Effects of sodium nitroprusside upon cardiac work, efficiency, and substrate extraction in severe left ventricular failure

Abstract

In nine patients with severe left ventricular failure caused by coronary artery disease or congestive cardiomyopathy, left ventricular pressure, cardiac output, coronary sinus blood flow, and myocardial substrate extraction were measured before and during infusion of sodium nitroprusside. Infusion of 55 μg/min reduced left ventricular systolic and end-diastolic pressures, peripheral resistance, coronary sinus flow, and myocardial oxygen consumption. Left ventricular minute work increased slightly, but the large improvement in myocardial efficiency was mainly the result of decreased oxygen consumption. Doubling the infusion rate to 110 μg/min caused no further decrease in ventricular pressures despite further reduction of peripheral resistance; as myocardial oxygen consumption did not change, the further improvement in efficiency was due to increased external work, the result of increased stroke volume. Thus, nitroprusside appears to improve efficiency in low dose by reducing load (ventricular pressure and volume decrease) and in high dose by increasing work at constant load. The large reduction in coronary sinus flow during the infusion of 55 μg/min did not provoke angina or lactate production, and thus reflected decreased myocardial oxygen demand and not inadequate perfusion pressure. The improvement in efficiency during nitroprusside infusion was not accompanied by significant changes in arterial concentration, extraction ratio, or oxygen extraction ratio of any of the substrates measured, and substrate consumption fell in proportion to coronary flow. Six patients performed isometric exercise during and after nitroprusside infusion. Off nitroprusside exercise increased heart rate, systolic pressure, and peripheral resistance; end-diastolic pressure rose in five patients. Myocardial oxygen consumption increased proportionately more than work to produce a small but significant decrease in efficiency. Exercise during nitroprusside infusion did not increase heart rate and peripheral resistance significantly, but end-diastolic pressure and myocardial oxygen consumption rose substantially, causing a large reduction in efficiency. Thus, though nitroprusside improves the performance of the failing heart, the gain in efficiency is precarious, and is substantially eroded by the stress of isometric exercise.