Thermodilution studies of ventricular volume changes due to isoproterenol and bleeding

The effects of intravenous isoproterenol on left ventricular stroke and on end-systolic and end-diastolic volumes were studied in anesthetized dogs before and after bleeding. Volumes were measured by a thermodilution technique. In this indicator-dilution method a small amount of cooled blood is rapidly injected into the ventricle, and the washout of cold from the ventricle is sensed by a thermistor catheter at the root of the aorta. Control values showed that approximately two-thirds of the end-diastolic volume remained in the ventricle at the end of systole. Bleeding decreased all three volumes. Isoproterenol consistently increased ventricular emptying, as shown by the fall in the proportion of the end-diastolic volume which remained at end systole. This effect did not depend on an increase or decrease in the end-diastolic volume itself. End-systolic force-circumference relationships were derived from a consideration of idealized ventricular dimensions. A linear relationship between these calculated values was not altered by isoproterenol.