In operations of the open heart inadequate cardiac output can lead to the so-called syndrome of a low cardiac output. The main indicator of cardiac function is the cardiac index. In patients in a critical state reduction of the cardiac index below 1.9 l/min q.m. suggests the presence of this syndrome. The submitted paper deals with the early detection of a low cardiac output which makes it possible to select optimal treatment before clinical signs of failure as well as further complications develop. The submitted paper deals with a group of 115 operated patients incl. 53 operated on account of IHD and 62 on account of rheumatic heart disease. Treatment was according to haemodynamic values divided into four groups: 1. Insurance of an optimal intravascular volume without pharmacological support, 2. pharmacological support of vasodilatating agents (nitroglycerine, sodium nitroprusside), 3. positive inotropic support by dobutamine, 4. support by a combination of vasodilatating agents and positive inotropic pharmaceutical preparations (dobutamine, dopamine). The favourable effect of treatment in the above groups is apparent already after four hours, and after 20 hours the investigated values shift to an optimal zone. The authors emphasize the high diagnostic value of comprehensive extensive monitoring which influences and controls therapy by retrospective informations.