The black lung field

Published on 23/05/2015 by admin

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CHAPTER 5 The black lung field

5.1 Chronic obstructive pulmonary disease 106

5.2 Pneumothorax 110

5.3 Tension pneumothorax 112

5.4 Pulmonary embolus 114

5.5 Mastectomy 119

5.1 Chronic obstructive pulmonary disease (COPD)

This is the chest film of a patient with chronic obstructive pulmonary disease. The lungs look larger in volume than normal. The diaphragms are rather flattened. The right upper zone and many areas of the left lung are abnormally black. In these areas the blood vessels are difficult to see or are very thinned. This is because these regions of lung have developed emphysema.

The CT section shows how the lung contains black holes (1) and thin thready blood vessels (2).

When trying to decide the cause of bilateral black lungs you need to:

1. Check the penetration. Look at the vertebral bodies behind the heart. Remember that in a good quality X-ray the vertebral bodies become harder to see behind the heart shadow. If they are too clearly seen the film is over penetrated making the lungs appear black.

If you are satisfied with the technical quality of the film then the most likely cause is COPD. COPD is associated with large lungs due to air trapping and the development of bullae. You therefore need to:

1. Look at the shape of the diaphragm. In COPD the diaphragms are flat or even scallop shaped instead of concave upwards. This is a more reliable sign of hyperexpansion than rib counting.

2. Count the number of ribs you see anteriorly. If the lungs are enlarged you should be able to count more than seven. Be careful, however, because you can sometimes count more than seven ribs in normal patients if they are tall and slim.

3. Look at the shape of the heart. The enlarged thorax of COPD appears on the X-ray to elongate and narrow the heart, elevating the lower border. The heart, instead of sitting on the diaphragm, often appears to ‘swing in the wind’. It will also appear small unless there is also an element of cardiac failure in which case it will be normal in size or large.

4. Look for bullae. These are densely black areas of lung, usually round, surrounded by fine curvilinear shadows. Bullae distort the surrounding vasculature so to help find them look out for areas of distortion of vascular markings.

5. Look at the distribution of lung markings. The black lungs of COPD are due to reduced size of blood vessels. The lung markings are reduced bilaterally and fan out in straight lines from the hilum, starting off chunky but stopping two-thirds of the way out – peripheral pruning.

Cigarette smoking causes damage to the parenchyma of the lung. Cigarette smoke in a room tends to float upwards, and similarly smoking related lung damage tends to occur in the upper lungs. This image shows how the normal smooth lung texture has been destroyed with the formation of air cysts (bullae – 1) and small black holes in the lung (centrilobular emphysema – 2). The blood vessels through these areas can also become very narrowed, as the body will not perfuse areas of the lung where gas exchange is significantly impaired.

5.2 Pneumothorax

This patient has a left-sided pneumothorax with partial collapse of the left lung. The outer left lung field is black. You can see the lung edge (arrowed).

When you see a unilateral black lung you need to:

1. Check the technical quality of the film. A rotated film may make one side less dense than the other.

2. Determine which side is abnormal. This is usually easy since the side with reduced lung markings will be the abnormal side.

You must now decide the cause of the blackness. Lung markings are made up of bronchi and blood vessels and it is their absence that makes the lung look black. Vascular shadows will disappear if the lung is replaced by air, which will occur with a pneumothorax or bullous or cystic lung disease or if the vessels are deprived of blood as in a pulmonary embolus. Therefore think pneumothorax, bullae/cyst or pulmonary embolism and:

1. Look for a lung edge. In a pneumothorax you will see the edge of lung which is not normally seen. Look carefully at the upper zone where air will accumulate first. Your eye is trained to see horizontal lines better than vertical so it is sometimes easier to detect the lung edge with the X-ray turned on its side.

2. Look at the mediastinum. Obvious mediastinal shift away from the black lung suggests that a tension pneumothorax is developing. This is a medical emergency and you need to urgently reassess the patient (see also p. 112).

3. Look at the rest of the lungs. Bullous disease is more likely if bullae or emphysematous changes are seen in the rest of the lung.

4. Differentiating between a pneumothorax and a bulla can be difficult and often impossible. Look at the distribution of the blackness. In a pneumothorax it will be peripheral and upper zone, or lateral and even underneath the lung. Bullae are within the lung and have curvilinear convex margins. In a pneumothorax the edge of the blackness will run parallel to the chest wall, which will not be the case with a bulla lying within the lung.

5. Look carefully for lung markings. If you see them crossing the area of blackness, you are probably looking at a bulla. If you see lung markings peripheral to the blackness, it is also probably a bulla.

Some causes of a pneumothorax

Iatrogenic/trauma, e.g. pleural tap, transbronchial biopsy, central venous line insertion, mechanical ventilation

Obstructive lung disease, e.g. asthma, COPD

Infection, e.g. pneumonia, tuberculosis

Cystic fibrosis

Connective tissue disorders, e.g. Marfan’s, Ehlers–Danlos

5.3 Tension pneumothorax

This chest film shows the potentially fatal condition of a tension pneumothorax. The build-up of pressure in the thorax on the side of the air leak can obstruct cardiac venous return. This can be potentially fatal if the pleural cavity is not urgently drained.

If you suspect a pneumothorax as a cause of a black lung field (see p. 110) you must consider carefully whether it may be under tension since this is a medical emergency.

1. Look at the size of the blackness. In a tension pneumothorax the black lung is usually very large.

2. Look at the position of the mediastinum. In a tension pneumothorax it will be shifted away from the affected lung.

3. Look at the shape of the mediastinum. Look at the edge on the side of the blackness. If it is concave to the side of the blackness you should suspect a tension pneumothorax.

