Why can polycythemia potentially cause a heart problem?

It is more likely that a heart problem will cause polycythemia than the other way around.
But because polycythemia causes your blood to thicken and slows blood flow, it increases your risk of developing blood clots. If a blood clot occurs in your head, it can cause a stroke. If they reach the coronary arteries that feed the heart muscle, a heart attack will be the result.