[New evidence for cardiac biomechanics]

Acute and chronic experiments have indicated that myocardial layers contract asynchronically. The subendocardial and subepicardial are the first to contract. Their contractions change the shape of a ventricle (from ellipsoid to spherical), shorten the inflow pathway, displace stroke volume to the arterial cone, and expand the circular layer. The circular layers begins contracting 20-49 msec later. Its contraction is accompanied by a rapid rise of intraventricular pressure, elongation of the outflow pathway, an increase in cardiac length, and a slight stretching of the subendocardial and subepicardial layers. Simultaneous contraction of all three layers develops to reach T wave. At this point the phase of the maximum ejection ends. During the phases of reduced ejection, isovolumic decline of intraventricular pressure and rapid filling, the subendocardial layers and papillary muscles continue to contract whereas the relaxed circular layer extends. As a result, intraventricular pressure decreases, the ellipsoidal shape of ventricular changes to the circular one, the cups of the mitral and tricuspid valves open, and the sucking action of the ventricles develops. In other words, diastolic phases are active through the contraction of the subendocardial and subepicardial layers rather than through active relaxation. ECG shows U-wave at this moment.