The maximal extent of disk opening of the Lillehei-Kaster and Bjo¨rk-Shiley aortic disk prosthetic valves was evaluated in vitro in relation to the characteristics of flow across the valve and valve orientation within the test section. The maximal opening angle of the disk of both types of prostheses was related to the mean systolic ejection rate, cardiac output, and maximal instantaneous aortic flow. Full opening of all valves occurred throughout the entire hydrodynamic range tested when the orientation of the valve was such that the disk opened in the direction of gravitational forces. However, when the valve was oriented to open against the direction of gravitational forces, less than full opening was observed at low flow rates. At such low rates of flow, the extent of opening was also dependent upon valve size. These studies indicate that full opening of the Lillehei-Kaster and Bjo¨rk-Shiley disk valves appears to be affected by the orientation of the valve relative to gravitational forces. When less than full opening occurs, it is associated with abnormally low mean systolic ejection rates and low levels of cardiac output and aortic flow. Therefore, irrespective of valve orientation, less than maximal opening with normal flow may indicate a mechanical impediment to opening.

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