Effect of Increased Positive End-expiratory Pressure on Intracranial Pressure in Different Respiratory Mechanic in Acute Respiratory Distress Syndrome

There are concerns that the use of positive end-expiratory pressure (PEEP) for the treatment of pulmonary complications in patients with brain injury may potentially elevate intracranial pressure (ICP), and deteriorate neurological status. It is suggested that both respiratory system compliance and ventricular compliance would contribute to the elevation of ICP when PEEP increases. In theory, PEEP may cause elevation of ICP by increasing intrathoracic pressure and diminish venous return. However, the transmission of PEEP into thoracic cavity depends on the properties of the lung and chest wall. Experimental study showed that when chest wall compliance is low, PEEP can significantly increases intrathoracic pressure; whereas low lung compliance can minimize airway pressure transmission. It is generally recognized that the lung compliance decreases in acute respiratory distress syndrome (ARDS) patients due to extensive alveolar collapse. However, it has been report that the elastance ratio (the ratio between elastance of the chest wall and the respiratory system, where elastance is the reciprocal of compliance) may vary from 0.2 to 0.8. Therefore, it is important to distinguish the compliance of the chest wall and the lung when investigating the effect of PEEP on ICP. Because intrathoracic pressure (pleural pressure) is difficult to measure in clinical situations, esophageal pressure (Pes) is considered as a surrogate of intrathoracic pressure. In the present study, the investigators determine the effect of PEEP on intrathoracic pressure and ICP by Pes measurement.