Portable ultrasound is a powerful bedside tool used to evaluate lung pathology in patients in whom pneumothorax, congestive heart failure, and interstitial lung disease are considered in the differential diagnosis
Portable ultrasound has several advantages over more traditional radiographic imaging modalities used in the evaluation of lung pathology. Thoracic ultrasound lacks radiation, provides real-time and dynamic imaging, and can be taken to the bedside. Thoracic ultrasound is especially useful in Emergency Department or Critical Care settings where a patient’s inability to lie flat due to hemodynamic instability or respiratory distress, creates suboptimal positioning and may impair the utility of other imaging modalities.1 Despite its advantages, a major disadvantage is that thoracic ultrasound is operator dependent. Therefore, performing bedside thoracic ultrasound requires supervised training of physician operators to ensure competency criteria is met.2-5
An additional disadvantage is that air is a poor medium for ultrasound waves due to its low density and slow propagation velocity. Healthy lungs contain air and are surrounded by the bony ribcage. Ribs, like any bony surface, reflect ultrasound waves back to the transducer. Rather than visualising lungs directly, pulmonary ultrasound utilises the “disadvantages” of air and bone densities to create various artefacts and detection of movement. These artefacts are discussed below and are key to the identification of lung pathology.
Instrumentation and Technique
Several commercial bedside ultrasound machines are widely available, with varying screen and machine sizes, keyboard layouts, battery capacity, Doppler and Motion mode capabilities, and abilities to store clips or still images. Machines with M-mode or Motion mode capabilities and colour or power Doppler functions are useful for a thoracic ultrasound. A high-frequency 7.5-12 MHz linear probe is optimal for chest wall interrogation, however, a phased array lower frequency 2-5 MHz probe with a flat footprint enabling visualisation between rib spaces may also be used. The lower frequency probe allows for visualisation of deeper structures, which may be an advantage in morbidly obese patients with excessive amounts of tissue overlying bony thoracic landmarks. In most patients, however, the higher frequency probe will allow for the best lateral and axial resolution of pleural and parenchymal abnormalities. Patients may be scanned sitting or supine, and care should be taken with patients in extremis that their positioning does not contribute to a deterioration of respiratory effort or vital signs. Posterior lung fields are optimally imaged with the patient sitting up, and anterior lung fields may be visualised with the patient sitting or supine.
Before scanning, the probe marker should be noted. Each transducer has a notched or otherwise indicated marker, corresponding to the screen marker. Screen markers are often a coloured dot or the machine company logo on the proximal field (top) of the screen. The transducer marker should be oriented towards the patient’s head, so that the transducer is parallel to the sternum, resulting in a scanning field that visualises the ribs on each side of the pleura. Water-based ultrasound gel applied to the probe allows for propagation of the ultrasound wave from the transducer surface to the target tissues.
Anatomic Landmarks
To identify lungs with ultrasound, the acoustic shadowing of the ribs marks the space or margin where the pleural line may be identified. In Figure 1, the transducer is held parallel to the sternum in this long-axis view. The acoustic shadow of the ribs (R) is created by the strongly reflective bony cortex. The pleural line (*) lies between the rib shadows. Higher-resolution probes demonstrate 2 echogenic lines between and deep to the rib cortex. The 2 lines correspond to the visceral and parietal pleura, which appear to slide over each other. Generally, the visceral pleura appears thicker.1, 6, 7
Many bedside ultrasound machines, including machines used by this author in a busy Emergency Department, lack the axial resolution differentiate between visceral and parietal pleura. The pleura line is seen as one line, up to 2mm thick.8
Ultrasound Appearance of Artifacts and Pathology
The pleura surface itself creates an ultrasound artefact. A reverberation artefact is created when the ultrasound beam encounters a strongly reflective surface and there is a difference in acoustic impedance between two tissues, such as the pleural line and the less dense lung. The ultrasound beam is transmitted back to the transducer several times, each time the signal is slightly weaker. This gives the appearance of the reflective surface being distal to, or in the far field from the actual structure. Lines are the reverberation artefact of the pleural line.
