ESTIMATION OF LEFT-VENTRICULAR ELASTANCE WITHOUT ALTERING PRELOAD OR AFTERLOAD IN THE CONSCIOUS DOG

Objective: The aim was to determine the slope (E(ES)) of the left vent ricular end systolic pressure-volume line (ESPVL) without altering pre load or afterload in conscious dogs. Methods: Dogs (n=10) were instrum ented to determine left ventricular volume from ultrasonic left ventri cular internal dimensions, and to measure left ventricular pressure us ing a micromanometer. Studies were performed one to two weeks after in strumentation while the animals were conscious. ESPVL was determined f rom variably loaded left ventricular pressure-volume (P-V) loops gener ated by the vena caval occlusion. Contractile state was increased by i ntravenous dobutamine (8 mug.kg-1.min-1) and decreased by intravenous verapamil (10 mg) given after autonomic blockade. From a single normal ly ejecting beat, we calculated E(ES)-single beat (mm Hg-ml-1) as peak isovolumetric pressure (P(max)) minus end systolic pressure divided b y stroke volume. Sunagawa's technique was used to estimate P(max) by f itting the pressure during the isovolumetric contraction and relaxatio n as: P(t)=1/2XP(iso)[1-cos(omegat+c)]+LVEDP, where P(iso)=peak isovol umetric developed pressure, LVEDP=left ventricular end diastolic press ure, c=constant accounting for variations in phase angle, and omega=2 pi/T in which T is duration of contraction. Results: After dobutamine, E(ES) increased, from 8.9(SEM 0.8) to 12.5(l.0) mm Hg.ml-1 (p<0.05), and E(ES)-single beat increased from 9.1(0.9) to 12.0(1.4) mm Hg.ml-1 (p<0.05). Conversely, after verapamil, E(ES) decreased, from 11.1(1.2) to 6.3(l.1) mm Hg-ml-1, (p<0.05), and E(ES)-single beat also decrease d, from 9.6(l.0) to 7.3(l.2) mm Hg.ml-1, (p<0.05). Conclusions: E(ES) calculated from one beat is similar to E(ES) determined from variably loaded left ventricular loops and responds appropriately to inotropic stimulation. This technique provides a reasonable method to calculate E(ES) from left ventricular pressure and stroke volume without alterin g preload or afterload.