Cerebrovascular reactivity of the middle cerebral artery velocity (CVR MCAv) to carbon dioxide (CO2) is a common method to assess cerebrovascular function. Yet, the approaches used to calculate CVR outcomes vary. The aim of this study was to explore the within and between-day reliability of traditional CVR outcomes. The second aim was to explore the reliability of novel kinetic-based analyses. Healthy adults (n = 10, 22.3±3.4 years) completed assessments of CVR over four minutes using a fixed fraction of inspired CO2 (6%). This was repeated across four separate visits (between-day), and on one visit measures were repeated 2.5 hours later (within-day). No mean biases were present between assessments for traditional CVR metrics, expressed as absolute (cm/s/mmHg) or relative (%/mmHg) outcomes (minute-3, minute-4, peak 1 second, peak 30 second) (between-day: P>0.14, ηp2<0.20, within-day: P>0.22, d>0.27). Absolute, rather than relative CVR, yielded the most reproducible parameters (coefficient of variation: 8.1-13.2% versus 14–83% respectively). There were significant differences between CVR outcomes (P<0.001, ηp2>0.89) dependent on the time point used to determine CVR, as a steady state MCAv response was rarely observed. Furthermore, the MCAv response was not reproducible within an individual (kappa = 0.15, P = 0.09). No mean differences were present for novel kinetic outcomes (amplitude, time-delay, time constant) (between-day: P>0.05, d<0.33, within-day: P>0.38, d<0.25). The results support the need for standardisation and indicate CVR should be defined as a dynamic peak, rather than a set time point for increased reliability. For novel kinetic outcomes variability was greater (CV: 8.7-120.9%) due to the nature of time-based metrics.

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