Evaluation of Variable Pressure Support Ventilation in the Therapy of Acute Lung Injury

Inclusion Criteria: * age 18 to 75 yrs * 150 mmHg \< PaO2/FIO2 \< 300 mmHg * mechanical ventilation with PSV or BIPAP * less than 15 days of mechanical ventilation * at least 20% of minute ventilation originated from spontaneous breathing Exclusion Criteria: * BMI \> 35 * esophageal disease * neuromuscular disease * instable thorax * pneumothorax * head trauma * brain injury * increased intracranial pressure * agitation * increased need for vasoactive drugs * chronic lung disease * acute coronary disease * participation in another clinical trial within the last 4 weeks at enrollment Interruption criteria: * acute change of mental status * SaO2 \< 92% * pHa \< 7.30 * respiratory rate \> 30 or \< 6 /min * dyspnea * diaphragm/thorax antagonism * diaphoresis * abnormal use of respiratory muscles * increase of the heart rate above 120% of baseline or absolute value \< 60 or \> 130 /min * mean arterial pressure above 120% of baseline or absolute value \< 70 or \> 110 mmHg