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CLINICAL STUDY: HYPERTROPHIC CARDIOMYOPATHY

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

Manuscript received December 8, 1999; revised manuscript received March 20, 2000, accepted May 1, 2000.

Reprint requests and correspondence: Dr. Mark V. Sherrid, Division of Cardiology, 3B-30, 1000 Tenth Avenue, New York, New York 10019
msherrid{at}slrhc.org

OBJECTIVES

The purpose of this study was to determine whether the dynamic cause for mitral systolic anterior motion (SAM) is a Venturi or a flow drag (pushing) mechanism.

BACKGROUND

In obstructive hypertrophic cardiomyopathy (HCM), if SAM were caused by the Venturi mechanism, high flow velocity in the left ventricular outflow tract (LVOT) should be found at the time of SAM onset. However, if the velocity was found to be normal, this would support an alternative mechanism.

METHODS

We studied with echocardiography 25 patients with obstructive HCM who had a mean outflow tract gradient of 82 ± 6 mm Hg. We compared mitral valve M-mode echocardiogram tracings with continuous wave (CW) and pulsed wave (PW) Doppler tracings recorded on the same study. A total of 98 M-mode, 159 CW, and 151 PW Doppler tracings were digitized and analyzed. For each patient we determined the LVOT CW velocity at the time of SAM onset. This was done by first determining the mean time interval from Q-wave to SAM onset from multiple M-mode tracings. Then, CW velocity in the outflow tract was measured at that same time interval following the Q wave.

RESULTS

Systolic anterior motion began mean 71 ± 5 ms after Q-wave onset. Mean CW Doppler velocity in the LVOT at SAM onset was 89 ± 8 cm/s. In 68% of cases SAM began before onset of CW and PW Doppler LV ejection.

CONCLUSIONS

Systolic anterior motion begins at normal LVOT velocity. At SAM onset, though Venturi forces are present in the outflow tract, their magnitude is much smaller than previously assumed; the Venturi mechanism cannot explain SAM. These velocity data, along with shape, orientation and temporal observations in patients, indicate that drag, the pushing force of flow, is the dominant hydrodynamic force that causes SAM.

Abbreviations and Acronyms   CW = continuous wave   HCM = hypertrophic cardiomyopathy   LV = left ventricle, left ventricular   LVOT = left ventricular outflow tract   PW = pulsed wave   SAM = systolic anterior motion

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