Mechanism of systolic anterior motion Systolic anterior motion is the mechanism by which outflow of obstruction usually occurs in patients with HCM. Figure 2 shows this motion as it appears on an echocardiographic apical "5-chamber" view. Certain anatomic features predispose HCM patients to obstruction. In obstructive HCM the mitral valve leaflets are found to be relatively large; increased leaflet area is found in 60% of valves removed at surgery or necropsy (14). Residual portions of leaflets extend past the coaptation point and protrude into the outflow tract (15). Importantly, the mitral valve is situated anteriorly in the left ventricular cavity (16). This is due to several factors including relatively large leaflets, displacement of the papillary muscles anteriorly, a small left ventricle and a bulging septum (17, 18). The anterior displacement puts the mitral valve into the flow stream of left ventricular ejection, subjecting the mitral valve to the hemodynamic force of ejection flow. Figure 2 From the two-dimensional echocardiogram, four frames were identified: initial mitral leaflet coaptation, just before mitral-septal contact, mitral-septal contact and immediately after mitral-septal contact. The protruding mitral leaflet moves in an arc toward the septum until its tip contacts the septum. In the frame after contact, a portion of the body of the leaflet usually comes into apposition as well. Reprinted from Sherrid et al (20) by permission from the Journal of the American College of Cardiology 1993;22:816-25 © 1993 The American College of Cardiology. There is agreement that that systolic anterior motion is caused by the action of left ventricular flow on the protruding mitral valve leaflet (16). However the nature of the hemodynamic force on the leaflet is a subject of ongoing debate (19). Initially, investigators hypothesized that anterior motion is caused by a Venturi mechanism, whereby high velocity flow in the outflow tract lifts the mitral valve towards the septum. More recent data has shown that drag, the pushing force of flow, initiates the anterior motion by pushing the protruding mitral leaflet into the septum (20). Flow drag is the component of force on a body that is in the direction of the flow - examples are the familiar force of rushing water or the wind. Figure 3 presents the difference between lift and drag and introduces the idea that the magnitude of flow drag on a surface is related to the angle between flow and the surface. As a surface moves from a position parallel to flow to a position more angled to flow, drag increases and lift decreases (21). The angle between flow and a surface is referred to as the angle of attack. This figure also reviews flow separation that occurs when a surface is angled more that 15° relative to flow. To evaluate in patients with obstructive HCM which of these two forces - Venturi or drag - cause SAM we reported the analysis of echocardiograms from 24 patients with obstructive HCM (20). Similar to the example in figure 3, the magnitude of flow drag on the protruding leaflet is directly related to the angle between the protruding leaflet and the direction of left ventricular outflow. By measuring the angle the protruding leaflet makes with flow we can quantitatively assess the hydrodynamic effects of flow on the mitral valve. The angle we measured was between "upstream" ejection flow, before the mitral valve and the plane of the protruding leaflet. This angle is shown in figure 4. We measured this angle between flow and the protruding mitral leaflet in three different echocardiographic views and at different moments in systole - at coaptation, just before mitral septal contact, and at mitral contact (figure 2). Figure 3 In this example of an airfoil, lift is the dominant force up to an angle of about 15° relative to air flow. Above 15°, flow separation occurs because air flow cannot follow the contour of the surface because of its momentum. Lift is greatly attenuated because the rapid flow and its local decrease in pressure are no longer in contact with the surface of the leaflet. At angles greater than 15°, drag forces are dominant. Lift is lost and the airfoil stalls (21). Figure 4 Measurement of the angle between the direction of left ventricular Doppler color flow and the protruding portion of the mitral valve in the apical five-chamber view. The direction line of Doppler color flow is indicated by an arrow and the angle is indicated by a [alpha]. IVS = interventricular septum; LA = left atrium; LV = left ventricle; PW = posterior wall. Adapted from Sherrid et al (20) by permission from the Journal of the American College of Cardiology 1993;22:816-25 © 1993 Journal of the American College of Cardiology. We found that the angles were positive and high. For example, in the apical five chamber view, angles at coaptation were a mean of 21°; angles just before septal contact increased to a mean of 45°. Angles at septal contact showed a further increase to a mean of 58°. At these high angles relative to flow, drag, the pushing force of flow is the dominant hydrodynamic force on the leaflet. The dominance of drag at high angles of attack has been shown for a wide range of surfaces over a wide range of Reynolds numbers (21). Ejection flow pushes on the underside of the protruding leaflet. The protruding leaflet is swept by the pushing force of flow into the septum. As the mitral valve moves towards the septum the angle relative to flow increases and constricting drag forces are correspondingly increased as the leaflet presents a greater surface relative to flow (Figure 2). An example of this phenomenon is a widely open door in a drafty corridor. The door starts by moving slowly; it then accelerates as drag increases because the door presents a larger area to flow, until it slams shut. Indeed, systolic anterior motion has been referred to as anterior mitral valve prolapse, an analogy that has merit since in both conditions the mitral valve is often large and is pushed by flow from its normal systolic position. What are the factors that contribute to the positive angle of attack between flow and the leaflet? First, the protruding mitral valve leaflet is anteriorly positioned into ejection flow. In obstructed HCM there is an overlap between the inflow and outflow portions of the left ventricle. This would not occur in the right ventricle because here, there is an infundibulum that separates the inflow and outflow portions. In the left ventricle there is no infundibulum and no separation. (22). What role does the septal bulge play if it is not causing a narrowed outflow tract and a Venturi mechanism? The septal bulge forces ejection flow to sweep from a relatively posterior and lateral direction in the left ventricle. When viewed in the echocardiographic apical 5 chamber view, flow comes from "right field" or "one oÍclock" direction. This contributes to a high angle of attack relative to the protruding leaflet. Finally, the last ingredient necessary for SAM is chordal slack. Without a reduction in chordal tension no SAM would occur since the leaflets would be tethered. Decreased chordal tension is likely caused by anterior positioning of the papillary muscles in the left ventricle (18, 23) and by a progressive disproportion between the small left ventricle and the larger mitral valve apparatus (14). In summary, we believe that the necessary conditions for SAM are: Anterior position of the mitral coaptation point (that positions the mitral valve in the ejection flow stream), positive angle of attack between ejection flow and the protruding leaflet and chordal slack. PREVIOUS PAGE     NEXT PAGE