The time constant of the cerebral arterial bed: exploring age-related implications

The time constant of the cerebral arterial bed (τ) represents an estimation of the  transit time of flow from the point of insonation at the level of the middle cerebral artery to the arteriolar-capillary boundary, during a cardiac cycle. This study assessed differences in τ among healthy volunteers across different age groups. Simultaneous recordings of transcranial Doppler cerebral blood flow velocity (CBFV) and arterial blood pressure (ABP) were performed on two groups: young volunteers (below 30 years of age), and older volunteers (above 40 years of age). τ was estimated using mathematical transformation of ABP and CBFV pulse waveforms. 77 healthy volunteers [52 in the young group, and 25 in the old group] were included. Pulse amplitude of ABP was higher [16.7 (14.6–19.4) mmHg] in older volunteers as compared to younger ones [12.5 (10.9–14.4) mm Hg; p < 0.001]. CBFV was lower in older volunteers [59 (50–66) cm/s] as compared to younger ones [72 (63–78) cm/s p < 0.001]. τ was longer in the younger volunteers [217 (168–237) ms] as compared to the older volunteers [183 (149–211) ms; p = 0.004]. τ significantly decreased with age (r_S =  − 0.27; p = 0.018). τ is potentially an integrative marker of the changes occurring in cerebral vasculature, as it encompasses the interplay between changes in compliance and resistance that occur with age.