Heat stroke dissected

Heat stroke: A problem of physiology, not fluid or environment

Continuing on from our post two days ago, we are looking at heatstroke, a condition where the body temperature rises above 41 degrees celsius (this cut-off is somewhat arbitrary, it has to be said, at least in the exercise literature).

In that post, we introduced some of the paradoxes of heatstroke. The classic teaching on heatstroke is that body temperature rises excessively thanks to excess heat production which cannot be matched by heat loss. Heat production is thus a result of high exercise intensity, which means that this theory holds that you quite literally exercise yourself to death by generating so much heat that you overwhelm your body's capacity for heat loss. What it fails to account for is that humans usually slow down long before this limit is reached, or they stop exercise altogether once they hit a certain temperature, and so it's difficult to explain why they run themselves into heat stroke unless there is some "malfunction", which is where we're ultimately headed with all this.

Of course, the fluid-pundits climbed on board and advocated that the biggest problem would happen if you failed to drink enough water, because then your body temperature would rise even more rapidly and heat stroke would be a very real possibility. This particular post is not about the fluid-hyperthermia myth - we covered that in great detail in our series on dehydration, for those who are interested. Instead, we're interested in the physiology of body temperature regulation (and fluid, quite frankly, is barely involved).

It does get quite technical, but we'll do our best to speak logically, rather than mathematically! As a result, we will skim over the specifics of the equations, but I'd encourage you to check out this paper (which inspired this series, it was published earlier this year), where the equations are presented and discussed in more detail. As always, if you can't get the paper, drop us an email request and we'll send it along...!

Body temperature balance

The figure below is a (very) oversimplified schematic of the two halves of heat balance. It says that heat storage (which can be negative/heat loss), is equal to heat production minus heat loss. We can quite easily calculate and predict the two sides of the scale using mathematic formulae because we know what factors affect the heat production and heat loss components. These are convective, evaporative and radiative heat loss/heat production.


So for example, we know that heat production is a function of exercise intensity (cycling or running speed), body mass and a constant, which varies depending on whether you assume that the person has a high or low level of efficiency. The larger you are, and the faster you run, the more heat you will produce, which is why smaller runners have an advantage in hotter conditions. For example, the equation for an inefficient runner reads:

Heat Production (Watts) = mass x [(5.89 x speed) - 4.69]

On the right side, we have heat loss, which is largely influenced by the environment. Here, it's convection and evaporation that are mostly responsible, which is why air velocity and sweating are so important. Note that sweating by itself does not remove heat, only evaporation, which is why humidity is so vital - if sweat drips off, it does nothing for temperature, as our readers in the East and tropical regions will testify! (Also, note that body surface area, which is a function of mass and height. The larger the athlete, the greater their capacity to lose heat, but it doesn't quite manage to offset the fact that they also produce more heat)

Introducing the mathematical equations - conceptualizing the limits of exercise

Because we know how these factors interact and influence heat production and heat gain, it's possible to take that basic equation and refine it a little more. It now becomes:

Heat storage = Heat production - Convective heat loss/gain - radiative heat loss/gain - evaporative heat loss

Note that in all cases, the option exists to either gain heat or lose heat. For example, convective heat LOSS happens when the skin is warmer than the surrounding air, but as soon as the air becomes hotter than the skin (at about 35 degrees celsius), then convective heat loss falls to zero, and then eventually switches around - you start GAINING heat from the environment.

Now, we are in a position to make some interesting calculations regarding heat stroke, because we know that the body temperature will rise when heat is stored. And if we know how much heat is stored, we can calculate how much body temperature will rise. That is, we know that every 3.47 kJ per kilogram will raise body temperature by 1 degree celsius, and so if a man weighing 80 kg gains 278 kJ in one hour, his temperature will increase by one degree celsius in that time. To extend this further, if he wants to run into heat stroke, he'd have to raise his temperature by 4 degrees celsius, which would require him to store 1111 kJ.

So the approach we can now take is the following:

• We can calculate the rate of heat production (thanks to knowing the running speed and mass of the person);
• We can calculate the rate of convective cooling if we know the air temperature
• We can calculate the rate of radiative heat gain if we know cloud cover
• We can calculate the maximum capacity for evaporative heat loss if we know the humidity

These four variables are all we need to be able to say whether the possibility of heat stroke exists, because:

• If the capacity for heat loss is greater than the calculated heat gain, then heat stroke is not possible (mathematically, anyway. More on this a little later)
• If the capacity for heat loss is lower than the calculated heat gain, then our scale tilts towards heat storage, and the result is that our athlete will gain heat, his temperature will rise, and in theory, heat stroke is possible.

