Effects of acute changes in load and inotropic state on the exponential rate of fiber shortening and other indices of myocardial contractility in the anesthetized intact dog

Abstract

The effects of an acute increase in preload, afterload, and inotropic state on several indices of left ventricular contractility were studied in 20 anesthetized intact dogs. The behaviour of the exponential rate of fiber shortening (ERFS), a newly described index, which is based on the instantaneous fiber length – time relationship through ejection, was compared with other classical ejection and isovolumic indices of left ventricular contractility. Acute volume overload by dextran 40 infusion produced a significant increase in preload as reflected by a 103% (p < 0.01) increase in left ventricular end-diastolic pressure and a 121% (p < 0.001) increase in end-diastolic circumferential wall stress. There was also a smaller but significant increase (p < 0.05) of heart rate (30%) and of peak systolic circumferential wall stress (24%). None of the left ventricular contractility indices showed any significant change. Acute pressure overload, produced mechanically by an aortic balloon, increased the afterload significantly as reflected by a 33% (p < 0.05) rise of end-systolic circumferential wall stress and a 43% (p < 0.001) increase in systemic resistance. Stroke volume decreased significantly by 23% (p < 0.05). All ejection indices, including ERFS, were significantly diminished by 30 – 37%; all isovolumic indices showed no significant changes. Positive inotropic intervention was induced by dopamine infusion, which caused a significant 28% (p < 0.05) increase in cardiac output. End-diastolic and end-systolic circumferential wall stress were significantly diminished. All indices of left ventricular contractility increased significantly and ERFS showed the quantitatively greatest change. This study suggests that ERFS is an ejection index of myocardial contractility, which is quite sensitive to inotropic intervention, independent of preload but sensitive to afterload.