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 Table of Contents  
ORIGINAL ARTICLE
Year : 2018  |  Volume : 5  |  Issue : 2  |  Page : 110-114

Lung ultrasound in intensive care unit: a prospective comparative study with bedside chest radiography using computed tomography of chest as a gold standard


1 Department of Critical Care Medicine, Cairo University, Giza, Egypt
2 Department of Radiology, Cairo University, Giza, Egypt

Date of Submission03-May-2017
Date of Acceptance26-Nov-2017
Date of Web Publication28-Jun-2018

Correspondence Address:
Ali Mohsen
Department of Critical Care Medicine, Cairo University, Giza, 11742
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/roaic.roaic_52_17

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  Abstract 

Background Bedside chest radiography (CXR) is routinely performed on a daily basis to assess lung status for the critically ill patients. However, the technical difficulties during the procedure have led to the incorrect assessment of most common lung pathologies diagnosed in the ICUs, like pneumothorax (PTX), pleural effusion, lung consolidation, and acute interstitial syndrome (AIS). Recently, lung ultrasound (LUS) has become a new reliable, accurate, and attractive tool for diagnosing most of these lung pathologies.
Objective Our study was designed to find out if LUS could be a more reliable, accurate, and sensitive bedside tool in diagnosing most of the common chest diseases encountered in the ICUs, in comparison with bedside CXR, using thoracic computed tomography (CT) as a gold standard.
Patients and methods Forty critically ill patients scheduled for CT chest were prospectively studied with a standard LUS protocol. Four pathologic entities were evaluated: consolidation, AIS, PTX, and pleural effusion. Each hemithorax was evaluated for the presence or absence of each abnormality. Eighty hemithoracies were evaluated by the three imaging techniques.
Results In comparing bedside CXR with CT chest, bedside CXR detected three cases of PTX of 12 by CT (P=0.02), 11 cases of pulmonary consolidation of 21 by CT (P=0.04), 12 cases of pleural effusion of 19 by CT (P=0.03), and 27 cases of AIS of 36 by CT (P=0.11). On the contrary, comparing LUS with CT chest, LUS detected 11 cases of PTX of 12 by CT (P=0.12), 21 cases of pulmonary consolidation of 21 by CT (P=1.0), 17 cases of pleural effusion of 19 by CT (P=0.32), and 32 cases of AIS of 36 by CT (P=0.02). The sensitivity, specificity, and diagnostic accuracy of CXR were found to be 62, 89, and 73%, respectively, for consolidation; 50, 73.6, and 60%, respectively, for AIS; 25, 100, and 78%, respectively, for PTX; and 46, 90, and 78%, respectively, for pleural effusion. The corresponding values for LUS were 100, 81.4, and 93% for consolidation; 100, 58, and 78% for AIS; 100, 86, and 90% for PTX; and 100, 97, and 98% for pleural effusion.
Conclusion In our general ICU population, LUS has shown a considerably better diagnostic performance and is more reliable than CXR for the diagnosis of common pathologic conditions and may be used as an alternative to chest CT.

Keywords: computed tomography chest, chest radiography, lung ultrasound


How to cite this article:
Mohsen A, Samy W, El-Azizy H, Shehata MA. Lung ultrasound in intensive care unit: a prospective comparative study with bedside chest radiography using computed tomography of chest as a gold standard. Res Opin Anesth Intensive Care 2018;5:110-4

How to cite this URL:
Mohsen A, Samy W, El-Azizy H, Shehata MA. Lung ultrasound in intensive care unit: a prospective comparative study with bedside chest radiography using computed tomography of chest as a gold standard. Res Opin Anesth Intensive Care [serial online] 2018 [cited 2018 Nov 19];5:110-4. Available from: http://www.roaic.eg.net/text.asp?2018/5/2/110/235489


  Introduction Top


Bedside chest radiography (CXR) is routinely performed on a daily basis and is considered as a reference for assessing lung status in critically ill patients. Limited diagnostic performance and efficacy of bedside portable CXR have been reported in several previous studies, which lead to incorrect assessment of pleural effusion, lung consolidation, alveolar-interstitial syndrome, and pneumothorax (PTX) [1],[2],[3],[4].

