DIFFERENT RESPONSES OF EXTENT AND VELOCITY OF CONTRACTION TO DOBUTAMINE IN CONSCIOUS SHEEP

The shortening- and shortening rate-preload-afterload relations, based on the concept of the myocardial end-systolic stress-strain relation (ESSSR), are a newly developed load- and size-independent assessment of myocardial contractility. The purpose of this study was to apply this assessment to compare extent and velocity of myocardial contraction during graded infusions of dobutamine. Seven chronically instrumented unsedated sheep were studied at rest and during graded infusions of dobutamine (2.5-20 mug.kg-1.min-1). The ESSSR were linear over a wide range of load alterations whereas the end-systolic pressure-diameter relations (ESPDR) were generally nonlinear. Midwall shortening rate (SR(m)) at common preload and afterload representing contraction extent increased with each dose of dobutamine through 20 mug.kg-1.min-1. In contrast, midwall shortening (S(m)) increased through dobutamine 5 mug.kg-1.min-1 but not at higher dobutamine infusion rates. Conventional endocardial shortening and the slope of the ESPDR, fitted to a linear model, exhibited responses similar to S(m). The velocity of circumferential endocardial fiber shortening (V(cf,c)), V(cf,c)-afterload relation, and maximum first derivative of left ventricular pressure exhibited responses similar to SR(m). Thus both the extent and velocity of contraction increased at low doses of dobutamine, whereas only the velocity increased at high doses. Potential mechanisms for the saturated response of the extent of contraction include 1) shorter systolic time for contraction due to earlier onset of relaxation and 2) the utilization of myocardial contractile energy for left ventricular wall deformation at small cavity volumes at high doses of dobutamine.