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 restin g left ventricular diastolic pressure stems from forces external to th e left ventricle (such as right-heart filling) in normal and chronical ly diseased hearts. Design: Nonrandomized study with single interventi on. Setting: University hospital. Patients: 29 patients referred for c ardiac catheterization who had normal left ventricles and ejection fra ctions (n = 12); chronic heart disease due to idiopathic dilated cardi omyopathy (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 ve na 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 me asured 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 the se external forces to resting left ventricular diastolic pressure. Res ults: The decline in pressure when external forces were released avera ged -19% +/- 13% with minimal change in left ventricular end-diastolic volume (-3.66% +/- 6.7%) and cardiac output (-5% +/- 8%). In all pati ents combined, the decline in pressure when external forces (Delta P-d ) were released correlated with resting left ventricular diastolic pre ssure (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 wi th only minimal compromise to ventricular filling and cardiac output. Conclusions: A substantial proportion of measured resting left ventric ular diastolic pressure stems from forces extrinsic to the left ventri cle 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 th e Starling law.