Response to Determinants of the Ambulatory Arterial Stiffness Index Regression Line
We thank Schillaci and colleagues for their continued interest1–3 in the ambulatory arterial stiffness index (AASI). Schillaci et al1 reported that the inverse association between AASI and nocturnal dipping is stronger for diastolic than for systolic blood pressure. This observation has no repercussion on r2, which is a measure of fit of the regression line. When A (diastolic blood pressure) is regressed on B (systolic blood pressure) or vice versa, estimates of r2 are exactly the same.
With regard to the proposed threshold value of r2 (0.36), Schillaci and colleagues1–3 might have misunderstood the perspectives of our article.4 We made the distinction between risk stratification in individual subjects and clinical outcome research in groups. We proposed to apply the 0.36 threshold for individual risk prediction, because for a patient, prognostic accuracy is a key issue. For clinical and epidemiological research in groups, we stated that it is not good practice to exclude subjects from analysis based on an arbitrary threshold. We suggested that the results from a primary analysis involving all of the available subjects be substantiated in a sensitivity analysis, from which subjects would be excluded with an AASI r2 value <0.36.4 The overall analysis on all of the subjects excludes the introduction of bias. The secondary analysis does not exclude a large group, but only a maximum of 20% of subjects.4
The last argument of Schillaci et al3 refers to the so-called “spurious” association between AASI and the nocturnal fall in diastolic blood pressure. We do not consider this inverse association, which in fact was first reported by Schillaci et al1 and confirmed by us,4 to be spurious. In statistical terms, a spurious association may arise if the same variables are used to define the dependent and explanatory variables in a regression equation. AASI involves the 24-hour systolic and diastolic blood pressures. The nocturnal blood pressure fall is the difference between the daytime and nighttime ambulatory blood pressures.5 The qualifier “spurious,” in our opinion, does not, therefore, apply to the inverse relation of AASI with the nocturnal fall in either systolic or diastolic blood pressure.
Finally, we did not find a consistent association between r2 and the nocturnal fall in systolic or diastolic blood pressure. Figure 14 of our article illustrates 3 subjects with the same value of AASI (0.50). The nocturnal fall in blood pressure, expressed as a percentage of the daytime level, was 10.5% systolic and 13.1% diastolic in the subject with an r2 of 0.23. For the subject with an r2 of 0.45, these percentages were 5.3% and 6.0%, respectively. For the subject with an r2 of 0.86, these values were 14.4% and 19.9%.
Sources of Funding
The study was funded in part by the European Union (grants IC15-CT98-0329-EPOGH, LSHM-CT-2006-037093 InGenious HyperCare, and HEALTH-F4-2007-201550 HyperGenes).
Schillaci G, Parati G, Pirro M, Pucci G, Mannarino MR, Sperandini L, Mannarino E. Ambulatory arterial stiffness index is not a specific marker of reduced arterial compliance. Hypertension. 2007; 49: 986–991.
Schillaci G, Pucci G, Mannarino MR, Pirro M, Parati G. Determinants of the ambulatory arterial stiffness index regression line. Hypertension. 2009; 53: e33.
Adiyaman A, Dechering DG, Boggia J, Li Y, Hansen TW, Kikuya M, Björklund-Bodegård K, Richart T, Thijs L, Torp-Pedersen C, Ohkubo T, Dolan E, Imai Y, Sandoya E, Ibsen H, Wang J, Lind L, O'Brien E, Thien T, Staessen JA, on behalf of the International Database on Ambulatory Blood Pressure in Relation to Cardiovascular Outcome (IDACO) investigators. Determinants of the ambulatory arterial stiffness index in 7604 subjects from 6 populations. Hypertension. 2008; 52: 1038–1044.
Li Y, Wang JG, Dolan E, Gao PJ, Guo HF, Nawrot T, Stanton AV, Zhu DL, O'Brien E, Staessen JA. Ambulatory arterial stiffness index derived from 24-hour ambulatory blood pressure monitoring. Hypertension. 2006; 47: 359–364.