Spotlights on ambulatory measures of arterial stiffness.

To the Editor: Arterial stiffness is an independent predictor of cardiovascular morbidity and mortality. 1 It should come as no surprise that researchers are currently looking for measures of arterial stiffness that are easy to record even under ambulatory conditions. In this Journal, Gosse and colleagues 2 recently compared three indexes, derived from individual ambulatory blood pressure (BP) recordings, namely, the QRS-Korotkoff delay index standardized to a systolic BP of 100 mm Hg and a heart rate of 60 beats per min (QKD 100–60), 2 the ambulatory arterial stiffness index (AASI), 3 and the slope of pulse pressure on mean BP (PP/MBP). Gosse and colleagues confirmed that AASI provides an indirect estimate of arterial stiffness, but they also proposed that QKD 100–60 might be a better index of arterial stiffness than AASI. To date, several cross-sectional studies 4 and three prospective cohort studies 5–7 demonstrated association of AASI either with signs of target organ damage in never-treated hypertensive patients, 4 or with the incidence of cardio-vascular mortality and morbidity. 5–7 AASI is particularly predictive of stroke, 5–7 even at levels of BP within the normotensive range. 5,6 When adjusted for PP 5–7 or aortic pulse wave velocity, 8 AASI retained its pre-dictive value, in particular for stroke. 8 We believe that over-interpretation of weak results and methodological issues go against Gosse's proposition that the QKD 100–60 outperforms AASI in the prediction of cardiovascular events. 2 The first argument of the French investigators in favor of the QKD 100–60 was the higher repro-ducibility of this index. 2 In 38 volunteers undergoing repeat ambulatory recordings within 2 weeks, the coefficient of variability was 4% for QKD 100–60 , 25% for AASI, and 61% for PP/MBP. 2 The authors did not present any test statistics or associated P-values for the comparison of the coefficients of variability between the three ambulatory indexes of arterial stiffness. Moreover, the coefficient of variability is the s.d. of the differences between paired measurements (SDD) divided by the mean of all measurements. It does not account for the possible range of biological variation in the ambulatory measures of arterial stiffness. We, therefore, proposed to express the SDD as a percentage of near maximal variation in a measurement as given by four times the s.d. 9 Had reproducibility been expressed in this way with the application of a proper test statistic , the conclusion of Gosse's reproducibil-ity study 2 might …