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Hemodynamic and biomechanical properties of stentless

Jonas Amstrup Funder

Summary 

The only definitive treatment of aortic valve stenosis is valve replacement therapy. Although implantation of a new heart valve clearly reduces mortality and morbidity, no artificial heart valve is perfect. The challenge is therefore to select the best available heart valve prosthesis for the individual patient. Biological valve prostheses have several hemodynamic advantages, but the durability is limited due to calcification. Calcification can be caused by mechanical wear of the heart valve leaflets due to shear stress or from the heart valve stent restricting leaflet movements. This was sought prevented when introducing biological heart valves without stents. Excluding the stent would minimise obstruction of the blood flow as well as allow for a more physiologic leaflet movement. This should result in less calcification and thereby increased durability. 

However, there is still a lack of detailed knowledge of the stentless heart valves. Whether the stentless valves actually preserve aortic root distensibility after implantation remains to be proven. Furthermore, we need specific information about blood flow through the valve in order to evaluate the valves in detail.

 In substudy I we investigated to types of stentless valves (Solo, Toronto SPV) and compared them with a stented valve (Mitroflow) and with native valves in pigs. Using cardiac magnetic resonance imaging (MRI) we were able to display blood velocities and velocity profiles. We found that all three artificial valves had a higher mean velocity gradient and peak velocity than the native valves. No statistically significant difference was found between the artificial valves. All valves showed maximum velocity gradients in the circumference of the aorta, but the Toronto valve in particular also had high gradients in the vessel center. The Solo valve - and especially the Mitroflow valve - displayed velocity profiles most like the native valves, whereas the Toronto valve had a more irregular asymmetric velocity profile. In this study we did not find the stentless advantageous compared with the stented design. 

In substudy II we investigated the same valves as in substudy A. We performed MRI scans showing the opening area as well as the distensibility of the aortic root. We found the opening area of the stentless Solo valve significantly larger than the other artificial valves. We furthermore found that the stented Mitroflow valve completely fixed the aortic annulus compared with stentless and native valves and thereby facilitating a physiologic leaflet movement. 

In substudy III we again investigated aortic root distensibility but using a method that could map movements more detailed and precise (sonomicrometry). Twelve sonomicrometry crystals were fixed at the exterior of the aortic root. These crystals transmit and receive high frequency ultrasound and thereby tracking their individual location. Again we found that the stented Mitroflow valve fixed the aortic annulus compared with stentless and native valves. We, furthermore, displayed that aortic root expansion commenced later for the Toronto SPV above the implantation site (sino-tubular junction). A delayed expansion indicates a more restrictive outflow tract requiring additional force to expand. 

In substudy IV we investigated Reynolds normal stress (RNS) as a measure of turbulence using a perivascular Doppler technique. We did not find a difference between stentless and stented heart valves in terms of lower maximum or mean RNS values. All valve prosthesis had higher mean RNS values than native porcine valves. We, furthermore, found that the amount of turbulence increases with increasing blood pressure. 

We, therefore, conclude that we did not find blood velocity or turbulence profiles of stentless valves advantageous compared with stented valves. Especially the Toronto SPV did display a suboptimal velocity profile. We also found that stented valves fixed the aortic root whereas stentless valves preserved the distensibility, probably resulting in an improved stress distribution.