Isotonic exercise testing imposes a physiological stress on the cardiopulmonary unit. Accordingly, monitoring of oxygen, carbon dioxide and air flow during an exercise test (i.e. a cardiopulmonary exercise test) can be used to assess heart function in patients with chronic heart failure. Specifically, an incremental treadmill cardiopulmonary exercise test represents a non-invasive means to determine aerobic capacity, or maximal oxygen uptake (VO2max ml min-1 kg-1), and anaerobic threshold (AT, ml min-1 kg-1). These objective measures of cardiopulmonary function are then used to grade the severity of failure and the functional capacity of the patient. In addition, they may be used to predict the cardiac reserve, or maximal cardiac index (CImax, l min-1 m-2) during exercise. That is, the severity is considered to be mild (class A) when AT greater than 14 or VO2max greater than 20, mild to moderate (class B) when AT falls between 11 and 14 or VO2max between 16 and 20, moderate to severe (class C) when AT ranges between 8 and 11 or VO2max between 10 and 16, and severe (class D) when AT less than 8 or VO2max less than 10. The predicted CImax for classes A, B, C and D are greater than 8, 6-8, 4-6 and less than 4, respectively. Finally, a major objective of medical therapy in patients with heart failure is to improve cardiac output and oxygen delivery to working skeletal muscle and thereby enhance effort tolerance. This therapeutic endpoint can be gauged by cardiopulmonary exercise testing from the response in AT and VO2max.