Sub-component analysis of the directional sensitivity of dynamic cerebral autoregulation.

The origin of the directional sensitivity (DS) of dynamic cerebral autoregulation (dCA) is not known. In 140 healthy participants (67 male, 27.5 ± 6.1 years old), middle cerebral artery velocity (MCAv, transcranial Doppler), arterial blood pressure (ABP, Finometer), and end-tidal CO2 (EtCO₂, capnography) were recorded at rest. Critical closing pressure (CrCP) and resistance-area product (RAP) were obtained for each cardiac cycle, as well as mean MCAv and ABP (MAP). The integrated positive and negative derivatives of MAP (MAP_‫D, MAP_-D) were used as simultaneous inputs to an autoregressive moving average model to generate two distinct MCAv step responses. Similar models allowed estimation of corresponding MAP-CrCP and MAP-RAP responses to step changes in MAP_‫D and MAP_-D. The strength of directional sensitivity (ΔDS) was expressed by the difference in mean values of the step responses for the time interval 12-18 s. ΔDS was significant for MCAv (8.5 ± 46.9% vs 26.7 ± 42.0%, p<0.001) and RAP (-93.9 ± 48.1 vs -74.5 ± 43.0%, p<0.001), respectively for MAP_‫D and MAP_-D inputs, but not for CrCP (2.2 ± 48.1% vs 0.72 ± 42.9%, p=0.76). Compared to males, female participants had higher MCAv (63.9 ± 15.6 cm/s vs 55.4 ± 12.9 cm/s, p<0.001), but lower EtCO₂ (p<0.001) and RAP (p=0.015). Sex did not influence ΔDS for any of the three step responses. The presence of directional sensitivity in the RAP, but not in the CrCP transfer function, suggests the origin could be solely myogenic, without metabolic involvement.