ABSTRACT
Global cardiac function has been studied in small animals with methods such as echocardiography, cine-magnetic resonance imaging (MRI), and cardiac catheterization. However, these modalities make little impact on delineation of pathophysiology at the tissue level. The advantage of tagged cine-MRI technique is that the twisting motion of the ventricle, referred to as torsion, can be measured noninvasively, reflecting the underlying shearing motion of individual planes of myofibrils that generate wall thickening and ventricular ejection. Thus we sought to determine whether the mechanism of ventricular ejection, as measured by torsion, was the same in both humans and mice. Nine mice and ten healthy humans were studied with tagged cine-MRI. The magnitude and systolic time course of ventricular torsion were equivalent in mouse and humans, when normalized for heart rate and ventricular length. The end-systolic torsion angle was 12.7 +/- 1.7 degrees in humans vs. 2.0 +/- 1.5 degrees in mice unnormalized and 1.9 +/- 0.3 degrees /cm vs. 2.7 +/- 2.3 degrees /cm when normalized for ventricular length). These results support the premise that ventricular torsion may be a uniform measure of normal ventricular ejection across mammalian species and heart sizes.
Subject(s)
Magnetic Resonance Imaging/standards , Ventricular Function, Left/physiology , Adult , Animals , Biomechanical Phenomena , Cardiac Volume/physiology , Diastole/physiology , Female , Humans , Magnetic Resonance Imaging/statistics & numerical data , Male , Mice , Mice, Inbred Strains , Observer Variation , Reference Values , Reproducibility of Results , Species Specificity , Systole/physiology , Torsion Abnormality , Ventricular FunctionABSTRACT
Myofibril shortening and the oblique fiber orientation of the left ventricular myocardium results in a twisting motion of the left ventricle. Advances in cardiac magnetic resonance imaging (MRI) have made it possible to label the myocardium noninvasively and track this motion (twist) through the cardiac cycle, but little data exist on its complete systolic time course. The purpose of this study was to delineate the normal human systolic time course of ventricular twist using tagged cine-MRI. Tagged cine-MRI was performed in 10 healthy subjects. The mean systolic twist angle relative to the short axis centroid for the 10 volunteers was calculated. Interstudy and intra- and interobserver variability were assessed. During isovolumic contraction, all ventricular twist was counterclockwise. Later in systole, the basal segments changed direction and rotated in a clockwise direction, whereas the apical segments continued counterclockwise rotation. The midpoint for rotation was 45+/-8% of ventricular length. The mean short axis net ventricular twist (apex-base) at 80% systole was 12.6+/-1.5 degrees. The four wall segments showed heterogeneity in twist (lateral wall, 20.6+/-1.7 degrees; anterior wall, 17.5+/-5.1 degrees; inferior wall, 8.8+/-4.9 degrees; septum, 3.5+/-2.4 degrees). The anterior and lateral walls demonstrated significantly higher twist than the other walls (p < 0.01). Torsion increased steadily throughout systole after isovolumic contraction, whereas twist displayed rate changes. The mean interstudy and intra- and interobserver differences were less than 2.1 degrees. The close similarity in twist between subjects and the low interstudy and inter/intraobserver variation indicates that twist is a robust parameter of myocardial function. Torsion varies smoothly during systole, which may play a role in minimizing oxygen consumption. These data can serve as a baseline from which to compare alterations in regional myocardial function in disease.
