Heatstroke

38 Heatstroke



Theory Refresher


A spectrum of severity is seen in dogs suffering from heat-induced illness in terms of both pathophysiological derangements and clinical consequences. It may therefore be more accurate to employ a gradation of severity from heat stress to heat exhaustion to heatstroke. Nevertheless, the general term ‘heatstroke’ will be used here.


Heatstroke results from severe hyperthermia, although a specific threshold above which heatstroke occurs has not been defined in dogs. Cellular injury may occur above 41°C and permanent brain damage is reportedly possible. Enzyme dysfunction, protein denaturation and severe organ damage may occur above 43°C and are associated with a significant increase in mortality. Further information regarding the prognosis in dogs with heatstroke is provided in Box 38.1.



BOX 38.1 Prognostic considerations in dogs with heatstroke


The prognosis in heatstroke depends on a number of factors including:


• Duration of exposure

• Highest core body temperature reached

• Pre-existing medical conditions

• Development of secondary complications

• Speed and intensity of treatment

Unfavourable prognostic indicators include:


• Coma or hypothermia at presentation

• Progressive neurological deterioration

• Persistent/refractory hypoglycaemia

• Persistently unstable cardiovascular status

• Worsening azotaemia and oliguria despite appropriate fluid therapy

• Disseminated intravascular coagulation (DIC)

In fatal cases, death usually occurs within 24 hours. Death is unlikely to occur in patients surviving more than 48 hours.



Pathogenesis


Heatstroke occurs when heat gain markedly exceeds heat loss and this is predominantly due to the cooling mechanisms stimulated by the hypothalamus becoming overwhelmed. These mechanisms include:


• Direct stimulation of panting to maximize evaporative heat loss – this is the predominant cooling mechanism in dogs

• Increased cardiac output and peripheral vasodilation to increase delivery of blood to the body surface – this increases heat loss via convection and radiation.

Heatstroke may occur more readily in dogs suffering from a lack of acclimatization to warmer weather. The acute phase response and heat shock proteins may also be involved in the pathogenesis of heatstroke.



Risk factors


A variety of predisposing factors exist that may be classified as endogenous or exogenous. Most of these factors impair heat loss and include:


• Exogenous
– Confinement and/or poor ventilation

– Increased humidity

– Water deprivation

• Endogenous
– Brachycephalic anatomy

– Laryngeal dysfunction

– Obesity

– Darker and/or thicker coat.

Some factors affect heat gain, for example exercise, pyrexia, and especially external ambient temperature. Measures designed to try to prevent heatstroke are summarized in Box 38.2.



BOX 38.2 Preventative measures for heatstroke in dogs


Heatstroke can be avoided in the majority of cases if appropriate preventative measures are employed. Preventative measures are especially important in dogs predisposed to heatstroke (e.g. brachycephalic breeds, dogs with laryngeal dysfunction, obese dogs) and include:


• Controlling exercise to allow acclimatization as summer approaches

• Avoiding exercise during the hottest times of the day

• Avoiding confinement in a car or other poorly ventilated space

• Ensuring shade is available if the dog is tied up outside

• Ensuring water is always available

• Keeping the coat short during warmer months


Pathophysiology


Heatstroke may be divided into two types. Classic heatstroke occurs due to exposure to high environmental temperature, for example being locked in a car with shut windows on a warm day, or being tied up outside without shade/water. Exertional heatstroke is the result of strenuous exercise. Many pathophysiological derangements are possible in heatstroke due to both direct heat-induced cytotoxicity and acute physiological changes due to hyperthermia. These are summarized in Box 38.3. Multiple organ dysfunction syndrome (MODS) (Box 38.4) is the most serious potential sequela of heatstroke.



BOX 38.3 Potential pathophysiological derangements in heatstroke



• Increased metabolic demand

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Heatstroke
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