Fluid mechanics of artificial heart valves

Abstract

1. Artificial heart valves have been in use for over five decades to replace diseased heart valves. Since the first heart valve replacement performed with a caged-ball valve, more than 50 valve designs have been developed, differing principally in valve geometry, number of leaflets and material. To date, all artificial heart valves are plagued with complications associated with haemolysis, coagulation for mechanical heart valves and leaflet tearing for tissue-based valve prosthesis. For mechanical heart valves, these complications are believed to be associated with non-physiological blood flow patterns. 2. In the present review, we provide a bird's-eye view of fluid mechanics for the major artificial heart valve types and highlight how the engineering approach has shaped this rapidly diversifying area of research. 3. Mechanical heart valve designs have evolved significantly, with the most recent designs providing relatively superior haemodynamics with very low aerodynamic resistance. However, high shearing of blood cells and platelets still pose significant design challenges and patients must undergo life-long anticoagulation therapy. Bioprosthetic or tissue valves do not require anticoagulants due to their distinct similarity to the native valve geometry and haemodynamics, but many of these valves fail structurally within the first 10-15 years of implantation. 4. These shortcomings have directed present and future research in three main directions in attempts to design superior artificial valves: (i) engineering living tissue heart valves; (ii) development of advanced computational tools; and (iii) blood experiments to establish the link between flow and blood damage.

Fig. 1

Towards the ideal prosthetic heart valve. Timeline of significant milestones in the history of prosthetic heart valve development. (*Image from Ramstack et al.; all other valve images are obtained from manufacturers’ websites: http://www.edwards.com, http://www.medtronic.com and http://www.sjm.com, www.carbomedics.com).

Fig. 2

Schematic of a bileaflet mechanical heart valve implanted in the aortic position during the leakage flow phase. Shown are the blood cells damaged from the high shear environment experienced within the leakage gaps (not to scale). Top panel: forward flow phase; bottom panel: leakage flow phase. Grey arrows, leakage flow; red arrows, bulk flow direction; Ao, aorta; LV, left ventricle.

Fig. 3

Flow fields downstream of selected valve designs during the forward flow phase (left) and the leakage flow phase (right).

Fig. 4

Complexity of the flow fields in four recessed hinge designs.

Fig. 5

Schematic of a computational coupling scheme between a large-scale solver and a small-scale solver implemented at the hinge and leaflet surfaces.

Fig. 6

Schematic of (a) blood cell paths inside the hinge recess computed from coupled computational approaches and (b) stress environment experienced by a cell along its trajectory (left) and residence time of the cells within the hinge recess (right).