Analysis of time delay between non-invasively measured pulse oscillations in cerebral macro- and microcirculation

<i>Objective.</i>Integration of cerebral blood macrocirculation and microcirculation form a crucial aspect of global cerebral blood flow. Our study aimed to investigate time delay between pulse oscillations of cerebral blood flow velocity (FV) and total hemoglobin concentration (tHb) acquired via transcranial Doppler ultrasonography and functional near-infrared spectroscopy, respectively. Additionally, we compared time-related characteristics with cerebral arterial time constant (τ).<i>Approach.</i>The study involved monitoring of FV, tHb, arterial blood pressure and end-tidal CO₂(EtCO₂) during 5 min of rest (normocapnia) and 5 min of controlled hypercapnia in 36 healthy subjects (age: 24 years,<i>Q</i>1-<i>Q</i>3: 21-27 years, 15 females). Onset time delay (onset TD) was defined as the time offset between onsets of FV and tHb pulses, and time to tHb max (TTM) as a time difference between the FV pulse onset and tHb pulse maximum.τwas calculated as a product of cerebrovascular compliance and resistance. Onset TD and TTM were compared between normocapnia and hypercapnia, and their associations withτwere assessed.<i>Main results.</i>Onset TD was consistently positive (0.075 s,<i>Q</i>1-<i>Q</i>3: 0.065-0.108 s). Onset TD andτwere significantly shorter in hypercapnia (0.075 s vs 0.071 s,<i>p</i>< 0.001; 0.107 s vs 0.099 s,<i>p</i>< 0.001). Changes in onset TD andτbetween normo- and hypercapnia were significantly correlated (<i>R</i>_S= 0.452,<i>p</i>< 0.01). TTM was correlated withτin normocapnia (<i>R</i>_S= 0.364,<i>p</i>= 0.03), hypercapnia (<i>R</i>_S= 0.407,<i>p</i>= 0.01) and in terms of relative changes (<i>R</i>_S= 0.421,<i>p</i>= 0.01).<i>Significance.</i>There is a time delay between cerebral macro- and microcirculation, which becomes shorter as the cerebral vasculature dilates in hypercapnia.