Ultrasound of the Lung: Clinical Applications

Massimiliano Meineri

Abstract

Lung ultrasound can be used like X-ray in the detection of life-threatening conditions, such as pneumothorax. It can be performed with any standard ultrasound probe and relies mostly on the interpretation of pattern of artifacts. Lung ultrasound can also provide quantitative information on pleural effusion and guide safe drainage. Similarly, it provides quantitative information on lung edema and interstitial water. Finally, it accurately displays consolidation and atelectasis. Its role in the emergency and critical care setting has been well established.

Keywords

point of care ultrasound; lung ultrasound; pneumothorax; pleural effusion; interstitial syndrome

Introduction

Lung ultrasound (LUS)¹ has become standard of care in many clinical settings and has proven feasible at the bedside for detecting life-threatening conditions, such as pneumothorax, with a sensibility and specificity often superior to standard chest radiography.² For some time, the use of ultrasound (US) to assess the lung has been questioned because of the inherent physics of US and the structure of the chest and respiratory system. A normally aerated lung is constituted by more than 80% air and is surrounded by the ribcage. US waves are almost entirely reflected by the interface between soft tissue, air, or bone therefore making these two tissues impenetrable to US, and making imaging their structure not feasible.

In fact, chest US, displays images of the soft tissues between the probe and the parietal pleura and only artifacts there beyond (Fig. 54.1). The lung becomes visible only when the air is replaced by fluids (edema), tissue (infection), or the alveoli are emptied from the air (atelectasis).

• Fig. 54.1 Lung ultrasound image of the pleura using a curved array probe, showing the subcutaneous tissue and artifacts beyond the pleural line. (Modified from http://pie.med.utoronto.ca/POCUS/POCUS_content/lungUS.html.)

For these reasons, LUS is based on the interpretation of patterns of artifacts.

Probe Selection and Scanning Protocols

LUS can be performed using any type of the US probe. The most commonly available include linear (typically used for vascular access), curved (typically used for abdominal scanning), and phased array (cardiac) probes. The microconvex array probe is a curved probe with a smaller footprint and, although not commonly available, it has been suggested to offer the best compromise for LUS.³ The main difference among probes is the US frequency, being the highest for the linear probe and the lowest for the phased array. Probe frequency is directly related to the image resolution but inversely related to the penetration into soft tissue. For this reason, the linear probe can only be used to assess superficial structures (Fig. 54.2).

• Fig. 54.2 Commonly available ultrasound probes and their characteristics. (Modified from http://pie.med.utoronto.ca/POCUS/POCUS_content/lungUS.html.)

LUS is performed by placing the probe perpendicular to the ribs and pointing the probe marker toward the patient’s head. LUS images can be displayed using the cardiology (image marker on the right upper corner) or the radiology (image marker on the left upper corner) convention (Fig. 54.3). This means that the marker indicates the cranial orientation.

• Fig. 54.3 Probe orientation, convention, and scanning zones. (Modified from http://pie.med.utoronto.ca/POCUS/POCUS_content/lungUS.html.)

Several protocols have been described for a complete LUS examination,⁴ which include a variable number of views. Each side of the chest has been conveniently divided by the anterior and posterior axillary lines into anterior, lateral, and posterior. The anterior and posterior quadrants are further divided in half into superior and inferior, making a total of five quadrants (Fig. 54.4). Current guidelines⁵ recommend a complete examination to obtain at least one US view of all quadrants on each side. Although air tends to move up and fluid follows gravity, in an emergency scenario the highest portion of the chest would be scanned to rule out pneumothorax and the lowest to rule out an effusion.

• Fig. 54.4 **Lung ultrasound scanning zones**. The four chest areas per side considered for complete eight-zone lung ultrasound examination. These areas are used to evaluate for the presence of interstitial syndrome. Areas 1 and 2 denote the upper anterior and lower anterior chest areas, respectively. Areas 3 and 4 denote the upper lateral and basal lateral chest areas, respectively. *AAL*, Anterior axillary line; *PAL*, posterior axillary line; *PSL*, parasternal line. (Modified from Volpicelli G, Elbarbary M, Blaivas M, et al. International evidence-based recommendations for point-of-care lung ultrasound. *Intensive Care Med*. 2012;38(4):577–591.)

Anatomic Correlation and Normal Physiologic Findings

LUS comprises two standard views: the pleural view (Fig. 54.5) (obtained from all scanning points above the diaphragm) and the diaphragm view (Fig. 54.6) (at the level of the diaphragmatic dome). The pleural view is used to assess pleural sliding and underlying pleura and lung parenchyma. The diaphragm view is used to assess diaphragmatic movement and rule out pleural effusion.

• Fig. 54.5 **Pleural view**. Probe orientation, ultrasonographic-anatomic correlation, and “ostrich” sign. (Modified from http://pie.med.utoronto.ca/POCUS/POCUS_content/lungUS.html.)

• Fig. 54.6 Right and left pleural views and negative spine sign. The vertebral bodies can only be seen on lung ultrasound below the diaphragm. (Modified from http://pie.med.utoronto.ca/POCUS/POCUS_content/lungUS.html.)

To obtain the pleural view, the probe is positioned perpendicular to the ribs. It is first rotated to bring the ribs close together and then tilted until the pleura is best displayed. Using the linear probe, the ribs will appear on US as two black round structures, casting a shadow underneath (see Fig. 54.5). The pleural line can be identified with certainty as the first bright white line below the ribs upper contour. The space between the probe and the pleural line is filled by the intercostal muscles. This US image may be compared with an ostrich head close up where the ribs are the eyes and the pleural line the beak. This view has also been associated with the shape of a bat whose wings would correspond to the upper edge of the ribs and the chest to the pleural line. A similar view can be obtained with all probes (see Fig. 54.2), however, with an increased sector depth (more than a few centimeters), a series of horizontal lines, parallel to the pleural line can be appreciated underneath. These are called A lines; they are considered a normal finding and are the result of a reverberation artifact. Given that the pleura is a strong reflector, the same US waves come back to the probe still very strong, bounce back off the probe toward the pleura, and travel back and forth several times. Given the duration of travel from the probe to the pleura, and back is constant, A lines are equidistant from each other at the same exact distance between the probe and the pleural line (Fig. 54.7).

• Fig. 54.7 A lines are caused by reverberation. The pleural line is a strong reflector and the same ultrasound waves travel multiple times (*red, blue, yellow*)back to the probe and are reflected by the probe head and return to the pleura. (Modified from http://pie.med.utoronto.ca/POCUS/POCUS_content/lungUS.html.)

The thickening of the interstitial space, as well as the presence of water in the alveoli, is responsible for the generation of reverberation of US between the most superficial alveoli and the parietal pleura resulting in vertical laser-like line originating from the pleural line, moving with the pleural sliding, and travelling to a depth of 15 cm.⁵ These are called B lines (Fig. 54.8) and can only be displayed using a curved or a phased array probe. B lines are non- specific signs and do not correspond to a single pathology. A single B line in one US view has no pathologic meaning and may be because of interstitial irregularities or localized atelectasis.