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Wednesday, October 29, 2008

Heatstroke continued

Heatstroke part 3: Abnormal heat production, or failure of heat loss?

Yesterday, in our second post on heatstroke, we introduced the concept that the attainment of a body temeprature 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 occuring 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 degrees 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!



Andrew said...

Regarding "case 3" from Two Oceans, has anyone tried to study folks such as Wim Hof who might be naturally more heat productive? Do these folks have higher "resting" base core temperatures?

Anonymous said...

Thank you Ross, that was a very interesting read.

I have a question though, something that i don't understand. You said, and explained very well that heat stroke is NOT dependant on evironmental conditions. You also said that it COULD BE dependant on evaportation. Using the "humidity" example you used, in high humidity conditions, the sweat rolls off and doesn't evaporate, so then the subject is no longer loosing as much heat, he/she COULD overheat and get heatstroke, right?

So in this case heat stroke would be dependant on environmental conditions, right?

I think I missed something important...

Anonymous said...

Stunning work, many thanks!

Anonymous said...

Great post folks. One question - what role does environmental condition play in determining heat storage (other than determining heat loss potential) ? Does environmental condition influence physiology ?

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.

Would this imply that last October during the Chicago marathon - those who suffered from heat stroke would still suffer regardless of the outside temperatures and lack of availability of fluid ? (similar to some of the examples in your post).


ps : the Usain Bolt video linked on the rhs of the page, is no longer available on youtube. cheers

Ross Tucker and Jonathan Dugas said...

Hi everyone

First, to Derek:

Good question. The answer is that those calculations that work out heat loss potential already take into account the humidity. And so therefore, when one calculates the potential for heat loss, you are already factoring in that on a humid day, sweat will roll off. This obviously reduces the calculated heat loss potential. HOwever, as you see from the ratio graph in this post, the heat loss potential remains substantially higher than the heat production, even when you have this high humidity problem. I hope that makes sense? Point is, the heat loss potential is GREATER than heat production even on a humid day in these 17 cases of heat stroke.

second, to Vikram

Yes, with regards to availability of fluid. There is zero evidence that drinking more or less has any impact on the likelihood of heatstroke. There isn't even a theoretical basis for it, because fluid intake has no effect on body temperature when you exercise outdoors. It's different indoors in a laboratory, as we explained in our series on dehydration. But outside, fluid intake makes little difference.

With regards to the environment, that's a different story. As I said, the environment is clearly involved in the problem, though not the cause. If it was the cause, then of course far more people would develop heatstroke. So I suspect that on hot days, far more people are going to FEEL terrible and think they've got heatstroke, but the "true" condition is going to happen regardless of temperature, as demonstrated perhaps by some of these cases in the table of this post.

As I said in the very first post of this heatstroke series, how you FEEL is NOT how you ARE! Therefore, I question how many of the runners in the Chicago Marathon actually had heatstroke. I think a lot of them felt dreadful, and were hot and uncomfortable, but until those temperatures are measured at about 41 degrees, it's not heatstroke.

ONe final thing, also mentioned above, is that the environment could well be a 'trigger' for whatever it is that causes heatstroke. In otherwords, it's possible that on a hot day, the body reacts perhaps with an elevated sympathetic response to start generating these "unphysiological" amounts of heat. In this case, the environment of Chicago 2007 may have been a catalysed that pushed those "susceptible" individuals into heatstroke, but they still had a malfunction of normal physiology. So it's not caused by the environment, but by some underlying problem, initiated by hotter than normal conditions and a stress response. THat's theory anyway. Now to find proof.

Finally, to Andrew

Unlikely that they're higher at rest, but certainly, we do have evidence that people who are adapted to cold water are able to elevate their temperature through a variety of means. We (Jonathan and I) both worked on Lewis Pugh, the Polar swimmer, and he was able to regulate his body temperature quite well in cold water, and even elevate in the hour or so before he entered the water.

This happens thanks to hormones like thyroid hormone which increase metabolic rate - that's actually another candidate for why these people develop heatstroke actually. I left it off the list in this original post.

But it seems that regular exposure to cold water stimulates the body to produce more heat through what is called "non-shivering thermogenesis", or hormonal means. That's actually quite a well documented finding, and maybe of interest for these heatstroke cases - maybe they're doing the same during running!

THanks for the comments and compliments!


Anonymous said...

superb post thank you. Just to clarify the record, I'm the " someone ... involved with the marines". You wrote "(with respect to the marines), they're not exactly exercising that hard when they develop heat stroke. So something else must go wrong" ... a very typical heat stroke situation is not during "physical training" per se (e.g. running 3 miles for time) but instead during events consisting of multiple hours of work (perhaps 3.5 mph walking) wearing full combat equipment (eg helmet and flak jacket) and carrying a 75-100 lb external load. Possibly two factors of note are (1) the difficulty of shedding body heat trapped by the kit and (2) the fact that often (though not always) the pace of activity is being set by an external factor (instructor; bad guys) not the 'exerciser'.

