A controller is provided to determine a ventricular filling phase slope as an indicator of high pulmonary capillary wedge pressure and/or cardiac index. Flow rate values describing a blood flow rate through a ventricular assist device are received. A ventricular filling phase segment is identified from a portion of the received flow rate values. A slope of the received flow rate values during the identified ventricular filling phase segment is determined. The determined slope is compared to a predetermined threshold value. When the determined slope exceeds the predetermined threshold value based on the comparison, a warning is triggered regarding an elevated pulmonary capillary wedge pressure or a low cardiac index value.