Introduction to HYPOthermia and survival in the cold
As promised (a while back, admittedly), today we kick off a series on exercise in the cold. A challenging one to write, mostly because most of you reading this will (hopefully) never be exposed to conditions that are so extreme that your physiology is challenged to the point where it can't cope. Because most of you reading this are runners or cyclists, and it's only when you get into the water that a potentially lethal drop in body temperature is a real possibility. For the most part, exercise in the cold on land is limited by sensation and perceptions, which are of course controlled by wearing appropriate clothing. The biggest dangers of exercise in the cold on LAND? Frost-bite and other medical conditions, and not actually hypothermia.
But, there are still some fascinating studies and insights into exercise in the cold, and it is one of the most amazing areas of physiology to discuss. As I mentioned yesterday, Jonathan and I both worked with the polar swimmer Lewis Pugh when he prepared to swim 1km in the Arctic Ocean in 2005. Jonathan subsequently worked on his trip to the Antarctic as well. So we have a tendency towards COLD WATER physiology, which will make up the first couple of posts. Then we'll move onto the studies of running, cycling and muscle function during cycling and running.
So bear with us as we move through the series in a few parts:
- How cold is cold? What are the limits to survival in the cold? Some stories of cold exposure in sport
- The physiological responses to cold water immersion
- The effects of cold temperatures on performance during swimming, cycling and running
- Practical recommendations and insights
In terms of body temperature, let's begin with a few basics - your body temperature as you read this is somewhere between 36 and 38 degrees - most textbooks will tell you 37 degrees celsius (or 98.6 Farenheit). As with exercise in the heat, the regulation of body temperature within a fairly narrow range is critical, not only for performance, but for health and survival. When we exercise, the body temperature very quickly rises to about 39 degrees celsius - one of the most remarkable questions in physiology, in my opinion, is why the body would allow this when it has the ability to lose the heat? It can reach 40 degrees celsius with few ill-effects, as found by numerous studies by the Danish group of Nielsen and Nybo.
The body regulates this temperature in a number of different ways. When we wrote about heat, we spoke of sweating, sending blood to the skin etc. In the cold, the opposite happens - you shift blood away from the skin, you shiver and release hormones that help keep the temperature up. The body is naturally insulated by skin, muscle and fat, and it's no co-incidence that on land and in water, the lean athlete, with low body fat percentage, is likely to get colder sooner!
What determines whether hypothermia happens ON LAND?
A drop in body temperature in recreational athletes is rarely seen ON LAND, even in quite cold conditions, provided they are not forced into prolonged exposure (discussed below). Rather, the main factor predicting body temperature is metabolic rate, and so provided you are exercising, your body temperature will rise, regardless of how cold it is...up to a point.
Remember that your body temperature will only fall if you lose heat faster than you produce it. When you are exercising in cold conditions, the fact is that you're usually:
- Dressed very warmly and therefore not likely to lose heat too rapidly - in this case, your body temperature is a function of heat production; or
- Dressed very inadequately, with skin exposed, or not waterproof. In this case, you'll FEEL so cold within minutes of starting, that you're likely to be unable to run altogether. You'll turn home and get warm in no time!
Interestingly, in water, the situation is quite different. Because as we'll see tomorrow, you actually lose MORE HEAT when you exercise in the water than when you don't! So it's in water that hypothermia becomes a real threat - hypothermia, incidentally, is a fall in body temperature below about 35 degrees celsius - by this time, you'd be shivering uncontrollably.
But for today, we'll look at survival in the cold, and just what the risks are.
Some stories of cold exposure
In 1964, during the Four Inns Walking Competition in Derbyshire, England, tragedy struck when 3 hikers died as a result of cold conditions combined with poor preparation and planning for the cold. In this race (which is described by Pugh and later Noakes), competitors set out in temperatures between 0 and 4 degrees celsius, with wind speeds of about 45km/hour.
Seven hours into the event, the first reports of hikers in distress were received. In total, three hikers died as a result of the extreme cold and wind. In an analysis of these events, the following three characteristics were common to all three deaths:
- Inadequate clothing, which was not water-proof. As a result, when it began to rain, the water, combined with the cold and wind would have meant zero insulation for the hikers
- All the hikers had low body fat percentages. This was insufficient to make up for the loss of insulation by clothing once the clothes were wet
- The three hikers fatigued within a few hours of starting. As a result, they were walking very slowly, often collapsing, and hence unable to keep their rates of HEAT PRODUCTION high. The hikers who survived were those who finished faster.