Several reasons account for the limited reliability of bedside CXR. First, during the acquisition procedure, the patient and the thorax often move, decreasing the spatial resolution of the radiological image. Second, the film cassette is placed posterior to the thorax. Third, the radiograph beam originates anterior, at a shorter distance than recommended, and quite often not tangentially to the diaphragmatic cupola, thereby hampering the correct interpretation of the silhouette sign. These technical difficulties lead to incorrect assessment of pleural effusion, lung consolidation, and alveolar-interstitial syndrome [2],[3],[4].

Computed tomography (CT) of the chest is now considered as the gold standard not only for the diagnosis of PTX, pleural effusion, lung consolidation, atelectasis, and alveolar-interstitial syndrome but also for guiding therapeutic procedures in critically ill patients. However, to perform a CT scan of the chest requires transportation to the department of radiology, which a risky procedure necessitating the presence of trained physicians and sophisticated cardiorespiratory monitoring [5].

Until recently, the role of ultrasound has most notably been as an anatomic imaging test, confined to a specialized department in specialized hands. In the past decade, as technology has continued to improve, ultrasound has made a move from the department to the bedside. First introduced in the emergency department, ultrasound was initially used as an adjunct in the trauma bay for the detection of pericardial or free intra-abdominal fluid. Its role has since expanded to the diagnosis of basic intra-abdominal pathology, safer line and tube insertion, and confirming pregnancy, among many other uses [6].

Because ultrasound waves are nearly completely reflected by an air-tissue interface, it was originally concluded that ‘ultrasound imaging is not useful for evaluation of the pulmonary parenchyma’. However, recent evidence reveals that an examination of the artifacts produced by lung ultrasound (LUS) results in a wealth of information about the underlying lung. Normal lung parenchyma is not visualized because it is composed primarily of air, which scatters and impedes the transmission of sound waves. The dramatic difference in the acoustic characteristics of soft tissues and the lung makes the lung surface particularly strong reflector of ultrasound waves and is responsible for creating a number of reverberation artifacts that lend valuable information about the lung’s current pathophysiology [7].

However, the ICU patient is either supine or in semierect position and unable to suspend respiration. The focus length is often less than 50 cm, Consistency in technique and positioning is critical for optimal evaluation of the patient on serial examinations. Therefore, it may be desirable to obtain a baseline at full inspiration, while recording the patient’s position, the exposure factors, and respiratory assistance being given [8].

Even when obtained under optimal conditions, portable CXRs are usually technically inferior to those obtained in the radiology department, primarily owing to longer exposures and motion-induced blurring and scatter. Scatter is the major reason for low contrast on portable images [9]. Moreover, portable CXRs subject the patient to a radiation dose two to four times that of a radiograph made on standard stationary equipment [8],[9].


  Aim Top


The aim of the study is to compare the diagnostic performance of LUS and bedside CXR for the detection of various pathologic abnormalities in unselected critically ill patients, using thoracic CT as a gold standard.

Primary goal

The primary goal is to compare the sensitivity, specificity, and diagnostic accuracy of bedside CXR versus that of LUS using CT chest as gold standard.

Secondary goal

The secondary goal is to find out if LUS could be a reliable bedside diagnostic tool for common lung pathologies in ICU.

Patients’ population and data collection

After the approval of our ethical committee, we conducted our study between February 2014 and May 2016. Forty critically ill patients were admitted to ICU Department of El Agouza Hospital. They underwent CT of the chest as requested by the primary physician for diagnosing respiratory distress or respiratory failure. After performing the initial bedside CXR and within 24 h of performing CT chest, we examined them with a standard bedside LUS protocol.

