Quantification of the cerebral pressure-flow relationship directional sensitivity: Reliability of shorter repeated...

The magnitude of changes in middle cerebral artery mean blood velocity (MCAv) is attenuated when mean arterial pressure (MAP) increases compared with when MAP decreases. This directional sensitivity has been characterized using a time-corrected ratio calculated on MCAv and MAP changes induced by repeated squat-stands (RSS) at 0.05 and 0.10 Hz for 300 s (∆MCAv_T/∆MAP_T). Herein, we examined the reliability of the metric within reduced RSS durations. Ninety-nine (25 females) healthy human participants (26 ± 5 years) performed 300 s RSS at 0.05 Hz (20 s cycles) and 0.10 Hz (10 s cycles) while MCAv and MAP were measured continuously. The ∆MCAv_T/∆MAP_T was calculated for each transition [increase (INC); decrease (DEC)] of MAP for 60, 120, 180, 240 and 300 s. A two-way ANOVA was completed, and the absolute (Bland-Altman plot) and relative (coefficient of variation and intraclass correlation coefficient) reliability were calculated to compare shorter-duration recordings with 300 s (reference standard). At 0.05 Hz, ∆MCAv_T/∆MAP_T was similar between INC and DEC and comparable to the 300 s reference from 120 s onwards. At 0.10 Hz, ∆MCAv_T/∆MAP_T was lower during INC (p < 0.0001). Bland-Altman plots indicated that differences trended towards zero (greater agreement) with increasing duration. Averaged coefficients of variation were <10% from 120 s (0.05 Hz) and 60 s (0.10 Hz) onwards. All intraclass correlation coefficients were >0.90 for recordings of ≥180 s in both frequencies. Although the 300 s reference is optimal, RSS duration could be shortened to 180 s, if needed, to identify this hysteresis-like pattern reliably using ∆MCAv_T/∆MAP_T.