Two opposite radiologic presentations, corresponding to different lung morphologies, can be observed.

In patients with focal computed tomographic attenuations, frontal chest radiography generally shows bilateral opacities in the lower quadrants and may remain normal, particularly when the lower lobes are entirely atelectasis (Fig. 23.4).

In patients with diffuse computed tomographic attenuations (Fig. 23.5), the typical radiologic presentation of “white lungs” is observed. If these patients lie supine, lung volume is preserved in the upper lobes and reduced in the lower lobes, although the loss of aeration is equally distributed between the upper and lower lobes.

In fact, interstitial edema, alveolar flooding, or both, not collapse, are histologically present in all regions of the lung in acute respiratory distress syndrome. Compression atelectasis is observed only in caudal parts of the lung, where external forces (such as cardiac weight, abdominal pressure, and pleural effusion) tend to squeeze the lower lobes.

Ultrasound can visualize a succession of B-lines, sign of interstitial involvement, with spared areas, and posterobasal consolidations for atelectasis areas (Fig. 23.6).

When a positive intrathoracic pressure is applied to patients with focal acute respiratory distress syndrome, poorly aerated and non-aerated lung regions are recruited, whereas lung regions that are normally aerated at zero end-expiratory pressure tend to be rapidly overinflated, increasing the risk of ventilator-induced lung injury, like pneumothorax.

Selection of the optimal positive end-expiratory pressure level should not only consider optimizing alveolar recruitment, it should also focus on limiting lung overinflation and counterbalancing compression of the lower lobes by maneuvers such as appropriate body positioning. Prone and semirecumbent positions facilitate the reaeration of dependent and caudal lung regions by partially relieving cardiac and abdominal compression and may improve gas exchange.

For these reasons, CT is essential to monitoring all exchanges [21] (Fig. 23.7); computed tomography allows an accurate assessment of the volumes of gas and lung tissue, respectively, and lung aeration. If computed tomographic sections are contiguous from the apex to the lung base, quantitative analysis can be performed either on the whole lung or, regionally, at the lobar level.

Analysis requires a manual delineation of lung parenchyma and is facilitated by software, including a color-coding system that allows direct visualization of overinflated, normally aerated, poorly aerated, and non-aerated lung regions.

In addition, lung recruitment can be measured as the amount of gas that penetrates poorly aerated and non-aerated lung regions after the application of positive intrathoracic pressure.

In ARDS, the radiographic abnormalities follow a sequence by tending to mirror the histopathological changes; in the first 24 h following insult, when there is a little alveolar edema, the chest X-ray is generally normal. As ARDS evolves, widespread ground-glass opacification invariably becomes apparent and, during the next 36 h, there is a frank consolidation on the chest radiograph (Fig. 23.8); obviously this is a nonspecific radiological sign.

The radiographic changes of uncomplicated ARDS usually plateau after initial catastrophic burst and remain unchanged for a variable time thereafter. Knowledge of this radiographically stable phase can be of practical value to the radiologist; any significant change in serial radiographic appearances like the development of new focal areas of air space opacification may be the harbinger of nosocomial pneumonia.

In the final phase, the chest radiographic abnormalities begin to resolve. Although the lungs may revert to normal, there may be coarse reticular opacities and cysts, likely to be a consequence of the effects of both lung repair and barotrauma; this phase will already start after 8–10 days and requires CT for diagnosis and follow-up [22].

Interstitial emphysema, pneumothorax, and pneumomediastinum may be relatively frequent iatrogenic complications of mechanical ventilation support; thoracic ultrasound permits an accurate and early diagnosis of pneumothorax but not of the two other conditions that require CT.

An important finding of earliest CT studies was that lung involvement in ARDS is totally inhomogeneous, a “patchy” distribution intermittent with normally aerated lung (Fig. 23.9); furthermore the dependent (dorsal) region are more densely opacified than the nondependent (ventral) lung, with a gradient of density [23].

It’s a common thinking that appearance of nondependent opacities may represent foci of organizing pneumonia. ARDS appears to have a far more diffuse opacity than that associated with congestive heart failure; there are many signs to differentiate the two conditions and the principle is pulmonary picture really inhomogeneous in ARDS.

