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Dyspnoea and hypoxaemia 

Dyspnoea and hypoxaemia
Chapter:
Dyspnoea and hypoxaemia
Author(s):

Ashley Miller

DOI:
10.1093/med/9780198749080.003.0022
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date: 26 October 2021

Introduction

LUS can be used to diagnose most pathologies which cause respiratory failure. Pneumothorax, interstitial syndrome, and pleural effusion are all visible at the pleural line with US, while approximately 90% of alveolar consolidations extend to the pleura and will be detectable with US. This chapter will outline a systematic approach to assessing the patient presenting with acute respiratory failure. In addition, the role of US in assessing ventilated patients with established respiratory failure, monitoring fluid balance and pleural effusions, identifying ventilator-associated pneumonia (VAP), and monitoring lung recruitment will be reviewed.

Acute respiratory failure and the BLUE Protocol

A seminal study (Lichtenstein and Mezière, 2008) described how a protocolized LUS examination that took 5 minutes to perform was able to correctly identify the cause of acute respiratory failure in 90% of patients presenting with acute dyspnoea and hypoxaemia. This compared to a figure of around 75% with chest radiography and clinical examination. It was named the BLUE Protocol which stands for Bedside Lung Ultrasound in Emergency. This diagnostic accuracy was achieved solely using the blinded US findings, without taking into account any history, examination, blood results, or other imaging. This highlights the power of LUS as a bedside tool for assessing the patient presenting with acute respiratory failure. How this can be adapted for ventilated patients is described on p. 194 and in Figure 22.1. Details of the technique of LUS, including where to place the probe, are described in Chapter 14.

Technique

A systematic approach to assessing the patient in acute respiratory failure is outlined in Table 22.1.

Table 22.1 Examination of the upper and lower anterior points

Step 1

Look for lung sliding

Present: pneumothorax can be ruled out

Absent: consider pneumothorax, ARDS, pneumonia, one-lung intubation, pneumonectomy

Step 2

Look for anterior A-lines or B-lines or consolidation

Bilateral A-lines with sliding: normal. Proceed to leg vein US to look for thrombus

Bilateral B-lines: cardiogenic pulmonary oedema

Unilateral B-lines: pneumonia

A-lines without sliding: possible pneumothorax. Look for lung point to confirm pneumothorax. If absent, probable pneumothorax. Consider CT scan to confirm

A diagnosis may well have been reached by this point. Pulmonary oedema and pneumothorax are ruled in or out with anterior scanning, and pneumonia can be ruled in (but not out).

Step 3

Examine postero-lateral points

Consolidation: pneumonia

Normal: asthma or chronic obstructive pulmonary disease likely diagnosis (equivalent to a normal chest X-ray)

DVT scanning

If anterior scanning is normal (i.e. bilateral sliding and A-lines), then venous US should be performed to examine for a DVT. This is quick and simple to perform, using a modified 2-point compression test, which focuses on the areas of greatest probability for a thrombus and has a very high sensitivity and specificity for detecting a DVT (see Chapter 18 for further details). Briefly, the patient is positioned in a 30–40° reverse Trendelenburg position, with the hip externally rotated and flexed. Using a linear probe, the femoral vein is imaged 2 cm proximal to the junction of the saphenous and femoral veins. The examination should extend distally, encompassing the bifurcation of the saphenous and then deep femoral veins. All veins should be seen to compress with enough force to slightly compress the femoral artery. The second site, with the patient’s knee flexed, is from the distal 2 cm of the popliteal vein to its trifurcation. Complete compression of the vein rules out a thrombus at the examination point, while incomplete compression rules one in. If a DVT is discovered, then the combination of acute respiratory failure and a normal anterior LUS makes a PE highly likely. Absence of a DVT means that the US examination of the lungs should continue to step 3. It is worth noting that in Lichtenstein’s study, while 20 out of 21 patients with a PE had a normal anterior LUS, half of them had postero-lateral consolidation (coexisting infection was common). This demonstrates the importance of the DVT examination.

Assessment of the ventilated patient

The BLUE Protocol was designed and validated for patients presenting acutely with respiratory failure. Most of the patients were spontaneously breathing; only a few (35 out of 260) had been intubated before the US exam took place. What does this mean for patients in critical care who are intubated and ventilated, perhaps for some time? The diagnostic accuracy of the BLUE Protocol will clearly be altered in these patients. The post-laparotomy patient with atelectasis would be identified as having pneumonia by the BLUE Protocol. In fact, it is unusual to perform an LUS on a ventilated patient and not find at least small amounts of pleural fluid and basal atelectasis. Also inflammation acute lung injury and fluid overload are common in ventilated patients. These will generate B-lines on US that are not the result of CPO. However, the BLUE Protocol, without necessarily drawing the same conclusions from its findings, can be extremely useful for assessing ventilated patients. LUS has been shown to have a higher diagnostic accuracy than chest radiography for interstitial syndrome, effusions, and consolidation and approaches the accuracy of the gold standard CT scanning. Most published assessments of LUS in ventilated patients have used six examination points on each hemi-thorax, but interestingly, region analysis confirmed that the diagnostic accuracy would have been just as good if only the three points from the BLUE Protocol had been used.

