Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol

Abstract

Background: This study assesses the potential of lung ultrasonography to diagnose acute respiratory failure.

Methods: This observational study was conducted in university-affiliated teaching-hospital ICUs. We performed ultrasonography on consecutive patients admitted to the ICU with acute respiratory failure, comparing lung ultrasonography results on initial presentation with the final diagnosis by the ICU team. Uncertain diagnoses and rare causes (frequency<2%) were excluded. We included 260 dyspneic patients with a definite diagnosis. Three items were assessed: artifacts (horizontal A lines or vertical B lines indicating interstitial syndrome), lung sliding, and alveolar consolidation and/or pleural effusion. Combined with venous analysis, these items were grouped to assess ultrasound profiles.

Results: Predominant A lines plus lung sliding indicated asthma (n=34) or COPD (n=49) with 89% sensitivity and 97% specificity. Multiple anterior diffuse B lines with lung sliding indicated pulmonary edema (n=64) with 97% sensitivity and 95% specificity. A normal anterior profile plus deep venous thrombosis indicated pulmonary embolism (n=21) with 81% sensitivity and 99% specificity. Anterior absent lung sliding plus A lines plus lung point indicated pneumothorax (n=9) with 81% sensitivity and 100% specificity. Anterior alveolar consolidations, anterior diffuse B lines with abolished lung sliding, anterior asymmetric interstitial patterns, posterior consolidations or effusions without anterior diffuse B lines indicated pneumonia (n=83) with 89% sensitivity and 94% specificity. The use of these profiles would have provided correct diagnoses in 90.5% of cases.

Conclusions: Lung ultrasound can help the clinician make a rapid diagnosis in patients with acute respiratory failure, thus meeting the priority objective of saving time.

FIGURE 1.

Ultrasound areas. Stage 1 defines the investigation of the anterior chest wall (zone 1) in a supine patient (1′ in this semirecumbent patient). Stage 2 adds the lateral wall (zone 2) [left panel]. Stage 3 adds the posterolateral chest wall using a short probe, moving the patient only minimally (zone 3) [right panel]. Each wall is divided into upper and lower halves, resulting in six areas of investigation. Note the shape of the microconvex probe, which allows satisfactory analysis of the intercostal space, and satisfactorily controlled compression maneuvers at the veins investigated in this study: internal jugular, subclavian, iliofemoropopliteal veins, and as far as possible, inferior vena cava and calf veins.

FIGURE 2.

Normal lung surface. Longitudinal scan of an intercostal space. Left panel: Pleural line and A line (real-time). The pleural line is located 0.5 cm below the rib line in the adult. Its visible length between two ribs in the longitudinal scan is approximately 2 cm. The upper rib, pleural line, and lower rib (vertical arrows) outline a characteristic pattern called the bat sign. The horizontal lines arising from the pleural line (horizontal arrows) are separated by regular intervals that are equal to the distance between the skin and the pleural line. These were called A lines. A lines are usually large (see upper line) but can be shorter (lower line), which has no clinical significance. Right panel: M mode. An obvious difference appears on either side of the pleural line (arrow). The motionless superficial layers generate horizontal lines. Lung dynamics generate lung sliding (sandy pattern). This pattern is called the seashore sign.

FIGURE 3.

Interstitial syndrome. These vertical comet-tail artifacts arise strictly from the pleural line, are well defined (laserlike), hyperechoic, move with lung sliding, spread to the edge of the screen without fading, and erase A lines (dotted arrows indicate their theoretical location). This pattern defines B lines. Several B lines in a single view, reminiscent of a rocket at lift-off, are called lung rockets, or B + lines (featuring here, B3 lines). Diffuse lung rockets indicate interstitial syndrome. One or two B lines in a single view, referred to as the b line, have no pathologic significance. This patient had cardiogenic pulmonary edema.

FIGURE 4.

Pneumothorax. Left panel (real-time): one significant item is the complete absence of the B line. Lower arrows: A lines; upper arrow: pleural line. Right panel (M mode): this succession of horizontal lines indicates complete absence of dynamics at, and below, the pleural line (arrowheads). This pattern is called the stratosphere sign. The lung point (not featured here) confidently rules in the diagnosis.

FIGURE 5.

Pleural effusion and alveolar consolidation; typical example of PLAPS. Left panel: real-time, stage 2. The quad sign: a pleural effusion on expiration (E) is delineated between the pleural line (upper white arrows) and the lung line, always regular, which indicates the visceral pleura (lower white arrows). The shred sign: a lower-lobe alveolar consolidation (LL) yields a tissular pattern, characteristically limited by the lung line (or the pleural line when there is no effusion) and in depth by an irregular border (black arrows), the shred line, as in connection with aerated lung. Below, air artifacts are displayed. Between consolidation and spleen (S) is the diaphragm, a basic landmark in stage 2. Right panel: time-motion demonstrates the sinusoid sign, a basic dynamic sign of pleural effusion. The sign will not be generated by alveolar consolidation, which behaves like a solid lesion.

FIGURE 6.

Ultrasound profiles. Left panel: The A profile is defined as predominant A lines plus lung sliding at the anterior surface in supine or half-sitting patients (stage 1/1′). This profile suggests COPD, embolism, and some posterior pneumonia. Pulmonary edema is nearly ruled out. Middle: The B profile is defined as predominant B + lines in stage 1. This profile suggests cardiogenic pulmonary edema, and nearly rules out COPD, pulmonary embolism, and pneumothorax. Right panel: an A/B + profile, massive B lines at the left lung, A lines at the right lung. This profile is usually associated with pneumonia.

FIGURE 7.

A decision tree utilizing lung ultrasonography to guide diagnosis of severe dyspnea.