Bronchial dilatation is seen frequently on CT in patients with ARDS; airway dilatation persists in the majority of survivors and is generally associated with ground-glass opacification. The most common abnormality on CT in survivors is a reticular pattern seen in about 85% of patients [24]. The extent of the reticular pattern, that has a predilection for nondependent lung, at follow-up was negatively correlated with the extent of dense parenchymal opacification (in dependent lung) during acute phase; maybe, atelectasis somehow protects dependent regions from the long-term effects of barotrauma (see Fig. 23.5).

One of the principal advantages of CT is that the global extent of parenchymal abnormalities can be quantified accurately and followed-up; furthermore, the effects of PEEP (Positive End-Expiratory Pressure), of recruitment maneuvers, of prone positioning, can be studied using CT.

One of the most frequent etiologies of non-resolving PNX is chest drains/tubes related, as its malposition or its obstruction. These conditions occur usually in emergency department, especially in traumatized patients in which the chest tube placement can be difficult and incomplete, determining a poor drainage of the air from the pleural space [40].

Nowadays, in fact, chest tube placement represents a cornerstone in acute chest trauma management, usually performed as a primary diagnostic procedure, often life-treating decompressing pneumothorax and the underlying collapsed lung, with a success rate ranging between 79 and 95% [41].

The British Thoracic Surgery (BTS) guidelines [42] have described the correct placement of a chest drain in a standardized technique preferably performed by surgeons or emergency physicians, after a prior physical and radiological basic examination with RX or e-FAST or, rarely, CT scan.

Although in emergency care could be difficult,, the chest drain should be inserted, in supine patient’s position, within the “safe triangle,” namely a thoracic area characterized by a low risk of complications like vascular injuries, circumscribed between the pectoralis major and the latissimus dorsi muscle, with the apex of triangle localized below the axilla and the basis as a superior line passing through the nipple; furthermore, it’s also preferable to avoid incisions below the level of the nipple, because of the high risk of intra-abdominal malposition or intra-abdominal organ injuries.

Chest tube should be inserted, after a transverse skin incision, in the 4th–6th intercostal space in the mid-axillary line [43], although, in case of anterior or apical pneumothorax, the 2nd–3rd intercostal space in the medial clavicular line is suitable for chest tube placement, always on the superior rib margin to avoid intercostal neurovascular bundle.

The chest drain insertion in emergency is usually performed percutaneously and requires the correct progression in the pleural space, so that its most peripheral hole, called “sentinel hole,” results all inside the pleural space, to avoid false position or insufficient drainage, and its attachment to the skin with a stitch, to avoid its dislodgement and its inefficiency.

After chest drain placement, it’s important to check its correct position through the execution of a chest radiograph, as well as rule out any complications, occurring between 2% and 10% [44], or iatrogenic injuries, especially in case of residual PNX, or pleural/mediastinal abnormalities.

An incorrect placement of chest tube, as false position or inadequate drainage, in fact, can occur and, sometimes, requires another chest drain to insert. A suboptimal drainage can occur, in fact, in case of kinking of the chest tube involving the intra- or extra-thoracic portion. The incomplete insertion of the chest drainage, detected with a chest radiograph performed after its placement to ensure the correct positioning, or with computed tomography scan during follow-up of a persistent air leak, is a typical etiology of non-resolving PNX, usually characterized by an abnormal seating of the tube, as outside the pleural cavity or even the chest wall; in the first case, the evacuation of air is inadequate; in the second case, instead, a backflow of atmospheric air into the pleural space can lead to suboptimal drainage and persistent PNX.

Risk factors for improper chest drainage placement, as in the chest wall outside the pleural cavity, are, especially in emergency department in traumatized patients, the presence of chest wall emphysema, or obesity or chest habitus like anomalies or deformities too, as well as the presence of scars of previous chest surgeries suggesting extensive intrathoracic adhesions and the risk of complications, especially if performed without any imaging guide.

Also a wrong seal of tissue around the chest tube insertion site could cause a persistent air leak because of the backflow of atmospheric air into the pleural space.

Other malposition or obstruction etiologies are due to intraparenchymal placement of the chest drain, usually due to traumatic pleuro-parenchymal injury or to a wrong and forced insertion, causing pleuro-parenchymal lesions too.

A more rare occurrence is the mediastinal placement of chest drainage, with high risk of severe complications as cardiovascular injuries or esophagus perforation.

Likewise, intrafissural positioning of the chest tube could lead to inadequate evacuation of air from pleural space.

The radiological features of chest drains/tubes malposition are several and detectable on RX or CT scan [29, 45].

