Chapter 42. Heat illness
Heat illness does occur in the summer months and is frequently associated with strenuous activities such as sport or military exercises.
As with other environmentally produced disorders, there is a range of problems progressing through minor conditions (e.g. muscle cramps) to the life-threatening illness of heat stroke.
Physiology
The centre for heat regulation is sited in the hypothalamic region of the brain. Heat is produced through metabolism, either as a byproduct (e.g. of muscle contraction) or directly as a heat-producing mechanism.
In humans, the core temperature is regulated to remain constant at around 37°C.
| System | Effects |
|---|---|
| Respiratory | Increased respiratory rate – with increased fluid lossa |
| Cardiovascular | Dilated skin capillary beds |
| Increased heart ratea | |
| Increased cardiac outputa | |
| Relative or actual hypovolaemia | |
| Reduced renal blood flow | |
| Fluid and electrolytes | Dehydration |
| Hyponatraemia (especially if fluid loss replaced by water only) | |
| Skin | Warm and red – increased blood flow |
| Increased sweating | |
| Other | Decreased liver function |
| Impairment of coagulation | |
| aExercise-related condition. | |
Acclimatisation
Prolonged exposure to a hot environment results in acclimatisation. Many of the changes that occur with this process are an attempt to reduce salt loss.
Types of heat illness
• Heat cramps
• Heat syncope
• Heat exhaustion
• Heat stroke.
Heat cramps
Heat cramps usually occur in the muscles of the lower limbs and are related to exercise. They occur in people in whom significant fluid losses due to sweating have been replaced with fluid with an insufficient salt content. As a result the individual becomes hyponatraemic and it is this electrolyte disturbance that is thought to cause the muscle cramps. Adequate salt replacement relieves the problem.
Heat syncope
Fainting related to the heat is not infrequently seen in Emergency Departments during hot weather. Elderly people seem particularly prone to this condition.
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