This studies´ objective is to describe how different PEEP levels affect dead space measured by Bohr´s formula.

In ARDS patients PEEP has contradictory effects on ∆P (driving pressure of the airway) hat depends on the balance between lung recruitment and overdistension. Some authors showed that incremental levels of PEEP did not much affect dead space in ARDS patients. These studies used different methods for the assessment of dead space.

Such technique also overestimates the dead space. We believe that the right way to measure dead space is applying the original Bohr's equation using the mean PACO2 value. Therefore, the aim of this observational study was to describe the effect that PEEP has on Bohr's dead space and its sub-components in mechanically ventilated patients with ARDS.

This observational study was performed in the Intensive Care Unit of a University Hospital.

Protocol design The protocol started recording the data during baseline protective ventilation. After 15 minutes of data recording, we studied four levels of PEEP - 0, 6, 10 and 16 cmH2O - which were randomly assigned by a randomization table.

Respiratory, hemodynamic, arterial blood gas analysis and volumetric capnographic parameters were recorded. We analyzed the last 2 minutes of each protocol steps consisting in more than 30 breaths or data-points.

Inclusion Criteria:

• age older than 18 years
• requirement of invasive mechanical ventilation for more than 4 hours and less than 12 hours
• severe hypoxemia (PAFI < 200) secondary to ARDS or post-operatory atelectasis

Exclusion Criteria:

-requirement of invasive mechanical ventilation prior to enrollment in this study

ARDS patients