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The danger is always to try to reduce a complex phenomenon (the development of heat stroke) to one simple mechanism ie dehydration. To conclude that dehydration is the cause you have at least to exclude a number of other possibilities. Also if the activity was less than a few hours and the subject had been drinking, then what level of dehydration would have been present? And would that level of dehydration not have been precisely the same as that measured in many other athletes completing the race on the same day in the same conditions without any evidence for "heat illness".
Clearly, at least to me, heat stroke is multifactorial. In the absence of illness and genetic abnormalities (malignant hyperthermia susceptible) dehydration seems to be a necessary component of this multifactorial disease. If you have evidence to the contrary I would love to see it.
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But more to the point, much of the early work on heat stroke was done in the South African gold mines in the 1920's and 1930's when an epidemic of heat stroke began to develop as the mines went ever deeper. (The temperature of the rock face at 3-4km below the earth surface is 50 degrees C). The main risk for heat stroke were the environmental conditions, the amount of work performed, the level of heat acclimatization and most importantly the presence of intercurrent illness. These were not healthy people who suddenly developed the condition. In laboratory testing on the mines it was soon observed that those who started exercise with an elevated temperature (because of an intercurrent illness of which they might or might not have been aware) were unable to regulate their body temperatures appropriately and had to be stopped from exercise when their temperatures reached abnormally high levels. So in an case of heat stroke you have to be certain that there was not an intercurrent illness at the same time. But most importantly in my view whether or not there is also some underlying muscle disorder that causes an explosive production of heat (thermogenesis) which is the real cause of the heat stroke (according to the theory that we are currently evaluating).
Of course there is an underlying muscle disorder that could cause heat related illness. However, it is not likely that MH is the source of most of these problems as I am unaware of a single case where dantrolene was necessary to stop the reaction at an athletic event, once triggered. But, some of these people might have the trait. It would be easy to find out, simply do an MH diagnostic muscle biopsey on all those with heat related illness at the finish line (sometime after the event, not necessary to do the biopsey at the finish line).
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Thank you for mentioning postural hypotension as the cause of your collapse. This is by far the most common cause of post-exercise collapse in all endurance athletes. Unfortunately it is labelled as "heat illness" and "dehydration" but both diagnoses are wrong. First the body temperature is not seriously elevated in these athletes and second they recover the instant their legs and pelvis are elevated above the level of the heart. This restores their blood pressures. Their symptoms as you correctly state are due to low blood pressure when standing (postural hypotension) which is corrected when the blood volume in the legs is redistributed to the centre of the body correcting a low filling pressure of the heart and again allowing a normal circulatory function.
Ugh, seems to me that you think all the doctors at the finish line of these events are complete idiots, unable to tell transient postural hypotension from dehydration related hypotension. This difference is relatively easy to discern clinically. Perhaps you have never heard of the tilt test. It don't take much blood volume to keep the body alive when one is supine with the legs elevated. Perhaps you should spend a few days hanging around with some anesthesiologists (or ER physicians) and learn how they clinically assess blood volume/hydration status and the consequences of those conditions. It is a problem that comes up almost every day. We think we have gotten pretty good at it.
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Frank's interpretation is also dependant on the catastrophe model that he evokes for exercise in the heat. I mentioned this model in an earlier post - Basic Physiology 3 - the one to do with the sports drink industry's model of how the body works.
I don't believe I have ever heard this model referred to as the "catastrophe" model. I just thought it was basic physiology. Anyhow, how does the sport drink industry's model differ from the model taught in medical school as to how the body works?
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Thus according to his model humans will continue to exercise without drinking until they suddenly collapse from heat stroke. When this does happen it is the exception not the rule. The rule is that if you become too hot or if you don't drink enough during exercise the brain will eventually take over and change your behavior by slowing you down. When you slow down your rate of heat production is reduced and you start to cool down (since it is your rate of heat production and not your level of dehydration that is the primary determinant of your body temperature during exercise - presuming that you do not have an intercurrent illness). in this way dehydration (caused by the absence of drinking which causes thirst which causes you to slow down and to go and search for fluid to alleviate your symptoms of thirst) should actually protect from and not cause heat stroke.