Sometimes chest radiographs show an abnormal seating of the tube, as outside the pleural cavity or even the chest wall, leading to inadequate drainage of air and persistent PNX.

If the tube is displaced across the lateral chest wall, chest radiograph is able to identify its abnormal seating, while in case of anterior or posterior chest wall dislocation of drainage, RX is not satisfactory, requiring a CT evaluation.

In case of intraparenchymal tube placement, chest radiograph and/or chest CT identify the etiology of non-resolving PNX; the first modality imaging, in fact, although it could be unremarkable, usually reveals the presence of an opacity along the course of the intrathoracic portion of the chest tube, due to the formation of a hematoma.

CT scan, instead, especially with post-processing modality, as multiplanar reconstruction (MPR), is able to demonstrate the chest tube crossing the lung parenchyma, usually affected by contusive or lacerative injuries too.

CT, in fact, can demonstrate not only an abnormal seating of chest tube, as outside the pleural cavity or the chest wall or within the lung parenchyma or the mediastinum, or a pathological kinking of the chest drain involving the intra- or extra-thoracic portion, or an incomplete seal, often difficult to identify especially in case of subcutaneous emphysema along chest wall. In case of major subcutaneous emphysema, in fact, CT is able to identify correctly a minimum PNX too, which, instead, is very difficult with the RX scan.

CT can also detect any pleuro-parenchymal complications as lung contusions or lacerations, resulting in persistent tubular opacities along the course of the chest drain or any pathological air leak as a bronchopleural or esophageal-pleural fistula or even any cardiovascular injuries.

Bronchopleural fistula (BPF) or persistent air leak (PAL) is an abnormal communication between the bronchial tree, usually a main stem, lobar, or segmental bronchus, or lung parenchyma and the pleural space [46] classified as either central or peripheral, the latter described by some authors as alveolar-pleural fistula (APF), potentially incurring in chest trauma [33].

Another, although more rare entity, is a fistula developing between the esophagus and the pleural space, named esophageal-pleural fistula.

Luckily BPF is not a common entity as it is an important cause of significantly increased morbidity, prolonged hospitalization and mortality, reaching between 18% and 50%, because of the high risk of aspiration pneumonia and acute respiratory distress syndrome, therefore requiring an early diagnosis and a proper management.

BPF can recognize several etiologies, including lung infection (as abscess, tuberculosis, etc.), traumatic injury, neoplastic invasion or iatrogenic causes (as lung biopsy or chest tube insertion or surgery) [46]. Our attention is focused on traumatic bronchopleural fistulae; the reported incidence of BPF after chest trauma, in fact, is extremely variable with a changeable time of onset distinguishable as early (1–7 days), in which the clinical condition is often very unstable needing a timely management with surgical repair and assisted ventilation, or intermediate (8–30 days), or late (more than 30 days), mainly caused by impaired healing of the bronchial leak, often associated with empyema, therefore requiring a pleural drainage too.

Major thoracic trauma, in fact, especially in presence of displaced fractured ribs, can easily cause pleuro-parenchymal injuries as pulmonary lacerations or BPF, because the elastic return of the lungs.

The presence of a bronchopleural fistula would be suspected by clinical signs and confirmed by imaging or endoscopic visualization. Nowadays, in fact, multidetector computed tomography (MDCT) is not only able to detect the fistula tract, but it can also evaluate the number, size, and location of the fistulas as well as identify any underlying traumatic diseases, sometimes avoiding further invasive diagnostic procedure such as bronchoscopies, whose role seems limited to therapy.

The radiological findings suggestive of BPF are aspecific on a plain chest radiograph, usually used as screening modality; in fact, BPF can be suspected in presence of an increased pneumothorax, or an increased opacity of the chest, usually due to pleural effusion or other pleuro-parenchymal complication, like hemo- or pyothorax.

CT scans, in fact, is the best choice in detecting BPF because it can demonstrate the airway injury and the site and the size of fistula as well as help in the surgical planning [47, 48].

CT is an effective noninvasive tool, considered superior to bronchoscopy [47], identifying, on thin slices and by image reconstruction algorithms, the defect of visceral pleura, usually recognized thanks to the presence of extraluminal air bubbles adjacent to a dehiscent bronchial stump.

CT images can demonstrate the direct communication between the bronchial tree, usually a main stem, lobar, or segmental bronchus, or lung parenchyma and the pleural space, as a dehiscent bronchial stump, distinguishing as either central or peripheral BPF.