Today’s case was a cat under surveillance for hypertrophic cardiomyopathy (HCM). In some animals, the level of hypertrophy and/or the geometry of the mitral valve apparatus can cause obstruction to blood ejection during systole. This is called hypertrophic obstructive cardiomyopathy (HOCUM).
The best way to screen for HOCUM is using Doppler echocardiography. From an apical view, the pulsed wave cursor is placed in the left ventricular outflow tract, about 1cm away from the aortic valve leaflets. With significant obstruction, the Doppler trace will likely alias (wrap around) on the scale, because the velocity will be too high to be measured by pulsed wave Doppler. Continuous wave (CW) Doppler will allow the maximum velocity to be measured accurately.
The waveform will have a late peaking, dagger-shaped profile, as the obstruction – and therefore the velocity of blood flow – increases as systole progresses. By moving the PW Doppler gate back into the ventricle in a step-wise fashion, it is possible to identify the point of obstruction. It may also be possible to spot this using Colour Doppler, on a system with a suitable colour map and sufficiently high colour frame rate.
Above: Late-peaking PW Doppler trace taken from mid-cavity level in the left ventricle, demonstrating an intercavity gradient at this point. The Doppler cursor can be moved up towards the apex, and further down into the LVOT, to identify the point (or points) of obstruction. In this instance, the degree of obstruction is clinically insignificant.
Obstruction in the case of HCM can be intercavity, at chordal level (chordal systolic anterior motion, or chordal SAM), or at mitral valve level (classic SAM of the mitral valve).
In this cat, the anterior mitral valve leaflet does not appear to make contact with the interventricular septum during systole, but chordal SAM cannot be ruled out:
A continuous wave trace through the aortic valve would be invaluable in this patient to rule HOCUM in or out. A more thorough examination could then be performed with PW Doppler in order to pinpoint the cause of obstruction.