B line artefacts, also known as “comet tails”, start from the pleural line, and are hyperechoic, or brighter than the surrounding field. B lines move with lung sliding, whereas A-lines are static. In normal lung, the B lines appear to wipe side to side over the stationary appearing A-lines. The lack of B line movement indicates pneumothorax.
B lines are also key to identifying interstitial lung diseases including pulmonary oedema (cardiogenic and noncardiogenic), neonatal and adult respiratory distress syndromes.9 Several authors have identified different anatomic and causal mechanisms for the sonographic appearance of B lines. In 2009, Soldati et al conducted a 3 part study that included a retrospective analysis of pulmonary ultrasound images in patients with the interstitial syndrome, a literature analysis, and an experimental model of artificially made lung tissue. This study concluded that reverberation artefact creating “ring-down” phenomenon is responsible for the appearance of B lines and this acoustic phenomenon is likely created by the proximity of air bubbles with a critical radius.10 Comet tails are useful in identifying patients with shortness of breath due to congestive heart failure versus chronic obstructive pulmonary disease. The absence of comet tails indicates the presence of chronic obstructive pulmonary disease.7, 11
Due to the pleural traction created from underlying fibrotic lung and thickening of the interlobular septa, B lines appear at least 7mm apart in interstitial lung disease.7, 12, 13 Ground glass appearing lung on chest tomography appear on ultrasound as B lines that are at least 3mm apart.14
The lack of B lines is seen in pulmonary consolidation due to the replacement of the alveolar air with fluid or blood. The consolidated lung may appear with homogenous or heterogeneous echotexture on ultrasound. Acute Respiratory Distress Syndrome (ARDS) demonstrates rib spaces with multiple B lines, few B lines or no B lines. ARDS, while a diffuse lung disease, on CT imaging, demonstrates areas of normal-appearing lung interspaced with focal areas of oedema. These “skip lesions” on CT imaging correspond to the presence of varying numbers of B lines per rib space in a patient with ARDS.
In contrast to ARDS, Respiratory distress syndrome (RDS) of the neonate lacks areas of normal lung. RDS is identified by a high density of B lines, also described as “white lung”, pleural line abnormalities, and the absence of “spared areas”.15
In addition to the aforementioned pathologies, comet tails identify the pleural line for procedural guidance in central venous cannulation, paracentesis, and other thoracic procedures. Comet tails also help to identify the lack of pleural sliding seen in post-procedure pneumothorax.16 Ultrasound is more accurate than supine chest x-ray in the detection of pneumothorax.17, 18 After interjugular or subclavian central line placement, the lung apices may be scanned to evaluate pneumothorax immediately post-procedure, or for unexplained hypotension or ventilation difficulties. After paracentesis, lung fields inclusive of, above and below the puncture site should be imaged for pneumothorax.
In summary:
- A lines: stationary, reverberation artifacts of the pleural line
- B lines: a quick ‘rule of thumb’ is that > 3 per rib space is indicative of interstitial lung disease
- Pneumothorax: No pleural or B line movement across A lines
- Pulmonary edema: B lines < 7mm apart (or > 3 per rib space)
- ARDS: “Skip lesions” with varying numbers or absence of B lines
- RDS: high density of B lines (white lung) with no “skip lesions”
- No B lines: Consolidated lung due to infection, neoplasm, or other cause
References:
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16 Gillman LM, Blavias M, Lord J, Al-Kadi A, Kirkpatrick AW. Ultrasound confirmation of guidewire position may eliminate accidental arterial dilation during central venous cannulation. J Trauma Resusc Emerg Med 2010;18:39: Epub 2010 Jul 13.
17 Brennan, J.M., et al., Reappraisal of the use of inferior vena cava for estimating right atrial pressure. J Am Soc Echocardiogr, 2007; 20(7):857-61.]
18 Kirkpatrick, A.W., et al., Hand-held thoracic sonography for detecting post-traumatic pneumothoraces: the Extended Focused Assessment with Sonography
Virginia M Stewart, MD RDMS RDCS RDMSK
Emergency Ultrasound Director, Emergency Ultrasound
Fellowship Director
Riverside Medical Group
Department of Emergency Medicine
Tel: (757) 594 2000
