I assume you will be using ultrasound for this.
Your received frequency will swing either side of the transmitted frequency as the direction of heart motion changes.
The problem will be that the heart is not a simple single reflective moving plane, but moves in three dimensions, and all four heart chambers (and respective valves) are doing very different and complex motions together at any instant in time.
So I would expect the overall reflected wave will be a confused jumble of many frequency components, some will be additive, and others cancel.
I am sure you could define overall heart rate from that mess, but I doubt if you could resolve much more detail than that.