Indian Journal of Respiratory Care

REVIEW ARTICLE
Year
: 2020  |  Volume : 9  |  Issue : 2  |  Page : 141--148

Lung ultrasound in critical care


Souvik Chaudhuri, Maddani Shanmukhappa Sagar, Sirish Gauni, Vedaghosh Amara, Tushar Mittal 
 Department of Critical Care; Department of Anaesthesiology, Kasturba Medical College, Manipal, Karnataka, India

Correspondence Address:
Dr. Maddani Shanmukhappa Sagar
Department of Critical Care, Kasturba Medical College, Manipal, Karnataka
India

Abstract

Point of care ultrasound for critically ill patients in intensive care unit has enabled clinicians to expedite the process of diagnosis and treatment without exposing the patient to any of the harmful radiations. Lung ultrasound (LUS) ensures an accurate assessment of the disease pathology, is easy to learn, widely accessible and can be performed at the bedside of hemodynamically unstable patients or those with high ventilator support, where shifting to a computerized tomography (CT) room involves substantial risk. Bedside lung ultrasound in emergency protocol has been formulated to guide intensivists and emergency physicians to enable a systematic approach in the diagnosis of various lung pathologies using ultrasound within a time of 3 min. LUS also provides a guide to fluid challenge requirements in acutely ill patients. It is a modality of immense help to physicians in the treatment and management of acutely ill patients who need emergent care. For the preparation of this review article, Medline search was done to assess previous review articles on LUS and the current clinical trials comparing the efficacy of LUS to other modalities of pulmonary imaging like chest radiograph and CT thorax. Review articles comparing the sensitivity and specificity of bedside LUS to CT thorax were also searched to establish the utility of LUS in diagnosing various lung pathologies.



How to cite this article:
Chaudhuri S, Sagar MS, Gauni S, Amara V, Mittal T. Lung ultrasound in critical care.Indian J Respir Care 2020;9:141-148


How to cite this URL:
Chaudhuri S, Sagar MS, Gauni S, Amara V, Mittal T. Lung ultrasound in critical care. Indian J Respir Care [serial online] 2020 [cited 2020 Oct 25 ];9:141-148
Available from: http://www.ijrc.in/text.asp?2020/9/2/141/289083


Full Text



 Introduction



Point-of-care bedside ultrasonography (USG) of the lung has been a boon for the acute care physician, especially in situ ations where the patient is critically ill and cannot be shifted to the computerized tomography (CT) room without taking significant risks. The numerous advantages of the lung ultrasound (LUS) have made its use popular and imperative, and it has now become an indispensable tool for the intensivist. LUS can be performed promptly, has a higher diagnostic accuracy than a combination of physical examination and chest radiography.[1],[2] Ultrasound examination does not lead to exposure to ionizing radiation, can be repeated whenever required, may be used for bedside therapeutic procedures, and is easy to learn.[2] USG has been referred to as the “third eye” of physicians in the acute care setting and is becoming a gold standard in critical care with its application in the demonstration of lung sliding.[3],[4] LUS encompasses both direct visualization of structures and a systematic interpretation of artifacts for accurate diagnosis of the clinical condition.[2]

 Lung Ultrasound Principle



LUS uses the property of acoustic impedance (Z) for the interpretation of images and artefacts.[2] Acoustic impedance is the resistance provided by particles an any medium to the passage of ultrasound waves through it. Resistance to the passage of ultrasound increases as the density of the medium increases.[2] When ultrasound falls on the boundary of two mediums with different impedances, a part of the ultrasound waves are transmitted, and a part is reflected back as an echo. The more the difference in Z of the two mediums, the more is the reflection, the whiter or hyperechoic is the appearance.[2],[3] On the basis of this acoustic impedance difference, O'Brien et al. had developed a model of the lung as a simple two-component tissue consisting of air and bulk material.[5],[6] As the lung becomes denser, the Z matches that of soft tissue, whereas more aeration leads to a difference in Z between the lung and soft tissue.

At the boundary of the soft tissue and rib, due to a large difference in Z, the ultrasound waves are almost completely reflected back, and so rib appears as a white hyperechoic line with an acoustic showdown below the posterior boundary.[3] Similarly, at the boundary of visceral pleura and lung containing air, there is also a large difference in Z, and about 99.9% of the beam is reflected.[1],[3],[7],[8],[9] Hence, the pleural line appears white, hyperechoic measuring 2 mm width[3],[10] [Figure 1]. Beyond the pleural line in a normally aerated lung, multiple reflections of this white hyperechoic pleural line is visualized due to repeated bouncing back of the ultrasound beam between the pleural line and the ultrasound probe.[3] These lines are called A-lines, which represent normal aerated lung tissue [Figure 2].[3] Structures below the pleural line, in a normally aerated lung, cannot be visualized, and only artifacts are seen. The artifacts vary depending the proportion of air and fluid. However, if the lung is de-aerated or fluid-filled, it can be visualized directly.[2]{Figure 1}{Figure 2}

 Probes for Lung Ultrasound



Most US machines have three probes – the linear probe, the curvilinear probe and the phased array or the echo probe [Figure 3]. Each of these probes is used in LUS depending on the clinical condition, which is of particular interest.{Figure 3}