Ugh, where did I ever say that continuing "to exercise without drinking" would sort of go unnoticed until the athlete "suddenly collapased"? I don't think that ever happens. As dehydration begins to get clinically significant performance will start to degrade as filling pressure will drop and cardiac output cannot be maintained at the previous high level. If the muscles are continued to be stressed at the same level though, one compensatory mechanism the body has to meet the demand is to divert blood from the skin to the muscles. This would be a good survival mechanism is being chased by a lion so we can see why evolution might have selected for this ability. It is an awful survival mechanism is in a hot marathon.
Slowing you down when dehydrated does not require a single neuron of brain activity. It is simply a matter of being able to maintain cardiac output and deliver oxygen to all the demands. If you don't slow down you will overheat. If you overheat, the proteins don't work so good anyhow and you will slow down. It is all a matter of rather simple physiology.
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Interestingly in the literature there are hundreds of individual cases of subjects who exercise in the heat without drinking. Often these subjects are acting as the controls in studies to show important it is to drink during exercise. But in none of these are there reports of ill health in the group who do not drink during exercise. The exception are a group of studies in which athletes exercise in such extreme heat that they are unable to regulate their body temperatures so that they will have to stop eventually because they are becoming too hot. But even in those experiments subjects usually stop because they develop postural hypotension with temperatures well below those measured in cases of heat stroke or even in winners of 10, 21 and 42 km races run in the heat.
Ugh, me thinks that is because the Human use committees would never approve a study that allowed the subjects to exercise long enough to risk any injury, let alone any substantial injury. Any body temperature more than a degree or so from the optimal 37º will adversly affect performance. That is a different story than heat stroke which is life threatening. You don't seem to be able to put this "heat issue" into perspective. It is a continuum, just as water intoxication is.
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These findings are best explained by a model in which the brain is clever enough not to want to kill itself every time its owner chooses to exercise in the heat. For the reality is that the brain also dies if the athlete dies from heat stroke. Furthermore it is only the brain that can determine how much heat your are producing (since the brain drives the muscles to exercise) and how much heat you are losing (by regulating the amount of sweat that is being secreted - this control is via the sympathetic nervous system).
Phooey. It is not necessary to invoke any "brain cleverness" to explain any or all of these findings other than we learn from prior experience so are better able to gauge our efforts. Ugh, and most athletic activity, like running does not involve any "brain activity" as it is purely a spinal reflex activity, never getting any higher than the cerebellum except when the brain is used to initiate the activity or change the intensity. People do not have to think about moving those muscles.
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Since it controls both heat production and heat loss the brain must determine whether or not to kill itself. Why would it choose suicide when it can simply choose to increase the rate of heat loss (by increased sweating) or if that fails simply reduce the rate of heat production by slowing the athlete down.
The only way the brain "controls" the periphery is in its ability to put out hormones that will divert blood from other organs to itself in cases of extremis. During ordinary activity the brain has no control over heating (except, perhaps, for shivering and initiating muscle activity), cooling, or anything else. If the brain is involved in this peripheral physiology the entire body is in extremis.
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The rarity of heat stroke suggests (i) that these controls work very very weil in most individuals and (ii) when heat stroke does occur something has gone seriously wrong since the brain has chosen suicide over survival.
Well, I concur that heat stroke implies something has gone seriously wrong but I disagree that it was the primary fault of the brain unless we blame it for signing up for the race or whatever.
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And evolution would not have allowed such a brain to survive.
That is correct. That is why the physiology works the way it does. Luckily for us our brains are used mostly for thinking and not for making sure the muscles work right when exercising which allows us to study and figure some of this stuff out.
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Frank,
An original Ironman and the Inventor of PowerCranks
