ObjectiveTo determine the influence of positive end-expiratory pressure (PEEP) on intracranial pressure and cerebral perfusion pressure.DesignNeurosurgical intensive care patients requiring intracranial pressure monitoring and mechanical ventilation were studied in a randomized, controlled study.SettingTertiary care, neurosurgical intensive care unit.PatientsEighteen patients were enrolled in the study. Patients had posttraumatic head injuries (n = 9), subarachnoid hemorrhage (n = 7), obstructive hydrocephalus (n = 1), and intracerebral hemorrhage of unknown cause (n =1).InterventionsPatients had PEEP levels of 5, 10, and 15 cm H2O applied to their lungs.Measurements and Main Results15 mm Hg). PEEP at 5 cm H2O had no effect on intracranial pressure in the group with normal intracranial pressure. However, PEEP at 10 and 15 cm H2O produced a significant (p < .05) increase in intracranial pressure (1.9 and 1.5 mm Hg, respectively). In the group with increased intracranial pressure, no significant change in intracranial pressure occurred at any of the PEEP levels used. In both groups, cerebral perfusion pressure was unchanged throughout.ConclusionsIn patients with normal intracranial pressure, PEEP at 5 cm H sub 2 O did not significantly alter intracranial pressure. The clinical relevance of the intracranial pressure increase at PEEP levels of 10 and 15 cm H260 mm Hg. In patients with increased intracranial pressure, higher levels of PEEP did not significantly change intracranial pressure or cerebral perfusion pressure. (Crit Care Med 1997; 25:1059-1062)