What is the Best Path Length for Aortic Pulse Wave Velocity? Preliminary Answer to a Stiff Question

A compliant (nonstiffened) aorta enables optimal cardiac function and appropriate transfer of stroke volume without major elevations in left ventricular afterload. Increased aortic stiffness, on the other hand (which commonly occurs with aging and hypertension), impairs ventricular–vascular interaction, decreases cardiac performance through raised afterload, and is a powerful independent predictor of cardiovascular mortality. The gold standard, noninvasive and widely applicable means to measure aortic stiffness is by the speed of pressure pulse wave travel (with each cardiac contraction) from the carotidto-femoral arteries. This measure is termed aortic pulse wave velocity (PWV), and a stiffer aorta equates to higher PWV. An aortic PWV value >12 m/s is recognized in guidelines as indicative of hypertension-related subclinical organ damage and is an influential factor regarding risk stratification and approach to antihypertensive therapy.1 There are many problems with a singular cut-off value to denote pathological aortic stiffness. Recent publication of normal ageand blood pressure-specific reference values2 is an advancement toward resolving issues and should lead to refinement of future hypertension guidelines relating to aortic PWV. Nonetheless, a critical issue remaining to be resolved is the most appropriate method to assess aortic path length. This is commonly estimated using a tape measure (or calipers) over the body surface between the carotid and femoral recording sites. There are several possibilities for determining path length and, depending on which one is used, may result in marked differences (up to 30%) in aortic PWV. This disparity has obvious clinical and research implications, and highlights the need for consensus on the use of a standard path length. Since most prognostic studies have utilized the direct (carotidto-femoral) path length, it has been argued that this should be the method routinely used,3 but apart from convenience, there does not appear to be data to support this contention. Indeed, the direct path length method overestimates true aortic length and, therefore, PWV.4 In this current issue of the Journal, Németh et al.5 present the first data to compare the prognostic utility of three aortic path lengths for predicting cardiovascular mortality in highrisk patients with renal disease. Subtraction of the sternal notch-to-carotid distance from the sternal notch-to-femoral distance (probably the most anatomically relevant method to estimate aortic length)4 was the strongest independent predictor of outcome, above and beyond the widely used direct path length method. Interestingly, this observation did not hold true in taller patients with a longer torso, regardless of which path length was used. The authors provide plausible possible reasons for this anthropometric discovery, but the unknown cause (which may relate to improved ventricular– vascular interaction favoring the tall), is certainly worthy of further investigation. Németh et al.5 acknowledge their study is limited by a relatively small, selected study sample, with only minor differences in hazard ratios between the different aortic PWV path length methods. Nevertheless, the findings provide the first evidence for preferential use of the subtracted, rather than direct, path length and represent an extremely important step toward standardizing the methodology for assessment of aortic stiffness.