Heart failure occurs when the heart is unable to maintain a cardiac output sufficient to satisfy the oxygen requirements of the body despite adequate blood volume and hemoglobin content. Regardless of the initial cause of heart failure and in spite of compensatory mechanisms, patients often follow a course of worsening heart failure that is characterized by a low cardiac output, high filling pressures, and increased peripheral vascular resistance. In addition to persistence of the initiating event, cardiac deterioration may be caused or aggravated by a variety of factors including depletion of cardiac norepinephrine stores, down-regulation of myocardial beta-adrenergic receptors, microvascular spasm with resultant further cellular necrosis, and subendocardial ischemia perpetuating myocardial failure
