Tag Archives: moderate hypothermia

HOW TO: Understanding hypothermia in swimmers – Moderate & Severe Hypothermia

Previous article: Understanding hypothermia in swimmers: Mild hypothermia.

Moderate hypothermia is obviously more serious than Mild hypothermia which I covered previously in the first part of this series. I’m always a bit bemused by the medical terminology of hypothermia. Many serious open water swimmers will have experienced moderate hypothermia and can tell you there’s nothing moderate about it. Moderate is from 35° C. down to 32.5° or even 32° Celsius. Experienced Sandycove swimmers will regularly swim into or experience Moderate Hypothermia during Channel training, (but they are too tough to call it that, they’s just say it was a bit cold).

Something I’ve noticed is huge variation in defining the differences between mild and moderate hypothermia. Hypothermia itself is body temperature below 35° Celsius. In Part 1, Understanding hypothermia for swimmers, Mild hypothermia, I said mild hypothermia was 35° C. to under 37°C (normathermia). Yet I have seen medical articles that define Mild Hypothermia as 32° to 35° C. and Moderate as 29° to 32° C., with Severe Hypothermia as under 29° C. We could argue this point but here’s my specific point for choosing the higher ranges: hypothermia is dangerous. You want to play around with more risky definitions in cold water? More power to you but that’s your own call.

I made the point in the last article, worth repeating, that you can’t become hypothermic in water instantly.  Your body is a heat reservoir, so hypothermia develops over time.

Take a good look at that chart. 0° C. is a safe zone (assuming you exit, and excluding any cardiac problems). In fact the chart indicates lethality occurs post one hour at freezing temperatures.

At a temperature of 10° Celsius you are in the marginal zone of survival until about three hours. Equate this with moderate to severe hypothermia. Also, that doesn’t mean that you definitely can survive three hours at that temperature, nor that you are definitely going to die. Think of it as a realm of possibility.

Now, if you have one, put on your cold water swimming cap, and look further along the x-axis. We know however that some Sandycove swimmers (Lisa, Finbarr, Rob, Ciaran, myself at least, no disrespect to other SISC swimmers who’ve done the same) have swum six hours at under 11° C. Lisa has swum nine hours at that temperature. So wear  an SISC cap (actually we don’t have those), and the zone of lethality moves. And for these articles I’m talking about hypothermia in the spectrum of swimmers. You can see it’s a moveable target.

Another point from this chart; hypothermia is inevitable at temperatures up to about 15° C. I can’t quantify that from any other sources. Theoretically, at any temperature under body neutral, hypothermia should always result. This would exclude acclimatisation, the amount of bioprene (body-fat), using food to offset the extra calories requires or thermogenesis (heat generation) from swimming. Think of 15° Celsius as an average English Channel solo temperature and we see that there is a variability in resulting hypothermia cases among soloists.

I keep stressing experience, because that means acclimation to cold, which really means as I’ve said previously, we/they are PHYSIOLOGICALLY adapted to tolerate cold easier and for longer. This is NOT just a matter of will-power or a psychological advantage or strength. You cannot think your way into cold adaptation. You may be the world’s most confident person, a 5k per hour swimmer, with no self-doubt, but without physiological adaptation due to repeated cold immersion it will mean little.

You train to swim longer and further. You train to swim faster. And you train to swim colder. The human body responds to physical stress by adapting. This is the Training Effect. Adaptation to cold from training in cold, is another less discussed training effect particularly important to open water swimmers.


One of the most dangerous effects of hypothermia is the slowing of thought processes and the person’s consequent inability to gauge their own condition. As blood cools in the body, bracycardia (slower hear rate) ensues. The blood undergoes partial coagulation, in effect becoming thicker and more viscous, and so less oxygen gets carried to the brain. Therefore, like the previously headlined item above, you cannot think your way out of hypothermia.This makes hypothermia deceptive, it sneaks up on you, those not familiar with cold shock and initial responses imagine hypothermia as a prolonged intense cold whereas it enfolds the sufferer gently, lulling them into a gradually more dangerous state. This is partly what makes hypothermia so dangerous, with thermoceptors overloaded, the sense of cold is reduced or eliminated, the cognitive functions are impaired and the person won’t or can’t realise the danger.

Violent shivering in or more commonly out of water is a symptom of moderate hypothermia. Violent shivering is the bodies attempt to warm up through exercise, by muscular contraction. Almost every distance Sandycove or Irish or UK swimmer has had happen this post-swim. This happens five to ten minutes after exiting the water, when the cold blood in the periphery reaches the core (Afterdrop). How do you tell violent shivering from normal shivering? Normal shivering can be stopped voluntarily, violent shivering can’t. We all know what it’s like to stand talking after a swim with severely chattering teeth, difficulty standing still and straight and pain in the lower back.

When swimming distance in cold water, one point of danger is when the person starts to feel warm, after being cold for a long time. This is not just acclimatization, this is paradoxical undressing, which on land can cause people to shed clothes when they are freezing. The swimmer will have passed through violent shivering to get to this point but it may also arrive for swimmers without violent shivering . Again, many distance swimmers will have experienced getting this feeling of warmth while swimming, but those who understand what they are doing will recognise it as an immediate warning sign to exit the water as soon as possible. As swimmers we have to recognise and deal with the problem that we have the Immediately option, having to swim to a safe exit point. There’s no point getting out where you have no access to clothes. About 25% of the deaths caused by hypothermia arise from poor judgement and decisions made by the hypothermic person.

