Using cardiac chamber catheterization and isotope methods, the systemic and intracardiac hemodynamics was studied in 232 patients who had undergone heart surgery under conditions of extracorporeal circulation. The syndrome of a low cardiac output was detected in 46.9% of the patients and was caused by various forms of heart failure, by hypovolemia, a postoperative shock and pericardiac tamponade. The functional state of the right and the left ventricles of the heart in various types of circulatory insufficiency was shown to vary in different directions. The volume of the circulating blood in the postoperative period was determined by the adequacy of the blood loss compensation, by blood deposition, the redistribution of fluid between the vascular, interstitial and cellular spaces of the body and also by the functional condition of the myocardium. The syndrome of a low cardiac output associated with cardiac tamponade is due largely to the diastolic insufficiency of the myocardium. The shock noted in the open heart surgery patients is a polyetiological syndrome in which the impaired circulation is secondary to myocardial failure, deficit of the circulation blood volume, and microcirculatory disorders.