OBJECTIVE: To determine, even during postoperative period, the confiability of the cardiac index correlate with the data data given by a central atrial venous blood gasometry in patients who underwent cardiac surgery. METHODS: From the sample of arterial and venous blood of right atrium gathered in postoperative of cardiac surgery, it was determinated the hemoglobin concentration and the gasometric study through what was observed of the venous oxygen saturation (SvO2) and the partial pressure of oxygen from venous blood gathered in right atrium (PvO2), add to the calculation of artery-venous difference of the oxygen content - radial artery / right atrium (C( a-v )O2). Afterwards, these parameters were compared with the cardiac index determined by thermodilution. RESULTS: There was good correlation between SvO2, C(av)O2 of the venous right atrial blood and cardiac index meansured by termodiluition method, with sensibility and especificity good and high positive predict value and negative predict value. The PvO2 demonstrated poor sensibility in the estimative of low output. CONCLUSION: In cardiac surgery postoperative, the SvO2and the C(a-v)O2 were safe parameters correlated with low cardiac output. The PvO2 demonstrated poor sensibility in the estimative of low output in postoperative cardiac surgery.