Myocardial Relaxation, Restoring Forces, and Early-Diastolic Load Are Independent Determinants of Left Ventricular Untwisting Rate

Abstract

Peak left ventricular (LV) untwisting rate (UTR) has been introduced as a clinical marker of diastolic function. This study investigates if early-diastolic load and restoring forces are determinants of UTR in addition to the rate of LV relaxation. In 10 anesthetized dogs we measured UTR by sonomicrometry and speckle tracking echocardiography at varying LV preloads, increased contractility, and myocardial ischemia. UTR was calculated as the time derivative of LV twist. Because preload modified end-diastolic twist, LV systolic twist was calculated in absolute terms with reference to the end-diastolic twist configuration at baseline. Relaxation rate was measured as the time constant (τ) of LV isovolumic pressure decay. Early-diastolic load was measured as LV pressure at the time of mitral valve opening. Circumferential-longitudinal shear strain was used as an index of restoring forces. In a multivariable mixed model analysis a strong association was observed between UTR and LV pressure at the time of mitral valve opening (parameter estimate [β]=6.9; P<0.0001), indicating an independent effect of early-diastolic load. Furthermore, the associations between UTR and circumferential-longitudinal shear strain (β=-11.3; P<0.0001) and τ (β=-1.6, P<0.003) were consistent with independent contributions from restoring forces and rate of relaxation. Maximal UTR before mitral valve opening, however, was determined only by relaxation rate and restoring forces. The present study indicates that early-diastolic load, restoring forces, and relaxation rate are independent determinants of peak UTR. However, only relaxation rate and restoring forces contributed to UTR during isovolumic relaxation.