Exclusion criteria

The exclusion criteria were as follows:
  1. Age younger than 18 years.
  2. Patients with history of lung cancer.
  3. Patients who underwent lung surgery.
  4. Patients with interstitial lung fibrosis.



  Patients and methods Top


Patients were subjected to the following
  1. Full clinical evaluation:
    • It includes history taking and physical examination comprising vital signs [blood pressure, heart rate, temperature, and respiratory rate (RR)] and chest and cardiac auscultation.
  2. Investigations
    1. Arterial blood gases.
    2. Bedside plain CXR.
    3. CT chest without contrast (including high-resolution CT).
    4. Bedside LUS.


A two-dimensional scanner was used (GE vivid S6) in this study, and a range of frequencies (4–12 MHz) was used to visualize the lungs. Probes of high frequencies were used to look at the periphery of the lung with a high resolution as in looking for ‘lung sliding’ and other signs of PTX, whereas probes of low frequencies help with the imaging of deep lung tissues as in looking at consolidation and pleural effusion.

Bilateral scanning of the anterior and lateral chest walls was done with the patient in the supine, sitting, or lateral decubitus position. Using anterior and posterior axillary lines as anatomical landmarks, each chest wall can be divided into six lung regions that should be systematically analyzed: upper and lower parts of the anterior, lateral, and posterior chest wall. In a given region of interest, all adjacent intercostal spaces offer acoustic windows that allow the assessment of the lung surface by moving the probe transversally. The following signs can be seen:
  1. The pleural line: bright longitudinal line is identified 0.5–1 cm from the rib shadow.
  2. Lung sliding (normal lung): it is a movement visible at the pleural line, synchronized with respiration.
  3. The A-lines (normal lung surface): this is a reverberation artifact of the pleural line located deeper at the same distance as the skin-pleural line distance.
  4. The B-lines (interstitial lung syndromes): it is a vertical narrow-based artifact that spreads out to the edge of the screen.
  5. Abolished lung sliding and the stratosphere sign: the absence of lung sliding is a basic first step for the diagnosis of PTX. The abolition of lung sliding generates the stratosphere sign on M-mode.
  6. The lung point (PTX): when abolished lung sliding and the A-line sign are detected, ultrasound confirmation is possible using the lung point.
  7. The quad sign (pleural effusion): pleural effusions are traditionally defined as anechoic collections. The pleural line and the acoustic shadows of ribs make three constantly straight borders.
  8. The tissue-like sign (lung consolidation): the tissue-like sign indicates an image is echogenic like a liver, and which behaves like a tissue, that is, does not generate any sinusoidal sign.


Statistical analysis

Data were statistically described in terms of range, mean±SD, frequencies (number of cases) and percentages when appropriate. Comparison of quantitative variables between the study groups was done using Mann–Whitney U-test for independent samples. For comparing categorical data, χ2-test was performed. Exact test was used instead when the expected frequency is less than 5. A P-value less than 0.05 was considered statistically significant. All statistical calculations were done using computer program Microsoft Excel 2003 (Microsoft Corporation, New York, New York, USA) and statistical package for the social sciences (SPSS Inc., Chicago, Illinois, USA) version 15 for Microsoft Windows.


  Results Top


Forty patients were conducted in our study, with mean of age 59+13. There were 19 (47.5%) males and 21 (52.5%) females. Hypertensive pulmonary edema and iatrogenic PTX were the most frequent etiologies leading to their respiratory manifestations in 15% of the cases whereas pulmonary embolism and other systemic diseases were the least (5%). Among the studied patients, 26 were mechanically ventilated, where nine of them were on noninvasive ventilation [continous positive airway pressure (CPAP)/bilevel positive airway pressure (BIPAP)], and the last 14 cases received oxygen via mask or nasal cannula.

CT pathological findings in the right 40 hemithoracies included acute interstitial syndrome (AIS) in 42.5%, pleural effusion in 32.5%, pulmonary consolidation in 40%, and PTX in 25%; no abnormality was detected in three (7.5%) cases. On the contrary, the left 40 hemithoracies included AIS in 47.5%, pleural effusion in 15%, pulmonary consolidation in 12.5%, PTX in 5% of scanned hemithoracies; no abnormalities were detected in 13 (32.5%) cases.

