Simultaneous macrovasculature and microvasculature cerebral autoregulation derived from the transfer function analysis of integrated cerebral haemodynamic data in older adults

Cerebral autoregulation (CA) maintains stable cerebral blood flow (CBF) despite mean arterial pressure (MAP) fluctuations. Transcranial Doppler ultrasonography (TCD) and Near-Infrared Spectroscopy (NIRS) report CBF surrogates and facilitate CA assessment. This study aimed to investigate the distinct information they provide about dynamic CA (dCA) responses to MAP step changes Approach: Simultaneous TCD-NIRS was performed on 28 healthy older participants alongside continuous measurements of beat-to-beat and breath-to-breath MAP, heart rate (HR) and end-tidal CO2. Transfer function analysis (TFA) was performed, varying input/output metrics including MAP, middle and posterior cerebral artery blood flow velocity (MCAv/PCAv) and oxyhaemoglobin (HbO2), comparing macro- and microvasculature dCA, respectively. Main Results: No differences in regional MAP step change HbO2 responses were found across eight pre-frontal (p=0.14) or four averaged regions (p=0.69). There was a significant effect of time in HbO2 responses to MAP step change (p<0.001), and to MCAv step change (p=0.016). There were also significant differences between HbO2 and MCAv and PCAv responses (p<0.001). Distinct TCD and NIRS step responses suggest much slower dCA responses in the microvasculature, compared to MCA and PCA, without regional differences. Significance: Further investigation into regional dCA differences is needed alongside potential benefits of simultaneous TCD-NIRS in pathological states.