Why Heat Waves Kill So Quickly

June 22, 2016

An intense heat wave that sent temperatures in Phoenix to 118 degrees Fahrenheit (47.7 degrees Celsius) this weekend has killed four people — and the heat could be worse today.

Those killed so far were all hiking or biking outdoors, but heat waves can kill close to home, too. In 2003, during a major European heat wave, 14,802 people died of hyperthermia in France alone.

Most were elderly people living alone in apartment buildings without air conditioning, according to Richard Keller, a University of Wisconsin-Madison professor of medical history and bioethics and author of “Fatal Isolation: The Devastating Paris Heat Wave of 2003” (University of Chicago Press, 2015).

So how does heat kill? When core body temperature rises too high, everything breaks down: The gut leaks toxins into the body, cells begin to die, and a devastating inflammatory response can occur.

Part of the insidiousness of heat-related deaths is how quickly they can happen. According to ABC15 News, a mountain biker who died near Phoenix was a fit 28-year-old who had consumed plenty of water and was biking with two doctors. Her pulse stopped at around 9 a.m. on Sunday (June 19). Despite immediate resuscitation efforts, she could not be saved.

Sudden death

The deaths so far in Arizona aren’t typical heat deaths, Keller told Live Science. Rather, they’re “like shots across the bow telling you that something is coming,” he said. Outdoorsy types and outdoor workers like roofers might suffer first, but it’s the elderly and the mentally ill who make up the majority of deaths.

The medical term for excessive body heat is hyperthermia. The first phase is heat exhaustion, a condition marked by heavy sweat, nausea, vomiting and even fainting. The pulse races, and the skin goes clammy. Muscle cramping can be an early sign of heat exhaustion, according to the Centers for Disease Control and Prevention (CDC).

Heat exhaustion can be reversed by moving to a cool location, loosening clothing and applying cool, wet washcloths to the body. But when people with heat exhaustion can’t find relief, they can quickly advance to heat stroke.

This condition happens when a person’s core body temperature rises above 104 degrees F (40 degrees C). (This number is something of an estimate; there are a few degrees’ variability among people as to how much internal heat they can tolerate.)

In heat stroke, sweating stops and the skin becomes dry and flushed. The pulse is rapid. The person becomes delirious and may pass out. When trying to compensate for extreme heat, the body dilates the blood vessels in the skin in an attempt to cool the blood.

To do this, the body has to constrict the blood vessels in the gut. The reduced blood flow to the gut increases the permeability between the cells that normally keep gut contents in, and toxins can leak into the blood, according to a book chapter in the textbook Wilderness Medicine (Mosby, 2011).

These leaky toxins trigger a massive inflammatory response in the body, so massive that the attempt to fight off the toxins damages the body’s own tissues and organs. It can be hard to tell what damage is caused directly by heat and what is caused by the secondary effects of toxins, according to Wilderness Medicine. Muscle cells break down, spilling their contents into the bloodstream and overloading the kidneys, which in turn start to fail, a condition called rhabdomyolysis.

Proteins in the spleen start to clump as a direct result of heat; they’re essentially cooked. The blood-brain barrier that normally keeps pathogens out of the brain becomes more permeable, allowing dangerous substances into the brain. Autopsies of people killed by heat stroke often reveal microhemorrhages (tiny strokes) and swelling, and 30 percent of heat stroke survivors experience permanent damage in brain function, according to Wilderness Medicine.

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