Reduced orthostatic tolerance in athletes: it's all in their heads !

Apr 06 2011 Published by under Uncategorized

Dear athletes, did you ever experience light headedness or dizziness following a sudden change in position, i.e. from the sitting to the standing position? These symptoms of brain hypoperfusion are known to be a consequence of orthostatic hypotension (sudden reduction in arterial pressure when standing up), and surprisingly, aerobic fitness seems to be associated with reduced orthostatic tolerance !

While the higher prevalence of orthostatic hypotension in athletes could be explained by factors such as the presence of a more important amount of blood volume in the legs due to improved leg compliance (change in volume for a given change in pressure), it may also be the consequence of an impaired capacity of the cerebral vasculature to efficiently deal with changes in arterial pressure.

A group of Danish researchers was interested by this specific issue and recently published interesting results in the Journal of Applied Physiology.

The aim of the study was to investigate whether trained individuals have less efficient regulation of cerebral vascular resistance compared to untrained individuals. They studied cerebral autoregulation (that reflects dilatation or constriction of resistance vessels of the brain to a reduction or an elevation in perfusion pressure, respectively, in order to maintain cerebral blood flow relatively constant) in 9 highly-trained subjects and 9 untrained subjects with similar baseline characteristics, except for body weight and maximal oxygen uptake (VO2max). Cerebral autoregulation was evaluated by the thigh cuff method (see below) and transfer function analysis. For the need of this post, we’ll focus on the section of the article using the former method.

Several parameters such as mean arterial pressure, heart rate, cardiac output, cerebral blood flow and cerebrovascular conductance (ratio of cerebral blood flow and mean arterial pressure) were continuously measured for a period of 5 min before (baseline), 2.5 min during and 5 min following the inflation of thigh cuffs at a pressure of 300 mmHg. In the figure below, you can see (no matter what line you’re looking at) that the sudden release of the cuffs (0 s) leads to a reduction in both mean arterial pressure (MAP) and cerebral blood flow (MCA Vmean). Following a few seconds, the cerebrovascular conductance (CvCi) starts to increase back to baseline values.

After the release of the cuffs, the rate of decline in MAP and MCA Vmean was faster in trained (black lines) compared to untrained subjects (dotted lines). In addition, the time at which CvCi started to increase (that represents the onset of cerebral autoregulation) was delayed in trained compared to untrained subjects. Once initiated, this autoregulatory response was similar between groups. Consequently, recovery of MCA Vmean and CvCi to baseline values was slower for the trained subjects.

According to the authors, the post-ischemic legs of the athletes may require a more important proportion of the cardiac output, compromising blood flow to other organs such as the brain. Also, a delayed onset of autoregulation in the trained subjects could be related to higher cerebrovascular tone (basal state of vasoconstriction), which could make the cerebral vessels of these subjects more vulnerable to a rapid reduction in blood pressure (although that higher cerebrovascular tone could, on the other hand, protect them from surges in arterial pressure such as during exercise).

Although exercise training is associated with an impressive amount of benefits, further studies are clearly warranted to better understand the long-term effects of exercise on the brain vessels. While researchers have focused on the increased ability to pool blood in the lower legs and/or the reduced ability of baroreceptors to maintain MAP to explain the higher prevalence of symptoms related to orthostatic hypotension in athletes, this study suggests that aerobic fitness affects cerebral autoregulation and could increase the risk of cerebral hypoperfusion during a sudden lowering in arterial pressure.

So, dear athletes, next time that you are feeling dizzy or light headed in response to a sudden change of position, well, it’s all in your head !

Patrice

Lind-Holst M, Cotter JD, Helge JW, Boushel R, Augustesen H, van Lieshout JJ, Pott FC. Cerebral autoregulation dynamics in endurance-trained individuals. J Appl Physiol 2011 (Epub ahead of print) PMID: 21372098

4 responses so far

  • Travis says:

    I've come close to blacking out after a number of hard races, but it always passes very quickly, and I've noticed that lowering my head seems to help. Is this likely to be a similar process? I imagine that there would be a huge increase in compliance once your leg muscles stop contracting?

    I've always found it ironic that in the finishing chute of cross-country races many people are light-headed, and yet the volunteers scream at you if you do anything to lower your head (they seem worried that you'll have some sort of cardiac event), which almost always fixes the problem.

  • Patrice Brassard says:

    You have excellent questions Travis ! The end of a hard race can certainly be a challenge for the brain circulation, and it could represent (to some extent) an example of what is going on following thigh cuffs release (at least in terms of systemic and cerebrovascular hemodynamics). You will definitely experience a reduction in blood pressure and cerebral blood flow if you suddenly stop exercising, especially if that muscle pump isn't in action anymore !

    In addition, you are most likely hyperventilating at the end of such races. The elevation in arterial carbon dioxide tension is a potent vasodilator, while its reduction lowers cerebral blood flow...So, the reduction in arterial carbon dioxide tension induced by hyperventilation will also be responsible for the reduction in cerebral blood flow at the end of intense exercise. Still, this could become complicated since hypocapnia improves cerebral autoregulation (but let's cover the latter issue in another post !)

    If you feel light-headed but it passes very quickly, this could be a consequence of that delay (few seconds) in the onset of dynamic cerebral autoregulation...some sudden changes in blood pressure are just to quick for the brain !

    In regards to the head lowering issue, quite frankly, I've never read any papers related to its influence on cerebral blood flow (within that kind of context). However, it makes sense ! Gravity will be there for your brain circulation ! Then, the question would be: is it dampening the reduction in cerebral blood flow or is it increasing cerebral blood flow ??

    This discussion definitely gives me a lot of ideas for research 🙂

    Again, thanks Travis!

  • Travis says:

    That's fascinating! Thanks for the detailed follow-up!

  • [...] posts, the impact of exercise training on cerebral hemodynamics remains ambiguous. I once wrote a post over at Scientopia describing a study that reported a less effective dynamic cerebral [...]