The effect of increasing preload on ventricular output and ejection in man. Limitations of the Frank-Starling Mechanism.

The effects of preload augmentation on left ventricular output (Frank-Starling mechanism), ejection fraction, and the relationship between pulmonary wedge pressure and end-diastolic volume were studied in 15 patients immediately after myocardial revascularization. The patients were studied under morphine sulfate anesthesia with the chest temporarily closed. Sequential measurements of radionuclide ejection fraction, thermodilution cardiac output, and systemic and pulmonary arterial pressures were made before and after infusion of 500, 1000, 1250 and 1500 ml of whole blood. Cardiac performance was assessed by means of (1) ventricular function curves (stroke volume and stroke work vs end-diastolic volume); (2) sequential changes in ejection fraction and end-systolic volume; (3) the relationship between pulmonary capillary wedge pressure (PCW) and end-diastolic volume; and (4) changes in estimated wall tension. Volume transfusion increased the left ventricular end-diastolic volume index (EDVI) from an average control value of 41 ml/m2 to 88 ml/m2 (after infusion of 1500 ml of whole blood), and increased PCW from 2 mm Hg to 8 mm Hg. Although cardiac index increased (2.1 to 3.6 1/min/m2) and left ventricular stroke work index increased (20 to 38 g-m/m2), the largest increases occurred when EDVI was less than 70 ml/m2 and PCW less than 7 mm Hg. Above these values the ventricular function curves were relatively flat. In contrast, ventricular ejection progressively decreased with increasing preload. Ejection fraction decreased from 0.70 to 0.49 and end-systolic volume index increased from 13 mi/m2 to 45 m]/m2, despite a decreasing systemic vascular resistance. Thus, in these patients: (1) the Frank-Starling mechanism plays little role in augmentation of stroke volume above a PCW of 7 mm Hg or an EDVI of 70 ml/m2; (2) increases in ventricular output achieved by increasing preload were accompanied by decreases in ventricular ejection fraction; and (3) the filling pressure (volume) associated with maximal ejection fraction was significantly lower than that associated with maximal ventricular output.