Calibration of the esophageal balloon catheter (Pes catheter) is important for the right measurement of the esophageal pressure (Pes) and subsequent interpretation of the derived transpulmonary pressures during mechanical ventilation. Both relative changes and absolute values of Pes can be affected by inappropriate filling of the esophageal balloon and by the elastance of the esophagus wall. Therefore one should calibrate the balloon to determine the best filling pressure. Calibration of the Pes catheter has only been validated in mandatory ventilation but not in support modes in which the patient triggers the ventilator and is supported by the mechanical ventilator. Because the forces in the thoracic cage behave differently in comparison with a controlled mode, it is to be expected that the calibration process in a support mode yields different filling volumes in comparison with the calibration process in a controlled mode. This would lead to a more reliable filling volume in support mechanical ventilation and a more reliable derivation of transpulmonary pressure and therefore to a better treatment of patients.

In mechanical ventilation the esophageal pressure (Pes) is used to calculate the transpulmonary pressure (PL). The transpulmonary pressure is used to set the positive end expiratory pressure (PEEP) and the inspiratory pressure. In spontaneous mechanical ventilation the transpulmonary pressure swing (which is the difference between the maximum and minimum transpulmonary pressure) is used as a measure of patient effort. Too much patient effort is perhaps detrimental in patients with early, severe ARDS, leading to additional lung damage and possibly a prolonged ICU stay.

The Pes is measured using a separate or integrated (feeding tube and Pes catheter) esophageal balloon catheter. The catheter is placed in the right position and has to be inflated to produce a readable pressure signal. The Pes balloon needs calibration before measurement to be sure that the readings are trustworthy. When calibrating one should take the compliance of the esophagus into account, together with the calibration curve, and with that, the filling volume of the patient changes from patient to patient. A calibration study of the Pes catheter has only been performed in passive mechanically ventilated patients, i.e. in patients without any breathing efforts.

In spontaneous breathing patients who still needs support from the ventilator, the forces within the thorax differ considerably in comparison with passive patients. When initiating a breath the diaphragm of the patient will move downwards and the Pes will become negative. In passive patient the ventilator initiates the breath and the diaphragm will be pushed downwards but the Pes will become more positive. It is probable that the calibration methods used in passive patients is not feasible in patients who exhibit a breathing effort. It is therefore necessary to study the calibration of the esophageal balloon catheter in patients that breath spontaneously with the support of a mechanical ventilator.

The hypothesis is that the calibration method, and therefore the filling pressure of the balloon is different from the values when the patient would be ventilated in a passive mode. Wrong calibration leads to wrong values, which could possibly lead to wrong interpretation of the obtained values with possible harm for the patient.

Inclusion Criteria:

• 18 years or older.
• Mechanically ventilated in spontaneous mode.
• Sedated (Richmond Agitation Sedation Scale (RASS) between -3 and -5).
• Pes catheter in situ according to the protocol used in the LUMC (Insertion of a Pes catheter is mandatory if it is suspected that te patient will be ventilated for more than 24 hrs.).

Exclusion Criteria:

• Medical condition that excludes the placement of a Pes catheter.
• Allergic reaction against rocuronium in the past.
• Pregnant.
• RASS > -3

ICU patients who are on mechanical ventilation with a esophageal balloon catheter in situ according to the current LUMC protocol. Expected mechanical ventilation time > 24 hrs. RASS between -3 and -5;

Data will be shared upon request with researchers.