Acute respiratory distress syndrome is a severe complication of critical illness. The diagnosis of ARDS is difficult, and it could be important to differentiate ARDS from other causes of acute respiratory failure. Innovative probe-based imaging techniques such as 'Confocal Laser Endomicroscopy' (CLE) and Optical Coherence Tomography (OCT) are high resolution optical techniques that, combined with conventional bronchoscopy, have been found to provide non-invasive, real-time near-histology information about the alveolar compartment in non ventilated non-critically ill patients.

It is important to identify the underlying cause of respiratory failure, in order to determine the appropriate treatment. Histopathology would help treatment decisions, however is in the fast majority of this critically ill patient-group not available. CLE is enables near histology/microscopic analysis during bronchoscopy, by tissue illumination with a low-power laser. Optical Coherence Tomography is the optical equivalent of B-mode ultrasonography, that consists of a small rotating optical fibre. Both the CLE and OCT techniques are minimally invasive and little time consuming. Therefore different areas of the lung can be sequentially imaged. With this pilot study the investigators aim to describe normal alveolar areas and areas with abnormalities in critically ill patients with non resolving acute respiratory failure mandating a standard bronchoscopy or laryngoscopy with or without bronchoalveolar lavage.

Inclusion Criteria:

• Admitted to intensive care unit of academic medical center in Amsterdam
• Indication for a procedure to investigate the airways that can be combined with pCLE/OCT

Exclusion Criteria:

• Inability and willingness to provide informed consent by family-members
• Inability to comply with the study protocol

Mechanically ventilated patients mandating a procedure that includes inspection of the airways and can be combined with pCLE/OCT. Patients can only be included when the research physician decides they are eligible. (based on the heterogeneity of the disease and the suspected etiology).

We plan to share the data.