5.2 Differential Characteristics Between Aortic Pressure Augmentation and Carotid Flow Augmentation: Clinical Implications for Cerebral White Matter Hyperintensities

Abstract

Background

Aortic stiffness and pressure wave reflection have been found to be associated with age-related cerebral microvascular disease, but the underlying mechanism remains obscure. We hypothesized that cerebral (carotid) flow augmentation potentially mediates these associations.

Methods

Doppler waveforms were recorded in 286 patients with hypertension to measure the carotid flow augmentation index (FAIx) as the late/early systolic velocity amplitude ratio. Tonometric waveforms were recorded to estimate the aortic pressure augmentation index (PAIx), aortic compliance, and carotid-femoral and carotid-radial pulse wave velocities (PWVs). Additionally, white matter hyperintensities (WMH) on brain MRI were evaluated using the Fazekas scale.

Results

With increasing age, the carotid late-systolic velocity increased whereas the early-systolic velocity decreased, although the aortic augmented pressure increased in parallel with the incident wave height (P<0.001). Both FAIx and PAIx increased with age, but the age-dependent curves were upwardly concave and convex, respectively. FAIx increased exponentially with increasing PAIx (r = 0.71). Compared to PAIx, FAIx was more closely (P ≤ 0.001) correlated with the aortic PWV, aortic compliance, and aortic/peripheral PWV ratio. FAIx was associated with WMH scores independently of confounders including age, gender, diabetes, hypercholesterolemia and aortic PWV (P = 0.02), and was more predictive of WMH presence than PAIx.

Conclusions

Carotid FAIx had closer associations with age, aortic stiffness and cerebral WMH than did aortic PAIx. These results indicate that carotid flow augmentation (enhanced by aortic stiffening and pressure wave reflection from the lower body) causes microcerebrovascular injury potentially through increasing cerebral flow pulsations, but this detrimental effect is even greater than that estimated from PAIx.