Mechanical Ventilation and Respiratory Muscle Work of Breathing in Acute Respiratory Distress Syndrome (ARDS) Patients

• Pressure-time product of the diaphragm [ Time Frame: average of last 15 minutes of each study arm ] [ Designated as safety issue: No ]
  The pressure-time product of the transdiaphragmatic pressure (Pdi) during inspiration is obtained for each breath by multiplying the corresponding mean inspiratory Pdi signal above the end-expiratory baseline by the inspiration time. Breath-by-breath data are ensemble-averaged over the last 15 minutes of each study arm.
• Patient - ventilator asynchrony index [ Time Frame: average of last 15 minutes of each study arm ] [ Designated as safety issue: No ]
  Ventilator asynchrony is determined as the sum of the triggering and cycling-off delays per breath, expressed as a percentage of the total breath duration. The trigger delay is measured as the time difference between the onset of neural inspiration and the ventilator inspiratory flow, and the cycling delay as the time difference between the end of neural inspiration and the end of ventilator inspiratory flow. Breath-by-breath data are ensemble-averaged over the last 15 minutes of each study arm.

• Transpulmonary pressure [ Time Frame: average of last 15 minutes of each study arm ] [ Designated as safety issue: No ]
  Transpulmonary pressure is determined as the difference between mouth pressure and esophageal pressure during inspiration. Breath-by-breath data are ensemble-averaged over the last 15 minutes of each study arm.
• Transdiaphragmatic pressure [ Time Frame: average of last 15 minutes of each study arm ] [ Designated as safety issue: No ]
  Transdiaphragmatic pressure is determined as the difference between gastric pressure and esophageal pressure during inspiration. Breath-by-breath data are ensemble-averaged over the last 15 minutes of each study arm.
• Oxygenation index [ Time Frame: at the end of each study arm ] [ Designated as safety issue: No ]
  Oxygenation index is determined as the ratio between arterial oxygen tension and fraction of inspired oxygen. Arterial oxygen tension is obtained at the end of each study arm.
• Dead space ventilation [ Time Frame: average of last 15 minutes of each study arm ] [ Designated as safety issue: No ]
  Dead space ventilation is determined each breath using the Bohr equation: (PaCO2-PeCO2/PaCO2)*Vt. Here Vt is tidal volume, PaCO2 is the partial pressure of carbon dioxide in the arterial blood, and PeCO2 is the end-tidal carbon dioxide tension in the expired air. Breath-by-breath data are ensemble-averaged over the last 15 minutes of each study arm.

The purpose of this study is to demonstrate that mechanical ventilation guided by the diaphragm EMG signal (also know as neurally adjusted ventilatory assist [NAVA]) is superior compared to pressure support and pressure control ventilation.

• Active Comparator: Pressure support ventilation, ARDSnet
  Mechanical ventilator is set to pressure support ventilation (6 ml/kg) for 30 min with positive end expiratory pressure (PEEP) set according to the "higher arm" of the ARDS network consensus.
  Intervention: Device: Mechanical ventilation
• Active Comparator: Pressure control ventilation, ARDSnet
  Mechanical ventilator is set to pressure control ventilation (6 ml/kg) for 30 min with PEEP set according to the "higher arm" of the ARDS network consensus.
  Intervention: Device: Mechanical ventilation
• Active Comparator: Neurally adjusted ventilatory assist, ARDSnet
  Mechanical ventilator is set to NAVA for 30 min with PEEP set according to the "higher arm" of the ARDS network consensus.
  Intervention: Device: Mechanical ventilation
• Active Comparator: Neurally adjusted ventilatory assist, titrated
  Mechanical ventilator is set to NAVA for 30 min with PEEP titrated using the diaphragm EMG signal.
  Intervention: Device: Mechanical ventilation

Inclusion Criteria:

• intubated, mechanically ventilated patients
• meeting criteria for ARDS
• mean arterial blood pressure > 65 mmHg (with or w/o vasopressors)

Exclusion Criteria:

• pregnancy
• increased intracranial pressure
• contra-indication naso-gastric tube
• diagnosed neuro-muscular disorder
• recent (<12 hours) use of muscle relaxants
• exclusion from sedation interruption protocol as used in our institution
• open chest or- abdomen
• very high inspiratory flow rate during supported ventilation
• inability to obtain informed consent