Visit-to-Visit Reproducibility of Cerebral Vascular Reactivity to CO2 in Healthy Young Humans

Partial pressure of arterial CO2 (PaCO2) is an important regulator of cerebral blood flow. The magnitude of change in cerebral blood flow per unit change in PaCO2 represents the cerebral vascular responsiveness to a CO2 stimulus, and is used as an index of cerebrovascular health. Accordingly, it is important to assess the reproducibility of this technique for clinical diagnoses. Purpose: To assess visit-to-visit reproducibility of a cerebral vascular reactivity to CO2 test in healthy young humans. Methods: Healthy adults (n=6, 25±2 y) performed a 5-min cerebral vascular reactivity to CO2 protocol (+5 mmHg end-tidal CO2 (etCO2) above eucapnic baseline) on two separate visits (7-234 days between visits). EtCO2 and middle cerebral artery velocity (MCAv) were measured continuously. Coefficient of variation (CV) was calculated for cerebral vascular reactivity to CO2 ((%Δ MCAv)/Δ etCO2), etCO2, and mean MCAv responses between the two visits. Results: While the CO2 stimulus between visits was similar (Visit 1: 5.3±0.2 mmHg vs. Visit 2: 5.1±0.3 mmHg, p=0.54) with a CV of 2.8%, there was a difference in the MCAv response to this stimulus (Visit 1: +15±1 cm/s vs. Visit 2: +12±2 cm/s, p=0.09; CV=19%). This resulted in a large variation in cerebral vascular reactivity to CO2 between visits (Visit 1: 4.6±0.3 %/mmHg vs. Visit 2: 3.6±0.5 %/mmHg, p=0.06; CV=18%). Conclusion: These findings suggest that despite a similar magnitude of change in the CO2 stimulus, physiological variations in cerebral vascular reactivity to CO2 occur in young healthy adults between two experimental visits.