Hypertropic Obstructive Cardiomyopathy (HOCM)
©Walter Rasmussen, R.D.C.S.
Above: Apical four chamber view: The interventricular septum is hypertrophic and the anterior mitral valve leaflet encroaches upon the LVOT, causing restriction of blood flow during systole.
Hypertrophy of the basal interventricular septum and systolic anterior motion of the anterior mitral valve leaflet cause a narrowing of the left ventricular outflow tract, resulting in restriction of blood flow in the left ventricular outflow tract during systole. Patients are routinely referred to echocardiography for HOCM diagnosis and for serial follow-up studies. There are many technical difficulties and pitfalls that sonographers encounter while trying to arrive at the proper diagnosis.
In hypertrophic cardiomyopathy, color Doppler of the LVOT from the apex will display bright, aliased colors (mixing of blues, reds and yellows) between the anterior mitral valve leaflet and inter-ventricular septum.
Placement of the PW Doppler sample volume within the LVOT will confirm the higher than normal velocity and the point of obstruction. Significant obstruction typically causes a gradient over 30 mmHG, which usually cannot be completely recorded within the spectral Doppler graph in PW Doppler without shifting the baseline and using high pulse repetition frequency (High PRF) Doppler.
Whenever high PRF Doppler is used, several sites along the ultrasound beam are being sampled simultaneously, thus introducing uncertainty of where the high velocity blood flow is actually located.
Because the mitral valve is in close proximity to the cursor position and because one of the three or four high PRF sample volume sites typically falls within the left atrium, mitral regurgitation, which is almost always present in HOCM, can be and often is mistaken for the velocity profile of LVOT obstruction.
The primary means of diagnosis of significant outflow of obstruction is based-upon the unique, “dagger shaped” velocity profile and the peak velocity, combined with systolic anterior motion of the mitral valve and hypertrophy of the basal septum.
Because the diagnosis of LVOT obstruction is a serious condition which can result in further testing and medical and surgical treatment, accurate assessment with echocardiography is of great importance. Most LVOT gradients associated with hypertrophic cardiomyopathy are within the range of 30 to 60 mmHg at rest however; any gradient over 100 mmHg should be suspected to be originating from mitral regurgitation, unless proven otherwise.
The following steps can be used to help assure that LVOT obstruction is not confused with mitral regurgitation.
- 2-D Echo: Observe and record the LVOT for narrowing, septal hypertrophy and systolic anterior motion of the mitral valve apparatus.
- Color Doppler: Record color Doppler over the LVOT, observing for brightness, aliasing and turbulence.
- Assess for the presence of mitral regurgitation and its location.
- CW Doppler: Record spectral Doppler of mitral regurgitation, obtaining the best possible velocity profile. Note the contour of the recording and its peak velocity. It is often the case that the obstructed LVOT velocity or a mid-ventricular velocity profile is seen buried within the mitral regurgitation profile at lower velocities.
- PWD: Interrogate from the mid-ventricle to the aortic valve for obstruction. If an obstruction is located, move the baseline to the top of the graph and switch to high PRF Doppler if necessary, to record the peak velocity. This velocity should correlate with the CW Doppler peak velocity however if it exceeds the Nyquist limit of the PW Doppler graph it will not be fully recorded. Above: PW Doppler from the Mid VentricleAbove: PW Doppler of LVOT blood flow, low PRFAbove: PW Doppler LVOT blood flow at high PRF, demonstrating peak LVOT velocity
- CW Doppler: Record Spectral Doppler of the LVOT. Avoid placing the cursor in proximity to mitral regurgitation. Observe the contour of the outflow tract velocity profile and measure the peak velocity. If significant LVOT obstruction is present, it is usually easily detected with CW Doppler, producing a strong, sharp, late-peaking signal as illustrated aboveAbove: LVOT using CW Doppler, Demonstrating LVOT velocity buried within the MR Doppler.
- An effective maneuver to help discriminate LVOT obstruction from mitral regurgitation is to start recording the CW Doppler MR contour at a chart speed of 25mm/sec and then sweep to the LOVT, demonstrating its contour. Freeze the image when the sweep is finished. This will demonstrate both velocity contours side by side, and can be labeled according to where they are located. Above: CW Doppler sweep from LVOT to Mitral Valve
- Left ventricular mid-cavitary obstruction mimics LVOT obstruction, but occurs instead at the mid-ventricular level. In this condition, the left ventricle appears hypertrophic and the left ventricular chamber will be small at end-systole or may have an hourglass appearance. Color Doppler demonstrates aliasing within the mid-ventricle. CW and PW Doppler display the typically late-peaking velocity profile but usually at a lower velocity than seen with moderate to severe HOCM.
- If the obstruction is not present or is of low velocity in patients and the anatomy is typical of HOCM, CW Doppler of the LVOT can be performed during and after a Valsalva maneuver. Refer to workplace protocols which vary in technique for details related to evoking obstruction.