BACKGOUND: The purpose of this study was to examine the effect of various levels of positive end-expiratory pressure (PEEP) on the intraocular pressure in the patients receiving positive pressure ventilation. METHODS: Twenty, critically ill sedated and hemodynamically stable patients without history of glaucoma were placed on controlled positive pressure ventilation. Measured variables included intraocular pressure (IOP), mean arterial pressure (MAP), central venous pressure (CVP), peak inspiratory pressure (PIP) and arterial blood gas analysis (ABGA), and were recorded at zero end-expiratory pressure (ZEEP), and at 5, 10, 15, 20 cmH2O PEEP, applied in random order. RESULTS: IOP increased significantly from 13+/-3 to 16+/-3 mmHg at 15 cmH2O PEEP and from 14+/-4 to 17+/-6 mmHg at 20 cmH2O PEEP. CVP increased significantly from its corresponding ZEEP measurements at all PEEP levels and from 14+/-4 cmH2O at 5 cmH2O PEEP to 21+/-4 cmH2O at 20 cmH2O PEEP. There was a positive correlation between PEEP levels and PIP or CVP but no relationship between PEEP levels and IOP was observed. CONCLUSIONS: The application of PEEP levels > or = 15 cmH2O resulted in a significant increase in the IOP of patients with normal basal ocular tonometry. This study suggests that further increase in IOP may occur in the mechanically ventilated patients with already increased IOP or normal-tension glaucoma, when higher levels of PEEP are used.