Quick Rundown
As fluid moves through a narrowed section of piping, the speed will increase, the pressure will decrease, and a suction is created. This phenomenon is called the Venturi Effect and it is used to bring gasoline into our car engines, clean our swimming pools, and complicate our echocardiographic assessment of hypertrophic cardiomyopathies.
Relevance in Hypertrophic Cardiomyopathy (HCM)
In obstructive hypertrophic cardiomyopathy (HOCM), the interventricular septum is thickened, narrowing the left ventricular outflow tract (LVOT). During systole, this narrowed pathway increases flow velocity, creating a Venturi Effect that can suction the anterior mitral valve leaflet toward the septum. This is known as systolic anterior motion (SAM) of the mitral valve, and contributes to to dynamic LVOT obstruction that we assess with echocardiography.
Using Pulsed Wave Doppler (PWD)
Pulsed wave Doppler is used to measure flow velocities at a specific locations, like the LVOT.
When flow velocity is very high, it can exceed the Nyquist limit, leading to aliasing.
In HOCM, pulsed wave Doppler should be used to march down the left ventricular cavity until we notice the “wrap around” of the waveform as this indicates that the Nyquist limit has been reached.
Aliasing & Nyquist Limit
The Nyquist limit is half the pulse repetition frequency (PRF). It’s the maximum velocity that can be accurately measured with PWD.
When flow exceeds this limit (common in HOCM due to the narrowed outflow tract), aliasing occurs, producing an artifact where the velocity appears to "wrap around" on the spectral display.
This is why continuous wave Doppler is often preferred for investigating high velocity jets—it doesn't alias
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