The elastic modulus of canine aortic valve leaflets in vivo and in vitro.

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.