Comparison of valve resistance with effective orifice area regarding flow dependence

Aortic valve resistance has been proposed to represent the severity of aort ic stenosis because some studies observed that it was less affected by chan ge in flow than the valve-effective orifice area, but this issue remains co ntroversial. The objective of this study was to systematically analyze the theoretical and practical determinants of these parameters in relation to c hanges in flow. Valve area and resistance in different valves were studied in vitro in a pulse duplicator system at different flow rates and in vivo i n 90 subjects referred to either exercise or dobutamine infusion. Theoretic al analysis and experimental results both demonstrated a unique relation be tween resistance (RES), valve-effective orifice area (EOA), and flow rate ( Q): RES = K x (Q/EOA(2)). Accordingly, in fixed stenoses or in mechanical v alves, resistance increased markedly with flow rate both in vitro (+0.88 +/ - 0,26%/% of flow increase) and in vivo (mechanical valves: +2.09 +/- 4.61, fixed stenotic valves: +0.59 +/- 0,32%/%), whereas valve area did not chang e significantly (<0,2%/%). In contrast, in valves with a flexible orifice ( bioprostheses and some patients with aortic stenosis), resistance was less increased due to the increase in valve area. Thus, both from a theoretical and a practical standpoint, valve resistance is much more flow dependent th an valve area, particularly in fixed stenoses. Situations in which resistan ce does not increase with flow rate are unpredictable and are found in flex ible valves when there is a concomitant increase in valve area. (C) 2001 by Excerpta Medico, Inc.