Directional sensitivity of dynamic cerebral autoregulation in squat-stand maneuvers

Dynamic cerebral autoregulation (CA), the transient response of cerebral blood flow (CBF) to rapid changes in arterial blood pressure (BP), is usually modelled as a linear mechanism. We tested the hypothesis that dynamic CA can display non-linear behavior resulting from differential efficiency dependent on the direction of BP changes. CBF velocity (CBV, transcranial Doppler), heart rate (HR, 3-lead ECG), continuous BP (Finometer) and end-tidal CO2 (capnograph) were measured in 10 healthy young subjects during 15 squat-stand maneuvers (SSM) with a frequency of 0.05 Hz. The protocol was repeated with a median [IQR] of 44 [35-64] days apart. Dynamic CA was assessed with the autoregulation index (ARI), obtained from CBV step responses estimated with an autoregressive moving-average model. Mean BP, HR, and CBV were different (all p<0.001) between squat and stand, regardless of visits. ARI showed a strong interaction (p<0.001) of SSM with the progression of transients; in general, the mean ARI was higher for the squat phase compared to standing. The changes in ARI were partially explained by concomitant changes in CBV (p=0.023) and pulse pressure (p<0.001), but there was no evidence that ARI differed between visits (p=0.277). These results demonstrate that dynamic CA is dependent on the direction of BP change, but further work is needed to confirm if this finding can be generalised to other physiological conditions, and also to assess its dependency on age, sex and pathology.