Instantaneous Blood Flow Velocity Profiles After Aortic Valve Replacement

Abstract

Modern artificial heart valves show good hydrodynamic characteristics when evaluated under in vitro conditions [14, 17, 18, 19, 32]. In vivo, however, after implantation in man — independent of whether they were inserted in a tricuspid, mitral, or aortic position — the hemodynamic properties of heart valve prostheses are less satisfying. Despite sufficiently large geometric orifice areas, important transvalvular pressure gradients were measured, especially during exercise [5, 9, 10]. The “effective orifice area” calculated from the hemodynamic data according to Gorlin’s formula [8] was shown to be only 50%–70% of the geometric value [5, 6, 12, 27]. Aside from the discussion of whether the constant in the Gorlin formula is appropriate for the different types of artificial valves [26], the presence of pressure gradients indicates a hemodynamieally effective stenosis, which cannot be explained on the basis of a small geometric opening area alone. According to the results of in vitro studies in the pulse duplicator system, this energy loss across the valve is most probably caused by eddies, splitting vortices, and turbulence, which originate from the sharp edges of the valve ring, the flat disc, the cage or struts or by heavy distortion of flow by a centrally moving obstacle [1, 3, 14, 17, 28, 33]. This fact is further supported by the clinical observation that in patients with heart valve prostheses even small pressure gradients at rest can increase sharply during exercise despite only modestly elevated transvalvular flows [5, 9].