Ultrasound waves are unable to penetrate aerated lung tissue. Historically, this has limited lung ultrasound almost exclusively to the evaluation of pleural effusions. More recently, the recognition that analysis of ultrasound artifacts arising from the pleura can provide valuable diagnostic information about underlying lung pathology has led to the wider application of lung ultrasound. In the intensive care setting, increased use of lung ultrasound has been facilitated by the availability of point-of-care machines and the short learning time required to become proficient in the technique. Compared to chest radiography and CT, ultrasound is rapid and inexpensive, and it avoids exposure to ionizing radiation. A number of algorithms have been published that incorporate lung ultrasound findings in the diagnostic evaluation of patients with respiratory failure.
Equipment
The ideal machine for lung ultrasound in the ICU should be easily transportable and robust, allowing multiple bedside examinations. It should also be able to withstand repeated disinfection procedures, for which a compact machine with a waterproof keyboard is required. A strict disinfection procedure should be followed after each examination to avoid transmission of nosocomial skin flora among patients. The probe should be wiped clean of gel after each use and then disinfected (according to manufacturer’s instructions) before being hung back on the ultrasound cart. Single patient packages of coupling gel should be used instead of multiuse bottles. The ECG leads and keyboard should also be cleaned (e.g., with alcohol-based wipes) after each patient.
Several transducers can be used for chest ultrasound. The standard phased array, low frequency (3-5 MHz) transducer used for transesophageal echocardiography (TTE) provides good depth penetration and sufficient resolution for most lung ultrasound applications. Second harmonic and Doppler imaging is not required. Processing filters (dynamic noise reduction) should be disabled to maximize artifact generation. This transducer is ideally suited for evaluating pulmonary edema and pleural effusions and, therefore, can be used to complement standard TTE assessment of cardiac function.
A microconvex 5-8 MHz curved array probe provides better artifact visualization than lower frequency probes, but depth penetration may be insufficient for large patients. This probe is probably the most versatile for a complete evaluation of the chest by ultrasound.
A high frequency linear array probe (6-13 MHz) allows detailed pleural line analysis and is optimal for pneumothorax detection (e.g., after venous cannulation) but otherwise has limited applicability. As artifacts appear different with different transducers, the trainee in lung ultrasound should initially become familiar with the use of a single probe (either a cardiac probe or a microconvex 5 MHz probe).
Examination technique and sequence
Mechanically ventilated patients can usually be satisfactorily examined in the supine position. Visualization of the posterior lungs is achieved by scanning over the posterior axillary line with the arm lifted out of the way over the anterior chest. A thorough examination of the lungs involves scanning bilaterally over four quadrants of the anterior chest wall (upper and lower zones laterally and medially), the upper and lower lateral chest wall (bounded by the anterior and posterior axillary lines), and the upper, middle, and lower zones of the posterior chest wall. For a complete study, each intercostal space, along multiple vertical lines, should be examined (see the section on alveolar–interstitial syndrome ). The findings for each space scanned can then be documented in tabulated form. Only the dorsal lung segments behind the scapulae cannot be examined by ultrasound.
Fluid-filled chest pathology (e.g., pleural effusions or atelectasis) is gravity dependent and thus lies inferiorly. In contrast, aerated pathology (i.e., pneumothoraces) is nondependent and lies anteriorly. Detection of specific pathology should thus be directed accordingly, noting that in the supine patient the least dependent region is the basal anterior chest, which is where a small pneumothorax will collect.
The examination sequence should commence with scanning of the lower lateral chest. Here, identification of the diaphragm provides a useful landmark for further scanning. Subdiaphragmatic structures (liver, spleen, and kidneys) may be identified to confirm the location of the diaphragm. Right-sided subdiaphragmatic structures are usually easier to visualize than left-sided structures. Diaphragmatic movement with tidal ventilation should be identified. The scanning depth should initially be set to 15 to 20 cm to evaluate basal lung pathology. The scanning depth can then be reduced to 10 cm (and use of a higher-frequency probe can be considered) to facilitate artifact analysis and complete the examination over the whole chest wall.
