An echocardiographic study of the fluid mechanics of obstruction in hypertrophic cardiomyopathy.

OBJECTIVES

The goal of this study was to investigate the hydrodynamic cause of mitral-septal contact and obstruction in patients with hypertrophic cardiomyopathy.

BACKGROUND

Mitral-septal apposition has been shown to be the cause of obstruction in patients with hypertrophic cardiomyopathy. With obstruction, characteristic continuous wave Doppler tracings show an increasing acceleration of flow. (Tracing is concave to the left.)

METHODS

We studied 24 consecutive patients who had a Doppler echocardiographic pressure gradient > or = 36 mm Hg. We pursued two lines of inquiry. 1) Before the onset of obstruction, we systematically measured the angle between the direction of left ventricular Doppler color flow and the protruding mitral leaflet in early systole. 2) After the onset of obstruction, we qualitatively analyzed the concave contour of the continuous wave Doppler tracings in our patients and developed a hydrodynamic theory of the obstruction phase to explain the characteristic tracings. We present a mathematic model to support this concept.

RESULTS

We measured 129 angles. Just before mitral-septal contact, the protruding mitral leaflet projects at a mean 40 degrees and 45 degrees relative to flow in the apical long-axis and apical five-chamber views, respectively. At mitral-septal contact, the obstructing leaflet projects at a mean 52 degrees and 58 degrees relative to flow in the same respective views. Even very early in systole, at leaflet coaptation, 11 of 23 patients had angles > 15 degrees relative to flow. After mitral-septal apposition, obstruction across a cowl-shaped orifice begins. During this stage, the obstructing leaflet projects at a mean 55 degrees and 63 degrees relative to flow. In 22 patients, the continuous wave Doppler tracing of the left ventricular outflow jet showed an increasing acceleration of flow.

CONCLUSIONS

Just before mitral-septal contact, the protruding leaflets project at high angles relative to flow. At these high angles, flow drag, the pushing force of flow, is the dominant hydrodynamic force on the protruding leaflet and appears to be the immediate cause of obstruction. The high angle between flow direction and the protruding leaflet precludes significant Venturi effects. Even earlier in systole, at leaflet coaptation, flow drag is dominant in half of the patients, with angles relative to flow > 15 degrees. After obstruction is triggered, it appears from our data and model that the leaflet is forced against the septum by the pressure difference across the orifice. The increasing acceleration of Doppler flow is explained by a time-dependent amplifying feedback loop in which the rising pressure difference across the orifice leads to a smaller orifice and a higher pressure difference.