Background: It is well-known that cerebral and ocular vessels are highly sensitive to changes in the partial pressure of CO2 (PETCO2). 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 PETCO2-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.
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