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# (p. 133) Pleural assessment

Pleural assessment
Chapter:
Pleural assessment
DOI:
10.1093/med/9780198749080.003.0015
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date: 24 September 2021

## Introduction

This chapter describes the use of US to assess pleural disease in the ICU setting. The US signs of pneumothorax are described and the specificity reviewed, with the pitfalls highlighted. The assessment and quantification of pleural effusions are outlined, together with the limitations and pitfalls.

## Benefit of ultrasound

Thoracic US is a very powerful tool for the assessment of pleural pathology. US is the gold standard method for the assessment of pleural effusions and is more sensitive than chest radiography for the diagnosis of pneumothorax. US is very sensitive in the diagnosis, assessment, and quantification of pleural effusions, as well as guiding safe diagnostic aspiration and therapeutic drainage. US is the imaging modality of choice when it comes to identifying complex loculated effusions. Pneumothorax can be accurately and rapidly ruled out or diagnosed using specific US signs in a time frame that is impossible to achieve with chest radiography or CT scanning.

## Normal sonoanatomy

### Pleural sliding

The adjacent parietal and visceral pleura can be visualized directly, even in the absence of pathology (Video 1.11.1 ). Lung sliding or gliding describes the US appearance of the movement of the two adjacent pleural layers seen throughout the respiratory cycle in normal lung. It is a shimmering back-and-forth movement seen at the pleural line and is an important US sign to recognize. The presence or absence of pleural sliding can provide useful and accurate information about the underlying lung.

With a postero-lateral scan at the level of the hemidiaphragm, pleural sliding is easy to visualize because, during inspiration, the curtain of aerated lung appears to ‘slide’ over the hemidiaphragm, obliterating it and the underlying structures. During expiration, the lung ‘slides’ back, once again, revealing the hemidiaphragm (Video 1.11.2 ). When scanning above the hemidiaphragm or over the anterior chest wall, sliding may be harder to identify. In these circumstances, reducing scan depth and changing probe will make pleural sliding easier to appreciate. The high-frequency linear array probe is better placed to visualize more shallow structures, and it will give a more detailed view of the pleura.

### Seashore and stratosphere signs

M-mode can be used to confirm or rule out lung sliding, as it makes the presence or absence of movement at and below the pleural line easier to visualize. The seashore sign (Figure 15.1) confirms the presence of lung sliding at the rib space being visualized. This is compared to the stratosphere sign (Figure 15.2), which reflects the absence of lung sliding at the rib space being visualized. The differentiation between these two M-mode signs can be demonstrated by asking a normal model to hold their breath in inspiration while you scan their anterior chest wall. In normal respiration, the seashore sign should be apparent, and during a breath-hold, the stratosphere sign should be seen.

Figure 15.1 The seashore sign. Normal lung sliding. The subcutaneous tissue and musculature create straight lines representing the ‘sea’. Below the pleural line, the continually moving aerated lung creates the speckled ‘sand’.

Figure 15.2 The stratosphere or barcode sign. Absent lung sliding. The straight lines created by the static subcutaneous tissue and musculature continue beyond the pleural line. The absence of movement deep to the pleural line creates multiple straight lines.

## Ultrasound assessment of the pleura

The pleura is visualized over the entirety of the chest wall, excluding subscapular and substernal areas. Particular pleural pathologies are more likely to be found in specific areas of the thoracic cavity, which will influence where you prioritize your scanning. A pneumothorax is most often present in the apical region of the thoracic cavity in a seated or semi-recumbent patient and can be identified by scanning the anterior chest wall. In contrast, pleural effusion, unless loculated or confined by tethered lung, is affected by gravity and is most usually seen in the postero-lateral areas. There are, of course, exceptions to these rules of thumb, and you are encouraged to take a systematic and thorough approach to scanning the lungs for pleural pathology.

## Common pathophysiology

### Pneumothorax

The optimum place to scan for a pneumothorax is the anterior aspect of the chest wall. Usually the collapsing lung will move down and in, towards the mediastinum as the pneumothorax expands, meaning the US signs of a pneumothorax are first found anteriorly. When the diagnosis of a pneumothorax is suspected, the scan should start with the upper and lower anterior points bilaterally. Perform a full assessment of the pleura beneath the anterior chest wall, before moving to the lateral and posterior areas.

