[The left ventricular force-length relationship in patients with heart failure].

A new contractility index (Ec) is proposed based on the slope of the left ventricular (LV) end-systolic force-length relationship. The Ec values of normal human hearts are relatively constant. The present study investigated the Ec values of 16 cases of heart failure in 11 patients with old myocardial infarction and 5 with dilated cardiomyopathy in NYHA functional class 1 to 3. The LV end-systolic pressure (Pes) and dimension (Des) were estimated simultaneously by intraarterial cannulation and LV echocardiography. The LV pressure decreased from 111 +/- 17 to 90 +/- 14 mmHg after intravenous infusion of nitroglycerin. The LV Fes-Des relationship was found to be nearly linear and the slope (Ec) and extrapolated dimension intercepts (Do) were obtained as 60 +/- 22 g/cm and 2.7 +/- 1.2 cm, respectively. The values of Ec and Do were both proportional to the baseline Des value. Our model predicts that the Ec value reflects intracellular peak Ca2+ concentrations ([Ca2+]) of the myocardium and/or myosin ATPase activity. The present results suggest that the working myocardium of a failing heart increases [Ca2+] and/or myosin ATPase activity to compensate for depressed LV pump function due to myocardial damage.