noninvasive methodologies for use in children and adolescents,173,174 the significance of arterial stiffening in the young is also being increasingly understood Measurement of Arterial Stiffness in vivo Noninvasive methods for the determination of local, regional, and systemic arterial stiffness and the quantification of wave reflections in vivo are available For meaningful interpretation of these indexes, their fundamental limitations have to be taken into account First, the relationship between pressure and arterial diameter is nonlinear due to progressive recruitment of the stiffer collagen as transmural pressure increases Second, modulation of smooth muscle tone by sympathetic nervous activity, hormones, or endothelium-derived vasoactive substances as previously mentioned can alter arterial stiffness Finally, spontaneous vasomotor changes in the muscular arteries can alter arterial diameter and stiffness.175 Local Arterial Stiffness Local arterial stiffness is obtained by relating pressure changes to arterial diameter or cross-sectional area changes at the site of interest Arterial stiffness can be expressed as compliance, distensibility, Peterson elastic modulus, Young's modulus, and stiffness index (Table 74.1).153,176,177 Among the various indexes of local arterial stiffness, the stiffness index is considered relatively independent of systemic blood pressure.178 Table 74.1 Indexes of Local Arterial Stiffness Term Definition Compliance Absolute change in diameter or area during systole for a given pressure change Relative change in diameter or area during systole for a given pressure change Inverse of distensibility (i.e., the pressure change required for a given relative change in diameter or area) Distensibility Peterson elastic modulus FORMULA Change in Vessel Diameter ΔD/ΔP Change in CrossSectional Area of Vessel Lumen ΔA/ΔP ΔD/(D • ΔP) ΔA/(A • ΔP) ΔP • D/ΔD ΔP • A/ΔA Stiffness index (β) Young's modulus or incremental elastic modulus Ratio of ln (systolic/diastolic pressure) to relative change to vessel diameter Elastic modulus per unit wall thickness or area; provides information on intrinsic elastic properties of the arterial wall ln (Ps/Pd)/(ΔD/D) ΔP • D/ΔD • h [3(1 + A/WCSA)]/crosssectional distensibility A, Diastolic cross-sectional area; ΔA, difference in systolic and diastolic cross-sectional area; D, diastolic diameter; ΔD, difference in systolic and diastolic diameter; h, wall thickness; ΔP, difference in pulse pressure; Pd, diastolic blood pressure; Ps, systolic blood pressure; WCSA, wall cross-sectional area = π(D/2 + intima-media thickness)2 − π(D/2)2 For superficial arteries such as the brachial, femoral, and carotid arteries, the diameter and its change from end-diastole to systole can be assessed by ultrasound and echo-tracking techniques Two-dimensional ultrasound assessment is limited by the precision of measurements By contrast, echotracking devices process radiofrequency signals to track the displacement of the anterior and posterior arterial walls with high precision (Fig 74.4).179,180 The precision in determining the change in diameter has been estimated to be as small as 1 µm for echo-tracking devices and about 150 µm for video-image analysis of ultrasound images.153,179,181 For deeper arteries such as the aorta, cine magnetic resonance imaging182 and transesophageal echocardiography183 have been used to determine diameter change during the cardiac cycle FIG 74.4 Echo-tracking technique (A) Tracking of radiofrequency signals of the anterior (green) and posterior (red) arterial walls (B) Average variation in arterial diameter based on selected heart beats (orange) throughout the cardiac cycle