Below are two important papers recently published challenging the assumption that the diameter of the middle cerebral artery remains constant in response to hypocapnia and hypercapnia (using MRI technology) as previously reported.
Cerebral blood flow velocity underestimates cerebral blood flow during modest hypercapnia and hypocapnia – Coverdale et al.
Assessment of middle cerebral artery diameter during hypocapnia and hypercapnia in humans using ultra high-field MRI – Verbree et al.
Overall, these results suggest that the middle cerebral artery vasodilates in response to hypercapnia and vasoconstricts in response to hypocapnia, and that changes in blood flow velocity monitored with transcranial Doppler methodology (which assumes a constant diameter of the middle cerebral artery) underestimate changes in cerebral blood flow with some important implications for the evaluation of cerebrovascular reactivity to carbon dioxide. These results also question the validity of transcranial Doppler ultrasound, at least during changes in carbon dioxide…
Because science is not that simple and the fact that these two studies do not agree on everything, I suggest you to read the invited editorial related to the two above-mentioned papers:
Transcranial Doppler ultrasound: Valid, Invalid, or Both ? Ainslie and Hoiland
Fascinating readings !
Maybe we could further discuss the results/methodology related to these papers over here ?
What do you think ?