Differential cerebrovascular CO2 reactivity in anterior and posterior cerebral circulations

- Cerebrovascular reactivity between anterior and posterior circulations was compared.
- TCD and hyperoxic rebreathing were used to test cerebrovascular reactivity.
- The PCA can be used as a surrogate for BA measures of CO2 reactivity.
- The absolute CO2 reactivity is larger in the MCA compared to the PCA.

The potential differences in cerebrovascular responses between the anterior and posterior circulations to changes in CO2 are unclear in humans. Using transcranial Doppler ultrasound, we compared the CO2 reactivity of the (1) BA and PCA and (2) MCA and PCA during hyperoxic rebreathing in supine position. The reactivity in the BA and PCA was similar in both absolute (1.27 ± 0.5 and 1.27 ± 0.6 cm/s/Torr; P = 0.992) and relative (3.98 ± 1.3 and 3.66 ± 1.5%/Torr CO2; P = 0.581) measures, suggesting that the PCA is an adequate surrogate measure of reactivity for the BA. The MCA reactivity was greater than the PCA in absolute (2.09 ± 0.7 and 1.22 ± 0.5 cm/s/Torr CO2; P < 0.001), but not relative measures (3.25 ± 1.0 and 3.56 ± 1.6%/Torr CO2; P = 0.629). Our findings (a) confirm regional differences in the absolute reactivity in the human brain and (b) suggest that in cerebrovascular studies investigating functions mediated by posterior brain structures (e.g., control of breathing), the posterior vasculature should also be insonated.