P5.8 TOWARDS IN VIVO BIAXIAL CHARACTERISATION OF CAROTID ARTERY MECHANICS

Abstract

Background: The relative content and loading of collagen and elastin in the arterial wall determine in vivo longitudinal pre-stretch. Assessment of arterial distensibility at varying longitudinal pre-stretch could improve characterisation of arterial collagen-elastin matrix properties. We introduce a technique to impose extra longitudinal pre-stretch (Δstretch) to the carotid artery in vivo and investigate whether we can predict Δstretch from ultrasound distensibility measurements using a constitutive model. Methods: In 11 healthy volunteers (22±3 yrs, mean±SD, 6m/5f) we obtained right common carotid artery bifurcation-to-bifurcation length by phase-contrast MR-angiography in two positions: head facing forward (relaxed, R) and head facing up and rotated to the left (stretched, S). We estimated Δstretch from the MR images by two independent operators (Bland-Altman inter-operator bias = 0.5±3%, mean±2SD). Additionally, we obtained brachial blood pressures and carotid diameter-distension (considered as cross-sectional area) by ultrasound echo-tracking. We fitted a constitutive model [Spronck et al., AJP-Heart 2015] to average single-exponential pressure-area curves for both R and S states. We predicted Δstretch using the model, assuming constant axial force over the cardiac cycle. Results: MRI-estimated Δstretch was +2% while constitutive model-predicted Δstretch was +6%. To check these estimates, we additionally predicted Δstretch based on echo-derived intima-media-thickness measurements, yielding +6%. Conclusion: Measured and predicted Δstretch values were of the same order of magnitude. Assessment of carotid artery distensibility at varying longitudinal pre-stretch could improve model-based in vivo assessment of arterial wall collagen-elastin matrix properties, which are relevant in age- and disease-related arterial remodelling.