Atelectasis Workup

Atelectasis of a significant size can result in hypoxemia as measured on arterial blood gas determinations. Arterial blood gas evaluation may demonstrate that despite a low PaO₂. The PaCO₂ level is usually normal but may be low as a result of the increased minute ventilation.

Atelectasis in the mechanically ventilated patient can result in the reduction of both dynamic and static compliance, and increased shunt physiology. ^[7] Complete left- or right-lung atelectasis in a mechanically ventilated patient can result in decreased lung compliance manifested by a decline in static compliance. Therefore, when monitoring peak and end-inspiratory plateau pressures, one should observe for a rise in both pressures.

Chest radiographs and CT scans may demonstrate direct and indirect signs of lobar collapse. ^[8] Direct signs include displacement of fissures and opacification of the collapsed lobe.

Indirect signs include displacement of the hilum, mediastinal shift toward the side of collapse, loss of volume on ipsilateral hemithorax, elevation of ipsilateral diaphragm, crowding of the ribs, compensatory hyperlucency of the remaining lobes, and silhouetting of the diaphragm or the heart border.

Complete atelectasis of an entire lung (see images below) is when (1) complete collapse of a lung leads to opacification of the entire hemithorax and an ipsilateral shift of the mediastinum and (2) the mediastinal shift separates atelectasis from massive pleural effusion.

With right upper lobe (RUL) collapse, the collapsed RUL shifts medially and superiorly, resulting in elevation of the right hilum and the minor fissure. Rarely, the RUL may collapse laterally, producing a masslike opacity that may look like a loculated pleural effusion. The minor fissure in RUL collapse is usually convex superiorly but may appear concave because of an underlying mass lesion. This is called the sign of Golden S. It is so named because this sign resembles a reverse S shape. It is created by a central mass obstructing the RUL bronchus and should raise suspicion for primary bronchogenic carcinoma, enlarged lymph nodes, or metastatic disease. Tenting of the diaphragmatic pleura juxtaphrenic peak is another helpful sign of RUL atelectasis. ^[9] Upon CT scanning, RUL collapse appears as a right paratracheal opacity, and the minor fissure appears concave laterally. Note the images below.

Right middle lobe (RML) collapse (see images below) obscures the right heart border on a posteroanterior (PA) film. Occasionally, a triangular opacity may be observed. The lateral view demonstrates a triangular opacity overlying the heart because the major fissure shifts upward and the minor fissure shifts downward. Upon CT scanning, the atelectatic RML appears as a triangular opacity against the right heart border with the apex pointing laterally and is termed the "tilted ice cream cone sign."

In right lower lobe (RLL) collapse (see images below), the collapsed RLL shifts posteriorly and inferiorly. A triangular opacity obscuring the RLL pulmonary artery may be observed. The major fissure, which normally is not visible, is seen with RLL collapse. The superior mediastinal structure shifts to the right, causing a superior triangle sign. Laterally, the collapsed RLL blurs the posterior third of the right hemidiaphragm. Upon CT scanning, a paraspinal masslike appearance is observed. Concomitant RML and RLL atelectasis may appear as an elevated right hemidiaphragm or a subpulmonic effusion. An attempt to identify the fissures usually leads to the accurate diagnosis.

In left upper lobe (LUL) collapse (see images below), an atelectatic LUL shifts anteriorly and superiorly. In half the cases, a hyperexpanded superior segment of the left lower lobe (LLL) is positioned between the atelectatic upper lobe and the aortic arch. This gives the appearance of a crescent of the aerated lung, called the Luft Sichel sign. On lateral views, the major fissure is displaced anteriorly and the hyperexpanded RUL may herniate across the midline. On PA views, an atelectatic LUL produces a faint opacity in the left upper hemithorax, obliterating the left heart border. A CT scan demonstrates the inferior location of the collapsed lobe and the shift of the RUL across the midline.

In left lower lobe (LLL) collapse (see images below), increased retrocardiac opacity silhouettes the LLL pulmonary artery and the left hemidiaphragm on frontal views. The hilum shifts downward, and the rotation of the heart produces flattening of the cardiac waist, which is known as the flat-waist sign. The superior mediastinum may shift and obliterate the aortic arch, the top of the aortic-knob sign. On lateral radiographs, opacity makes the posterior third of the left diaphragm indistinct. A CT scan shows the atelectatic LLL in the inferior posterior location.

Rounded atelectasis is a segmental or subsegmental atelectasis that occurs secondary to visceral pleural thickening and entrapment of lung tissue. Rounded atelectasis is usually located in the lower lobes, the lingula, or the RML. On chest radiographs, rounded atelectasis manifests as a subpleural mass, with bronchovascular structures projecting out of the mass toward the hilum. An associated parietal pleural plaque may be present. The swirl appearance of bronchovascular shadows is called the comet-tail sign. ^[10]

Flexible fiberoptic bronchoscopy can be a useful diagnostic and therapeutic procedure. Bronchoscopy helps evaluate the cause of bronchial obstruction. In addition, bronchoscopy helps clear mucous plugs when they cause bronchial obstruction. Bronchoscopy has limitations. Because only the subsegmental bronchi are visualized, a distal endobronchial lesion is not accessible through bronchoscopy.

In an evaluation of middle lobe syndrome, bronchoscopy may show an endobronchial etiology (mucus plugging/broncholithiasis/tumor), although inflammatory processes and defects in the bronchial anatomy and collateral ventilation have been designated as the nonobstructive causes of the syndrome.

During fiberoptic bronchoscopy, the washing, brushing, and biopsy specimens of any obstructing mass should be examined for evidence of malignancy or Aspergillus mucous plugging (ie, allergic bronchopulmonary aspergillosis).