Background: Vascular stiffness increases with age and is accelerated by disease. The arterial system also becomes more amenable to a Windkessel with increased pulse wave velocity and altered vascular structure and function with age. Pressure waveform derived indices of stiffness such as pulse wave velocity do not explicitly account for pressure dependence of stiffness, but depend on statistical means for comparisons. We present a new stiffness index, termed CPI, derived from a modified Windkessel model with pressure-dependent compliance C(P), that requires pressure waveform only and can explicitly account for pressure-dependence to permit isobaric comparisons.

Methods: A modified Windkessel model incorporating pressure-dependent compliance was used to analyze 19 pairs of aortic pressure and flow waveforms experimentally obtained from dogs. Various vasoactive states were induced using pharmacological interventions. Cardiac properties were altered by brief occlusion of LAD coronary artery. CPI was compared against C(P) normalized by stroke volume for each dataset.

Results: CPI evaluated at various pressures (range 49–177 mmHg) presented the same inverse exponential shape as C(P) values (range 0.194–1.31 mL/mmHg) obtained using both aortic pressure and flow waveforms. There was strong positive correlation between CPI and C(P) (r=0.981, p<0.001).

Conclusion: As with C(P), CPI evaluated at corresponding pressure levels in different conditions revealed how compliance was passively affected by changes in pressure and cardiac parameters, rather than a change in arterial wall property. Thus, comparing stiffness between and within subjects when pressure or cardiac properties are altered should be made at common pressure ranges and stroke volumes.