Ross Tucker and Jonathan Dugas said...

Hi anonymous

Thanks for coming back - i was wondering whether you'd be following and would contribute.

You make two good points. The first is the clothing. I guess that's a "necessary evil" given the situation the guys will find themselves in, they can hardly shed the gear and load and do it like marathon runners!

The second is the workload being imposed, though that's interesting. In the lab, the workload is also imposed (by the researcher) and people still stop when they hit about 40 degrees celsius. Physiology always wins the day, regardless of the level of motivation, or the incentive to keep going despite the fact that these soldiers must feel terrible leading up to the point where they collapse.

Just a note, I have a suspicion that the condition is over-diagnosed, and what you see is not necessarily heatstroke. You'll have noted in this post the one case of a man who took 10 hours to recover in a bath of ice. There are many people who finish sports events very hot (between 40 and 41 degrees), but recover almost instantly when they are cooled with cold water/ice. That, to me, is not heatstroke, it's just being very hot. And their body has intervened and saved their lives. Unfortunately, it's diagnosed as heatstroke for lack of a better diagnosis (because it is tricky, and this whole point may just be semantic anyway)

Having said that, I recall from your previous post that you did mention that people die, and that's certainly not up for discussion - if that happens, you have a serious problem.

I can think of two similar occurrences - about 5 years ago in France, a number of elderly people died during a heatwave. These were people who lived in homes where air-conditioning was not present, and they tend to "overdress" for the conditions. Under those circumstances, heat gain from the environment exceeds heat loss, and they simply cannot escape their situation. Also, their resistance is lower (as a result of age), and they are very susceptible to organ failure, possibly at lower temperatures than we would see in athletes. So then, heatstroke is "situational".

Similarly, every year, a couple of high school footballers die, and of course, everyone blames dehydration. I suspect it's a similar thing to the elderly - they find themselves unable to lose heat once they accumulate it because of their uniforms. And perhaps individuals who are susceptible are "triggered" once their temperature rises a certain amount. So take a case of Joe, a fictional footballer - he trains in the heat, he produces a great deal of heat without having the ability to lose it, and he hits about 39.5, 40 degrees celsius. And then his system goes haywire, and he's not able to lose that heat. Next thing you know, he's up above 41 and the systems and organs are failing. Given the number of footballers, it's statistically likely that these people, who are susceptible, exist.

And, I dare say, the same goes for the marines. So perhaps it's a "situational problem", brought about by the inappropriate clothing (for the environment) and once things start going wrong, well, they just keep going.

Sorry for the long ramble. Thanks again for your email!


Ross Tucker and Jonathan Dugas said...

Hi Anonymous,

Just to reinforce the points Ross made above. . .

There is heaps of research from the military about heat balance, and in fact I am pretty sure they coined the ever ambiguous "heat injury" and "heat illness" terms that are quite meaningless when it comes to describing the person's actual medical problem.

The issue is that they do not have a medical problem at all, and this is where heat stroke differs from just being too hot and collapsing from what is probably something we call postural hypotension. The use of the word "stroke" in the condition denotes pathology, and therefore it is not a situation that occurs on a regular basis.

I am sure the recruits or soldiers feel absolutely dreadful prior to collapsing, but providing that they cool off when stopping or when removing some of their kit, then it is not heat stroke as there is nothing wrong.

In normal and healthy (and well-trained) individuals it is possible to push very hard and reach a core temp of 41 C, but upon stopping we see that their core temps drop very rapidly even in hot conditions (assuming they are not fully covered in battle fatigues). It is when their temp does not drop, and active cooling is required, and their core temp remains elevated even while cooling, that we have heat stroke and something pathological is occurring.

So as Ross said, and as we have mentioned many times on the site before, physiology will always win the day, and even if the enemy or drill instructor is pushing the pace, the individual will collapse and "fail" (fatigue, that is) before dying. Now, in someone who is predisposed to heat stroke, then potentially we have something entirely different---pathology, and these people for sure will have problems.

I suppose we just must be careful at what we classify as heat stroke. Only a rectal temp of 42 C or in some circles 41 C will clearly differentiate heat stroke from something else, and so unless a rectal temp is taken we cannot say for sure that someone suffered from heat stroke. I am sure this is what leads to the over-diagnosis of this condition.

Thanks again for visiting and for your meaningful comments.

Kind Regards,