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 PaCO2 = 24.2

 PaO2 = 59.5

 PAO2 = (0.21 × (760 – 53)) – (24.2/0.9)

 PAO2 = 121.6

 P(A–a)O2 = 121.6–59.5

 P(A–a)O2 = 62.1 (indicates hypoxemia is due to pulmonary dysfunction)

Thoracic ultrasound, also known as thoracic focused assessment with sonography for trauma, triage, and tracking (TFAST), allows clinicians to assess for pleural and pericardial effusion, pneumothorax, and pulmonary parenchymal infiltrates [15–20]. It is particularly useful in patients that are not stable enough for thoracic radiographs, as well as a monitoring tool to assess for response to therapy. Thoracic ultrasound may be performed with the patient in sternal or lateral recumbency. Pleural effusion is generally visible in the cranial and/or caudoventral pleural space. Ultrasound guidance to localized fluid pockets can be helpful to guide thoracocentesis. When evaluating for the presence of pneumothorax, the caudodorsal thorax is evaluated for the lack of a “glide” sign, which is diagnostic for pneumothorax. A glide sign is created by the normal back and forth respiratory motion of the interface between the visceral and parietal pleura (Video 1.1). Free air in the thoracic cavity obliterates the glide sign [15–17]. Cellular or fluid infiltrate into the pulmonary parenchyma, as with edema, hemorrhage, and pneumonia can be assessed using ultrasound in four windows in each hemithorax (caudodorsal, cranial, middle lung lobe regions, and perihilar) for the presence of increased penetration of ultrasound, which manifest as hyperechoic lines (B‐lines) in parallel with the ultrasound beam, that can be individual or coalescing (Figure 1.4 and Video 1.2) [18–22].

Video 1.1 TFAST showing a normal glide sign, which is created by the respiratory motion of the visceral and parietal pleural interface sliding back and forth.

Video 1.2 TFAST showing coalescing B‐lines created by marked pulmonary infiltrates allowing ultrasound penetration into the pulmonary parenchyma.

Figure 1.4 TFAST ultrasonographic appearance (still image) of a B‐line, which is created by increased infiltrates in the pulmonary parenchyma allowing ultrasound penetration.