The heart is a muscular mechanical pump with an ability to generate both flow (cardiac output) and pressure. The product of flow output and systemic arterial pressure is the rate of useful work done, or power output. Cardiac pumping capability can be defined as the cardiac power output (CPO(max)) achieved by the heart during maximal stimulation. Cardiac reserve is the increase in power output as the cardiac performance is increased from the resting to the maximally stimulated state. Recently, several studies have shown that CPO(max), a direct indicator of overall cardiac function, measured directly or as an approximation, is a major determinant of exercise capacity and a most powerful predictor of prognosis for patients with chronic heart failure. The same holds true in acute heart failure, where it is also useful in subclassifying the conditions to select the appropriate treatment.