Heatstroke continued  //  Abnormal heat production, or failure of heat loss?

29 Oct 2008 Posted by
Yesterday, in our second post on heatstroke, we introduced the concept that the attainment of a body temperature above 41 degrees Celsius is NOT POSSIBLE due solely to environmental conditions, which is how you’ve probably always been told to think of it.

We explained how body temperature is a function of heat loss and heat production, and provided the potential for heat loss is greater than or equal to the heat production, there is zero chance of heat stroke occurring via purely “normal” physiological means. Therefore, when people do develop this condition, it is not as simple as saying “they didn’t drink enough and the conditions were too hot”, which was really the take-home message of yesterday’s post.

We illustrated this with one example of heatstroke from the published literature, that of a man who hit a body temperature of 40.8 degrees after only 16 minutes of running when the temperature was 22 degrees celsius. There is no “normal” explanation for this, it must be pathology, which is where we continue this discussion today.

Eighteen cases, and not one makes physiological sense

Below is a table showing you 18 documented (that is, published in scientific journals) cases of heat stroke during exercise. There are undoubtedly others (we received two very interesting stories from readers – thank you for those – explaining their own adventures. One had a body temperature of 42 degrees (incredible), the other was at 40.5 degrees, but more on that a little later), but these are the documented cases.


I’ve highlighted three particularly interesting cases. You’ll recognize the one in light green as the example of yesterday’s post – a runner developed heatstroke after only 16 minutes of running when the temperature was only 22 degreesĀ Celsius, and the runner was only doing 4:30/km – hardly fast enough to overheat in any conditions, let alone the mild conditions, and certainly not in only 16 minutes.

The example in yellow is even more spectacular. This was a runner who collapsed after 16 minutes of a race when the temperature was only 17 degrees celsius. His body temperature was an incredible 42 degrees celsius, and most amazing of all, he was only at 7.4km/h. That is a pace of 8 min/km, which is either a very fast walk, or a very slow jog. I’m sure I don’t need to emphasize just how spectacular a failure of physiology it is for this individual to overheat so quickly while exercising that slowly on that cool a day!

And then finally, the example highlighted red is a man who ran a half-marathon in air temperatures of only 4 degrees celsius, and made it to 88 minutes (he was on course for a 1:35 time), and his temperature was 41 degrees. I’m sure you can appreciate just how cold 4 degrees celsius is, and the next time you have to run in those conditions, ask yourself what the chances of overheating are, and you’ll have an idea of why this particular case warrants attention!

The problem with heatstroke – a glitch in the balance

These are three cases that punch holes in the normal theory for heatstroke. There are others – 15 of them in fact in that table above, and numerous others, including the two cases we received yesterday from readers. Our approach to these 18 cases of heatstroke is to calculate two things:

  1. The heat production as a result of exercise. As we described yesterday, heat is produced thanks to muscle contraction, and we can work out this value with fair accuracy
  2. The heat loss through convective, radiative and evaporative means. Again, these concepts were explained yesterday

Now, the key to understanding heatstroke is to recognize that when heat loss potential is greater than heat production, the athlete SHOULD NOT develop heatstroke. I guess the analogy here is that if you are saving more money per month than you spend, you should not ever have to file for bankruptcy!


So let’s take those 17 cases and simplify them to illustrate that heatstroke is very rarely a consequence of the environment. What we do is work out the ratio of heat production to heat loss.

  • If that ratio is equal to 1, then it means that heat production equals heat loss potential, and the athlete will be safe
  • If the ratio is greater than 1, then the athlete has a problem – they are producing MORE heat than they can lose, and therefore their body temperature will rise. They will thus either stop, slow down, or develop heatstroke
  • If the ratio is less than 1, then the athlete is safe – they are able to lose more heat than they produce, and so heat stroke should not happen

The graph below shows the result for 16 of the cases – 2 of them do not have precise weather data:


Clearly, every single one of these people had a potential for heat loss that exceeded the amount of heat they would produce from exercise. Therefore, these cases of heatstroke should never have happened, unless our equations are wrong (they aren’t!), or unless something else, unaccounted for by the concepts, is causing the problem.

And therein lies the crux. These mathematical models for predicting heatstroke are clearly not “complete” for these individuals. The fact that these 16 runners and cyclists did develop heatstroke means that somewhere, either heat production or heat loss has been incorrectly accounted for. Broadly speaking, there are two possibilities:

1) Heat production is actually a lot higher than is calculated by the equations

This is not because the equations are wrong, because in 99.99% of people, they are relatively accurate, and certainly, the calculation for heat produced during exercise is well-established. However, in these people, something has gone wrong, and it may be that they have produced heat in large quantities from NON-EXERCISE related sources. In our recently published paper in Medicine and Science in Sports and Exercise, we called this “excessive endothermy”, which basically means heat production from within.

