Central venous pressure (CVP) is characterized by a low-frequency pleural-dependent as well as a high-frequency cardiac pulsatile component. The aim of the study is to compare the low-frequency component of CVP with the esophageal pressure (Pes), as surrogate of pleural pressure, to estimate trans-pulmonary pressure (PL).

The investigators will enroll mechanically ventilated patients admitted to the intensive care unit with the diagnosis of respiratory failure monitored with a dedicated nasogastric catheter and a central venous catheter for measuring Pes and CVP, respectively. Electrocardiogram trace, Pes, CVP and airway pressure (Paw) will be recorded at the end of inspiratory and expiratory pauses. The CVP waveforms will be analyzed off-line using a dedicated low-pass digital filter to obtain the low-frequency component of CVP (filtered CVP, fCVP). Paw, Pes and fCVP will be used to calculate PL using transpulmonary driving pressure formula (PL-Pes and PL-fCVP, respectively). The PL values obtained with fCVP and Pes will be compared to assess the correlation of the two methods.

Inclusion Criteria:

• adult patients receiving invasive mechanical ventilation
• diagnosis of acute respiratory failure (defined as the ratio of partial oxygen pressure and fraction of inspired oxygen below 200 mmHg)
• esophageal balloon catheter
• central venous catheter

Exclusion Criteria:

• age under 18 years old
• Hemodynamic instability
• Esophageal diseases (varices, stenosis..)
• Refusal of the patient
• evidence of active air leak from the lung (pneumothorax, pneumomediastinum, existing chest tube....)
• history of lung/abdominal surgery
• pregnancy
• severe coagulopathy

Patients admitted to ICU treated with mechanical ventilation.