New Blood Pressure Index Estimates Arterial Stiffness from Routine 24-Hour Monitoring

Arterial stiffness, the progressive hardening of the body’s large elastic arteries, is one of the most reliable warning signs of future cardiovascular trouble. The gold-standard way to measure it, carotid-femoral pulse wave velocity (cfPWV), involves specialized tonometry equipment and trained staff, which is why it remains largely absent from everyday hypertension care. A new study published in Artery Research proposes a way around that bottleneck: a simple index, called tau-SI, that can be calculated from the ordinary numbers produced by a standard 24-hour ambulatory blood pressure monitor.

In the early twentieth century, Otto Frank described the arterial system as an elastic reservoir: during each heartbeat, the aorta and other large vessels stretch to store part of the ejected blood, then release it during diastole, converting intermittent ventricular pumping into nearly continuous flow through the periphery. This Windkessel behavior is governed by two variables, arterial compliance (C) and peripheral vascular resistance (PVR), whose product defines the arterial time constant, tau. Tau determines how quickly arterial pressure decays during diastole, and its deterioration is a hallmark of vascular aging.

The researchers, Dagnovar Aristizábal-Ocampo, Diego Álvarez-Montoya and Jaime Gallo-Villegas of institutions in Medellín, Colombia, reasoned that if tau captures the integrated behavior of compliance and resistance, an index built from tau might describe arterial function more physiologically than pressure-derived surrogates such as pulse pressure or the Ambulatory Arterial Stiffness Index (AASI). Using the Bramwell-Hill relationship, in which pulse wave velocity is inversely proportional to the square root of compliance, they derived a simplified expression: tau-SI equals the square root of heart rate multiplied by the natural logarithm of the ratio of systolic to diastolic blood pressure. All three inputs come directly from routine ambulatory monitoring.

To give the index clinical meaning, the team first built a normative cohort. From 29,743 consecutive ambulatory recordings in their institutional database, they selected 3,778 untreated normotensive adults who met strict quality criteria, including at least 40 valid measurements and no antihypertensive therapy. This group, 2,444 women and 1,334 men with a mean age of 50.2 years, provided age- and sex-specific reference percentiles for tau-SI. Women showed consistently higher values than men (5.77 versus 5.54 on average), along with higher resting heart rates, and the percentile curves were constructed separately for each sex using quadratic quantile regression.

The biological behavior of the index proved strikingly nonlinear. Median tau-SI values followed a U-shaped trajectory across adulthood, declining from early adult life to roughly the fifth decade and then rising with advancing age, a pattern seen in both sexes. In men, the median ranged from 5.88 at age 20 to 6.11 at age 90; in women, from 5.87 to 6.48 over the same span, with upper reference limits widening progressively, particularly among women. The authors attribute the sex difference partly to geometry: women have smaller aortic dimensions, and vessel size influences wave speed independently of wall degeneration, which is one reason they argue for sex-specific, and eventually body-size-adjusted, reference values.

The crucial validation came from a separate proof-of-concept cohort of 56 participants who underwent simultaneous 24-hour ambulatory monitoring and cfPWV measurement by applanation tonometry with the SphygmoCor system. Measurements were taken between November 2017 and November 2022 at a center in Medellín, 1,495 meters above sea level. In this mixed group of normotensive and hypertensive individuals, tau-SI correlated positively with cfPWV with a coefficient of 0.69 (95% confidence interval, 0.52 to 0.81; P < 0.001), a moderate-to-strong association indicating that higher index values track greater measured stiffness.

Diagnostic performance was assessed against the conventional threshold of arterial stiffness, a cfPWV above 10 meters per second. Tau-SI achieved an area under the receiver operating characteristic curve of 0.79 (95% CI, 0.52 to 0.94), with an optimal cutoff of 6.0 identified by the Youden index. For comparison, pulse pressure yielded an AUC of 0.74 (95% CI, 0.50 to 0.88) with a threshold of 53.3 mmHg. Both tau-SI and cfPWV showed similar age-related patterns in the validation subsample, with progressively higher values at older ages, supporting the index’s plausibility as a marker of vascular aging.

What distinguishes tau-SI from earlier ambulatory indices, the authors argue, is its mechanistic grounding. AASI, derived from the regression slope between systolic and diastolic pressures across a recording, is known to be influenced by blood pressure variability, nocturnal dipping, heart rate and statistical methodology rather than purely by arterial wall properties. Pulse pressure, meanwhile, reflects the combined effects of stroke volume, compliance, wave reflections, vascular geometry and pressure amplification. Tau-SI instead characterizes the dynamic behavior of the arterial reservoir itself, integrating compliance, peripheral resistance and the temporal profile of diastolic pressure decay into a single construct.

The authors also offer a physiological interpretation of what an elevated tau-SI might mean. Because arterial pressure-volume curves are curvilinear, rising distending pressure pushes arteries onto a steeper portion of the curve, reducing compliance through collagen recruitment even without structural remodeling. In early hypertension, increased peripheral resistance may offset pressure-dependent compliance loss, preserving tau; with true vascular aging, elastin fragmentation, collagen accumulation and calcification reduce compliance beyond what resistance changes can compensate, shortening tau and raising tau-SI. On this reading, the index may preferentially flag progressive loss of reservoir function rather than transient vasomotor change, though the authors stress this functional-versus-structural distinction was not directly tested with imaging or invasive data.

The limitations are substantial and the authors state them plainly. The validation sample was small, and external validation in larger, more diverse populations is required. The study was cross-sectional, so tau-SI’s prognostic value for cardiovascular events and longitudinal vascular aging remains unknown. The index derives from peripheral blood pressure measurements and may be influenced by central pressure amplification, stroke volume and body size. Reference values were derived from untreated normotensive adults, and performance in treated hypertension, diabetes, chronic kidney disease, heart failure and older high-risk groups has yet to be evaluated. The team has posted an online calculator for the index, but for now tau-SI stands as a promising, physiologically grounded candidate that complements, rather than replaces, established stiffness measures.

Subject of Research: A physiology-based arterial stiffness index derived from the arterial time constant and estimated from 24-hour ambulatory blood pressure monitoring

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

Article References: Aristizábal-Ocampo, D., Álvarez-Montoya, D., & Gallo-Villegas, J. (2026). A Physiology-Based Ambulatory Arterial Stiffness Index Derived from the Arterial Time Constant. Artery Research, 32(1), Article 42. https://doi.org/10.1007/s44200-026-00135-w

Image Credits: AI Generated

DOI: 10.1007/s44200-026-00135-w

Keywords: arterial stiffness, pulse wave velocity, 24-hour ABPM, arterial time constant, hypertension, vascular aging, arterial compliance, peripheral vascular resistance, pulse pressure, Windkessel model, ROC analysis, reference percentiles