Perhaps the main difference between scanning acutely presenting patients and ventilated ones on ICU is the interpretation of the interstitial syndrome (the sign of which is B-lines).

Interstitial syndrome in ventilated patients

In the acutely presenting patient, interstitial syndrome will nearly always be due to CPO, whereas in the ventilated patient, it is a very common finding where it can be due to left ventricular failure, fluid overload, ARDS, or a combination of these. Lung fibrosis will also demonstrate B-lines. Advanced knowledge of LUS helps to distinguish between cardiogenic and non-cardiogenic pulmonary oedema (Table 22.2), but even without these, the identification of B-lines is very useful as it suggests that limiting fluid administration and establishing a diuresis may be beneficial to respiratory function, whatever the cause. Table 22.2 summarizes the US features which may be used to differentiate cardiogenic from other causes of pulmonary oedema.

Table 22.2 Ultrasound features of cardiogenic and non-cardiogenic pulmonary oedema

Interstitial syndrome

Ultrasound features

Cardiogenic

ARDS/fibrosis

Lung sliding

Normal

Reduced or absent

Pleural line

Normal

Irregular, thickened, coarse

Often multiple small anterior subpleural consolidations

Distribution

Homogenous

Spread from postero-lateral to anterior, with increasing severity. No spared areas

Spared areas (normal pleura, no B-lines)

More severe in dependent areas

Consolidation

No consolidation

Dependent consolidation

Lung pulse

Absent

Present in areas of reduced sliding

Pleural effusion

Common

Less common

Assessment of fluid balance

Simply put, A-lines mean the lungs are dry, while B-lines mean the lungs are wet (the exception to this is lung fibrosis). An important point to remember about B-lines is that they appear with interstitial oedema before alveolar oedema has occurred—the interlobular septa become engorged with fluid before the alveoli flood. This means interstitial oedema can be identified before the patient develops severe respiratory failure by the presence of a few discrete B-lines in the anterior chest. The absence of anterior B-lines has been shown to have a positive predictive value of 97% for a pulmonary artery occlusion pressure (PAOP) of <18 mmHg. If the B-lines are cardiogenic in origin (high PAOP), the presence of interstitial oedema on US suggests that the patient is on the flat portion of their Starling curve. They therefore provide a warning that respiratory failure will ensue if fluid resuscitation is administered. As alveoli start to flood, the LUS picture progresses to a white lung pattern where the B-lines become more numerous and confluent. This is a sign that the patient’s condition will improve by using treatment to reduce lung water (diuresis, renal filtration, or PPV). Similarly, if the patient has B-lines from non-CPO, this is likely to get worse with fluid administration, regardless of the PAOP. The management here would be identical—avoidance of further IV resuscitation fluid, fluid removal, and PPV.

Pleural effusions

More detailed information on the assessment of pleural effusions with LUS can be found in Chapter 15. US appearance can help discriminate between transudates and exudates, allows volume estimation, reveals any septations (Figure 22.2), and allows guided thoracocentesis for diagnosis or drainage.


Figure 22.2 A septated parapneumonic effusion.

Figure 22.2 A septated parapneumonic effusion.

Ventilator-associated pneumonia

LUS can help guide both the diagnosis and treatment of VAP. When there is deterioration in respiratory function in a ventilated patient, the differential diagnosis is broad. Other investigations are useful, but not very specific. Raised inflammatory markers, shadowing on a chest radiograph, and pyrexia, while suggestive, all have other causes. LUS will clearly demonstrate any new consolidation (Figure 22.3), as well as rule other causes in or out. Because basal consolidation and atelectasis are common in ventilated patients, it is important to regularly assess with LUS, so it is known whether a finding is new. A recent study compared various US features to assess their performance in detecting VAP and showed it to be superior to the Clinical Pulmonary Infection Score (CPIS). Lobar and semi-lobar consolidations were poor predictors (presumably because these were pre-existing atelectasis, rather than consolidation or infiltrates from another cause). The two features most suggestive were dynamic air bronchograms and small subpleural consolidations, with dynamic air bronchograms performing the best. Demonstrating the presence of any associated effusion will allow a guided diagnostic tap to assess the nature of the effusion and provide a sample for microbiology.


Figure 22.3 Linear air bronchograms in a consolidated lung base in a patient with VAP.

Figure 22.3 Linear air bronchograms in a consolidated lung base in a patient with VAP.

