isovolumic contraction /-vəlo̅o̅″mik/ [Gk, isos + L, volumen, paper roll, contractio, drawing together] , the early phase of systole, in which the myocardial muscle fibers have begun to shorten but have not developed enough pressure in the ventricles to overcome the aortic and pulmonary end-diastolic pressures and open the aortic and pulmonary valves. During this period of muscle fiber contraction, the ventricular volumes do not change. See also afterload.