Example: Why heatstroke is not an environmental problem

This is best illustrated with an example:

Note that we're making "worst case scenarios" here - we assume he's inefficient, that there is no wind other than the wind he generates by running and that he is also running on a bright sunny day. We do this to make a point - by taking the "extremes", we want to see just how bad things need to be in order for him to develop heat stroke.

So, our calculations reveal the following:

To empahsize this further, we can work out that for our runner to keep his body temperature EXACTLY the same, he would have to evaporate 1.5 L of sweat per hour. But our calculations also reveal that it would be POSSIBLE to evaporate 1.6 L of sweat per hour. This means that he has no problem losing the heat he produces, and should NOT develop heatstroke (once again, for more detail on the calculations, refer to this paper)

Here's the catch: He did get heatstroke, in only 16 minutes!

Ah, but now, what if I told you that this man is one of the 18 cases reported in the literature. In fact, this runner, running in these conditions, was pulled out of the race after ONLY 16 minutes, with a rectal temperature of 40.8 degrees celsius! Therefore, despite the fact that there were no limitations in the environment, and the fact that he COULD have lost all the heat he produced, he failed. And the result was that he developed heat stroke after less than 4km of running!

If that does not strike you as extra-ordinary, nothing will. Your first thought might be that our maths is dodgy (and you have a reasonable case, as I'll explain at the bottom of the post), but really, consider those conditions: 22 degrees, and the humidity was high, sure, but they're not difficult running conditions. If you stood on the start line of a 10km race in those conditions, the thought of heat stroke would not cross your mind. How about after 16 minutes? You should be thinking that something serious went wrong with this runner. And you'd be right. The problem is that we don't quite know what it is!

Some pointed questions about heatstroke

In the interests of time, we'll tackle that question in the next post in this series. But what I want to leave you with are the following questions, which will hopefully give you reason to challenge what you know about heatstroke:

1. If heatstroke is purely due to the environmental conditions, then why is it so rare? In SA, for example, we have a cycle race with about 30,000 participants per year, and only 5 cases in the last 6 years have been reported. That's 1 in 30,000. And the prevalence seems about that low. Now, consider that 29,999 people will be exposed to the SAME conditions, and NOT develop heatstroke, and suddenly you realise that the environment is NOT the crucial variable. Obviously, it contributes, as we've shown above, but it's not the driver. Something else is...
2. Heatstroke cannot simply be a function of exercising so hard that you overwhelm your body's capacity for heat loss. The cases we showed in yesterday's post are representative of this, and that's what I'll write about tomorrow. But the point is, as we saw in the example above, heatstroke occurs even when the theoretical limit doesn't exist. It's not a function of heat production through any normal means.
3. Third, why is heatstroke more common in back-of-the-pack runners? According to every theory, heatstroke should be most likely in faster runners (especially larger ones). Yet this is not consistent with what is observed. We had an email from someone who is involved with the marines (which is where heatstroke does seem to occur, though it's rarely documented in scientific journals), and I dare say (with respect to the marines), they're not exactly exercising that hard when they develop heat stroke. So something else must go wrong.

In conclusion, heat stroke doesn't seem to be driven by the environment, though it's a contributing factor. It's also not explainable by the athlete's "high" workrate, because they are rarely actually producing that much heat. So the quest begins for the answer. Join us next time!

Ross

Disclaimer:

I've made use of mathematical equations in this post to illustrate the concepts. That's certainly a point of contention, because the human body is more complex than an Excel spreadsheet. So I don't mean to oversimplify or rely too heavily on the maths and calculations. However, what these equations do allow is a demonstration of the conceptual issues around heatstroke. We assume the worst (no wind, direct sun, poor efficiency) and then show that despite everything being "worst case", the capacity for heat loss exceeds heat gain. The equations are therefore useful to demonstrates concepts.

Where they would fail is if we tried to use them predictively or prescriptively. In other words, we can't say definitively that a 60kg man running at 15km/hour at 25 degrees with humidty of 60% will have a body temperature of 39 degrees after 45 minutes. That would be reckless use of the tool. So please, understand that we've illustrated concepts here, and hopefully made a strong point that the environment is rarely a key, and that actually developing heat stroke is extremely difficult according to "normal physiology".

We'll pick up on this point again tomorrow.

Ross

References:

For those who feel like sinking their teeth into the cases and the equations a little more, check out:

Rae, Knobel, Mann, Swart, Tucker and Noakes. Med Sci Sports Exerc, 40: 1193 - 1204, 2008