Dr. Gaudio covered the topics of thermoregulation and heat dissipation, acute heat stress and acclimatization, impaired thermoregulation, and specific heat related illnesses. Thermoregulation for humans is essentially the balance of heat load (generation or external source addition) with heat loss. The act of being alive (basal metabolism) at rest generates about 75 kilocalories of energy per hour, which undissipated would create a human temperature rise of approximately 1 degree Celsius (approximately 9/5 degrees Fahrenheit) per hour. The optimal temperature for human metabolism is between 36 and 37.5 degrees C (96 and 99 degrees F).
Humans shed heat by radiation, evaporation, conduction, convection and respiration. For radiation to be maximally effective, blood flow to the skin must increase. When this occurs, there is a compensatory decrease in blood flow to certain internal organs. Convective heat loss in humans occurs by sweating, which requires evaporation to be effective. Therefore, sweating becomes ineffective with high relative humidity (greater than 75%). The scalp, face and torso have more sweat glands than do the lower limbs. A very important fact is that a limiting factor for evaporation as a cooling mechanism for humans is the gastrointestinal tract, which can only absorb about a liter per hour of liquid. Taking your shirt off to allow sweat to evaporate is generally a good thing, with the following possible exceptions: (1) in strong sunlight, (2) in the absence of any cooling breeze when there is a high solar load (of heat) and (3) if skin is highly pigmented, and would therefore absorb more heat than non-pigmented skin.
Hats should be worn to reduce solar load, but be aware that they may decrease evaporative cooling. So, wear your lightweight hat in the hot sun, but remove it when you are in the shade or if there is a brisk breeze to promote evaporation. And you are sweating.
Acclimatization to heat generally occurs over 1 to 2 weeks, but may take longer, particularly if exposure is intermittent and inadequate for the purpose. Among many changes, it is interesting to note that acclimatized sweat glands have increased sweat capacity and conserve sodium (so the sweat is more dilute).
Pay particular attention to elders and infants, and persons with predisposing medical conditions, such as obesity, kidney disease, diabetes, cystic fibrosis, scleroderma, and Alzheimer’s dementia. Many drugs, both prescription and illicit, may impair heat dissipation.
Fluid replacement strategies for heat cramps, which are generally felt to be caused by water-without-electrolytes replacement, are numerous, but generally center around “sports beverages.” A good natural concoction is ½ liter orange juice combined with ½ liter of water and ½ to 1 teaspoon of table salt. This provides water, fructose, sodium, potassium, chloride, vitamins C and B6, thiamine and folate.
Heat edema (fluid retention and swelling) involves the hands and feet of persons during the first few days of heat exposure. Treatment is rest, elevation of the affected body parts and support hose for the legs and feet. Heat syncope (fainting) is caused by a brief drop in blood pressure associated with some combination of dehydration, dilation of blood vessels in the skin, drug effects, slow heart rate, and pooling of blood in the lower limbs during periods of standing. If there is no other obvious cause of swelling, heat edema may be treated with rest, elevation of the swollen legs and feet and support hose.
Heat syncope is caused by a brief drop in blood pressure and therefore in the pressure of blood delivered to the brain. It may come on quickly, and is usually seen early during an episode of heat exposure. Dehydration is a contributor, as are the effects of certain drugs. The treatment is to replenish fluids.
When someone suffers heat exhaustion, which may lead to full-blown heat stroke, the length of heat exposure has been longer than that which causes heat syncope. Physical findings occur weakness, fatigue, normal to mildly elevated body temperature, rapid heart rate and breathing, thirst, decreased urine output, low blood pressure and altered mental status (listlessness, agitation or confusion). The latter is very important. Anyone who is in the heat and acting abnormally is suffering from heat illness until proven otherwise. Additional physical findings include nausea, vomiting, headache and muscle cramping. Skin signs may be variable and show skin that is sweaty or dry, and hot and flushed or cool and clammy.
Heat stroke is a situation of extreme heat exhaustion with a failure to be able to control body temperature. This is life-threatening and is associated with a body temperature in excess of 40 degrees Centigrade (104 degrees Fahrenheit), severe neurologic signs (delirium, coma, seizures), and injury to many organ systems, such as the liver, gastrointestinal tract, kidneys and heart. It is essential to cool the victim as soon as possible.