Adaptation of cerebral pressure-velocity hemodynamic changes of neurovascular coupling to orthostatic challenge

Neurovascular coupling (NVC), analysed by a control system approach, was shown to be unaffected by orthostatic challenge, but data is lacking regarding the mechanism of this interplay and the behaviour of other cerebrovascular reactivity parameters. We investigated the changes in different pressure–velocity models during functional transcranial Doppler (TCD), under different orthostatic conditions.

Thirteen healthy volunteers performed a reading test stimulation task in sitting, supine and head-up tilt (HUT) positions. CBF velocity was monitored with TCD in the posterior cerebral artery, and blood pressure was monitored with Finapres. Cerebrovascular resistance index (CVRi) was compared to a two-parameter model including resistance-area product (RAP) and critical closing pressure (CrCP), in the maximal and in the stable phases of flow response to visual stimulation.

All cerebrovascular resistance parameters decreased with visual stimulation but the magnitude of their variation in each orthostatic condition was not similar. From supine to HUT, CrCP variation decreased (both maximal and stable phase p = 0.001). CVRi variation increased from sitting to HUT positions (maximal p = 0.039; stable phase p = 0.033). RAP variation to visual stimulation did not change between the three positions (maximal p = 0.077; stable phase p = 0.188).

A 2-parameter model of vascular resistance provided better discrimination for the effects of posture on NVC as shown by the adaptive changes in CrCP with orthostatic challenge, in comparison with the classical use of CVRi. These findings suggest that although NVC seemed unaffected by orthostatic challenge, more complex vasoregulative mechanisms are activated in different orthostatic conditions that could potentially be of diagnostic or prognostic value.