The US signs of a pneumothorax are:

• Absence of pleural sliding

• Absence of sliding B-lines

• Absence of a lung pulse

• Presence of a lung point.

The presence of pleural sliding, sliding B-lines, or a lung pulse reliably rules out a pneumothorax in the area of the lung being scanned. A lung point (described in Presence of a lung point, pp. 135–7) identifies the boundary between the empty, but air-filled, thoracic cavity and inflating and deflating lung and is a sign that rules in a pneumothorax. The combination of absence of pleural sliding, absence of B-lines, and presence of a lung point has been shown to have a specificity and positive predictive value approaching 100% in diagnosing a pneumothorax.

#### Absence of pleural sliding

This identifies that there is no movement of the visceral pleura over the parietal pleura with respiration (Video 1.11.3 ). It occurs in any condition that prevents ventilation of the lung immediately underlying the area being visualized and is not a sign that is specific to a pneumothorax. Absent pleural sliding is therefore seen in atelectasis, consolidation, endobronchial intubation, and breath-holding, in addition to pneumothorax. Pleural sliding may also appear to be absent in severe parenchymal lung disease that prevents lung expansion. Lung hyperinflation as a result of obstructive airways disease or application of high levels of positive end-expiratory pressure (PEEP) may also make pleural sliding difficult to identify, particularly in the apical and anterior regions.

Pitfall

Absence of pleural sliding does not diagnose a pneumothorax; the presence of pleural sliding excludes a pneumothorax in the underlying lung.

#### Absence of sliding B-lines

B-lines arise from subpleural interlobular septa and will only occur if the visceral and parietal pleura are directly adjacent. Multiple B-lines are a pathological finding, but a single B-line excludes the presence of a pneumothorax in the area of the lung being scanned.

#### Absence of a lung pulse

A lung pulse describes movement of the pleural line as a result of transmission of the cardiac pulsation through underlying static lung tissue (Video 1.11.4 ). Subtle pleural movement is seen to occur at the same frequency as the heart rate, while no movement is seen in time with respiration. For the phenomenon to occur, the parietal and visceral pleura must be adjacent and the lung tissue must be stationary. It is best demonstrated with M-mode, which reveals an underlying ‘stratosphere’ appearance with superimposed intermittent bursts of a ‘seashore’ appearance in time with the heartbeat (Figure 15.3). A lung pulse occurs with atelectasis, endobronchial intubation, and breath-holding. The presence of a lung pulse excludes a pneumothorax in the area of the lung being scanned.

Figure 15.3 Lung pulse. The cardiac pulsations (see arrows) transmitted through static lung are clearly visualized using M-mode. This ultrasound sign is known as the ‘lung pulse’.

#### Presence of a lung point

A lung point is a US sign specific to pneumothorax. It requires the US probe to be placed over a point on the chest wall where the border of the lung and pneumothorax crosses under the footprint of the US probe during the respiratory cycle (Figure 15.4).

Figure 15.4 The lung point. When the probe is placed over the lung point, the aerated lung will move under the probe on inspiration and back on expiration. This can be seen as intermittent pleural sliding in 2D mode or as a transition between the seashore and stratosphere sign in M-mode.

The boundary between the air-filled thoracic cavity and lung is identified by the presence of pleural sliding moving into and across the rib space with inspiration and back with expiration (Video 1.11.5 ). The US probe needs to be placed exactly, so that at the end of inspiration, the boundary of the inflated lung does not reach the upper rib. This is the lung point and is highly specific to the diagnosis of a pneumothorax. The lung point can occur at any point over the anterior chest wall and requires a systematic approach to ensure that it is identified. If the other three signs of a pneumothorax have been identified, the lung point is searched for by slowly moving the probe laterally and observing the point where lung sliding comes and goes with respiration.

M-mode is a useful modality to help with the identification of a lung point. Once a lung point has been confirmed using the 2D mode, M-mode can be used to identify the boundary between sliding and non-sliding pleura. By placing the scan line directly over the lung point, the M-mode trace will switch from the seashore sign to the stratosphere sign and back again as the lung slides through the scan line (Figure 15.5). The US features of a pneumothorax are summarized in Box 15.1.

