Carbon dioxide, intense exercise and cerebral blood flow and oxygenation

Once in a while, I will blog about studies related to cerebrovascular physiology (as I was doing over at http://www.physiologiste.blogspot.com). However, I will do so exclusively in English. Be indulgent !! My English skills should improve with every post !

If you have an impression of “déjà vu” with the following article, it’s because I have already blogged about it… but in French 🙂

Recent evidence suggests that the reduction in cerebral blood flow and oxygenation could limit exercise capacity. Indeed, if the amount of oxygen in the brain is reduced below a critical threshold, this could negatively influence the ability to generate muscle work.

Arterial carbon dioxide tension (PaCO2) is a potent modulator of cerebral blood flow at rest and during exercise. More precisely, CO2 dilates blood vessels. Accordingly, an important reduction in CO2 levels in the blood, usually observed during intense exercise, is associated with constriction of blood vessels (cerebral vessels included) and consequently, a reduction in cerebral blood flow. This lowering in CO2 levels in the blood is a consequence of hyperventilation.

A research group has therefore decided to study, in 10 athletes (age: 25.9 ± 7.7 years, weight: 71.2 ± 7.7 kg, height: 181.1 ± 7.4 cm), the impact of maintaining the amount of CO2 in the blood (measured by gas exchange: PETCO2) during a maximal exercise test, on middle cerebral artery flow velocity (CBFV;) and on oxygen levels in the brain (specifically in the frontal lobe) while measuring systemic hemodynamics. The measurements obtained during the exercise protocol were compared with those obtained during a second exercise protocol performed by the same subjects, without maintaining the amount of CO2 stable (1).

This figure shows changes in PETCO2, CBFV, frontal lobe oxygen level (delta TSI: Tissue saturation index) , cerebrovascular reactivity (which is the change in CBFV in relation to the change in PETCO2 ) and cerebral vascular conductance (which is the change in  CBFV / blood pressure ratio compared to the change in PETCO2) at different workloads during exercise with (black dots) and without maintaining PETCO2 stable (white dots).

As we can see, maintaining the amount of CO2 stable was associated with a higher cerebral blood flow velocity during intense exercise as compared to the control condition. However, no difference was observed regarding the other measured variables, including oxygen levels in the brain and maximal workload (watts). This study suggests that despite the fact that CO2 is an important modulator of brain perfusion during exercise, it is definitely not the only one!

Thus, preventing a decrease in CO2 during maximal exercise increases blood flow to the brain but doesn’t seem to have any benefit for the amount of oxygen in the brain (frontal lobe). However, as noted by the authors, it must be remembered that the entire blood flow measured in the middle cerebral artery is not completely directed to the frontal lobes of the brain. The latter issue could partly explain the lack of significant impact of CO2 manipulation on this specific variable. Finally, some mechanisms (not measured in this study) such as changes in sympathetic activity, acid-base balance and distribution of cardiac output could also partly explain these results.

Without discrediting the importance of CO2 variations on brain perfusion during exercise, this study demonstrates the complexity of the regulation of cerebral blood flow and oxygenation in exercising humans!

(1) Olin JT, Dimmen AC, Subudhi AW, Roach RC. Cerebral blood flow and oxygenation at maximal exercise: The effect of clamping carbon dioxide. Respir Physiol Neurobiol 175: 176-180 2011. DOI:10.1016/j.resp.2010.09.011

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