Heat wave: why can we die of heat?

A period of particularly intense heat is settling in France in mid-July 2022. Temperatures can reach 40°C over a large part of the territory. For the body, such high temperatures are not harmless. What happens in the body during a heat stroke? Why can we die of heat?

It will not have escaped any French or French that it is currently very hot in France. Temperatures are expected to rise to 40°C in a very large number of cities on July 18 and 19, 2022.

Last June, it was a “feather of heat” that fell on France. The weather for this summer 2022 in France resembles the dystopian prediction of the fake weather report that was presented by Évelyne Dhéliat in 2015, to raise awareness of climate change. From now on, we have to get used to talking about a heat wave, a dome, a plume or a heat wave more and more frequently – as well as seeing peaks in ozone air pollution. Due to climate change, France is expected to experience upheavals in the decades to come, including heat waves that are likely to multiply.

Such rises in temperature also remind us of the heat wave that occurred between June and August 2003 in Europe. According to Inserm, 15,000 deaths were caused by this extraordinary heat wave in France, compared to the mortality usually recorded at this time of year.

If we understand without difficulty that excessive heat is harmful for the human body – spending a few minutes in an open space without air conditioning in the middle of summer makes it possible to notice this -, the exact effects of this “heat stroke” on the organism are less known. Without claiming to be exhaustive, it is in any case possible to list certain bodily reactions linked to exposure to heat.

You should drink more during periods of high heat. // Source: Pexels/Karolina Grabowska (cropped photo)

There are two types of “heat strokes”

In 2012, Dr Pierre Hausfater, specialist in internal medicine, and Bruno Riou, head of the resuscitation department at Pitié-Salpétrière Hospital, explained how “heat stroke” occurs, which is ” a rare or even exceptional pathology in France under usual climatic conditions “.

Specialists differentiate between two types of heat stroke: environmental heat stroke, which occurs during ” an exceptional heat wave » and especially touches « the elderly population “, and ” exercise heatstroke which affects a young population, and occurs during physical tests carried out in very hot conditions.

The balance between heat production and heat loss by the body directly influences body temperature. As Pierre Hausfater and Bruno Riou explain, the body evacuates heat through the skin, through different processes (such as evaporation or conduction). To regulate its core temperature, the body has thermosensory neurons, which are found in the hypothalamus. It also relies on “thermal receptors” located in the skin and in the muscles.

How does the heat stroke affect this regulation? When the body accumulates exogenous heat, it can end up in hyperthermia: its temperature rises above its normal valuefrom 37 to 37.5 ° Celsius — above 41.5 ° C, the risk of death is possible, because irreversible problems can reach the brain.

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You're hot ?  Consider drinking.  // Source: Pexels/ Karolina Grabowska (cropped photo)

Why does the body no longer manage to regulate itself in the event of a heat wave?

During this hyperthermia, the specialists note that the heat has the effect of dilating the cutaneous vessels, which are under the skin – those which are responsible for its vascularization. The blood flow becomes higher, in order to promote even more heat exchanges in the process of sweating. For the body, this process is energy hungry, and happens to the detriment of other areas: the body then focuses less on blood circulation in the kidneys and digestive organs. In addition, hyperthermia causes the body to sweat more, and to seek to reduce its heat production.

Normally, these three mechanisms are sufficient to maintain a temperature that does not exceed its physiological limit (ie the body’s normal functions and reactions). According to the two specialists cited, two mechanisms must be invoked to understand why the body’s temperature can then continue to rise, and why the body can no longer regulate itself.

The first concerns the hypothalamus: this structure of the central nervous system is at risk of direct damage linked to the high ambient temperature. In fact, the hypothalamus then loses the capacity of thermostat and that of triggering thermolysis by sweating — thermolysis being precisely the systems implemented by the body to stabilize its internal temperature. However, this balance is mainly achieved by sweating.

The second mechanism concerns the cardiovascular system: it is also found in failure situation, because it also fails to favor this balance of the internal temperature of the body. To achieve thermolysis, the body needs to increase its heart rate (from 12 to 14 liters per minute). He must also increase the flow of blood in the cutaneous vessels (to reach 8 liters per minute): by doing this, the blood vessels which ensure the vascularization of the digestive tract undergo a vasoconstriction. This means that their diameter decreases, the objective being to regulate blood pressure.

In other words, the heart decreases the share of work that it usually grants to other organs, since it concentrates on its work dedicated to the skin, and that it must at the same time maintain the most stable possible flow. . Pierre Hausfater and Bruno Riou note here that failure of the cardiovascular system is the mechanism that most commonly occurs in the older population, due to ” physiological changes […] age related “.

When you are dehydrated, there is less liquid in the vessels, and this affects the entire cardiovascular system, that is to say the supply circuit of the organs of the human body. “Explains to Numerama Agnès Ricard-Hibon, emergency doctor.

Heat causes damage inside cells

At the level of the body’s cells, heat causes direct damage whose intensity can vary according to the temperature reached, and the duration of this rise in temperature. A temperature of 41.6°C or more, reached for a period of 45 minutes to 8 hours, can cause cell damage.

In the case of environmental heat stroke, specialists point out that it is often linked to a heat wave, that is to say ” at least three successive days of ambient temperature above 32°C “. This is what makes them say that ” a heat wave is defined less meteorologically than medically since it is the first predictor of heatstroke.

However, not all people are equal in the face of this ” thermal load » : depending on genetic factors, cases where people see their abilities diminished (for example, in a state of intoxication), and age, the situations can vary.

In the event of a heat wave, the government recalls that people over the age of 65, pregnant women, children under four, sick people, people in precarious situations, sportsmen and outdoor workers, are particularly vulnerable. to heatstroke. To avoid it, a few precautions are necessary, such as drinking water regularly, cooling your body and avoiding going out during the hottest hours of the day.

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