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 <i>within-day short interval</i> protocol (17 ± 2 min between trials), ten performed <i>a within-day long interval</i> protocol (145 ± 16 min between trials), and seventeen performed a <i>between-day</i> 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. <i>Within-day short interval</i>, %MCAv_mean slope demonstrated fair to excellent repeatability (intraclass correlation, ICC = 0.92 [95% confidence interval 0.72-0.98]; <i>P</i> < 0.001) while %CVCi slope showed more variability (ICC = 0.84 [0.47-0.95]; <i>P</i> = 0.002]). <i>Within-day long interval</i>, %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 (<i>P</i> < 0.001 for both). For <i>between-day</i> trials, better repeatability was observed for %CVCi (ICC = 0.85 [0.57-0.95]; <i>P</i> < 0.001) compared with %MCAv_mean (ICC = 0.76 [0.33-0.91]; <i>P</i> = 0.004) slope. These findings indicate repeatable <i>within- and between-day</i> CVMR responses to rebreathe-induced hypercapnia. However, a longer interval may be better for <i>within-day</i> repeat trials, particularly for CVCi measures.<b>NEW & NOTEWORTHY</b> 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.