Every cardiac pump has its own maximum performance, which denotes its pumping capability. The difference between the performance in the resting state and that at maximum is called pumping reserve. Cardiac pumping performance is therefore best quantified by its hydraulic power output. Cardiac pumping capability is predictive of the ultimate prognosis of patients in severe heart failure whereas pumping reserve is a major determinant of exercise capacity. The therapeutic efficacy of cardiotonic drugs used in the treatment of ambulant heart failure patients should be evaluated with reference to the way they alter the relation between cardiac pumping reserve and exercise capacity.