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- Tissue vs Mechanical Heart Valve
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The enhanced flexural strength of On-X carbon has enabled On-X LTI to incorporate sound fluid dynamic principles into the On-X valve design. The designers successfully maximized orifice area and optimized the shape, length and leaflet configuration. Through this process, the sources of destructive blood turbulence have been minimized.
Length is perhaps the least intuitive hemodynamic principle. Intuition tells us that a very long tube has more flow resistance than a very short one, and this is true. On closer analysis, however, the point of minimum resistance is not the shortest length but a length that is about 70% of the diameter. Shorter than this, an increase in turbulence impedes flow. Longer than this, increased wall resistance impedes flow. The human heart valve reflects this principle, as Swanson and Clark reported in 1974, with a height of about 70% of the diameter. The On-X valve is the only prosthetic valve to approach this ratio, at about 62%.
A straight cylinder tube will experience flow separation at the inlet, creating an effective reduction in tube diameter (the phenomenon known as vena contracta). Jet turbines alleviate this restriction by employing flared air inlets to maximize flow. Similarly, the On-X valve has a flared inlet to reduce turbulence for maximum efficiency.
For mitral valves, only relatively small gains in pressure drop are achieved by increasing the orifice diameter. However, there are significant increases in regurgitant flow, closing velocities and mechanical stresses accompanying increasing the orifice diameter. MCRI engineers analyzed the benefits and detriments of increasing size and made the important decision to limit the orifice diameter to an optimal size. That size for the On-X valves was determined to be 3.752. The result is that the 25mm On-X mitral valve flow dynamics are comparable to the SJM standard 29mm, without increasing stresses and regurgitation. The On-X valve design not only produces optimum orifice area, it prevents the subsequent reduction of that area due to pannus encroachment by positioning the thin carbon walls within the annulus.
A meaningful evaluation of orifice area must include leaflet obstruction within the valve. With the exception of On-X valve, all bileaflet mechanical valves stop short of fully open. This ensures reliable closure on reverse flow, but it also creates additional obstruction to flow. The On-X valve incorporates an innovative solution to minimize this problem. An ingenious sliding hinge design allows the leaflets to open fully to align with the flow. (See valve features for details). This strategy very effectively increases the geometrical orifice area and reduces turbulence. On-X valves can be seen to have a more natural motion in response to shifting pressures rather than being pinned against a stop, fighting the current.
In addressing fundamental hemodynamic issues, On-X valves achieve the lowest pressure gradients among prosthetic valves, particularly in small sizes. The benefits of efficient hemodynamics go beyond lowered pressure gradients to reduced hemolysis and, in combination with other On-X valve features, reduced thrombosis and thromboembolisms.