Load independence of radionuclide diastolic filling measurements in acute coronary occlusion

An accurate noninvasive method for measuring the effects of pharmacologic agents on active relaxation of the left ventricle would provide a valuable tool for monitoring the treatment of diastolic heart failure related to coronary artery disease.The time constant of isovolumic relaxation (T) and the left atrioventricular gradient were measured with micromanometer catheters and diastolic left ventricular filling variables were measured with radionuclide ventriculography in nine anesthetized, open-chest dogs with an acute coronary artery occlusion. Infusion of the positive inotropic drug dobutamine hydrochloride (5 to 10 micrograms/kg/min) resulted in a 35% shortening of T and a 37% increase in the radionuclide first-half filling fraction (both p0.05), but no change in the left atrioventricular gradient. Conversely, infusion of the alpha-adrenergic vasoconstrictor phenylephrine hydrochloride (20 to 40 micrograms/min) augmented left ventricular load, increasing the atrioventricular gradient by 45%, but had no significant effect on T or the first-half filling fraction. Infusion of the direct-acting vasodilator, sodium nitroprusside (80 to 120 micrograms/min), was accompanied by a 14% lengthening of T with a corresponding 28% decrease in the first-half filling fraction (both p0.05).In an anesthetized, open-chest canine model of acute myocardial ischemia, the radionuclide first-half filling fraction reflects pharmacologically induced changes in the lusitropic state of the left ventricle with relative independence of loading conditions.