Mechanical ventilation with low tidal volume (about 6 ml.kg-1) reduces mortality in ALI/ARDS patients respect to high tidal volume ventilation (about 12 ml.kg-1).

This finding is usually explained by alveolar tidal overdistension associated to high tidal volume. Stretch-induced lung injury may trigger a cytokine-mediated inflammatory response. This may contribute to the development of systemic inflammatory response and multiple system organ failure and death.

High tidal volume strategies might affect organ function by pathways not mediated by inflammatory response.

It is well recognized the inverse relationship between tidal volume and cardiac output during mechanical ventilation. Nevertheless there are no clinical studies about cardiac output changes induced by low (6 ml.kg-1) and high tidal volume (12 ml.kg-1) in ALI/ARDS patients.

The study hypothesis is that high tidal volume ventilation reduces cardiac output in ALI/ARDS patients respect to low tidal volume strategy. Thereafter reduced hemodynamic impact could explain beneficial effect of low respect to high tidal volume ventilation.

If study hypothesis is confirmed, other studies should define the main cause of mortality reduction related to low tidal volume strategies and if appropriate hemodynamic monitoring and support should be required when low tidal volume strategies are harmful (i.e. traumatic brain injury).

Inclusion Criteria:

• diagnosis of ARDS/ALI

Exclusion Criteria:

• mean arterial pressure lower than 65 mmHg
• beginning or change of vasoactive agents infusion rate in the last 2 hours