CONTRIBUTION OF EXTERNAL FORCES TO LEFT-VENTRICULAR DIASTOLIC PRESSURE - IMPLICATIONS FOR THE CLINICAL USE OF THE STARLING LAW

Objective: To test whether a substantial proportion of measured resting left ventricular diastolic pressure stems from forces external to the left ventricle (such as right-heart filling) in normal and chronically diseased hearts. Design: Nonrandomized study with single intervention. Setting: University hospital. Patients: 29 patients referred for cardiac catheterization who had normal left ventricles and ejection fractions (n = 12); chronic heart disease due to idiopathic dilated cardiomyopathy (n = 6); ischemic heart disease(n = 6); or left ventricular hypertrophy (n = 5). Intervention: Acute reduction of external forces imposed on the left ventricle using balloon obstruction of inferior vena caval inflow to the right heart. Measurements: Continuous catheter-derived left ventricular pressure-volume data before and after abrupt obstruction of inferior vena caval inflow. Diastolic pressures were measured at the same volume just before atrial systole before and after sudden decrease of external (right-heart and pericardial) forces. The resulting decline in pressure was a measure of the contribution of these external forces to resting left ventricular diastolic pressure. Results: The decline in pressure when external forces were released averaged -19% +/- 13% with minimal change in left ventricular end-diastolic volume (-3.66% +/- 6.7%) and cardiac output (-5% +/- 8%). In all patients combined, the decline in pressure when external forces (Delta P-d) were released correlated with resting left ventricular diastolic pressure (LVP(d)) given by: Delta P-d = 0.38 x (LVP(d) 6) [r = 0.86, P < 0.0001]. This indicates that when resting diastolic pressure was more than 6 mm Hg, almost 38% of the pressure was due to external factors. This percentage was similar among all subgroups. Furthermore, the left ventricular diastolic pressure could be reduced by this percentage with only minimal compromise to ventricular filling and cardiac output. Conclusions: A substantial proportion of measured resting left ventricular diastolic pressure stems from forces extrinsic to the left ventricle rather than from diastolic stiffness in the left ventricle itself. This markedly influences the dependence of cardiac output on filling pressure and has important implications for clinical application of the Starling law.