Normal aortic valve function in dogs

Abstract

Previous trileaflet aortic prostheses failed from fatigue and flexion stresses because their design was not based on the physiology of the normal valve. The dynamics of normal aortic valves were studied in terms of movement of aortic valve commissures and leaflets in vivo. Eight dogs were placed on total cardiopulmonary bypass; through an aortotomy, radiopaque platinum markers were placed on the commissures at the level of leaflet coaptation and at the center of the leaflets' free edge. Three dogs were studied immediately and five dogs after 10 or more days. Marker movement in the beating heart was recorded under fluoroscopy on videotape. The aortic root expands 12 ± 0.4 percent (mean ± standard error of the mean) during systole at a blood pressure of 120 80 mm Hg; it expands 8.5 to 24 percent at blood pressures ranging from 50 30 to 240 180 mm Hg. The relation between aortic root diameter and blood pressure is similar to a classic tension-radius relation of the aorta. The aortic root diameter is controlled by blood pressure and by the pressure gradient across the valve. The functional elastic modulus of the aortic root is 4.4 × 10 5dynes/cm 2. Expansion of the aortic root begins 20 to 40 msec before the valve opens. The velocity of leaflet opening is 97 cm/sec. The orifice of the opened valve is circular and maximal at the beginning, gradually decreasing to the point when the valve snaps shut. Commissural expansion gives a smooth opening, less shear stress on the leaflet surface, less flexion stress at the center of the leaflet and less fatigue strain on the leaflet, thereby maximizing the life and efficiency of the aortic valve. If these characteristics were incorporated into aortic prostheses, prosthetic failure due to fatigue and flexion stresses should be reduced.