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Systolic anterior motion begins at low left ventricular outflow tract velocity in obstructive hypertrophic cardiomyopathy

^a HCM Program, Division of Cardiology, St. Luke’s–Roosevelt Hospital Center, Columbia University College of Physicians and Surgeons, New York, New York, USA

View larger version (34K): [in a new window]   Figure 1 (Left) Continuous wave (CW) Doppler tracings were recorded in the five-chamber view through the LVOT as shown by the dashed line. Pulsed wave (PW) Doppler tracings were recorded in the LV at a point 2.5 cm apical of the mitral coaptation point and 1 cm from the septum, in LV outflow, as marked with the X. This location is the AMV point, apical of the mitral valve. (Right) The time interval Q-SAM is the interval from Q-wave onset to SAM onset. The velocity V-SAM is the CW velocity in the outflow tract at the time interval Q-SAM. Ar = atrial reflected wave.

View larger version (105K): [in a new window]   Figure 2 M-mode (top), continuous wave (middle), and pulsed wave Doppler (bottom) tracings of two patients. Patient 3 is on the left and patient 14 is on the right. In both patients, the left dashed line indicates time of Q-wave onset. The right solid line indicates the time of SAM onset. In both patients SAM onset begins at a time of low velocity both in the outflow tract and in the LV. In both patients SAM begins before the onset of ejection, during the Ar wave (16–19). Both M-mode and Doppler tracings were recorded on the same study, within a 5-min interval, and were selected for matched RR intervals. Vertical calibrations of CW and PW tracings are at 1-m/s and 20-cm/s intervals, respectively.

View larger version (113K): [in a new window]   Figure 3 Four patients with obstructive HCM. In all cases the M-mode echocardiograms showing SAM onset are on top and the Doppler tracings are below. For the two patients on the left, the M-mode is compared with the CW Doppler tracings in the LVOT. For the two patients on the right, the M-mode is compared with the PW Doppler tracings in the LV at the AMV point. In all, the left dashed line indicates Q-wave onset. The right solid line indicates SAM onset. In all four patients SAM onset occurs at a time of low velocity, before the beginning of ejection. In the two patients on the right, SAM onset begins during the Ar wave. Both M-mode and Doppler tracings were recorded on the same study, within a 5-min interval, and were selected for matched RR intervals. Vertical calibrations of CW and PW Doppler tracings are at 1-m/s and 20-cm/s intervals, respectively.

View larger version (33K): [in a new window]   Figure 4 The pushing force of flow. Intraventricular flow relative to the mitral valve in the apical five-chamber view. In obstructive HCM the mitral leaflet coaptation point is closer to the septum than normal (1). The protruding leaflets extend into the edge of the flowstream and are swept by the pushing force of flow toward the septum. Flow pushes the underside of the leaflets (arrow) (6–9). Note that the midseptal bulge redirects flow so that it comes from a relatively lateral and posterior direction; on the five-chamber view, flow comes from "right field" or "one o’clock" direction. This contributes to the high angle of attack relative to the protruding leaflets. Also note that the posterior mitral leaflet is shielded and separated from outflow tract flow by the cowl of the anterior leaflet. Venturi flow in the outflow tract cannot be lifting the posterior leaflet because there is little or no area of this leaflet exposed to outflow tract flow. Venturi forces cannot be causing the anterior motion of the posterior leaflet.

View larger version (27K): [in a new window]   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.

View larger version (95K): [in a new window]   Figure 6 Flow strikes the undersurface and lateral aspect of the mitral valve very early in systole, causing SAM. Two echocardiographic views of one patient with obstructive HCM are shown; resting gradient = 54 mm Hg. (Left) Apical five-chamber view. (Top left) The 2-D image shows the protruding mitral leaflet on the first frame in systole that showed mitral coaptation. Arrowhead points to mitral valve. O indicates outflow tract. (Bottom left) The same view showing the first systolic frame with color flow. Color flow is seen lateral to the leaflet tips (arrow). These images show the event graphically drawn in Figure 4. (Right) Apical three-chamber view. (Top right) The 2-D image again shows the protruding mitral leaflet on the first frame in systole that showed mitral coaptation. Arrowhead points to mitral valve. (Bottom right) This shows the same view of the first systolic frame with color flow. Color flow is seen posterior to the leaflet tip (arrow). Note that color flow velocity is low on both views. On 2-D imaging, the next systolic frames showed fully developed SAM on both views. On color flow, the next systolic frames showed aliasing in the outflow tract. The mitral leaflets are medially and anteriorly positioned into the edge of the flow stream. Low velocity flow strikes the undersurface of valve leaflets; they are swept toward the septum by the pushing force of flow.

View larger version (26K): [in a new window]   Figure 7 Evidence in the debate between Venturi (lift) and drag (pushing) force as the dynamic cause for SAM. References are enumerated in the text. ASH = asymmetric septal hypertrophy.