It is also important to always be cognizant of ambient conditions. Irish and UK open water swimmers training through the winter and swimming in five or six degree water will have similar air temperatures and at least in Ireland, wind is a fairly constant factor. (Ireland in fact is one of the windiest countries in the world and that’s not including all my writing here). High humidity and wind means heat stripped from the body faster, both in the water when shoulders and arms can get cold even more quickly, and especially outside the water when wind can cause violent shivering within minutes. For example, last Saturday Ciarán Byrne & I were swimming at Garrylucas, water temperatures were very variable from about 10 Celsius to 13 Celsius, the day was warm by Irish standards at about 20° C. but at the 90 minute feed on the beach, there was a howling offshore south-easterly and we were both shivering violently within less than two minutes. Swimming in cold water on the Northern European seaboard generally means constant cool or cold air temperatures.

Experience, and I mean significant experience, not just the initial four to six immersions required to blunt cold shock, does allow a swimmer greater awareness of their own hypothermia progression, though this is a variable process with a lot of external factors which vary every time, and therefore is not to be advised. Even the most experienced cold water swimmer can’t tell for sure exactly much more time they might have while swimming before moving further up the scale of hypothermia severity. However we live in a world where we are seemingly perpetually warned about the dangers of everything, and if this is something you choose to do, then you should choose to do it with knowledge and awareness. /End short rant.

Next article: Diagnosing and addressing hypothermia in swimmers.

Related articles

HOW TO: Understanding hypothermia in swimmers – Mild Hypothermia

When all fails and I am at a loss for something to write about, I can write about cold, my favourite subject. Especially in the context of Cork Distance Week coming in two weeks, when we had a few people pulled from the water with hypothermia last year.

For anyone involved in open water swimming in Ireland, the UK and other Northern Hemisphere cold water locations, being able to spot and diagnose dangerous hypothermia in a swimmer is an essential skill. To do that properly an understanding of hypothermia is useful.

It’s essential to understand that there is no such thing as sudden hypothermia. Most of us grow up hearing this myth, (for example I remember stories of survivors from the Titanic freezing to death in five degree water within fifteen minutes, and that fifteen minute myth is repeated all the time).

The heat in your body can’t instantly disappear. The Second Law of Thermodynamics is always the Universe’s governing and inviolate principle. Hypothermia is a developing situation over time. Your body has enough stored heat that even in zero degree water, you probably won’t develop severe hypothermia until about thirty minutes, though you will be subject to After-drop and potentially lethal consequences even if you emerge before that time. And Afterdrop itself isn’t a myth, as is sometimes inversely claimed to sudden hypothermia, it does exist.

Cold shock response is an entirely different thing to hypothermia, it’s the bodies response to sudden cold, with gasping reflex, hyperventilation and possible acute pain in hands, feet, face and head, and even cardiac events. The biggest danger in immersion is uncontrolled hyperventilation leading to sudden aspiration of water. You gasp and breathe water into your lungs and drown.

Breathing rate increases for the first 20 seconds in cold water

This is the main reason why a diving or jumping entry into cold water for people not cold-acclimated is absolutely a stupid thing to do, and not tough or macho. This response is attenuated in cold-adapted swimmers.

Definitions of Mild Hypothermia can vary depending on where you look but a core body temperature of between 35° and 36° when body-normal is 37° is a good measure, i.e. a drop of about two degrees is a good indicator. The hormone ADH, (anti-diuretic hormone) which controls urination in suppressed and some blood volume is shunted to the core so there is a decrease in blood volume and some dehydration also. There are no long-lasting effects of mild hypothermia, (such that it can be used as a medical procedure for brain protection during certain operations). Almost every serious open water swimmer in these waters will have experienced it as completely normal, and the body acclimates and adapts as we have seen before, by blunting initial response, reducing stress hormones, and increasing brown adipose tissue.

However, people with any diagnosed cardiac problems should avoid cold water swimming.

And also as we’ve often discussed previously, mild hypothermia leads to peripheral vaso-constriction, the reduction of blood flow in the periphery. With experienced open waters mild hypothermia is the completely normal and usual state, in Irish and UK waters. The swimmer will still be able to talk and will still retain motor control in the fingers, but often with reduced dexterity. Surface temperature will be decreased.

Mild hypothermia will of course lead to more severe hypothermia shgould the swimmer continue to be immersed or unprotected. Hypothermia will eventually result for everyone in temperatures under twenty degree is they stay swimming long enough.

There are no great  concerns in recovering from mild hypothermia, just get dry and dressed quickly, following the usual procedure of dressing the torso and head first, and warming up with a walk. Do NOT vigorously dry the extremities even in mild hypothermia.

In diagnosing mild hypothermia, simply seeing if there is some chattering or shivering out of the water. In the water is more difficult, but the swimmer might have clenched jaws and have a minor difficulty speaking freely, or maybe report lesser claw-like symptoms in the hands (lessening on full hand motor control).

In the next article we look at Moderate Hypothermia.

How we FEEL cold water (loneswimmer.com)

Peripheral Vaso-constriction in pictures (loneswimmer.com)

Where did my CLAW go? (loneswimmer.com)

Extreme Cold Adaptation in Humans Part 1 (loneswimmer.com)