A mechanistic analysis of the force-frequency relation in non-failing and progressively failing human myocardium

This review focuses on the role of the myocardial force-frequency relation (FFR) in human ventricular performance and how changes in the FFR can reduce cardiac output and, ultimately, can contribute to altering the stability of the in-vivo cardiovascular system in a way that contributes to the progression of heart failure. Changes in the amplitude, shape, and position of the myocardial FFR occurring in various forms of heart failure are characterized in terms of maximal isometric twitch tension, slope of the ascending limb (myocardial reserve), and position of the peak of the FFR on the frequency axis (optimum stimulation frequency). All three of these parameters decline according to severity of myocardial disease in the following order: non-failing atrial septal defect, non-failing coronary artery disease, non-failing coronary artery disease with diabetes mellitus, failing mitral regurgitation, failing viral myocarditis, failing idiopathic dilated cardiomyopathy. Evidence is presented supporting a sarcoplasmic reticulum Ca-pump based mechanism for this progressive depression of the FFR. Intracellular calcium cycling and concentration and Ca-pump content all diminish in proportion to degree of depression of the FFR. Additional evidence from myocyte culture studies suggests a cause of diminished Ca-pump content is sustained, elevated levels of plasma norepinephrine. A hypothesis is presented to explain the mechanism of myocardial failure and its progression in terms of changes in the cardiovascular feedback control system that are triggered by reduced myocardial reserve. Sustained elevation of plasma norepinephrine levels depresses expression of sarcoplasmic reticulum Ca-pump protein causing depression of the FFR and this causes a compensatory further increase in norepinephrine levels and a further depression of Ca-pump protein.