This occurs when the muscles that perform contraction and force the blood out of heart are performing weakly. Thus cardiac failures primarily arise from the reduced contractility of heart muscles, especially the ventricles. Reduced contraction of heart leads to reduced heart output but new blood keeps coming in resulting in the increase in heart blood volume. The heart feels congested. Hence the term congestive heart failure.
Congested heart leads to lowered blood pressure and poor renal blood flow.
There are several mechanisms by which this could be achieved. Cardiac steroids are perhaps the most useful and are being discussed here. Phosphodiesterase inhibitors, such as amrinone and milrinone, have also been explored and so are direct adenylate cyclase stimulants, such as forskolin. The cardiac glycosides are an important class of naturally occurring drugs whose actions include both beneficial and toxic effects on the heart. Throughout history these plants or their extracts have been variously used as arrow poisons, emetics, diuretics, and heart tonics.