The design parameters of the natural aortic valve in vivo were not known, which may explain why various bioprosthetic valves have been designed differently. The design of the aortic valve was studied in vivo by placing radiopaque markers in the valve. The marker movement revealed that, during a cardiac cycle, the design parameters of the valve were changing continuously with changing aortic pressure and ventricular geometry. During diastole decreasing radius of the commissures (Rc) and increasing radius of the bases (Rb) caused the leaflets to tilt toward the ventricle, thereby decreasing the bottom surface angle (alpha) and increasing the free-edge angle (phi) of the leaflet. During systole Rc increased, Rb decreased, and interleaflet distance decreased, causing a change in the geometry of the open valve from conical to cylindrical. In middiastole the design parameters were Rb/Rc = 1.2, H/Rc = 1.4, phi = 34 degrees, and alpha = 20 degrees, where H is sinus height. How a significant deviation from the design could compromise the efficiency and longevity of the valve is discussed.

Aortic valve leaflets undergo extraordinary flexion due to the complete reversal of their curvature during billions of cardiac cycles. The flexion stresses in the leaflet depend on its elastic modulus which we investigated in vivo and in vitro. In six dogs, we placed radiopaque markers on an aortic leaflet. Leaflet length was calculated from the marker positions recorded fluoroscopically. Aortic and ventricular pressures were recorded. Dogs were killed and leaflet stress-strain curves determined in vitro. Leaflet length in vivo decreased 10.4 +/- 4.7% from diastole to systole in each cardiac cycle. Using the law of LaPlace, pressure gradients across the leaflets were converted into the stresses in the leaflets. The leaflets had an initial "elastic phase" of low modulus in systole followed by an "inelastic phase" of high modulus in diastole. The elastic modulus was 2.4 +/- 0.7 x 10(6) dynes/cm2 in systole and 5.2 +/- 1.7 x 10(7) dynes/cm2 in diastole. These results were similar to those obtained in vitro. Since flexion rigidity is proportional to (elastic modulus) x (thickness)3, the lower modulus in systole greatly reduces flexion stresses in the leaflet and increases leaflet longevity. The higher elastic modulus in diastole prevents excessive bulging or prolapse of the leaflet while it is subjected to the diastolic pressure gradient. We conclude that a natural or prosthetic leaflet which is thickened or has a high elastic modulus throughout the cardiac cycle will have a greater flexion stress that could cause early failure.

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