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 min between trials), ten performed a within-day long interval protocol (145 ± 16 min 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 ([Formula: see text]) 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 with %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.NEW & NOTEWORTHY The cerebral vasodilator response to increases in arterial carbon dioxide concentration, termed cerebral vasomotor reactivity, provides an index of cerebral vascular function/health. Reduced responses are present in populations with elevated cerebral vascular and neurocognitive disease risk/overt disease. Cerebral vasomotor reactivity is often assessed during rebreathing-induced hypercapnia. This study determined that the day-to-day and between-day variability in this response is repeatable, thereby providing important methodological information to the scientific community.