Here's the dilemma of heart failure. When you have a mild form we tell you to exercise in the hope of building up the heart muscle with the stress. But, when it's really bad we say "take it easy." Because when it's really bad, the only way to get the heart to pump the blood forward, in the times stress, is to dehydrate you to decrease your pressure so the heart has to work less. This dehydration is done with diuretics. But with both sides having problem or with very very severe left side failure....the patients blood pressure is usually so low that we can't dehydrate patients...it will drop their blood pressure even further. During these times we need to force the heart to work harder with IV meds. Of course this is a gross over simplification....the type of stress (eg arrhythmia vs infection vs renal failure) changes the situation also. But, what i said above usually explains most situations.