Augmentation index as a specific marker of large arteries distensibility

Abstract

lthough high blood pressure has been undoubtedlyshown to be a major risk factor for cardiovascularmorbidity and mortality at all ages and in bothsexes, the suitability and accuracy of traditional bloodpressure measurements for cardiovascular risk staging isnow a matter of lively debate [1]. In this context, thepossible additional contribution carried by central bloodpressure values assessment and by pulse wave analysis hasbeen underlined by a number of studies [2,3].Indeed, the technique of pulse wave analysis, usingnoninvasive high fidelity arterial tonometers, has recentlybecome increasingly popular [4,5]. This method can pro-vide not only quantitative, although indirect, informationconcerning the levels of central blood pressure, but alsoqualitative data on the ascending aortic waveform. Analysisof such waveforms can, in fact, define the elastic propertiesof the arterial wall and can estimate the importance and thetransmission speed of reflected waves [6,7].In the last years, augmentation index (AIx) has beenfrequently used in the context of pulse waveform analysisand has been suggested to represent a parameter able toreliablyreflectthelevelofarterialstiffnessintheassessmentof cardiovascular risk. Assessment of AIx provides anindication of the role played by reflected waves in deter-mining pulse pressure. The contribution of the backwardwave to pulse pressure is related to the timing of its super-imposition onto the forward wave, as well as to its magni-tude and shape. AIx is calculated as the ratio between theaugmented central pressure, due to reflected waves, andpulse pressure. Conditions of markedly increased arterialstiffness are characterized by an early superimposition ofbackward waves onto the forward wave, causing anincrease in central SBP. Assessment of AIx has, thus, beenproposedasasimpleapproachtoquantifytheroleofwavereflection in determining an elevation of central bloodpressure values.However, it has to be acknowledged that several otherfactors, apart from the viscoelastic properties of the aortaand of large arteries [8], may affect AIx, in particular: themagnitude and variability of reflected waves, mainly inrelation to systemic vascular resistance; the length of theaorta (related to an individual’s height), because at anygiven level ofarterial distensibility, thenearer the reflectionsites are to the ascending aorta, the shorter is the timeneeded for the reflected wave to reach it; the participant’sheart rate, an increase in heart rate being accompanied byanincreaseinpulsewavevelocity,butalsobyadecreaseinaugmented pressure [3], that is in AIx; the phenomenon ofpressure waves attenuation while travelling along the arte-rial tree; and the role of gender, several studies havingshown significantly higher values of AIx in female than inmale individuals of comparable height (Fig. 1).Thus, given the contribution of several factors to AIxmagnitude, use of this parameter as a specific index of theviscoelastic properties of large arteries is incorrect and maylead to inaccurate estimates. In fact, in clinical practice it isnot uncommon to find a marked discrepancy betweenpulse wave velocity (considered as the gold standardmarker of arterial distensibility [9]) and AIx values, whichmay indeed be due to the multiple factors involved indetermining the latter parameter.The article by Cheng et al. [10], published in this issue ofthe Journal of Hypertension, was aimed at exploringthe potential contribution of cardiac motion and arterialproperties, respectively, to arterial pressure waveformparameters, in particular to AIx.The conclusion of this study is that AIx may not bepredominantly determined by arterial properties, but, onthe contrary, it largely depends on left ventricular systolicfunction. Thus, based on these data, the use of AIx as asurrogate index of arterial stiffness might need to be recon-sidered [10].These conclusions are based on data collected in 20healthy male individuals (median age 22.3 years), withoutcardiovascular risk factors and free of chronic diseases. Ithas to be emphasized that although the selective inclusionin this study of a healthy young male cohort has carried theadvantage of minimizing the possible confounding effects