Heat Related Illnesses

Normal Physiology

1. Increase work of muscle and metabolic rate produce heat and an increase in body temperature.
2.  
Vasodilatation and increased blood flow to the skin lead to a dissipation of heat.
3.  Adequate hydration is an important component for heat dissipation. 

4. Evaporation of sweat plays an important role in lowering body temperature.  The process of evaporation causes loss of fluids and electrolytes. 

5. High humidity and ambient temperature interfere with the body's mechanisms to lose heat
6. 
Children have greater heat production during exercise compared to adults 

7. Children have less efficient adaptation to exercise and less ability to lose heat by sweating. 

8. Children have a greater surface area to body ratio 

9. When the air temperature is greater than 35 C, children have lower exercise tolerance.

Risk Factors for Heat Related Illness

1. Hot and/or humid weather
2.  Poor preparation- not heat acclimatized, inadequate prehydration, little sleep, poor fitness
3. Excessive physical exertion- multiple same day sessions, insufficient rest in between exercise
4.  Clothing, uniforms or protective equipment that contribute to heat retention
5. Overweight individuals, very young, and elderly
6. Children with recent illnesses (esp gastrointestinal distress due to dehydration status)
7.  Children with chronic conditions affecting hydration status or thermoregulatory control:
   1. ​ Excessive fluid loss: fever, vomiting, diarrhea, diabetes insipidus, diabetes mellitus.
   2. Decreased sweat production: spina bifida, eczema, extensive sunburn
   3.  Excessive sweat production- cyanotic heart defects, cystic fibrosis
   4. Abnormal hypothalamic thermoregulatory function- anorexia nervosa, malnutrition, prior heat-related illness, obesity

Physiology of Heat Related Illness

1. Occurs when there is excessive heat generation and storage in the face of decreased heat dissipation because the body is no longer able to adequately rid itself of excess heat
2. Heat related illnesses constitute a spectrum of maladies ranging from very mild to life threatening and include:
   1. ​Prickly heat
   2. Heat edema
   3. Heat syncope
   4. Heat cramps
   5. Heat exhaustion
   6. Heat Stroke

Prickly Heat

• Heat rash, miliaria, rubra generally occurring on clothed or covered areas of the body
• Caused by blockage of sweat gland pores, may have secondary staph infection of pores
• Preventative measures: have children wear clean, loose-fitting clothing. Avoid talc, powders and creams. Cool baths may sooth itching. 

Heat Edema

• Swelling of the hands and feet.  More common in elderly but may occur in children
• Caused by peripheral vasodilation and aldosterone secretion
• Self-limited and transient.  Elevate extremities.  Do not use diuretics

Heat Syncope

• Usually occurs early in the warm weather season before acclimatization
• Orthostatic syncopal episode usually associated with prolonged standing, a sudden rise from a sitting position, or with vigorous activity.  There may be a prodrome of dizziness, visual blurring, tachycardia, nausea, sighing, or yawning
• Likely caused by peripheral vasodilation and pooling of blood in interstitial areas
• May be prevented by having the patient sit or lie down.  Treatment includes removal from the heat and encouraging rest. Dehydration is usually not severe and can be treated orally

Heat Cramps

• Heat cramps consist of brief (less that 1 minute), intermittent, excruciating muscle contractions which occur predominantly in the muscles of the lower extremities, abdomen, and shoulders
• Frequently occur after intense exercise, when relaxing, or showering
• Thought to result from electrolyte depletion. May be exacerbated in individuals drinking large quantities of hypotonic fluids
• Heat cramps are a warning sign to impending heat exhaustion
• Treat by supplying fluids WITH electrolytes and mild stretching

Heat Exhaustion 


• Most common heat illness in athletes. May be a precursor to heat stroke and can rapidly progress if not treated
• Can either be due to insufficient fluid intake resulting in hypernatremic dehydration or secondary to hyponatremic dehydration due to salt depletion in children who are rehydrating with hypotonic fluids
• Defined as reversible heat overload. Children with heat exhaustion should suffer no tissue damage or permanent sequelae
• Symptoms are nonspecific: any combination of isolated muscle or generalized weakness, fatigue, dizziness, headache, occasional syncope, nausea, vomiting, diarrhea, hyperventilation, tachycardia, orthostatic hypotension, vertigo, chills, visual disturbances, cutaneous flushing. Core body temperature is usually between 38.0-40.0 C. Mental status is normal
• Treatment is usually rest and observation. Rapid cooling techniques are usually not necessary. Provide fluid replacement with isotonic fluid. Measure serum electrolytes, BUN and hematocrit

​Heat Stroke

• A catastrophic, life-threatening medical emergency. Morbidity is high (17-70%)
• Temperature > 40.6. Body temperature is no longer under hypothalamic control
• Heat stroke triad: Hyperpyrexia, severe CNS disturbance, anhidrosis
• Results in permanent damage to end organs (liver, heart, kidneys, or CNS). Leads to cardiovascular collapse and coma
• Symptoms may include disorientation, muscle twitching, ataxia, anxiety with tachycardia, hyperventilation, dypnea, anhidrosis
• Differential diagnosis includes: DKA, status epilepticus, sepsis, CNS infection, anticholinergic poisoning, toxic ingestion, neuroleptic malignant syndrome, serotonin syndrome, thyroid storm, and pheochromocytoma
• There is release of endotoxins, and cytokines leading to end organ damage, possible DIC, and rhabdomyolysis
• Treatment of heatstroke must be prompt:
  • Transfer child to cooler environment
  • Remove clothing and spray skin with water and then fan the child to increase evaporative and convective heat loss
  • Apply ice to groin, axillae, and neck
  • Isotonic fluid replacement with electrolyte replacement as needed
  • Children may need to be intubated and given oxygen due to decreased respiratory drive and increased oxygen demand
  • Cardiovascular monitoring and support is essential
  • Rapid cooling should continue until temperature is 39 (102). The more rapidly the patient is cooled, the lower the mortality
  • Antipyretics (acetaminophen, asprin, NSAIDs) are ineffective
  • Diazepam may control shivering during cooling process

References

1. AAP. Climatic Heat Stress and the Exercising Child and Adolescent Pediatrics Jully 2000
2. Howe A. et al.  Heat-Related Illness in Athletes.  The American Journal of Sports Medicine.  Vol 35 No. 8 2007