Welcome back to Environmental Emergencies Theatre. In our last thrilling episode we saw Hypothermia.
It’s summertime now, so it’s time to talk about Heat Stress, aka Hyperthermia. Hyperthermia, like her twin sister Hypo, can kill you deader’n dirt by this time tomorrow.
We do best when our core temperature is within one degree either way of 98.6 degrees Fahrenheit (37 degrees Celsius). Below that, you’re into hypothermia. Above it, you have hyperthermia.
We generate heat all the time (the fancy name for this is “thermogenesis”) via three basic means. One is thermoregulatory thermogenesis, which is the endocrine system and the central nervous system working together to control the rate of cellular metabolism. The second kind is work-related thermogenesis: when the skeletal muscles contract, they throw off heat as a byproduct. The third kind is diet-related thermogenesis: heat generated by chemical bonds being broken, and complex sugars and proteins being reduced to simpler molecules, as we digest our food.
The name for keeping the interior environment of our body within certain narrow limits (pH, salinity, and so on) is homeostasis. The human body has several systems that are tasked with maintaining our internal temperature. It’s a mammal thing. We regulate our internal temperature by internal means so we don’t have to crawl into cracks or sun ourselves on rocks.
Regardless of the exact origin of the heat in the body, it has to go somewhere, because the body is very heat-sensitive. Even when it’s forty below and we’re wearing our parkas, we still need to bleed off excess heat. All we’re doing is controlling the rate. If the core body temperature hits around 105 degrees Fahrenheit, the proteins in the brain start to denature, and within ten to fifteen minutes … you’re in deep, deep trouble.
Convection is carrying away heat through the motion of a fluid over a surface (the fluid is warmed, expands, rises, and is replaced with cooler fluid). Conduction is carrying away heat through direct contact with a cooler object. Radiation is the direct loss of heat as infrared radiation. And evaporative cooling is using the latent heat of evaporation to cool things — it takes energy to move water from its liquid form to its vapor form at the same temperature.
As the core temperature of the body rises, the hypothalamus (it’s under the thalamus at the base of the brain) senses the rise both directly from the blood that’s moving by it, and remotely from temperatures sensors in the extremities and in the great vessels in the chest. The hypothalamus stops producing the hormones that stimulate cellular metabolism, and instead starts dilating the blood vessels (vasodilation) near the surface of the skin and stimulating perspiration.
The skin grows flushed, and wet. With the vasodilation in the skin and subcutaneous tissues, the skin gets hotter, which leads to more convective cooling and radiant cooling. The sweat evaporates, carrying away heat as liquid turns to vapor. The newly-cooled blood goes back into the core. All’s well.
That’s if things are working right.
The very young and the very old have a harder time dealing with heat stress. They have less-responsive thermoregulating systems, and have a lower tolerance to variations in core temperature. Folks with diabetes may have suffered damage to the parts of the autonomic nervous system that provide feedback to the hypothalamus, and may have nervous system damage that interfers with vasodilation and sweating. Some drugs, notably diuretics, beta blockers, and vasopressors, interfere with vasodilation and sweating. Antihistamines, and some psychotropics, can interfere with the central nervous system’s thermoregulation.
High humidity can interfere with evaporative cooling. High environmental temperatures and lack of ventilation can interfere with convective and radiant cooling. High heat, high humidity, and poor ventilation is the trifecta. Make it an elderly diabetic in a non-air-conditioned apartment where the windows don’t open, and it’s 9-1-1 time.
So, we’re now in the Land of When Things Go Wrong.
First up is heat cramps. The main causes of heat cramps are dehydration and loss of electrolytes (especially sodium). Sweat not only takes water out of the body, it takes out salt. You usually see heat cramps in folks who are working in a hot environment: work-related thermogenesis leading to vasodilation and sweating, leading to dehydration and hyponatremia. Heat cramps usually show up in the extremities (especially legs) and abdomen. This is nature’s way of telling you to stop exercising when it’s that hot out.
What to do about it: get out of the hot environment, stop using your large muscles, drink water, replace electrolytes.
Next up: Heat exhaustion (AKA heat prostration and heat collapse). This is the most common heat-related injury, and its basic mechanism is the same as heat cramps. The basic causes are heat exposure, stress, and fatigue. (It doesn’t have to be particularly hot before heat exhaustion is a possibility — wearing multiple layers of clothing that limit the effectiveness of sweating will do the job just fine. So, if you’re out hiking, take off layers; when you stop to rest, put on layers.)