Regarding CXR findings, there was no specific finding in 40% of the examined right hemithoracies and 52.5% of left hemithoracies. Exaggerated BVMs were the prominent findings in both hemithoracies representing 20 and 17.5%, respectively. Radiopaque shadow(s) in CXR was detected in eight right hemithoracies compared with only three left hemithoracies and only one case of air fluid level was detected in one left hemithorax. Pulmonary infiltrates were found in CXR of six right and left hemithoracies. An obliterated costophrenic angle was found in six right hemithoracies and five left ones.

On comparing bedside CXR with CT chest, bedside CXR detected three cases of PTX of 12 by CT (P=0.02), 11 cases of pulmonary consolidation of 21 by CT (P=0.04), 12 cases of pleural effusion of 19 by CT (P=0.03), and 27 cases of AIS of 36 by CT (P=0.11) ([Figure 1]).
Figure 1 Bar chart illustrating bedside chest radiography (CXR) versus computed tomography (CT) chest in diagnosing different lung pathologies. PTX, pneumothorax.

Click here to view


On comparing LUS with CT chest, LUS detected 11 cases of PTX of 12 by CT (P=0.12), 21 cases of pulmonary consolidation of 21 by CT (P=1.0), 17 cases of pleural effusion of 19 by CT (P=0.32), and 32 cases of AIS of 36 by CT (P=0.02) ([Figure 2]).
Figure 2 Bar chart illustrating bedside lung ultrasound (LUS) versus computed tomography (CT) chest in diagnosing different lung pathologies. PTX, pneumothorax.

Click here to view


Regarding sensitivity and specificity in diagnosing PTX, 25 and 100% for CXR versus 100 and 85.7% for LUS; for pulmonary consolidation, 61.5 and 88.9% for CXR versus 100 and 81.4% for LUS; for pleural effusion, 45.5 and 89.6% for CXR versus 100 and 96.5% for LUS; and for AIS, 50 and 73.6% for CXR versus 100 and 57.8% for LUS ([Table 1]).
Table 1 Bedside chest radiography versus lung ultrasound chest in diagnosing different lung pathologies

Click here to view



  Discussion Top


The bedside diagnosis of PTX is extremely important in ICU patients. It is known that supine CXR is not sensitive for diagnosis of PTX [10],[11]. Indeed in our study, bedside CXR did not identify nine of 12 PTX (sensitivity 25%, specificity 100% and diagnostic accuracy 76%). In our study, LUS identified 11 PTX compared with 12 by chest CT, having high sensitivity (100%). In our study, four tests were false positive, resulting in a specificity of 83%, positive predictive value of 73%, and diagnostic accuracy of 90%. These false-positive cases occurred because of clinical subcutaneous emphysema owing to chest trauma (i.e. complication of faulty endotracheal intubation) and in patients with severe COPD. The false negative result was due to a case with a small PTX for which no intervention was required. Nevertheless, because LUS did not miss any clinically significant PTX, our data indicate that this technique is a reliable tool for bedside diagnosis of this abnormality.

In agreement with our study, Soldati et al. [11] in their study found 25 traumatic PTXs in the 218 hemithoracies (109 patients; two patients had a bilateral PTX) evaluated by spiral CT scan; of these, only 13 of 25 PTXs (52%) were revealed by CXR (sensitivity, 52%; specificity, 100%), whereas 23 of 25 PTXs (92%) were identified by LUS with one false-positive result (sensitivity, 92%; specificity, 99.4%). Moreover, Xirouchaki et al. [12] studied 42 mechanically ventilated patients and found LUS to have sensitivity of 100%, specificity of 99%, and diagnostic accuracy of 89% for PTX, with the corresponding values of bedside CXR of 73, 93, and 92%.

