Techniques for studying arterial elastic properties.

Techniques for Studying Arterial Elastic Properties To the Editor: I applaud the effort of Rietzchel and colleagues1 to compare two techniques in wide use for studying arterial elastic properties. This is a study long in need of performance because of the controversial claims in the literature relating to the theory of systolic and diastolic waveform analysis. As we had anticipated and predicted,2 a reasonably good correlation was observed between the C2 measured by diastolic pulse wave analysis using a modified Windkessel analysis and late systolic augmentation index assessed using a transfer function previously described. The values from both of these techniques apparently relate, at least in part, to reflected waves from the peripheral small arteries. The augmentation index is also critically dependent on large artery pulse wave velocity that accounts for appearance of the reflected wave at the root of the aorta in late systole. The C2 also may have some dependency on large artery elasticity, but fortunately an independent assessment of this large artery elasticity is also available from the calculation of C1 in the modified Windkessel model. The authors of the study have, however, performed a disservice by their flawed analysis of their data. They claim that the variability of C2 (33.3% by their calculation) is much greater than the variability of augmentation index (6.7%), and they imply that this “wide gap” might make the C2 measurements of less diagnostic precision. But variability of a diagnostic tool can only be evaluated in relation to the range of absolute values to be anticipated. The range of AIx observed in their studies was from about 80% to 180%, a 125% range. The range of C2 was from about 1 to 14 mL/mm Hg 100, a 1300% range. The diagnostic precision for AIx could then be estimated as 125/6.7 or 19, whereas the diagnostic precision of C2 would be 1300/33.3 or 39. Because both of these tests provide a range of values from normal (young) to abnormal (old), a 33% variability in C2 would have even less impact on diagnostic precision than a 6.7% variability in AIx. The authors also demonstrate some bias in their interpretation of Figure 4, which analyzes the differences observed in regression lines describing C2 and AIx. They assume in their analysis that the C2 is inaccurate, whereas it is equally plausible that the inaccuracy is in the AIx calculation. Critical analyses of methodology require the absence of bias and a pristine assessment of data. We regret that the authors’ analysis appears to have fallen short of that standard and thus may have misled users of these instruments.