Diastolic Simple Elastic and Viscoelastic Properties

We performed simultaneous high-fidelity left ventricular pressure and echocardiographic dimension measurements in 30 patients with normal left ventricular function (group 1; n = 6), moderate-tosevere aortic regurgitation (group 2; n = 14) or congestive cardiomyopathy (group 3; n = 10). We determined diastolic stress, strain and strain rate in all 30 patients and fitted the data to a simple elastic and a viscoelastic stress-strain model. The intrapatient comparison of the simple elastic and viscoelastic stress-strain relationship showed a significantly better curve fitting (r = 0.93 vs 0.96) for the viscoelastic model. The correction by the viscoelastic model occurred mainly during the highly filling-rate dependent early diastole, whereas the correction during atrial filling was low. Early diastolic deviations from the simple elastic stress-strain relationship were especially pronounced in the patients with myocardial hypertrophy. The viscoelastic constants of myocardial stiffness, B and K, were significantly different from the corresponding simple elastic constants, b and k, indicating that the simple elastic stiffness constants include both elastic and viscous forces. Normalized stress-strain data are mandatory for interpatient comparison. We attempted normalization by using a diastolic circumferential reference length at a common wall stress of 15 dyn X 103/cm2. However, in 17 of the 30 patients, stress values of 15 dyn X 103/cm2 were either too high (controls) or too low (congestive cardiomyopathy). Another limiting factor was the loss of data points during early diastole by normalization, which resulted in an underestimation of viscous forces. The non-normalized and the normalized viscoelastic stress-strain relationships clearly have limitations the first is preload dependent, the second underestimates viscous forces. However, the viscoelastic constants of myocardial stiffness, K, as assessed by both methods, were increased in patients with congestive cardiomyopathy, but normal in patients with aortic regurgitation. The enhanced diastolic viscoelastic stiffness in cardiomyopathy does not seem to be related to muscle mass, because the angiocardiographically determined left ventricular muscle mass was similar in both groups of patients with myocardial hypertrophy.