A Physiology-Based Ambulatory Arterial Stiffness Index Derived from the Arterial Time Constant

Abstract Background Carotid-femoral pulse wave velocity (cfPWV) is the reference standard for the non-invasive assessment of arterial stiffness but requires dedicated vascular equipment and is not routinely available in clinical practice. Previous studies have shown that arterial stiffness can be estimated from 24-hour ambulatory blood pressure monitoring (24-hour ABPM)-derived parameters, creating the opportunity to incorporate vascular function assessment into routine hypertension diagnosis and risk stratification. Because arterial stiffness reflects the integrated behavior of arterial compliance (C) and peripheral vascular resistance (PVR), biomarkers based on arterial reservoir function may provide a more physiological characterization of large artery status than pressure-derived variables alone. We aimed to develop a novel arterial stiffness index, termed tau-SI, derived from the arterial time constant (τ) and suitable for integration into routine 24-hour ABPM. Methods This cross-sectional study comprised two complementary cohorts. A normative cohort of 3,778 untreated normotensive adults was used to derive age- and sex-specific reference values. An independent proof-of-concept cohort ( n = 56) underwent simultaneous 24-hour ABPM and cfPWV assessment. The proposed tau-SI was calculated from mean 24-hour systolic blood pressure (SBP), diastolic blood pressure (DBP), and heart rate (HR) as √[HR × ln(SBP/DBP)]. Its association with cfPWV and age-related behavior was subsequently evaluated. Results tau-SI demonstrated a significant positive correlation with cfPWV ( r = 0.69; 95% CI, 0.52–0.81; P < 0.001). For identifying arterial stiffness (cfPWV > 10 m/s), tau-SI achieved an area under the ROC curve of 0.79 (95% CI, 0.52–0.94), with an optimal threshold of 6.0. The tau-SI increased progressively with age, reproducing the expected trajectory of vascular aging, and age- and sex-specific reference percentiles were established for clinical interpretation. Conclusions This study introduces a physiology-based ambulatory arterial stiffness index derived from τ and provides proof-of-concept evidence supporting its association with cfPWV. Rather than representing another pressure-derived stiffness metric, tau-SI provides a mechanistically interpretable biomarker of arterial function that integrates C, PVR, and ambulatory blood pressure into a single physiological construct. This approach may facilitate routine assessment of arterial function during 24-hour ABPM and the identification of higher risk phenotypes in hypertension.