Trend towards noninvasive, easy to use monitor was always a challenge. Numerous indices have been used to monitor the progress of patients on positive pressure ventilation. The present study compares different indices in a mixed larger intensive care unit (ICU) population. In a prospective observational study Arterial blood gases (ABG) analyses were obtained from 225 patients under mechanical ventilation in a polyvalent adult ICU. Values of ideal body weight (IBW), Body mass index (BMI), PAO2, PaO2/FiO2 ratio (PFr), SpO2/FiO2 ratio (SFr), SpO2/PEEP ratio (SPr), SpO2/PaCO­2 ratio (SPCr), Oxygenation index (OI) and Ventillatory ratio (VR) were calculated; and further correlation analysis was conducted. In Pressure control ventilation mode a relative strong relation between PFr and SFr and OI was found; yet further regression analysis implies that no direct replacement of PFr with SFr can be made without limitations, in clinical setting. In Volume control ventilation mode moderate relation was found between SFr and PFr. In the present study a moderate relation was found between SFr and PFr. The results agree with previous published studies; the differences among them lie in the different design of each one of them. The authors believe that- given the fact that one still considers using broadly PFr as index of oxygenation- SFr can be used safely as a surrogate for PFr only for certain disease states. Larger series are needed in order to define those patients groups and these pathophysiological conditions.