Is it fine to assume that cerebrovascular reactivity to CO2 is similar between anterior and posterior cerebral circulations?

When we want to examine cerebrovascular function in humans (cerebral blood flow, cerebral autoregulation, cerebrovascular reactivity to carbon dioxide (CO2), the middle cerebral artery is usually insonated. We assume that autoregulation and reactivity are similar between anterior and posterior regions. But are we right ?

Skow et al. compared cerebrovascular reactivity to CO2 from both the anterior (middle cerebral artery) and posterior (basilar artery and posterior cerebral artery) circulations using the hyperoxic rebreathing method in healthy volunteers. The authors hypothesized that reactivity to CO2 would be comparable between basilar artery and posterior cerebral artery. However, the middle cerebral artery and posterior cerebral artery would have different reactivity to CO2.

Thus, they compared reactivity to CO2 in basilar artery and posterior artery in a first protocol (n=13) and compared reactivity in the middle cerebral artery and posterior cerebral artery in a second protocol (n=17).

The main findings of this study are that both absolute and relative cerebrovascular reactivity to CO2 between basilar artery and posterior cerebral artery were not different. Also, absolute, but not relative, cerebrovascular reactivity to CO2 of the middle cerebral artery was higher compared to the posterior cerebral artery.

In light of these results, there exist differences in cerebrovascular reactivity to CO2 between anterior and posterior circulations. In addition, the authors suggest that the posterior cerebral artery represents an adequate surrogate index of basilar artery reactivity in studies where the latter is difficult to insonate. This is interesting because the basilar artery measurements are usually obtained by a hand held probe while the posterior cerebral artery measurements are obtained by a probe installed on a headpiece. This is an important methodological consideration for transcranial Doppler, since it will allow for stable and continuous measurements.

Particularly appreciated in this manuscript are the methodological and analysis considerations. Usually, these sections are not necessarily explicit. However, in this manuscript, the authors provide an in-depth discussion of important issues related to the study and its calculations. They have to be congratulated for the inclusion of such a detailed section.

Reference

Skow RJ, Mackay CM, Tymko MM, Willie CK, Smith KJ, Ainslie PN, Day TA. Differential cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations. Respir Physiol Neurobiol 189(1):76-86, 2013

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