Assessing response to treatment

Just as LUS has a high diagnostic accuracy, it can also be used for assessing response to treatment. B-lines in CPO will be seen to resolve quickly to therapy, whether that is preload reduction (venodilatation or positive airway pressure) or fluid removal (diuresis or filtration). ARDS will also be seen to resolve by the reduction in B-lines (albeit more slowly than with CPO). Aeration will be seen to increase, and consolidation decrease, with resolution of pneumonia. Parapneumonic effusions may be seen to resolve or become increasingly complicated, necessitating video-assisted thoracoscopic surgery (VATS). Lung can be seen to re-expand after pleural drainage. The size of pneumothoraces can be monitored by the location of the lung point, either in patients with chest drains or in those with pneumothoraces too small to drain.

Lung recruitment

Lung recruitment is straightforward to assess with LUS. Chapter 16 describes how consolidation and collapse have different features, depending on the proportion of air and fluid in the lung (fully consolidated and hepatized or the ‘shred sign’). If alveoli are recruitable, then the US appearances will change to those of more aerated lung with increasing PEEP. Similarly, as pneumonia resolves, the lung will be seen to reaerate. A scoring system for assessing recruitment has been described by ascribing each area of the lung examined into one of four patterns of aeration: normal appearance, moderate loss of aeration (frequent B-lines), severe loss of aeration (confluent B-lines), and lung consolidation (see Chapter 23, table 23.1). The US score correlated well with lung recruitment induced by PEEP and quantified by changes to the pressure–volume curve and increases in PaO2. An US scoring system has also been used to assess lung reaeration in VAP treated with antibiotics over 7 days. Highly significant correlation was found between US- and CT-assessed reaeration. It is very important to remember that hyperinflation cannot be assessed with LUS, which is a significant limitation. Aerated lung looks the same, whatever its volume. The principles of lung-protective ventilation strategies should still be employed.

When significant pleural effusions are present, it can be unclear whether the underlying lung is consolidated or collapsed (atelectatic). While there are some distinguishing features, it is only by draining the effusion that the underlying condition of the lung is revealed. A lung base that reaerates was simply squashed, while one that remains hepatized is consolidated.

Conclusion

LUS is a powerful tool in the diagnosis of respiratory failure. In acute presentations, the use of the BLUE protocol will rapidly and accurately yield the diagnosis. In ventilated patients, LUS can be used for diagnosis, to assess response to treatment and guide interventions. Any worsening of respiratory function in a ventilated patient should prompt an LUS, and progress should be reassessed. LUS is therefore a key tool in the emergency department, acute medical unit, and critical care.

Chapter 22

MCQs

Questions

1. Regarding the BLUE Protocol:

  1. A The presence of lung sliding always rules out a pneumothorax at that point

  2. B Bilateral A-lines and lung sliding require examination for a DVT as the next step

  3. C It distinguishes between CPO and non-CPO

  4. D It can be used with a similarly high diagnostic accuracy in ventilated patients

  5. E It is designed to assess acute, rather than established, respiratory failure in spontaneously breathing patients

2. When assessing ventilated patients with US:

  1. A Widespread bilateral B-lines are a feature of wet lungs, regardless of heart function

  2. B The presence of lobar consolidation is highly predictive of VAP

  3. C US is useful in assessing lung hyperinflation when performing recruitment manoeuvres

  4. D Repetition of scans is useful to assess the response to treatment or changes in respiratory function

  5. E Pulmonary fibrosis and ARDS have the same sonographic appearance

Answers

1. Regarding the BLUE Protocol:

  1. A TRUE. There cannot be a pneumothorax if there is lung sliding.

  2. B TRUE. This is the next step in the protocol.

  3. C FALSE. No distinction can be made just going by the protocol.

  4. D FALSE. While the signs are the same, the conclusions are not.

  5. E TRUE. This is the patient group in which it was tested.

2. When assessing ventilated patients with US:

  1. A TRUE. They are caused by increased lung water.

  2. B FALSE. Dynamic air bronchograms and small subpleural consolidations are more suggestive.

  3. C FALSE. Hyperinflation cannot be assessed, as air-filled lung looks the same whatever its volume.

  4. D TRUE. Sonographic appearances will change over time.

  5. E TRUE. Reduced sliding, a thickened irregular pleural line, inhomogenous widespread B-lines, and spared areas are present in both.

Further reading

Bouhemad B, Brisson H, Le-Guen M, Arbelot C, Lu Q, Rouby J-J. Bedside ultrasound assessment of PEEP induced lung recruitment. American Journal of Respiratory and Critical Care Medicine 2011;183:341–7.Find this resource:

Lichtenstein DA, Mezière GA. Relevance of lung ultrasound in the diagnosis of acute respiratory failure: the BLUE protocol. Chest 2008;134:117–25. Find this resource: