Brain blood flow during exercise: towards a more complete picture
Regional brain blood flow increases up to moderate intensity exercise. At higher exercise intensities, blood flow velocity in the middle cerebral artery levels off (at around 50-60% of maximal exercise) and then decreases during heavy exercise. This blood flow pattern has also been observed in the internal carotid artery. Potential mechanisms underlying such a response include a reduction in partial pressure of arterial carbon dioxide (PaCO2) induced by hyperventilation or high exercise-induced sympathetic nerve activation. Interestingly, vertebral artery blood flow and common carotid artery blood flow increase from rest to heavy exercise.
In a study recently published in the Journal of Physiology, Sato et al. examined the relative contribution of different cerebral arteries (common carotid artery, internal carotid artery, external carotid artery, vertebral artery and middle cerebral artery) providing blood flow to the brain and head during two incremental cycle exercises (the incremental exercise protocol needed to be performed twice in order to have the complete picture due to insufficient space at the level of the neck and interference between Doppler beams from multiple probes) in healthy subject. The figure below shows blood flow responses in each artery at different exercise intensities.
As in several other studies, these authors observed transient elevations in blood flow velocity in the middle cerebral artery and blood flow in the internal carotid artery from rest to 60% of maximal exercise followed by a reduction in blood flow at 80% of maximal exercise. However, blood flow in the common carotid artery, external carotid artery and vertebral artery increased from rest to heavy exercise.
Interestingly, the elevation in external carotid artery blood flow from moderate- to high-intensity exercise was negatively correlated with the decrease in internal carotid artery blood flow. The latter suggests that a large elevation in external carotid artery blood flow may contribute to the decrease in internal carotid artery blood flow observed during heavy exercise. In other words, although we already know that PaCO2 is a primary contributor in the regulation of brain blood flow during exercise, this study suggests that the distribution of carotid blood flow could limit the elevation in internal carotid blood flow during heavy aerobic exercise.
Sato, K., Ogoh, S., Hirasawa, A., Oue, A., & Sadamoto, T. (2011). The distribution of blood flow in the carotid and vertebral arteries during dynamic exercise in humans. The Journal of physiology, 589(Pt 11), 2847–2856. doi:10.1113/jphysiol.2010.204461