Noninvasive measurements of aortic stiffness: methodological considerations.

This article overviews methods currently available for the non-invasive determination of aortic stiffness and critically appraises their strengths and weaknesses. Approaches are divided into indirect and direct (Windkessel models are not reviewed). Indirect techniques rely on the measurement of pulse wave velocity (PWV) to obtain information about the average stiffness of the vessel pathway being studied. Typically "foot-to-foot" transit time measurements are combined with transcutaneous length measurements to calculate PWV (as length/transit time). Applanation tonometry, acoustic transducers, Doppler ultrasound and magnetic resonance imaging (MRI) have all been used to measure transit times. Direct techniques rely on assessing the relative change in aortic diameter (or area) between systole and diastole (strain) and combine these data with pulse pressure (stress) measurements to calculate vessel stiffness (as stress/strain). Transthoracic echocardiography and MRI have been used to assess aortic strain. All the approaches--both direct and indirect--however have their limitations. For indirect techniques the greatest errors are likely to be introduced by the transcutaneous estimation of the aortic path length. For direct techniques stress and strain are generally measured in different portions of the vasculature. These and other methodological issues are considered and areas where further work is needed are highlighted.