Quite where this heat comes from is anyone’s guess – perhaps the runner’s efficiency is massively reduced for metabolic reasons? Perhaps there is a sudden increase in heat production thanks to unregulated opening of calcium channels? There is a condition known as malignant hyperthermia, where certain chemicals, most notably anesthetics, cause calcium channels in muscle to open. As this calcium is then transferred back into storage, it uses up a great deal of ATP and generates quite enormous quantities of heat.

The malignant hyperthermia linked has been looked at before – there are reviews on the topic. They distinguish between exertional heatstroke and malignant hyperthermia, but don’t rule it out, mainly because when we look at people who suffer from heatstroke, they tend, in many cases, to be susceptible to malignant hyperthermia as well! In other words, there is likely a genetic link that predisposes people to these conditions. It has been suggested that heatstroke sufferers have a skeletal muscle abnormality that is similar to malignant hyperthermia.

Is it possible that exercise-induced heatstroke involves a similar “wasteful” use of energy in order to correct some kind of channel disruption? And what are the triggers? Interestingly enough, caffeine is one of the chemicals known to cause calcium channels to open, and so may actually warrant a closer look as a potential “trigger” for heat stroke. I must confess that I don’t know the dosage that is required for this effect to exist. Other triggers may be central nervous system stimulants, like ephedrine (common in weight loss products), and the combined use of caffeine and ephedrine may be a real warning sign for this heat producing “spiral”. This was in fact reported in Case 1 from the table above – a weight loss supplement contained both caffeine and ephedra.

There is a few other candidate “pathologies” – it may be that there is excessive sympathetic nervous system activation, triggered by a metabolic condition or muscle myopathy. Another possibility is illness – a bacterial infection before exercise may increase the chances of overheating, though this has never been confirmed (for obvious reasons), and relies mostly on retrospective analysis of specific cases (and not all cases either, it’s worth noting).

Certainly, hot environmental conditions may be a trigger – we are not dismissing the role of the environment in all this, and it seems feasible that on a hot day, some “trigger” exists that may cause this same excessive endothermy to occur. The point we are making, however, is that the environment is merely the stage for the drama to be played out on – there is a pathlogical process at play here, and environment is not the CAUSE of heatstroke, merely a roleplayer.

A final illustration that heat production may be the cause comes from one case (case 3 above).


This person was admitted to the medical tent after the 56km Two Oceans Marathon with a temperature of 41 degrees Celsius. He was placed in an ice-water bath, and wore a cooling vest. His body was then surrounded with ice-packs after that. It took TEN HOURS of cooling to get his body temperature down to normal levels! So how does the human body manage to keep its temperature above 38 degrees Celsius when it’s surrounded by ice-packs? The only answer can be excessive heat production, so great that it overwhelms the heat loss to the ice water and packs.

2) Heat loss is lower than the calculations suggest

Of course, the other option in these cases is that heat loss fails. If that were to happen, then our scale would tilt to the left, because evaporation and convection would suddenly fail to deal with the heat production (the ratio would then jump above 1).

Of the avenues for heat loss, the most likely to fail is evaporation, and this would of course occur as a result of a failure of sweating. This is difficult to prove, however, because often, people with heatstroke are sweating profusely, and so seems unlikely. Interesting studies of soldiers in Iraq suggests that it can happen when people are exposed to dry heat for a prolonged period (though this study dates back to 1932, and the methods for research may have been limited back then!)


However, it seems more likely that the problem is excessive heat production, and not reduced heat loss. Or, alternatively (to sit on the fence), it is possible that heatstroke is a generic term that actually describes a SYMPTOM, and not a condition. If this is true, then it could be caused by all of the above, or any one of them! There may be no single cause, in fact, it’s highly unlikely. What does seem certain is that heatstroke is a failure of “normal physiology”, because you do NOT develop this condition simply by running on a hot day and failing to drink enough water.

Therefore, the point of this series on heatstroke has been to debunk some of the myths surrounding the condition, and to explain that it occurs more as a result of a physiological failure than an environmental problem. And it is most definitely not the result of dehydration, which is unfortunately what most people attribute it to! Does anyone seriously believe that our three cases highlighted in the table above were dehydrated within 16 minutes of starting to run in moderate conditions, or after 85 minutes of running at near-zero conditions?

No, heatstroke is a very complex, difficult to predict and even more difficult to explain condition. But hopefully we haven’t lost you in the maths of the series, and you now appreciate that heatstroke is pathology, not normal physiology, and does not happen simply because it’s hot outside.

Thanks for the emails on your cases and experiences, by the way. We will pursue those further!