Arterial stiffness index beta and cardio-ankle vascular index inherently depend on blood pressure but can be readily corrected

Objectives: Arterial stiffness index β and cardio-ankle vascular index (CAVI) are widely accepted to quantify the intrinsic exponent (β₀) of the blood pressure (BP)-diameter relationship. CAVI and b assume an exponential relationship between pressure (P) and diameter (d). We aim to demonstrate that, under this assumption, b and CAVI as currently implemented are inherently BP-dependent and to provide corrected, BP-independent forms of CAVI and β. Methods and results: In P = P_refe^β₀^[(d/d_ref^)-1], usually reference pressure (Pref) and reference diameter (dref) are substituted with DBP and diastolic diameter to accommodate measurements. Consequently, the resulting exponent is not equal to the pressure-independent β₀. CAVI does not only suffer from this 'reference pressure' effect, but also from the linear approximation of dP=dd. For example, assuming β₀ = 7, an increase of SBP/DBP from 110/70 to 170/120 mmHg increased β by 8.1% and CAVI by 14.3%. We derived corrected forms of β and of CAVI (CAVI0) that indeed did not change with BP and represent the pressure-independent β₀. To substantiate the BP effect on CAVI in a typical follow-up study, we realistically simulated patients (n = 161) before and following BP-lowering 'treatment' (assuming no follow-up change in intrinsic β₀ and therefore in actual P-d relationship). Lowering BP from 160 ± 14/111 ± 11 to 120 ± 15/79 ± 11 mmHg (p < 0.001) resulted in a significant CAVI decrease (from 8.1 ± 2.0 to 7.7 ± 2.1, p = 0.008); CAVI₀ did not change (9.8 ± 2.4 and 9.9 ± 2.6, p = 0.499). Conclusion: β and CAVI as currently implemented are inherently BP-dependent, potentially leading to erroneous conclusions in arterial stiffness trials. BP-independent forms are presented to readily overcome this problem.