Directional sensitivity of the cerebral pressure-flow relationship in middle and posterior cerebral arteries using the repeated squat-stand model: within-day reproducibility and impact of diurnal variation in young healthy men and women

We recently employed repeated squat-stands (RSS) to quantify directional sensitivity of the cerebral-pressure flow relationship (i.e. hysteresis) using a novel metric. Within-day reproducibility and diurnal variation impacts of this metric are unknown. We evaluated this metric for: 1) within-day reproducibility and the extent diurnal variation has in middle (MCA; ΔMCAv T /ΔMAP T ) and posterior cerebral arteries (PCA; ΔPCAv T /ΔMAP T ); 2) sex differences. Absolute (ΔMCAv T /ΔMAP T ; ΔPCAv T /ΔMAP T ) and relative (%MCAv T /%MAP T , %PCAv T /%MAP T ) metrics were calculated at seven time-points (08:00-17:00) in 18 participants (8 women; 24 ± 3 yrs) using the minimum-to-maximum MCAv/PCAv and MAP for each RSS at 0.05 Hz and 0.10 Hz. Reproducibility was evaluated with intraclass correlation coefficient (ICC). For all metrics, reproducibility was good (0.75-0.90) to excellent (>0.90). The metric in both arteries was impacted by MAP direction at 0.10 Hz (all p < 0.024). Time-of-day influenced ΔMCAv T /ΔMAP T (0.05 Hz: p = 0.0028; 0.10 Hz: p = 0.0009), %MCAv T /%MAP T (0.05 Hz: p = 0.035; 0.10 Hz: p = 0.0087), and ΔPCAv T /ΔMAP T (0.05 Hz: p = 0.0236). Sex differences in the MCA (p = 0.0028) vanished in relative terms and was absent in the PCA. These findings demonstrate within-day reproducibility of this metric in both arteries. Moreover, hysteresis is not impacted by sex.