Ultrasound of the Lung: Clinical Applications

Introduction

Lung ultrasound (LUS) 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. 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. 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, 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 guidelines 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.)

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