Segmental aortic wall stiffness from intravascular ultrasound at normal and subnormal aortic pressure in pigs

Segmental aortic wall stiffness was calculated from intravascular ultrasound images and intravascular pressures in six pigs at normal and subnormal aortic pressures (21 sequences of pressures and areas before and after boli of intravenous nitroglycerin). The wall stiffness was expressed as the pressure–strain elastic modulus (E_p). The E_p was calculated from the formula: E_p=ΔPRΔR^-1 (P, pressure; R, radius) in two different ways. First from maximal and minimal values of pressure and area. Second as the slope of linear regression line of ΔPR as a function of ΔR from 29 simultaneous recorded pressures and images. The average E_p value for all sequences in the different segments was 0.58 ± 0.55 10⁵ Pa (Method 1) and 0.50 ± 0.40 10⁵ Pa (Method 2). E_p increased with the distance from the heart at normal aortic pressures. At subnormal aortic pressures after intravenous nitroglycerin this relationship was not so evident. At subnormal aortic pressures the calculated E_p values were significantly reduced in the lower half of the abdominal aorta. The phase lag, i. e. hysteresis, between pressure and diameter was demonstrated. Our study shows the applicability of intravascular ultrasound as a tool to evaluate arterial wall stiffness.