Shortness of breath with activity is the most common symptom of heart failure. In many cases of heart failure the left ventricle of the heart, which pumps blood to the body, is rendered unable to properly eject its contents. As the left ventricle's abilities are diminished, blood tends to pool and back-up behind it in the pulmonary (lung) circulation. This back-up is under increased pressure, as the right side of the heart is still pumping into this circulation. The result is fluid leakage into the lungs, a condition called pulmonary edema. Fluid retention in the body by the kidneys can also be a culprit in developing pulmonary edema. This situation translates into shortness of breath. The diagram below illustrates just how apparent this problem can develop in some patients:

Individual genetic and other factors may influence how likely this is to occur at any given pressure backed up behind the left ventricle. Pulmonary edema can prevent the individual from breathing properly and comfortably. Furthermore, it can strongly impair one's ability to exercise and lead an active life. A primary way to improve this is the use of diuretics. Diuretics are a family of drugs that reduce the kidney's ability to retain the body's salt and water in favor of excreting it in urine. Shedding body salt and water allows for the reduction of symptoms associated with pulmonary edema as well as reducing the strain of volume overload on the heart.