In Paget's disease, the extensive bone turnover underlies an increase in blood flow. The increased vascularity in pagetic bone can lead to the development of an AV shunt and this is what leads to the high ouput cardiac failure. If you think about Paget's it's defined by excessive and disorganized bone remodeling, so as a consequence you can end up with small arteriovenous fistulas in the vascularity of the involved bone that eventually amount to an AV shunt.