Figure 15.5 Lung point (M-mode). Transition between seashore and stratosphere signs seen using M-mode at the lung point.

### Pleural effusion

Pleural effusions are normally found in the most dependent part of the thoracic cavity, relative to the position of the patient; therefore, the search for pleural fluid should start at the most postero-lateral point possible in the semi-recumbent patient. It is important to note, however, that tethered lung and loculations mean that effusions can be found anywhere, and a systematic approach to scanning the whole lung is important.

#### Ultrasound features of effusion

The key US sign that identifies a pleural effusion is an echo-free space that is clearly above the diaphragm and shows dynamic change with respiration (Box 15.2, Figure 15.6, and Video 1.11.6 ). Fluid within the pleural cavity can be a transudate, an exudate, or frankly purulent, each with characteristic US findings. Fluid, on the whole, is anechoic and is seen as a black space separating the parietal and visceral pleura. It is not uncommon to see a small pleural effusion at the costo-phrenic angle of critically ill ventilated patients without known respiratory pathology.

Figure 15.6 Pleural effusion. Large anechoic area with atelectatic lung seen lying deep within the effusion. The diaphragm is clearly visualized in its entire length.

Lung sliding is no longer present because the pleura are no longer adjacent. When the pleural effusion is large, the lung can be collapsed and seen freely floating as a non-aerated structure within the pleural fluid. The hemidiaphragm is clearly visualized, often in its entirety, if there is adjacent pleural fluid. A large effusion that contains collapsed consolidated lung transmits US waves and allows visualization down to the mediastinum, including cardiac structures lying closer to the surface than originally suspected.

Pleural effusions will vary in shape and size with respiration. The lung within a pleural effusion will move closer to the chest wall with inspiration and away with expiration. This can be demonstrated using M-mode by the sinusoid sign (Figure 15.7), which can be useful for differentiating small effusions from pleural thickening.

Figure 15.7 The sinusoid sign. By using M-mode, a pleural effusion can be seen to change in size with respiration, forming a sinusoid wave moving towards and away from the pleural line. This is in contrast to pleural thickening, which does not change in size with respiration.

### Assessment of size

The decision of whether to drain a pleural effusion should depend more on the clinical condition of the patient, rather than the size of the effusion. Infected effusions will need to be drained, independent of size. Uncomplicated effusions are frequently seen in critically ill patients, and the majority do not require drainage as they will resolve spontaneously. Large simple effusions (e.g. >800 mL) should be drained if there is evidence of respiratory compromise such as impaired gas exchange or ventilator dependence. Pleural US can provide information about diaphragmatic movement, the possible nature of the effusion, and the lung within the effusion. The depth of the effusion measured at the lung base in expiration can be used to provide an approximate estimate of the effusion size. A number of formulae have been published of varying complexity, with the simplest being:

$Display mathematics$

It is important to measure the pleural separation size at the lung base with a transverse scan in the posterior axillary line. A longitudinal scan angled posteriorly may overestimate the size of an effusion.

Another simple way of estimating effusion size has been described and requires measuring the depth of the effusion in expiration at the most postero-lateral alveolar/pleural syndrome (PLAPS) point (see Table 15.1).

Table 15.1 Estimating the volume of a pleural effusion using measured depth at the postero-lateral (‘PLAPS’) point

Depth of effusion at PLAPS point in expiration

Approximate size (mL)

1–3 mm

15–30

1 cm

75–150

2 cm

300–600

3.5 cm

1500–2500

Large effusions cause a significant increase in intra-pleural pressure that may result in inversion of the diaphragm and paradoxical movement with respiration (Figure 15.8 and Video 1.11.7 ).

Figure 15.8 Large pleural effusion. Note the inverted hemidiaphragm and paradoxical movement reflecting raised intra-pleural pressure (seen on Video 1.11.7 ).

#### Appearance of transudate, exudate, and loculated effusions

A transudate, which is caused by an imbalance of the normal forces resulting in the production and reabsorption of pleural fluid, appears anechoic on US. Common conditions associated with transudative effusions include heart failure, fluid overload, reduced serum protein, cirrhosis, and nephrotic syndrome.

An exudate can vary from anechoic to particulate and is secondary to an inflammatory process (infection, malignancy, pancreatitis). It can contain multiple particles seen swirling within the fluid or septations that float within the effusion and sometimes seem to tether the lung to the diaphragm or parietal pleura.

