The aim of the study is to examine the effects of two different levels of PEEP on subsequent standard recruitment maneuver.

In this study we will examine the effects of two levels of PEEP on subsequent recruitment maneuver. In the first group a recruitment maneuver will be performed in patients who are ventilated with PEEP level set according to the ARDS network algorithm.

Patients will then be crossed over to a study arm where another recruitment maneuver will be performed while PEEP is adjusted according to esophageal pressure measurements.

Inclusion Criteria:

20 Man and women older than 18 years will be recruited.

Patients with acute respiratory failure of any cause who are mechanically ventilated according to the ARDS network recommendations will be considered for inclusion to the study.

To be included in the study a prerequisite of high peak Inspiratory pressure (plateau pressure of 25 to 30 cmH2O) has to be present, and at least one of the following four severity inclusion criteria has to be met.

1 - Low Total Respiratory system compliance (CT), defined as less than 50ml/cmH2O. 2 - PaO2 /FIO2 ratio of less than 300. 3 - Need for a PEEP greater than 10 cmH2O to maintain SaO2 of > 90%. 4 - PCO2 over 60 mmHg, or PH less than 7.2 that is attributed to respiratory acidosis.

Exclusion Criteria:

Patients with any of the following will be excluded from the study. Previous lung or chest wall surgery, previous esophageal surgery, known Achalasia or any other esophageal motility or spasm disorder, presence of chest thoracostomy tube, and any significant chest wall abnormality such as kyphoskoliosis.