For each region of interest, the probe should be placed between the rib spaces and aligned with the long axis of the patient, with the orientation marker cephalad. By convention, the orientation marker is on the left side of the ultrasound screen as opposed to TTE, where the orientation marker is on the right side.
Ultrasound analysis and image interpretation
Interpretation of the lung ultrasound image involves the following steps, which should be performed in real-time during tidal ventilation (dynamic image analysis):
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Identification of the extrapleural landmarks, which “frame” the image.
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Identification of the diaphragm during interrogations of basal lung segments.
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Artifact analysis.
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Analysis for lung sliding using M mode.
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Evaluation for pathology that can be directly visualized, such as pleural effusions or consolidation.
Extrapleural landmarks
In 2-D mode with the probe aligned with the long axis of the body, the hypoechoic ribs above and below the pleural space cast a dark shadow that extends to the full depth of the image plane on each side. Between the ribs, the pleural line can be identified as a bright (hyperechoic) horizontal line located 0.5 cm deep to the outer surface of the ribs ( Figure 20-1 ).
Artifact analysis
Ultrasound waves cannot be transmitted through aerated tissue. Normal lung parenchyma is thus not visible beyond the visceral pleura. The interface between the pleura and the lung parenchyma reflects the ultrasound waves and is seen as a bright horizontal line (see Figure 20-1 ), the pleural line. All artifacts (except E lines) used for analysis of lung pathology arise from the pleural line:
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A lines (see Figure 20-1 ) are roughly horizontal repetitions of the pleural line due to reverberation artifacts and are a normal finding. The vertical distance between two adjacent A lines is the same as the distance between the skin and the pleural line. Usually, only one or two repetitions are visible depending on ultrasound gain settings and image depth. A lines are so called because they are reminiscent of the crossbar of the capital letter A framed by the diagonal shadows cast by the ribs.
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B lines ( Figure 20-2 ), also known as comet tail artifacts (as they are called by French authors), or ultrasound lung comets (as they are called by Italian authors), are seen as multiple vertical bright lines originating at the pleural line and fanning out to the bottom of the screen without fading. They arise from reverberations generated at the interface of fluid-filled or fibrosed interlobular septa abutting the visceral pleura. They are referred to as comet tails throughout the remainder of this chapter. The presence of multiple comet tails erases the A line artifact. With increasing loss of aeration from worsening edema, the comet tails become more closely spaced, or confluent (white out) ( Figure 20-3 ). Comet tails are equivalent to Kerley B lines seen on the chest radiograph, although they may be present before radiographic changes are visible. Isolated comet tails, or short, ill-defined vertical artifacts, are of uncertain significance.
Figure 20-2
Comet tails (B) demonstrated with an 8-5 MHz curved array probe (A) and with a 1-5 MHz phased array probe (B).
Figure 20-3
Confluent comet tails creating a white-out appearance demonstrated with a 1-5 MHz phased array probe.
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E line artifacts ( Figure 20-4 ) have a similar appearance to comet tails but arise superficial to the pleural line. They occur with subcutaneous emphysema, where pockets of air create an air–tissue interface that generates reverberation artifacts. It is important to clearly identify the origin of vertical artifacts in relation to the pleural line to avoid mistaking comet tails and E lines.
Figure 20-4
E lines demonstrated with a 1-5 MHz phased array probe. The vertical lines originate at the apex of the sector scan, above the pleural line, which is poorly defined.
Lung sliding analysis
With tidal inflation of the normal lung, the visceral pleura can be seen to slide against the parietal pleura. This is best appreciated using M-mode imaging, which shows an image reminiscent of the seashore; above the pleural line there is a series of horizontal lines created by extrapleural tissue static in time (the sea), and below the pleural line there is a grainy appearance (the beach) ( Figure 20-5 ). The probe must be held completely still. Diagnostic accuracy is improved if a higher-frequency probe is used. Lung sliding is absent if the visceral pleural cannot be visualized because of air in the pleural space (pneumothorax) ( Figure 20-6 ) or if lung movement is abolished (e.g., dense consolidation or fibrosis and atelectasis or apnea). The lung pulse sign (see the later section on atelectasis ) can only be present if lung sliding is absent ( Figure 20-7 ).