The signs and symptoms of heat exhaustion are:
What to do about all this: Take off any excessive layers of clothing, particularly around the head and neck. Get out of the hot environment (say, into the back of a nice air-conditioned ambulance). Drink a liter or so of water (slowly, so nausea doesn’t develop). Loosen restrictive clothing, lie down with your feet up, and use a fan for cooling.
Usually the symptoms resolve within a half hour. You should get worried if the symptoms don’t start to resolve, if the core temperature stays elevated or increases, or if the patient starts to lose consciousness. Be very cautious with the very young, the very old, and folks with underlying medical conditions (e.g. diabetes, heart disease).
Heat exhaustion, like heat cramps, is caused by dehydration and loss of electrolytes.
Like heat cramps, heat exhaustion is nature’s way of telling you to slow down.
Now comes the biggie: Heat stroke. This one will kill you, and kill you fast. When your brain is gone it’s game over, and that can take as little as ten to fifteen minutes from onset of symptoms.
In heat stroke your body has essentially given up on cooling. The hypothalamus is saying “See ya later.”
This is the one that kills kids who are locked in cars on sunny days. It kills old folks in poorly ventilated apartments during heat waves. It kills healthy thirty-year-old guys who are working in humid warehouses. It kills.
Most of your heatstroke patients aren’t sweating — the sweating mechanism has been overwhelmed. You may find ‘em with wet skins, though — because the sweat that was there before hasn’t dried off. Wipe ‘em down with a towel — if no new sweat forms, be very suspicious. The patient can be going into heat stroke even if the sweat is still pouring off him. The main thing is the core temperature: 105, 106, higher.
Signs and symptoms:
Not all of these signs and symptoms will be present in every case.
This one is a medical emergency. You have to act, right now. Your first and biggest objective is to lower the core temperature, and do it by any means available.
One minor caveat: Try not to put the patient into hypothermia. If he starts shivering he’s just going to build body temperature back up.
Death is nature’s way of telling you to slow down.
Prevention, Ounce of
Stay out of high heat/high humidity environments, particularly if you aren’t acclimated to them. Shopping malls, office buildings, movie theaters, are all air conditioned. Try to be in one of them during the heat of the day. Crank your home air conditioning to 70 or lower. If you must be in a high heat/high humidity environment, try to limit your exposure to three hours or less.
But the worst o’ your foes is the sun over’ead:
You must wear your ‘elmet for all that is said:
If ‘e finds you uncovered ‘e’ll knock you down dead,
An’ you’ll die like a fool of a soldier.
Wearing a hat is the simplest thing you can do if you must be out-of-doors in a heat-stress environment. Without a hat the only things between your brain and a 10,000 degree thermonuclear furnace are a layer of thin bone, a layer of thin scalp, and a (perhaps thinning) layer of hair. Carrying an umbrella or parasol isn’t a bad idea. Wear light-colored, loose-fitting, cotton clothing. Remove layers as necessary to allow sweat to dry on your skin.
A cool neck wrap, to keep the brain cool, can sometimes help (if the humidity is low enough to allow evaporation.
Try to avoid heavy meals (diet-related thermogenesis) and heavy physical labor (work-related thermogenesis) when it’s hot and humid. The siesta is a wonderful idea. So’s having the major meal of the day well after sundown.
Hydration. Water is your friend. How much water? Just like with hypothermia, drink water until your urine is frequent, copious, and clear. Drink water even if you aren’t thirsty. Line up eight to twelve half-liter bottles of water on your desk and drink one of them at the top of every hour.
This brings us to the subject of Water Intoxication. Every year you lose a frat pledge or two from this — being forced to drink large amounts of water over short periods. What happens is the electrolytes get washed out of the body, and Bad Stuff (like cardiac arrythmias) follow. So, drink your water over long periods of time, and keep up your salt intake. Pretzels, potato chips, lemonade, watermelon, bananas … but not salt pills. (Salt pills can rip your stomach and can send you into hypernatremia, which has its own constellation of not-fun signs and symptoms.)
As always, I am not a doctor and can neither diagnose nor prescribe. Nothing here is advice for your particular condition; it is presented for amusement only.
Please stay safe.
Copyright © 2006 by James D. Macdonald.
I am not a physician. I can neither diagnose nor prescribe. This post is presented for entertainment purposes only. Nothing here is meant to be advice for your particular condition or situation.
Heat Stress by James D. Macdonald is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.
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