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.
The probable mechanism is a degree of dehydration from sweating, combined with peripheral vasodilation.
If the person then stands for a long period, there is pooling of blood in the lower limbs (loss of the calf muscle pump) resulting in a drop in blood pressure and a subsequent syncope. Injuries can occur as a result of the fall.
The patient should be rested and provided with an oral fluid intake. In the elderly patient this diagnosis should only be made after other, more serious, diagnoses have been excluded.
Heat exhaustion
Heat exhaustion is a condition caused by water or salt depletion. It typically occurs in subjects who are not acclimatised and who undertake vigorous exertion, e.g. in military training.
Symptoms and signs
The following symptoms and signs may develop.
• Headache
• Dizziness
• General weakness
• Fainting
• Normal or mildly elevated core temperature (<40°C)
• Tachycardia
• Orthostatic hypotension.
It is very important that the patient is treated at this stage as, if left untreated or allowed to progress, heat stroke will occur.
Treatment
The patient should be placed in a cool environment and an oral electrolyte solution provided. Care needs to be taken when cooling the patient as the traditional tepid sponging and fanning can increase core temperature by causing capillary shutdown in the skin and by stimulating a shivering response. This is a particular problem with younger children. Cautious cooling with a fan is appropriate.
In patients who have significant symptoms, intravenous fluid and electrolyte replacement is required.
Heat stroke
Heat stroke is a serious life-threatening condition and requires rapid treatment. The condition normally occurs in hot, humid conditions where there is little wind and it can occur in the absence of exercise.
The condition occurs when the heat-regulating systems fail to keep up with heat production, are unable to function effectively or fail (e.g. loss of sweating).
It is recognised that there are two clinical forms of heat stroke: classic and exertional
• Classic heat stroke occurs during a period of sustained high environmental temperature and humidity. It tends to occur in older or debilitated people
• Exertional heat stroke is caused by overproduction of heat as a result of exertion and occurs primarily in young, fit subjects. Some people seem to be genetically prone to this condition.
Exertional heat stroke differs from classic heat stroke in that rhabdomyolysis and hypoglycaemia are a frequent problem. From the prehospital perspective, the conditions are very similar.
Symptoms and signs
The diagnosis is primarily clinical. The symptoms and signs are:
• Temperature usually 41°C or greater
• Skin is hot and dry, although sweating may still be present
• Weakness
• Nausea and vomiting
• Confusion progressing to lethargy and eventual coma
• Tachycardia and hypovolaemia
• Clotting abnormalities, including disseminated intravascular coagulation
• Hepatic damage – jaundice seen after 24 hours.
Treatment
Early initiation of treatment in the prehospital phase is very important as heat stroke, if not corrected, will result in rapid death due to damage to the central nervous system. This has been likened to frying eggs – the heat causes the protein to be denatured and irreversibly changed.
• Cool the patient carefully
• Remove clothes
• Consider immersion in water if there is likely to be a significant delay in transfer of the patient to hospital
• Otherwise rapid cooling can be commenced after urgent transfer to hospital
• Give high flow oxygen
• Protect the airway is consciousness is obtunded
• Give intravenous fluids
• Check BM stix and correct hypoglycaemia.
Immersion rapidly cools the individual owing to the high specific heat capacity of water (its ability to remove heat rapidly). Care needs to be taken if the conscious level of the patient is altered and in the unconscious patient protection of the airway is mandatory while performing this procedure. It is important to avoid the production of hypothermia.
Fits can occur and treatment is primarily aimed at airway control and maintaining oxygenation. Hospital management is usually undertaken on an intensive care unit. Cooling needs to be continued and electrolyte levels and the clotting status monitored.
For further information, see Ch. 42 in Emergency Care: A Textbook for Paramedics.
