Validity and reliability of diastolic pulse contour analysis (Windkessel model) in humans

The present study assessed (1) the impact of the measurement site (lower versus upper extremity) on the corresponding compliance variables and (2) the overall reliability of diastolic pulse contour (Windkessel-derived) analysis in normal and hypertensive subjects. Arterial tonograms were recorded in the supine position from the radial and posterior tibial arteries in 20 normotensive (116 ±12/68±8 mm Hg) and 27 essential hypertensive subjects (160±16/94±14 mm Hg). Ensemble-averaged data for each subject were fitted to a first-order lumped-parameter model (basic Windkessel) to compute whole-body arterial compliance (C_A) and to a third-order lumped-parameter model (modified Windkessel) to compute proximal compliance (C₁) and distal compliance (C₂). Despite high-fidelity waveforms in each subject, the first-order Windkessel model did not yield interpretable (positive) values for C_A in 50% of normotensives and 41% of hypertensives, whereas the third-order model failed to yield interpretable C₁ or C₂ results in 15% of normotensives and 41% of hypertensives. No between-site correlations were found for the first-order time constant, 2 of the 3 third-order model curve-fitting constants, or C_A, C₁, or C₂ (P>0.50). Mean values for all 3 compliance variables were higher for the leg than the arm (P<0.05 each). We conclude that differences in Windkessel-derived compliance values in the arm and leg invalidate whole-body model assumptions and suggest a strong influence of regional circulatory properties. The validity and utility of Windkessel-derived variables is further diminished by the absence of between-site correlations and the common occurrence of uninterpretable values in hypertensive subjects.