Regional differences in the vascular response to CO₂ among cerebral, ocular, and mesenteric vessels

Background: It is well-known that cerebral and ocular vessels are highly sensitive to changes in the partial pressure of CO₂ (P_ETCO₂). However, it is unclear whether there are regional differences in the magnitude of the vascular response to CO₂ (CO₂ reactivity) in various vessels of a single individual, and whether there is a relationship in the CO₂ reactivity of specific vessels between different individuals. Methods: We compared the CO₂ reactivity of the superior mesenteric artery (SMA), middle cerebral artery (MCA), retinal artery (RA), and retinal and choroidal vessels (RCV) during a 3-min inhalation of CO₂-rich air (5%) in 70 subjects. Results: The blood flow velocities in the MCA, RA, and RCV significantly increased by 21 ± 2% (mean ± SE), 9 ± 1%, and 7 ± 2%, respectively (P < 0.05) during hypercapnia, whereas that in the SMA did not change significantly (-17 ± 11%). The CO₂ reactivity-the relative change in blood flow to a given change in P_ETCO₂-was significantly greater in the MCA (2.2 ± 0.2%/mmHg) than in the RA and RCV (0.9 ± 0.1%/mmHg and 0.8 ± 0.2%/mmHg, respectively). There was a weak correlation in the CO₂ reactivity only between RA and RCV (r = 0.43, P < 0.05). Conclusion: These findings indicate that the CO₂ reactivity can vary even in cerebral and ocular vessels, although the correlation is weak.