Cerebral Vasomotor Reactivity to Carbon Dioxide Using the Rebreathe Technique: Assessment of Within-day and Between-day Repeatability.

The cerebral vasodilator response to increased arterial carbon dioxide (CO₂) concentration, termed cerebral vasomotor reactivity (CVMR), is used to assess cerebral vascular function. We sought to assess the within-day and between-day repeatability of CVMR to rebreathing-induced hypercapnia. Twelve healthy adults performed a within-day short interval protocol (17±2 minutes between trials), ten performed a within-day long interval protocol (145±16 minutes between trials), and seventeen performed a between-day protocol (5±2 days between visits). Repeatability of the slope of the percent change in middle cerebral artery mean blood velocity (%MCAv_mean) and cerebral vascular conductance index (%CVCi), to the change in partial pressure of end-tidal CO₂ (PETCO₂) between the two trials/days was assessed. Within-day short interval %MCAv_mean slope demonstrated fair to excellent repeatability (intraclass correlation, ICC=0.92 [95% confidence interval 0.72-0.98]; p<0.001) while %CVCi slope showed more variability (ICC=0.84 [0.47-0.95]; p=0.002]). Within-day long interval, %MCAv_mean (ICC=0.95 [0.80-0.99]) and %CVCi (ICC=0.94 [0.71-0.99]) slopes showed good to excellent and fair to excellent repeatability respectively (p<0.001 for both). For between-day trials, better repeatability was observed for %CVCi (ICC=0.85 [0.57-0.95]; p<0.001) compared to %MCAv_mean (ICC=0.76 [0.33-0.91]; p=0.004) slope. These findings indicate repeatable within- and between-day CVMR responses to rebreathe induced hypercapnia. However, a longer interval may be better for within-day repeat trials, particularly for CVCi measures.