[New findings on cardiac biomechanics]

Three series of acute and one series of chronic experiments were performed to study the functional interaction of muscle layers forming the ventricular walls. In the first series of acute experiments, the following parameters were recorded: ECG, tensograms of the subepicardiac, circular and subendocardiac layers of the left ventricle myocardium, pressure in the right and left ventricles and in the aorta. In the second series of acute experiments, label movement along the myocardial surface and ECG were recorded simultaneously by means of time-lapse photography. In the third series of acute experiments, electric potentials of myocardial layers were examined. In chronic experiments, systolic and diastolic displacement of X-ray contrast labels implanted into the myocardium was investigated. It was found that myocardial layers contracted asynchronously. The first to contract were the subendocardiac and subepicardiac layers and papillary muscles. This contraction modifies the ventricle geometry (from ellipsoidal to spherical), shortens the inflow pathway, displaces stroke volume to the arterial cone, and extends the circular layer. The circular layer begins contracting 20-40 msec thereafter. This contraction is accompanied by a rapid rise of intraventricular pressure, straightening of the outflow pathway, increase in the cardiac length, and a slight elongation of the subendocardiac and subepicardiac layers. This muscle contraction is also recorded at the onset of the ejection phase. However the release of the first portions of blood facilitates shortening of all muscle layers. At this point in time stroke volume gets the major part of kinetic energy. The simultaneous contraction of all muscle layers develops to reach T-wave. At this moment the phase of maximal ejection ends. During the phases of reduced ejection, isovolumic decline of intraventricular pressure and fast filling, the subendocardiac and subepicardiac layers and papillary muscles continue to contract, whereas the relaxed circular layer extends. As a result, intraventricular pressure decreases, the ellipsoidal shape of ventricles changes to the circular one, the cusps of the mitral and tricuspid valves open, and the sucking action of ventricles develops. In other words, diastolic phases are active at the expense of contraction of the subendocardiac and subepicardiac layers rather than at the expense of active relaxation. This is the point in time when U-wave is visible in ECG.