Background: Positive end-expiratory pressure [PEEP] is commonly used in mechanically ventilated patients to prevent alveolar collapse and improve oxygenation. However, optimal PEEP levels remain controversial, especially in non-acute respiratory distress syndrome patients in the intensive care unit [ICU].
The aim of the work: To compare the effects of different PEEP levels on oxygenation status and clinical outcomes in non-acute respiratory distress ICU patients requiring invasive mechanical ventilation.  
 Patients and Methods: This prospective randomized controlled trial included 80 ICU patients without acute respiratory distress syndrome who required invasive mechanical ventilation. Patients were randomized to receive low [4-8 cmH2O] or high [9-12 cmH2O] PEEP levels. The primary outcomes were Pao2 to Fio2 ratio, as an indicator for improvement of oxygenation parameter and number of ventilator-free days at day seven.
Results: The Fio2 values for the 4 to 8 cm H2O group averaged 0.42 [±0.19] and differed significantly from the 9 to 12 cm H2O group, which averaged 0.40 [±0.22] [p < 0.001]. However, no significant differences were found in respiratory rates, heart rates, mean arterial pressures, ARDS, severe hypoxemia, or ICU and hospital mortality. Driving pressures significantly differed [14.75 ± 1.56 vs. 12.5 ± 1.2; p < 0.001]. Ventilator-free days were similar, averaging 2.92 [±1.46] and 4.0 [±0.93] [p=0.056].
Conclusion: Ventilation with different levels of PEEP in ICU patients without ARDS at the onset of ventilation was associated with higher Po2/ Fio2 but not associated with decrease in ventilator free days nor lower in-hospital mortality nor a lower incidence of ARDS or pneumonia.