Lung Imaging in ARDS

Silvia Coppola, Sara Froio, and Davide Chiumello

Introduction

The definition of acute respiratory distress syndrome (ARDS) has been recently revised with the new Berlin definition [1]. According to this latest definition, the diagnosis is based on the onset of the hypoxemia and of bilateral radiological opacities within 1 week. Three mutually exclusive categories of ARDS based on the degree of hypoxemia were identified (mild, <300 mmHg PaO₂/FiO₂ > 200 mmHg; moderate, <200 mmHg PaO₂/FiO₂>100 mmHg; severe, PaO₂/FiO₂ <100 mmHg with a positive end-expiratory pressure >5 cmH₂O) associated with increased mortality and associated with median length of mechanical ventilation in survivors. The new definition updated the following concepts:

• The specific time frame identifying the acute onset as “ARDS developing within 1 week of a known clinical insult or new/worsening respiratory symptoms”
• The origin of edema judging ARDS, a respiratory failure not fully explained by cardiac failure or fluid overload
• The chest radiological criterion specifying that it should include bilateral opacities not fully explained by effusion, lobar lung collapse on chest radiograph, or on CT scan if available

S. Coppola • S. Froio

Dipartimento di Anestesia, Rianimazione ed Emergenza Urgenza, Fondazione IRCCS Ca’ Granda-Ospedale Maggiore Policlinico, Milan, Italy e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

D. Chiumello (*)

Director of Anaesthesia and Intensive Care Services, ASST Santi Paolo e Carlo, Department of Scienze della Salute, University of Milan, Milan, Italy e-mail: This email address is being protected from spam bots, you need Javascript enabled to view it

Table 10.1 Lung imaging techniques: clinical role and technical limitations

Imaging technique	Clinical role	Technical limitations
X-ray	Lung assessment	Poor contrast diagnostic resolution Quality of the exam affected by ICU conditions Radiation exposure
Computed tomography	Lung assessment Lung recruitment Overdistension evaluation Lung perfusion analysis	Risk of critically ill patient’s transport Radiation exposure
Lung ultrasound	Lung assessment Lung recruitment	Operator-dependent technique Physical shape such as obesity, subcutaneous emphysema, large rib cage
Positron emission tomography	Lung assessment	Risk of critically ill patient’s transport Time-consuming Short half-life of tracers
Electrical impedance tomography	Lung assessment Lung recruitment Lung perfusion analysis	Indirect acquisition Detection of relative changes in functionally active structure
Magnetic resonance imaging	Lung assessment	Risk of critically ill patient’s transport Required dedicated assistance, monitoring, and devices Time-consuming

From a pathophysiological point of view, ARDS can be considered an acute, diffuse inflammatory lung injury that leads to increased pulmonary vascular permeability, increased lung weight, and loss of aerated lung tissue. In this context of parenchymal abnormalities, lung imaging plays a role in the diagnosis and followup and can provide prognostic information.

Therefore, the aim of this chapter is to summarize the role of the different imaging techniques that have been developed throughout the years to better understand ARDS and to monitor ARDS patients. In particular we would focus on the role of each lung imaging technique in the diagnosis and in the evaluation over the time of lung recruitment, overdistension, and perfusion (Table 10.1).