As we all know, nothing is simple when it comes to heart valve surgeries. Cavitation bubbles are a phenomenon in fluid dynamics that are caused by the local acceleration of a fluid, such as blood, over a hinge, leaflet, or other rotational point.
This phenomenon is mainly associated with mechanical valves, and is related to a situation when velocities of the blood increase at a specific location, resulting in an unnatural flow of the blood--kind of a spinning or whirling--that can cause bubbles. If you've ever seen under-water pictures of a boat propellor spinning, with bubbles forming around the water flow, you've seen cavitation.
When these bubbles "pop" or implode, they can cause damage to blood, tissues and even the valve itself.
I imagine that in this case, they want to determine the magnitude of the cavitation, and if serious, they'll want to find out what has changed in the valve's hemodynamics (how the blood flows in the vicinity of the valve) that has resulted in cavitation bubbles being formed.
Best wishes, and hope this helped a little,
--John