Initial Phase of Ventricular Systole: Asynchronous Contraction

The initial phase of ventricular systole has been termed the phase of isometric contraction because all the cardiac valves are closed while the pressure is rapidly elevated. Cyclic changes in the dimensions of the left ventricle, recorded by gauges applied directly to the ventricular walls have consistently exhibited an abrupt expansion of the internal diameter, external circumference and external length of the chamber at the onset of systole. Apparently the longitudinal axis of the chamber is abruptly shortened by early contraction of papillary muscles and trabeculae carnae. The lateral walls bulge outward so that the chamber assumes a more spherical configuration as the internal pressure rises. It is doubtful that any of the myocardial fibers actually contract without a change in length, and the term "isometric contraction" is not appropriate for this phase. The initial stage of ventricular systole is actually a period of asynchronous contraction or sphericalization.