In marathon running - evidence of hypothermia
The story above deals with hikers, exposed to snow, rain, wind and cold for days at a time. What about marathon runners? There is some evidence of hypothermia in marathon running. And perhaps surprisingly, it doesn't take temperatures below freezing to cause this. For example, in Scotland in 1982, in the Aberdeen Marathon, temperatures were about 12 degrees celsius, it was quite windy (25 to 30km/hour), but there was no rain, and yet 4 out of 59 runners finished with temperatures lower than 37 degrees! Similarly, in South Africa in 1985, during a 56km race, a temperature of 19 degrees combined with wind and rain was enough to see 8 runners taken to the medical tent with body temperatures below 37 degrees celsius!
Once again, in these cases, the likely cause is a combination of inadequate clothing, combined with low heat production as the athlete becomes fatigued. One of the body's responses to cooling is to shiver, and use more energy in an attempt to keep your temperature up. This is hardly conducive to performance and so the exercise intensity drops and body temperature tends to fall. Also, most of the time, athletes, particularly in South Africa, would expect reasonably warm conditions - they'd dress for these conditions, wearing little more than running shorts and a vest, completely inappropriate for wet, cool conditions, once the wind starts blowing. The same would apply to cycling, perhaps even more so, since the higher air speeds move heat away from the body more rapidly.
Having been in the medical tent at the last three Comrades Marathon events (90km race), I can also testify that towards the end of the day (around 4 or 5pm), when temperatures have fallen to the low teens (not cold, by any means), many runners report to the medical tent feeling very cold, shivering uncontrollably. And a big part of it is that they are fatigued, exhausted from the preceding 80km. They walk more, which causes heat production to fall. At the same time, they are underdressed, and then unable to do two jobs at one time - provide energy for exercise and for keeping the body warm. So the athlete often pays by cooling down and feeling very cold.
More recently - a high cost for winning a race
So we've seen that on land, the biggest danger is that you'll be caught underdressed and unable to keep exercising hard enough to keep your rate of heat production up. Provided you combat these two potential problems, your risk of hypothermia is actually relatively low. However, there is still a risk of frost-bite, demonstrated as recently as this past weekend by the case of one Andrew Wells. Wells won the Frozen Otter Ultra Trek, a 64-mile trek on what is called the Ice Age Trail in the Northern Kettle Moraine State Forest.
Wells won the race not because he finished the 64 miles first, but because he was the only competitor to actually reach the fifth check-point of the race. Only two competitors reached half-way! That's how severe conditions were - it was nearly 15 degrees below freezing at times!
But Wells clearly kept warm enough to avoid the potential for hypothermia. But unfortunately, he was not as lucky when it came to frostbite. This quote comes from this news story on the race:
Wells said he never noticed the frostbite set in. "My feet were obviously frozen, so I couldn't feel them," he said. "And it was too cold to take my shoes off to check my feet. On my hands, I had mittens on, and just to take them off for 30 seconds, my hands got really cold, painful. I thought my toes were OK."It turns out that Wells competed in only running shoes, one pair of wool socks and a pair of waterproof socks! He'll now have two toes amputated! This story demonstrates that the danger of hypothermia may be lower, but frost-bite is a real possibility. Interestingly, none of the other competitors had this problem, because most, it is reported, bailed from the race when they'd hd enough. Wells, on the other hand, pushed past the initial pain until numbness set it. So it certainly is possible to voluntarily exercise into health problems, but for most, the sensation determines the safe limit.
But the record belongs to...
However, all these stories pale into insignificance when compared to the record for LOWEST SURVIVED body temperature. This report, published in Lancet in 2000, reports the remarkable story of a skiier, who holds the distinction of record cold-survivor:
“A twenty-nine year old woman fell while skiing (18h20), and was trapped and partially submerged under a waterfall. After 7 min, emergency rescue was summoned and, after 40 min struggling in the ice water, she stopped moving. The rescue team arrived (19h39), commenced cardiopulmonary resuscitation, and transported her to Hospital…Her Tc, 14.4oC at the start of rewarming, dropped to 13.7oC (afterdrop), and then returned to near normal after 3h. She recovered to normal function…”
So these stories and situations are obviously what we all wish to avoid! And in our next post in this series, we'll look at what happens to the body when it's suddenly exposed to a very cold environment - we'll look specifically at water immersion.
So join us then, and in the meantime, if you're in the Northern hemisphere, remember, it's all in the clothing!