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Figure 5 (Left) In the example of a wing, as it moves from a position parallel to flow to a more angled orientation, flow separates from the upper surface, lift decreases, and drag increases (10,11). (Top) Lift is the dominant force up to an angle of approximately 15° relative to air flow. (Middle and bottom) Above 15°, separation occurs and lift is greatly attenuated because the rapid flow and its local decrease in pressure are no longer in contact with the top surface. Lift is lost, drag forces are dominant, and the wing stalls (10,11). The angle between flow and a surface is referred to as the angle of attack. Streamlines are shown with continuous arrows; forces on the wing are shown with larger gray arrows. (Right) Similarly, in HCM the magnitude of flow drag on the protruding mitral leaflet is directly related to the angle between the local direction of LV flow and the protruding leaflet. (Middle) The angle between the direction of local flow and the protruding leaflet of the mitral valve is the angle of attack, . The anterior position of the protruding leaflet contributes to the angle of attack. Also, the midseptal bulge redirects flow so that local flow comes from a relatively posterior direction as it curves around the septum. Color flow Doppler defines local flow direction (6). Measured angle of attack was high at the moment of coaptation, mean 12° in the parasternal view, and mean 21° in the apical five-chamber view. High angles preclude significant Venturi forces and implicate drag. (Bottom) As the leaflet is pushed toward the septum, both the angle and drag forces increase markedly.