End-systolic pressure-volume, pressure-length, and stress-strain relations in canine hearts

End-systolic pressure-volume relationship (ESPVR) of the in situ heart in the dog was measured during changes in contractile state and was compared with end-systolic pressure-length (ESPLR) or stress-strain relationship (ESSSR). Circumferential segmental length and wall thickness at the equator and external long and short axis diameters of the left ventricle (LV) were determined sonomicrometrically, and LV volume was calculated by an ellipsoidal model. Circumferential wall stress at the equator was calculated by a very thin shell model. Contractile state was enhanced by an intravenous infusion of dobutamine and was suppressed by propranolol. ESPVR, ESPLR, and ESSSR were determined during a reduction of arterial pressure by occluding temporarily the inferior caval vein (IVC). ESPVR, ESPLR, and ESSSR during changes in end-systolic pressure from 108 +/- 3 to 71 +/- 2 mmHg were linear, irrespective of inotropic states (r greater than 0.92). Slopes of these relationships increased similarly in case of dobutamine and were reduced after propranolol, yet the extrapolated X-axis intercept of ESPVR, ESPLR, and ESSSR remained unchanged. Thus the slope of ESPVR is unique to the inotropic state, and both ESPLR and ESSSR are useful as a substitute for ESPVR when there is no regional wall motion abnormality.