Cerebral arterial time constant calculated from the middle and posterior cerebral arteries in healthy subjects.

The cerebral arterial blood volume changes (∆Ca BV) during a single cardiac cycle can be estimated using transcranial Doppler ultrasonography (TCD) by assuming pulsatile blood inflow, constant, and pulsatile flow forward from large cerebral arteries to resistive arterioles [continuous flow forward (CFF) and pulsatile flow forward (PFF)]. In this way, two alternative methods of cerebral arterial compliance (Ca ) estimation are possible. Recently, we proposed a TCD-derived index, named the time constant of the cerebral arterial bed (τ), which is a product of Ca and cerebrovascular resistance and is independent of the diameter of the insonated vessel. In this study, we aim to examine whether the τ estimated by either the CFF or the PFF model differs when calculated from the middle cerebral artery (MCA) and the posterior cerebral artery (PCA). The arterial blood pressure and TCD cerebral blood flow velocity (CBFVa ) in the MCA and in the PCA were non-invasively measured in 32 young, healthy volunteers (median age: 24, minimum age: 18, maximum age: 31). The τ was calculated using both the PFF and CFF models from the MCA and the PCA and compared using a non-parametric Wilcoxon signed-rank test. Results are presented as medians (25th-75th percentiles). The cerebrovascular time constant estimated in both arteries using the PFF model was shorter than when using the CFF model (ms): [64.83 (41.22-104.93) vs. 178.60 (160.40-216.70), p < 0.001 in the MCA, and 44.04 (17.15-81.17) vs. 183.50 (153.65-204.10), p < 0.001 in the PCA, respectively]. The τ obtained using the PFF model was significantly longer from the MCA than from the PCA, p = 0.004. No difference was found in the τ when calculated using the CFF model. Longer τ from the MCA might be related to the higher Ca of the MCA than that of the PCA. Our results demonstrate MCA-PCA differences in the τ, but only when the PFF model was applied.