Cold: when the body is hypothermic

The hypothalamus, an accurate thermoregulator

The control center for our body temperature is in the hypothalamus, a small area at the base of the brain. It acts as a highly accurate thermostat, reacting to information received via ultra-sensitive sensors located in the skin, spinal cord, abdomen, and large veins.

At the slightest deviation from 36.6°C, it kicks up our inner boiler – in this case, it increases our heat production and reduces heat loss thanks to involuntary mechanisms known to all: “bulging” (body hair, or what’s left of it, straightens out to expand our insulating layer of air), and shivering (our muscles contract to increase our metabolism up to five times and generate more heat) and redirect warm blood away from cold outer surfaces – hence our pale skin.

So, even if your toes and fingers are cold when walking on the ice, your central organs continue to benefit from that famous 36.6°C. Physiologists call this “homeostasis” the ability to maintain a constant internal temperature, regardless of external conditions.

Four stages of hypothermia

And that’s not all: the cold also triggers behavioral reactions. Besides our body, the hypothalamus also informs the higher regions of the brain about the situation, the management of reasoning and the search for solutions. This is what drives us to look for warmer places, to take shelter from the wind, to drink something warm…

But our ability to fight the cold has limits – clothing that is too light or damp in freezing weather, being outside in the cold for too long, indoors that are too cold… When these are exceeded, the body starts to get cold. We speak of hypothermia when our core temperature falls below 35°C.

Depending on the symptoms and the measured body temperature, four stages of hypothermia are distinguished (according to the Swiss system). A body temperature between 28 and 35 °C is indicated as mild to moderate hypothermia; below 28°C, severe hypothermia; Below 20°C, severe hypothermia.

Just two degrees below normal indoor temperature is enough to reduce the activity of our proteins and cause our cells’ metabolism to drop to levels so low that they threaten the functioning of vital organs. It is as if the engine of the body begins to sputter …

If these signals are not taken into account in time, the situation can quickly endanger our lives. The physical effects of hypothermia are multiple:

• Our heart pump beats slower and weaker, resulting in a weaker pulse. Movement and rapid motion can lead to an irregular heartbeat. At the same time, blood clotting decreases.

• Our metabolism, going down, weakens the muscles of the lungs. Our breathing (inhalation and exhalation) is slower and more shallow.

• Chills appear at first, but as our mitochondria become less efficient, their energy supply is impaired. After a certain stage, the severely undercooled person will no longer shiver, which is an important warning sign.

Cases of “paradoxical nudity”

Our minds are not spared either. The best indicators of hypothermia can be changes in behavior: people with hypothermia become disoriented, do not realize that they are cold (because it occurs gradually), talk more and more and act strangely.

For example, cases of “paradoxical nudity” have been reported. During the straying of the hypothalamus, the victims think they are hot and begin to undress… They are found naked, but curled up in a small space, the remnants of an animal reflex withdrawing into itself in order to protect themselves.

Then, when the level of attention changes, coordination weakens and risky behaviors increase. Finally, a loss of consciousness occurs, which can be fatal due to cardiac arrest and prolonged hypoxia to the brain.

In a last-ditch effort to stay alive, our internal control mechanisms cut off the flow of warm blood to the extremities, like our hands and feet. Body heat is saved for vital organs – heart, brain, etc.

Every mountaineer knows the signs of frostbite, when the tissues of the hands and feet begin to redden and millions of needles seem to be poking out to torment them. Lots of evidence that they’re depleting sugar and oxygen…to the point of eventually risking death. When a cold lasts for a long time, gangrene (that ominous darkening) can set in, making amputation inevitable.

This increases the risk of hypothermia

Various factors favor hypothermia and how quickly things can deteriorate.

Water conducts heat 24 times faster than air. It is therefore essential to avoid sweating (and thus intense physical activity) in cold weather, on pain of significant heat loss. It goes without saying that you should also avoid falling into an icy puddle …

Age is also a factor to take into consideration. Babies are more at risk, because their skin surface is relatively large compared to their body, which causes them to lose heat quickly. Their muscles, which are still underdeveloped, do not allow them to shiver so effectively. Younger people have a special type of fat called “brown fat” (rich in mitochondria) that produces excess heat, but it’s not very abundant.

In the elderly, the risks come from the fact that their body’s sensors are less able to assess changes in temperature. The interior thermostat, which is less efficient, no longer reacts optimally to the cold.

And the bad news for those over 18: You’ll be disappointed to learn that alcohol … increases heat loss by dilating blood vessels in the skin! Excessive consumption is known to alter thinking faculties and increase risky behaviour. Some medications and medications, especially antidepressants, can have similar effects.

Finally, some disorders such as anorexia or hypothyroidism can reduce our resistance to cold.

Every minute counts

Do not hesitate to provide first aid to a person who appears to be suffering from hypothermia. Every minute counts!

Call a doctor and move her to a warmer place, avoiding rapid and dangerous warming. If possible, remove wet clothing. Give her a warm non-alcoholic drink and cover her with a sheet of dry clothes. It is best not to rub the skin and not to expose it directly to extreme heat, such as a hot water bath (which can cause burns). In severe cases, with loss of consciousness, a visit to the intensive care unit is necessary.

In 1999, after a skiing accident, Swede Anna Bagnholm stayed 80 minutes in the icy water. When she was rescued, doctors measured the lowest body temperature ever recorded at 13.7°C. To everyone’s surprise, she recovered. Sometimes the sudden hypothermia appears to trigger “tissue cryoprotection,” leading EM to conclude that no one can be declared dead until they are warm and dead.

This knowledge is now being applied to improve the preservation of transplantable organs and better protect vital organs during prolonged heart surgery, by injecting cold fluid into the blood.

During cold snaps, be sure to protect yourself and vulnerable people, young and old. Stay covered in dry, windproof, and water-repellent clothing, especially around the face and head—which can get us very cold. For people who are homeless, alone or unable to care for themselves, social services can be of great help, and so can you.

Peter VanCamp is a postdoctoral fellow at the National Museum of Natural History (MNHN).