With HF the LV, right ventricle (RV), or both ventricles fail as pumps. Usually the left side of the heart fails first. In time, the right side fails as a result of the left-sided failure. HF can be classified as systolic HF resulting from ineffective pumping of the ventricles or as diastolic HF resulting from impaired filling of the ventricles. With either classification, cardiac output is decreased. HF can occur acutely, with onset over several hours or days, or it can be chronic, with onset over months to years.