Lung ultrasound in acute respiratory distress syndrome (ARDS)
Overview
Acute respiratory distress syndrome (ARDS) is a clinical entity characterized by acute inflammatory lung injury, increased pulmonary vascular permeability and lung weight, as well as loss of aerated lung tissue. The clinical hallmarks of ARDS are hypoxemia, bilateral opacities detected by chest radiograph or computed tomography (CT) scan, decreased lung compliance, and increased dead space. ARDS has different etiologies, among which the most common ones are sepsis, aspiration, pneumonia, trauma, major surgery, and multiple transfusions.1
A new definition of ARDS has been recently proposed based on the presence of the following criteria2:
1. Timing: Acute onset within 1 week of a known clinical insult or new or worsening respiratory symptoms
2. Chest imaging: Bilateral opacities detected by chest radiograph or CT scan
3. Origin of edema: Respiratory failure not explained by cardiac failure or fluid overload. Objective assessment by echocardiography or quantification of pulmonary capillary wedge pressure (PCWP) to exclude hydrostatic edema
4. Oxygenation: partial pressure of arterial oxygen (Pao2)/fraction of inspired oxygen (Fio2) ≤ 300
Past studies using chest CT scans have elucidated that, unlike acute cardiogenic pulmonary edema (ACPE), ARDS is characterized by inhomogeneous pulmonary injury, with some areas of the lungs severely affected, some mildly involved, and others completely healthy.3,4 However, CT cannot be used on a daily basis because of the high amount of ionizing radiation exposure, the high cost, and the difficulty in transporting patients to the CT scanner. Hence the diagnosis of ARDS may be challenging, especially during its acute phase, because it is not always feasible to perform a CT scan or invasive hemodynamic measurements (e.g., evaluation of PCWP using a pulmonary artery catheter) and because of the tenuous respiratory function and hemodynamic instability of patients. In addition, images from a simple portable chest radiograph are inaccurate in distinguishing an ARDS pattern from ACPE. Recently, lung ultrasound has emerged as a noninvasive, radiation-free, bedside technique with high sensibility in the detection of different lung and pleural disorders, as analyzed in preceding chapters.5
Lung ultrasound b-lines and extravascular lung water
Healthy aerated lungs are featured by an anechoic echotexture. Diseased lungs are characterized by varying amounts of reduced aeration and edema that correspond to progressively dense and/or hyperechoic sonographic patterns (Figure 22-1). The latter is observed mainly due to the increased number and thickness of artifacts (B-lines or comet-tails). B-lines are generated because of subpleural interlobular septal thickening resulting from hydrostatic or “lesional” edema, pulmonary inflammatory edema, and/or fibrosis.6 Previous work demonstrated that the number of B-lines is proportional to the amount of extravascular lung water, thus being a sensitive sign of alveolar-interstitial syndrome.7–10 A simple visual score based on the sum of the number of B-lines (in each scanning intercostal space) correlates with chest radiograph visual scores,6,7 lung weight and density (quantitative CT),10 PCWP evaluated by the thermodilution method using the invasive pulse-induced contour cardiac output (PiCCO) system (Pulsion Medical Systems, Munich, Germany),8 and also with lung weight assessed by gravimetry in experimental animals.11
Figure 22-1 Various stages of increasing severity in alveolar-interstitial syndrome: interstitial syndrome (panel A), alveolar-interstitial syndrome (panel B), more severe alveolar interstitial syndrome with spared area (star) and thickening of pleural line (panel C) and alveolar-interstitial syndrome with ground-glass attenuation, irregular pleural line and spared area (star) (panel D).
Ultrasonographic signs of acute respiratory distress syndrome
The main signs that can be recognized by lung ultrasound in cases of suspected ARDS are
1. B-lines: Vertical artifact, also called “comet-tail artifact,” extending to the edge of the screen, moving within pleural line synchronously with inspiration and effacing A-lines. In ARDS, B-lines should be bilateral and, in number, greater than 3 or more, to confluence in a completely “white lung.” In the anterior lung fields, B-lines are not homogeneously distributed, whereas in the posterior lung fields, the B-lines are more compact and homogeneous, producing the image of a global white lung (see Figure 22-1).
2. Spared areas: Areas of normal lung that are observed in at least one intercostal space, surrounded by areas of B-lines or white lung (usually in the anterior lung fields) (Figure 22-2).
Figure 22-2 Visualization of spared area (star) and thickened pleural line in an acute respiratory distress syndrome (ARDS) case.
3. Consolidations: Areas of hyperechoic echotexture with punctiform elements or “hepatization,” with presence of static or dynamic air bronchograms.12 In ARDS, consolidations may be located in the posterior lung fields, especially at the bases (Figure 22-3).
