Neurally adjusted ventilatory assist (NAVA) is a new concept of mechanical ventilation. NAVA delivers assist to spontaneous breathing based on the detection of the electrical activity of the diaphragm. We study the effect of changing ventilatory parameters on the patient's breathing pattern during NAVA.

Inclusion Criteria:

• Invasive mechanical ventilation (tracheally intubated or tracheotomized)
• Presence of an arterial pressure line.
• Subject itself or its next of kin has given written informed consent

Exclusion Criteria:

• Patient is less than 18 years or more than 85 years of age
• The attending physician refuses to allow enrollment
• The patient refuses informed consent
• Next of kin is unavailable or refuses informed consent
• Pregnant or breast-feeding female. A pregnancy test will be performed in all female patients less than 60 years of age.
• Any contraindication to insertion/exchange a nasogastric tube, including (but not limited to): severe oropharyngeal malformation or bleeding; esophageal varices, tumor, infection, stenosis, or rupture
• Presence or suspicion of diaphragm injury
• Hemophilia or other severe bleeding disorder
• Presence or suspicion of a central nervous system (CNS) disorder, including (but not limited to): CNS infarction, bleeding, tumor, or infection
• History of heart and/or lung transplantation
• Any mechanical cardiac assist device (excluding intraaortic balloon pump)
• Any contraindication to reduce sedation or to stop neuromuscular blockage in order to allow spontaneous breathing
• The patient needs to be ventilated with a mode of mechanical ventilation that targets a predefined tidal volume or airway pressure as per attending physician
• Severe hemodynamic instability as per attending physician
• Planned or anticipated intervention within the study period necessitating either transfer out of the ICU or requiring prolonged interaction with the patient.
• A fraction of inspired oxygen (FiO2) of > 0.8
• The patient currently participates in another interventional clinical trial