Heart failure develops when the heart muscle is not able to pump enough blood to meet the body's need. "Failure" does not mean that the heart is not pumping. It means that it is not pumping as well as it should.
Bones affected by Paget's disease contain extra blood vessels. When Paget's disease affects a large amount of bone tissue, the heart pumps faster and harder in order to supply blood to this extra bone, as well as to all tissues of the body. If the heart cannot pump enough blood, heart failure develops.
This is a very rare complication of Paget's disease.