The US appearance of a haemothorax will vary, depending on its age. Acutely, blood within the thoracic cavity will appear anechoic, but this will become more echoic over time, often accompanied by the formation of septations.

Frankly purulent pleural fluid can sometimes have a similar echogenicity to the liver or spleen (Figure 15.9). The turbid fluid can normally be seen swirling within the pleural cavity, and this can be confirmed using colour Doppler. The fluid can be seen to move and swirl freely within the pleural cavity and is not pulsatile (Video 1.11.8 ).

Figure 15.9 Massive purulent effusion. The fluid has a similar echogenicity to the liver or spleen but can be seen swirling around in the pleural cavity.

Loculated pleural effusions are clearly visualized using US. Each pocket of fluid is seen within septated margins. Given the nature of a loculated effusion, they can occur anywhere within the chest cavity and their position is independent of gravity and the position of the patient (Figure 15.10).

Figure 15.10 Loculated anterior pleural effusion. The locules of fluid are clearly visible and remain in the anterior position when the patient is upright.

### Pitfalls in interpretation

#### Pleural thickening or pleural effusion

A thickened pleura, secondary to plaque formation or malignant infiltration, can look very similar to a small pleural effusion. The use of M-mode scanning to demonstrate the sinusoid sign is useful to differentiate fluid from other pleural pathology. Pleural thickening is uniform and does not vary in size with respiration.

#### Pleural fluid or ascitic fluid

In the critically ill patient, pleural fluid is often accompanied by ascitic fluid. Ascites can often be seen above the abdominal viscera in the semi-recumbent patient. It is vital to be confident with the US appearance of the sub-diaphragmatic anatomy to accurately determine the position of free fluid. It is essential to clearly identify all sub- and supra-diaphragmatic anatomy in order to accurately determine the position of any fluid, prior to attempting thoracocentesis.

#### Complex exudative/purulent effusion or consolidated lung

The differentiation of a turbid pleural exudate and consolidated lung can be challenging. There are a few US characteristics that can help this differentiation. Turbid/particulate pleural fluid should move freely and non-uniformly within the pleural cavity throughout the respiratory cycle. If this free, swirling movement is not clear using the 2D mode, colour Doppler can be used. The use of colour Doppler will also identify the pulsatile nature of consolidated lung or abdominal viscera, which is not seen with pleural fluid. If there is any doubt, it is essential to undertake further imaging with lung CT scan to exclude the presence of consolidated lung, before considering pleural drainage.

Pitfall

Pleural thickening, sub-diaphragmatic fluid, or consolidated lung can be misdiagnosed as pleural fluid.

# MCQs

## Questions

1. The following are US signs associated with a pneumothorax:

1. A Absence of pleural sliding, B-lines

2. B Absent pleural sliding, with a lung pulse

3. C Absence of pleural sliding and presence of a lung point

4. D A-lines

5. E Sinusoid sign

2. In the assessment of a pleural collection:

1. A An exudate can be reliably distinguished from a transudate

2. B The volume can be estimated from 40 times the maximal pleural separation in millimetres

3. C Haemothorax may appear anechoic

4. D Consolidated lung and a complex, infected collection can be difficult to distinguish

5. E The sinusoid sign reflects the dynamic change associated with a pleural effusion

1. The following are US signs associated with a pneumothorax:

1. A FALSE. B-lines exclude a pneumothorax.

2. B FALSE. Lung pulse reflects cardiac pulsation in a motionless lung and excludes a pneumothorax.

3. C TRUE. The lung point is a specific finding in pneumothorax.

4. D TRUE. Will also be seen in normal aerated lung.

5. E FALSE. This is an M-mode appearance seen in pleural effusions.

2. In the assessment of a pleural collection:

1. A FALSE. They may appear identical.

2. B FALSE. The volume of an effusion is estimated by 20 times the maximal separation in millimetres.

3. C TRUE. A haemothorax typically appears anechoic initially.

4. D TRUE. Both have similar echogenicity and move with inspiration.

5. E TRUE. In M-mode, when separated by a pleural effusion, the visceral pleura moves towards the parietal pleura during a spontaneous inspiration.

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