Findings: PA chest film shows RUL air-space disease with an apparent cavitation (A, red arrow). The infiltrate is limited inferiorly by a descended minor fissure (A, white arrow). Two years earlier the fissure was in the same position (B, white arrow) and a rounded opacity was visible in the RUL (B, yellow arrow).

Expiratory film taken two years earlier depicted air-trapping in RUL (C). CT at that time showed increased lucency of the posterior segment of RUL (D, circle) with a large mucous impaction (D, arrow). This appearance is highly suggestive of congenital bronchial atresia.

Final diagnosis: pneumococcal pneumonia in a patient with congenital bronchial atresia

I am showing this case to discuss the subject of expiratory films. Standard chest radiographs are taken with the patient upright and in full inspiration. In certain circumstances, expiratory films may complement the information offered by the inspiratory film.

There are two main indications for acquiring an additional expiratory view:
When pneumothorax is suspected and the inspiratory film is not helpful
To confirm or exclude air-trapping of the lung

Here I’d like to discuss the diagnostic value of expiratory films.

The most common indication for an expiratory film is to confirm/exclude a suspected pneumothorax that is not seen in the inspiratory radiograph (Fig. 1). Negative findings on expiration are practically conclusive for ruling out pneumothorax (Fig. 2)

Fig. 1: 24-year-old man with sudden chest pain suggestive of a pneumothorax. The inspiratory film (A) is inconclusive. The expiratory film clearly shows a right pneumothorax (B, arrows).

Fig. 2: 79-year-old woman with a peripheral line that suggests a pneumothorax (A, arrows). The line disappears in the expiratory film (B), confirming that it is caused by a skin fold and pneumothorax is not present.

The second indication to obtain an expiratory film is to confirm/exclude air-trapping of the lung. Nowadays, CT on expiration is widely used to detect areas of small airway obstructive disease, but expiratory views are little used in conventional radiography. A pity, because they are a great source of information.

Remember that you cannot diagnose air trapping unless you have an inspiration/expiration pair.

Localised areas of air trapping within the lungs are easily demonstrated with expiratory films, and indicate further workup to establish an etiologic diagnosis (Fig. 3).

Fig. 3: 45-year-old asymptomatic man. PA chest film shows increased lucency of the left upper lung (A, circle) and an ovoid shadow within it (A and B, arrows). The expiratory film depicts air-trapping in this area (B, circle).

Axial CT depicts a large area of lucent lung in the apical-posterior segment of the LUL (C, circle) with a dilated bronchus filled with mucus and air in the center (C, arrow). Diagnosis: congenital bronchial atresia

When an entire lung traps air, the expiratory film confirms the diagnosis and identifies the affected lung as the one that fails to empty on expiration (Fig. 4).
It is important to emphasise that air trapping can only be confirmed with inspiration/expiration films, because a single inspiratory film can be deceptive (Fig. 5).

Fig. 4: 3-year-old child with suspected foreign body aspiration. Inspiratory film (A) looks normal. Expiratory film shows air-trapping of the whole right lung with mediastinal deviation towards the left (B, arrows). A foreign body was retrieved from the right main bronchus.

Fig. 5: patient with extrinsic asthma. Inspiratory film (A) shows increased lucency of the right lung and a smaller left lung. On expiration (B), the right lung lucency disappears and the left lung remains the same. This appearance is typical of obstruction of the left main bronchus: on inspiration, air enters the right (normal) lung, which becomes more lucent. On expiration, the right lung empties.

Note that the left hemidiaphragm does not move (it is superimposed on the 9th rib in inspiration and expiration).

Coronal and sagittal CT images show obstruction of the left bronchus by thick mucus (C and D, arrows). The patient was treated with mucolytics and bronchodilators, and the appearance of the chest returned to normal.

Broadly speaking, the two main causes of air-trapping are:

• Central bronchial obstruction, of any cause
• Peripheral obstructive disease of the small airways.

Figs. 4 and 5 are good examples of air-trapping due to a central obstruction. CT is diagnostic in these cases and is the technique of choice when a segmental bronchial lesion is suspected, but not visible on conventional radiography (Fig. 6).

Fig. 6: 42-year-old man with persistent cough. Inspiratory CT shows slight LLL volume loss (A, red arrows) with a suggestion of an endobronchial lesion (A, yellow arrow). The expiratory image shows LLL air-trapping (B, circle). Sagittal reconstruction confirms a small rounded lesion in the LLL bronchus (C, arrow). Diagnosis: carcinoid

Peripheral obstructive disease of distal bronchi may affect the entire lung or a part of it (Fig. 7), or be widespread, a sign of diffuse small airway disease (Fig. 8). Swyer-James/McLeod syndrome is a well-known cause of unilateral hyperlucent lung. In these patients, the main bronchi are permeable and the air-trapping is secondary to peripheral bronchiolitis following viral infection in childhood (Fig. 7).

Fig. 7: 47-year-old woman with MacLeod syndrome. PA film shows a hyperlucent right lung with decreased vascularity (A). Inspiratory CT shows decreased vascularisation and bronchiectasis (B, arrow). Expiratory CT confirms right air-trapping (C).

Expiratory CT is now widely used to detect small airway obstructive disease, characterised by the typical mosaic attenuation and air-trapping (Fig. 8).

Fig. 8: patient with diffuse bronchiolitis showing areas of mosaic attenuation on inspiration (A) and air-trapping on expiration (B). This study was done to prove to a skeptical colleague that the white areas on inspiration were normal and the dark areas abnormal. It dates from 1984, which makes me feel like a pioneer.

Lung hernias are uncommon. They may not be visible in inspiratory films, but can manifest in expiratory views. Lung hernia is secondary to weakening of the chest wall and is usually related to previous surgery. It can occur in the apex of the lung (Fig. 9), but most (80%) are intercostal (Fig. 10).

Fig. 9: 49-year-old asymptomatic woman. The upper lungs appear normal on inspiration (A). Expiratory film shows apical herniation of the right lung (B, arrow). Incidental azygos lobe.

Fig. 10: 41-year-old woman who underwent chest surgery in childhood. PA chest film shows a right extrapulmonary opacity (A, arrow). The corresponding intercostal space is widened. Coronal CT confirms the widened intercostal space and atrophy of the intercostal muscles (B, circle). The extrapulmonary opacity is not visible.

The patient was placed in the left decubitus position, and fat herniation was confirmed (C, arrow). Expiratory CT showed lung herniation at the same weak spot (D, red circle).

Follow Dr.Pepe’s Advice:

1. The most common use of expiratory films is to demonstrate a doubtful pneumothorax.

2. Expiratory films are necessary to confirm/rule out air-trapping.

3. The main causes of air-trapping are central bronchial obstruction and peripheral small airway disease.