Ideally the local pressure should be measured at the site of diameter measurements Applanation tonometry allows noninvasive recording of the arterial pressure waveform in the carotid and peripheral conduit arteries.184 The recorded pressure waveform can be calibrated against the cuff mean and diastolic blood pressures of the brachial artery.185,186 Derivation of the central aortic waveform from radial arterial tonometry has been made possible by the application of a transfer function, which has been validated in adults187,188 but not in children Although the cuff brachial artery pulse pressure is commonly used for the calculation of local arterial stiffness indexes, amplification of pressure pulse along the arterial tree constitutes a potential source of error Several groups have reported the use of oscillometric blood pressure measurements for the noninvasive assessment of local arterial stiffness.189,190 The magnitude of the oscillation as a result of the blood vessel pulsation reflects the volumetric change of the underlying blood vessel A stiffness parameter can be derived from the change in the cuff pressure in relation to the change in volume as represented by the accumulated magnitude of oscillation at each of the cuff pressure Recently tracking of vessel wall motion by ultrasound technologies has been used to interrogate vascular mechanics Two-dimensional speckle tracking of arterial wall motion enables the quantification of global circumferential strain and strain rate and time to peak strain (Fig 74.5).191 A stiffness index that relates global circumferential strain to blood pressure can be calculated by the formula: ln (systolic blood pressure/diastolic blood pressure)/circumferential strain.192 Integration of the tissue Doppler-derived velocity data of the anterior and posterior arterial walls also allows the derivation of regional arterial strain and strain rate.193 FIG 74.5 Two-dimensional speckle tracking analysis of global circumferential strain (top panel) and strain rate (bottom panel) of the right common carotid artery from the short-axis arterial view Circumferential expansion of the arterial wall to accommodate the blood flow during early systole leads to the positive strain and strain rate, whereas vascular recoil during the later phase of ventricular systole results in the negative circumferential strain rate Regional Arterial Stiffness Stiffness of an arterial segment, or regional stiffness, is assessed by measuring the pulse-wave velocity over the segment of interest Pulse-wave velocity is the speed at which the pressure or flow wave is transmitted along the arterial segment It is related to Young's elastic modulus (E) by the Moens-Korteweg equation: where PWV is pulse-wave velocity, h is wall thickness of the vessel, r is the inside radius of vessel, and ρ is the density of blood.140 The Bramwell and Hill194 equation relates pulse-wave velocity to arterial distensibility: where P is pressure, V is volume, ΔP • V/ΔV represents volume elasticity, and D is volume distensibility of the arterial segment Furthermore, pulse-wave velocity is directly related to characteristic impedance (Zc) by the formula147: Zc = PWV • ρ Pulse-wave velocity is hence related directly to arterial elasticity and characteristic impedance and inversely to arterial distensibility By providing an average stiffness of the arterial segment of interest, pulse-wave velocity may provide a better reflection of general vascular health Pulse-wave velocity is determined by dividing the distance of pulse travel by the transit time The arterial pulse may be registered using pressure-sensitive transducers,195 an oscillometric device,196 applanation tonometry,197 Doppler ultrasound,198,199 or photoplethysmography.200,201 Furthermore, the pulse wave can be detected using magnetic resonance imaging,202,203 which also allows accurate determination of path length and measurements to be made from