be absorbed across the skin and into the blood stream.
Examples of invasive methods of cooling
- bladder lavage with room temperature sterile saline
- cool water enemas
- gastric lavage with cool water
- downside: difficult to get an accurate body temperature
- can easily cause a patient to become hypothermic
At what temperature do you STOP active cooling?
Until the patient reaches a high normal temperature of 103F (39.4C)
Why should shivering be avoided in a patient?
Shivering is a normal physiological response to a rapid decrease in temperature as it serves to increase body temperature and can lead to rebound hyperthermia
Why should oxygen be provided at every hyperthermia patient at the emergent phase?
- Hyperthermia will cause an increase oxygen demand to the body because of increased metabolic demands
- Perfusion and oxygen delivery to the tissues will be impaired due to metabolic shock
- Supple oxygen via methods that will not cause additional heating - flow by or the use of nasal prongs or cannula
- oxygen cages and tents often increase body temperature as they can trap heat
- if airway compromise is present, incubate and ventilate
What are the minimal database for hyperthermic patients?
PCV/TS BG BUN Blood gas Electrolytes Coagulation test - ACT, or PT/APTT If possible: urine dipstick and USG
What are the blood abnormalities you expect to find in a heatstroke patient?
Elevated PCV and TP (both can be decreased if hemorrhage has occurred)
Hypoglycemia: secondary to acute hepatic insult and increased metabolic demands
Azotemia secondary to dehydration and/or acute renal failure
Coagulopathy due to DIC
Mixed acid base disturbance: typically a metabolic acidosis (from increased lactate reflecting poor tissue perfusion) and a respiratory alkalosis from excessive panting
What are the normal secondary insults that a heatstroke patient experience once the immediate crisis has passed?
DIC, ARF, GI bleeding, pulmonary edema, cerebral edema, cardiac arrhythmias
