Classification of Prosthetic Valve Types and Fluid Dynamics

Mechanical Valves

The three basic types of mechanical valves are ball-cage, tilting-disk, and bileaflet valves (see Fig. 109.1 ). Given that there are major differences in the design of these three types of valves, their flow pattern and hemodynamics differ markedly ( Fig. 109.2 ). Furthermore, all these types of mechanical valves have a normal regurgitant volume that includes a backflow related to the backward motion of the occluder(s) (i.e., the closing volume) and leakage backflow through the components of the prosthesis (leakage volume) ( Fig. 109.3 ). This “built-in” regurgitation theoretically prevents blood stasis and thrombus formation by a washing effect. As opposed to the pathologic regurgitant jets, the normal regurgitant jets are characterized by being short in duration, narrow, and symmetrical.

Ball-Cage Valves

The only ball-cage valve that is still encountered nowadays in patients is the Starr-Edwards 1260, which consists of a silastic ball with a circular sewing ring and a cage formed by three metal arches located at 120-degree intervals around the sewing ring (see Fig. 109.1 ). Antegrade blood flows around the ball, and a large wake is generated in the central part (see Fig. 109.2 ). Large regions of flow separation are responsible for thrombogenicity of the valves and the obstruction to flow caused by the valve results in high pressure gradients. The normal regurgitant flow is composed mainly of the closing volume (2–6 mL per beat) that can be visualized on color-flow Doppler imaging (see Fig. 109.3 ). This type of valve is no longer implanted, but owing to its relatively good durability, several thousands of patients still have ball-cage valves.

Monoleaflet Valves

Monoleaflet or tilting-disk valves use a single circular disk that rotates within a rigid annulus to occlude or open the valve orifice (see Fig. 109.1 ). The disk is secured by lateral or central metal struts. The opening angle of the disk relative to the valve annulus ranges from 60 to 80 degrees, resulting in two orifices of different size (see Fig. 109.3 ). The jet through the major orifice is semicircular in cross-section and flow through the minor orifice consists of one, two, or three jets. Depending on the number of struts, minor orifice jet velocities are 30% to 40% lower than those of major orifice (see Fig. 109.2 ). The nonperpendicular opening angle of the valve occluder tends to slightly increase the resistance to blood flow, particularly in the major orifices.

To minimize flow turbulence, the preferred orientation of monoleaflet valves in the aortic position is with the major orifice toward the right posterior aortic wall and in the mitral position is with the major orifice oriented toward the left ventricular free wall as opposed to the septum. Because of the eccentricity of the jet in the major orifice, it is also crucial to use multiwindow interrogation with continuous-wave (CW) Doppler to get the maximum transvalvular velocity. Normal regurgitant volume is low (5–9 mL per beat) and includes the closing volume as well leakage backflow through small gaps around the perimeter of the valve, and with the Medtronic Hall valves, there is a small amount of regurgitation around the central strut (see Fig. 109.3 ; Videos 109.1, Videos 109.2 ).

Video 109.1. Transthoracic echocardiography long-axis view of a Medtronic-Hall monoleaflet aortic prosthesis with normal valve function.

Video 109.2. Transesophageal echocardiography long-axis view at 121 degrees with color Doppler imaging of a Medtronic-Hall monoleaflet aortic prosthesis with a normal flow pattern.