A new study by Matsumoto et al. [13] on 159 trauma patients found that the sensitivity and specificity of oblique CXR for detecting occult PTX was 61.4 and 99.2%, respectively. The sensitivity and specificity of LUS was 62.9 and 98.8%, respectively.

Our results showed that the sensitivity and specificity of LUS in detection of pulmonary consolidation were comparable to CXR (100 vs. 62% and 81.4 vs. 88.9%, respectively). In agreement with our findings, a prospective clinical study carried by Cortellaro et al. [14] on 120 patients with suspected pneumonia in ER showed the superiority of LUS over CXR. The first CXR was positive in 54/79 patients (sensitivity 65%) and negative in 33/39 (specificity 82%), whereas LUS was positive in 78/79 (sensitivity 98%) and negative in 37/39 (specificity 95%). A CT scan was performed in 30 patients (26 of which were positive for pneumonia); in this subgroup, the first CXR was diagnostic for pneumonia in 18/26 cases (sensitivity 67%), whereas ultrasound was positive in 25/26 cases (sensitivity 96%) [14].

The ability of CXR in the diagnosis of pleural effusion was lower compared with LUS in our study, with sensitivity of 46 versus 100%, specificity of 90 versus 98%, and diagnostic accuracy of 76 versus 97%. These results agree with that of Lichtenstein et al. [15] who conducted a prospective study on 32 patients with ARDS and 10 healthy volunteers to compare the diagnostic accuracy of auscultation, bedside CXR, and LUS with that of thoracic CT. Auscultation had sensitivity of 42%, specificity of 90%, and diagnostic accuracy of 61% for pleural effusion; bedside CXR had sensitivity of 39%, specificity of 82% and diagnostic accuracy of 47% for pleural effusion; and LUS had a sensitivity of 92%, specificity of 93% and diagnostic accuracy of 93% for pleural effusion.

Our study agreed with the study of Xirouchaki et al. [12] regarding relatively lower sensitivity and diagnostic accuracy of bedside CXR than LUS in diagnosing AIS. Our results showed that CXR had sensitivity, specificity and diagnostic accuracy of 50, 72, and 60%, respectively, whereas those of LUS were 100, 58, and 76%, respectively. Xirouchaki et al. [12] had CXR values of 46, 78, and 58% and LUS values of 94, 93, and 94%, respectively. In concordance with our results, Volpicelli et al. [16] clarify the significance of comet tail artifacts (as a sign) in the diagnosis of AIS and showed that LUS is a highly sensitive and specific tool, as the artifact showed a sensitivity of 82.7% and a specificity of 97.7% in recognition of radiologic AIS.

Regarding the overall diagnosis of the four pathologies selected in our study, LUS was found to have higher sensitivity, specificity, and diagnostic accuracy (100, 97, and 97.5%, respectively) than CXR (63, 50, and 62.5%, respectively), denoting that LUS is considerably a better bedside tool for diagnosing common chest pathologies inside ICU, and this agree with a meta-analysis done by Alrajab et al. [17], who reviewed 13 studies and demonstrated a pooled sensitivity of 76.6% and specificity of 98.4% for LUS, whereas these rates were 39.8 and 99.3% for CXR, respectively.

Conclusion and recommendations

  1. In comparison with bedside CXR, LUS is found to be a more reliable, accurate, and sensitive bedside tool in diagnosing most of the common chest diseases encountered in critically ill patients.
  2. In comparison with CT scan, bedside LUS seems to be a valuable substitute in cases where performing CT is problematic
  3. We recommend starting the use of bedside LUS as routine tool to improve the diagnostic accuracy for most of the pulmonary presentations as well as designing training courses and hands-on training for the intensivists for the daily practice of LUS inside ICUs.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

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Cortellaro F, Colombo S, Coen D, Duca PG. Lung ultrasound is an accurate diagnostic tool for the diagnosis of pneumonia in the emergency department. Emerg Med J 2012; 29:19–23.  Back to cited text no. 14
    
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