Vasodilator Therapy for Heart Failure: The Influence of Impedance on Left Ventricular Performance

T HE ROLE OF THE arterial system in determining the performance of the diseased left ventricle has been relatively neglected in traditional thinking about heart failure. Attention has instead been focused on the influence of diastolic filling of the ventricle and of its contractile state. Therapy directed at altering these latter factors in cardiac performance has served as the cornerstone of the management of congestive heart failure. However, recent studies have demonstrated dramatic hemodynamic improvement during adm?nistration of vasodilator drugs in patients with left ventricular failure due to acute myocardial infarction, chronic ischemic heart disease, and cardiomyopathy.14 An understanding of this response requires consideration of the concept of impedance to left ventricular outflow. During ventricular systole the rise in aortic pressure is related to the stroke volume, its rate of ejection, and the impedance the blood faces as it crosses the aortic valve. Changes in total impedance during ejection are due to alterations in either the compliant component, which opposes a change in volume of the arterial vascular bed, or the resistive component, which opposes runoff of blood from the arterial tree and is determined primarily by the cross-sectional area of the arterioles and the viscosity of blood. If impedance increases because of arteriolar vasoconstriction or reduced arterial compliance, then aortic systolic pressure will rise more rapidly with any given ejection rate, and mean left ventricular systolic pressure will be increased for any given stroke volume. During ejection the rise in left ventricular pressure is accompanied by a reduction in left ventricular chamber size so that wall tension (pressure X radi-