4. Always remember the patient. A tension pneumothorax can develop at any time and if the patient suddenly becomes distressed the absence of tension on a previous X-ray does not exclude the diagnosis.

5.4 Pulmonary embolus (PE)

A large pulmonary embolus is a cause of black lung. However, a pulmonary embolism is very rarely detected on a plain chest X-ray and the main reason for doing a plain film is to exclude other causes of shortness of breath, such as pneumonia or pulmonary oedema. Of far more use for the detection of pulmonary emboli are ventilation/perfusion () scanning and CT pulmonary angiogram (CTPA).

scanning

The scan is a nuclear medicine test. This uses small low-dose radioactive particles to compare the pattern of perfusion with that of ventilation. For the ventilation part of the test the particles are suspended in a gas, which is then inhaled, and stick to the walls of the airways to show airflow. For the perfusion part of the study the particles are injected intravenously and lodge in the very small blood vessels in the lungs. The distribution of radioactivity within the lung is then monitored using a gamma camera. Normally four views are taken (anterior, posterior, and right and left posterior oblique).

In the normal lung ventilation and perfusion should match. In a pulmonary embolism the blood supply to a region of lung is reduced but the ventilation maintained, so-called ‘ventilation/perfusion mismatch’. In some lung diseases, e.g. pneumonia, the ventilation and perfusion may both be reduced giving a so-called ‘matched defect’. Other lung diseases, for example COPD, can result in a mismatch between ventilation and perfusion and in practice in a PE it is very common to see a mixture of both matched and mismatched defects. A scan is therefore only of value in a patient with otherwise normal lungs.

The scan does not give a definite diagnosis of pulmonary embolus (PE). Instead it gives you the probability of the patient having a PE. As such it must be interpreted alongside the clinical situation. A normal scan will virtually exclude a PE, certainly one of clinical significance.

In order to interpret a scan:

1. Look at the plain chest X-ray. Abnormalities on the chest X-ray may affect ventilation and perfusion and so may make interpretation of the scan difficult. A scan can be interpreted with much more confidence in the presence of a normal chest X-ray. Someone with multiple areas of lung pathology on their chest film, e.g. consolidation or extensive COPD, is likely to have an inconclusive

scan, and may be better investigated by CTPA.

2. Look at the perfusion scan (labelled Q). A dark speckled pattern is seen over the well-perfused areas, with under-perfused areas appearing as lighter holes. Compare these areas to the ventilation scan (marked V). If they also appear light then they are classified as matched defects. If ventilation is normal they are classed as mismatched defects.

3. Make sure that you have looked at both lungs. In the presence of a massive PE it is possible for a whole lung to be obliterated on the perfusion scan.

4. The test needs to be carefully interpreted by a radiologist who will determine the number and size of matched and mismatched defects to work out the radiological probability of a PE.

scan in patient with pulmonary emboli. Note how the perfusion image is more patchy than the smoother texture of the ventilation scan.

CT pulmonary angiogram

A CT pulmonary angiogram (CTPA) is essentially a contrast enhanced spiral CT scan in which the administration of contrast is timed to highlight the pulmonary arteries. Vessels blocked by clots will appear darker. It is a sensitive means of detecting emboli within central or segmental arteries, although it will not necessarily detect more minor peripheral emboli. Any pulmonary embolus that causes significant breathlessness should be detectable on a CTPA.

Think before you order a CTPA. These scans are often undertaken in young people and involve significant amounts of radiation. Make sure that you have stratified the clinical risk by taking an appropriate history and reviewing the D-dimer result, and consider whether a scan or ultrasound imaging of the peripheral veins to look for a deep vein thrombosis would be a reasonable alternative.

In order to interpret a spiral CT scan you need to:

1. Look at the contrast enhanced scans, usually labelled as such.

2. Look at the main pulmonary artery and then follow your way through the scans to examine the major pulmonary vessels. With the spiral CT scan you will be able to visualize the clot, which will appear as a filling defect – a dark area surrounded by whiter contrast. Clot tends to lodge across bifurcations in the vessels and may be seen extending down the vessel towards the periphery. The vessel may be expanded by acute clot compared to the equivalent vessel in the other lung.

3. Look for accompanying features of an embolus. Look on the lung windows for areas of consolidation, which may be wedge shaped, and also look for small pleural effusions.

4. Remember that even if the CTPA does not show a pulmonary embolus it may show another cause for the patient’s symptoms.

Changes of infarction

Although a PE is a cause of black lung the findings you will usually see following a PE are those due to subsequent infarction of the lung, leading to haemorrhage or lung necrosis. This may cause the following changes on X-ray:

Raised hemidiaphragm

Collapse and linear atelectasis

Pleural effusion

Wedge-shaped shadowing

This patient became suddenly short of breath. CTPA showed a pulmonary embolism. Clots can be seen in both the right pulmonary artery and the left lower lobe pulmonary artery. (arrows)

A plain film diagnosis of a pulmonary embolism is very rare.

This X-ray is of a patient who has sustained an acute large pulmonary embolus. Look carefully at the right upper zone. Immediately above the horizontal fissure there is an area (1) which is blacker than the left side at the same level. This is Westermark’s sign of reduced perfusion to that area of lung which indicates that the artery to this area contains a large clot. Note also an area of consolidation below the horizontal fissure (2) – this is a small focus of infarction.

5.5 Mastectomy

Film of a patient who has undergone a unilateral mastectomy on the right side.

It is important to remember that problems outside the lung can sometimes cause the lung fields to look too black (or too white). This is why it is important always to examine the chest X-ray for soft tissue markings.

A mastectomy will make the underlying lung look too black since there will be less soft tissue overlying the lungs on the affected side, compared to the normal side. Therefore, if one lung looks blacker than the other, look carefully for the breast shadows. There will be an absent breast shadow on the side of the mastectomy.

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