The haemodynamics of asymmetric stenoses

Objective: Occlusive arterial disease is usually irregular with both symmetric (concentric) and asymmetric (eccentric) stenoses. Knowledge of the effect of stenosis geometry on stenosis haemodynamics is necessary to correctly interpret tests that measure stenosis severity anatomically. The independent haemodynamic effect of stenosis asymmetry has not been described. Design: In vitro flow-rig measurement of the steady pressure/flow behaviour of rigid, 20 mm long, square-ended symmetric and asymmetric stenoses in a 6.7-mm diameter tube using a blood analogue (40% glycerol). Results: All stenoses, irrespective of geometry, had a linear resistance (R) to flow (Q) relationship such that R = k(1) + k(2)Q where k(1) and k(2) are constants (corr > 0.99, p < 0.05). Asymmetry was found to have a significant haemodynamic effect if the stenosis severity is expressed as a diameter reduction but no effect if stenosis severity is expressed as an area reduction. The maximum flow for an inflow pressure of 90 mmHg (Q(90)) fell from 2050 to 280 ml/min as stenosis area reduction increased from 80% to 96%. Conclusions: Arterial stenoses exhibit flow-dependent resistance irrespective of their geometry. The effect of stenosis asymmetry can only be ignored if anatomical severity is expressed as a percentage area reduction. A clinically useful measure of stenosis severity is the maximum flow for a given inflow pressure.