Linear probe

They are high-frequency probes used for visualizing superficial structures with great clarity. It is typically used to visualize lung sliding of the anterior pleura and when pneumothorax is a concern in the patient. It is also used to rule out pleural thickening and lung fibrosis. However, there is poor penetration with this probe, and deeper lung structures cannot be visualized.[2],[3]

Curvilinear probe

This probe is most commonly used for LUS and has deeper penetration. It is used to diagnose most of the lung conditions such as pleural effusion, consolidation, collapse, and pulmonary edema. However, due to its large footprint, insinuating it between the rib spaces may be difficult.[2]

Phased array probe

This probe has the advantage of a small footprint and can be well placed between the rib spaces. Even though the deeper structures can be visualized, but the clarity is lesser than the curvilinear probe.[2]

Scanning areas

For LUS in an acute or emergency setting, there are total of eight areas which are supposed to be scanned. They are the upper and lower anterior chest and upper and lower lateral chest on each side. The anterior axillary line divides the anterior and lateral chest regions, and the posterior axillary line is the margin of the lateral chest region.[11],[12],[13]

Another method of scanning the lung areas is the 3-point examination of each lung.[2],[14] It also has a high diagnostic accuracy. It is used for the bedside lung ultrasound in emergency (BLUE) protocol also.[14] Two hands are placed side by side without the thumbs over the anterior part of chest. The wrists of both hands are placed in the anterior axillary line, and the little finger of the upper hand abuts the lower clavicular margin. The lower little finger gets aligned with the lower border of the lung also called phrenic line. All the views should be taken with the probe kept in a longitudinal manner. The upper anterior point coincides with the base of the middle and ring finger of the upper hand. Moreover, it lies above the upper lobe of the lung. It is also called the upper BLUE point.[14] The lower anterior point is the midpoint of the palm of the lower hand, and it coincides with the middle or lingular lobe, also known as the lower BLUE point.[14] The third point is called the postero-lateral point. It is formed by the junction of the lines drawn from the lower anterior point or lower BLUE point laterally and horizontally towards axilla, and the vertical line over the posterior axillary line. It usually lies over the lower lobe.[2],[14] It is also known as the posterolateral alveolar or pleural syndromes (PLAPS) point [Figure 4].{Figure 4}

There are seven principles of LUS which have been enunciated.[14]

Lung (and critical) ultrasound is performed at best using simple equipmentIn the thorax, gas and fluid have opposite locations or are mingled by pathologic processes, generating artifactsThe lung is the most voluminous organ. Standard areas can be definedAll signs arise from the pleural lineStatic signs are mainly artifactualThe lung is a vital organ. The signs arising from the pleural line are foremost dynamicAlmost all life-threatening disorders about the pleural line, explaining the potential of LUS.

Pleural line – When the ultrasound probe is placed longitudinally over the chest rib shadows are visualized as the sound waves are not able to pass and are reflected. About 0.5 cm below the rib line with thickness of about 2 mm a bright white pleural line is seen, and its movement with respiration or “lung sliding” can be appreciated [Video 1]. This appearance two rib shadows on either side of the pleural line is called the “bat sign” [Figure 5].[2],[3]{Figure 5}

A lines – These are horizontal repetition lines below the pleura, demonstrating the presence of air below pleura, which may be either physiological or pathological as in pneumothorax.[2],[14] They have the same spacing as the distance between the probe and the pleural line [Figure 2].[2]

B lines – They are artifacts produced by the presence of both air and alveolar septal thickening either due to fluid or fibrosis. They always arise from the pleural line and are long vertical hyperechoic white lines which reach till the bottom of the image without fading away [Figure 6]. They always move with lung sliding and abolish the A lines.[2],[15] Hence, even one B line rules out pneumothorax.[14] Up to two B lines between two rib spaces are considered normal. Three or more B lines between two rib spaces is considered abnormal.[2] B lines indicate an increased acoustic impedance between the air and fluid or thickened tissue when the ultrasound beam passes. Thus, B lines indicate diseased interlobular septae thickened by either fluid, transudate or exudate.[15] Accordingly, interstitial syndrome (IS) encompasses pulmonary edema, interstitial pneumonia, or lung fibrosis.[2],[4]{Figure 6}

Z lines – It is like a comet tail artifact arising from the pleural line. However, that is the only similarity it has to the B-line. It is not hyperechoic, is not well defined, and does not reach the bottom of the screen and does not erase the A lines. They have no definite significance and should not be confused with B lines.[16]

E lines – It is also a comet-tail artifact, spreads up to the bottom of the screen and is well defined. The differentiating point from B line is that it does not arise from the pleura. Partial emphysema may have this artifact.[16] It is also seen in subcutaneous emphysema.[3]

Normal lung – It is characterized by the hyperechoic pleural line with the presence of lung sliding and A lines which are the reflections of the pleural line. In the time-motion mode (M-mode), the structures above parietal pleura appear as horizontal lines, and below it, it appears as the sandy pattern known as the “seashore sign” [Figure 7].[1],[3],[7],[9]{Figure 7}

IS – At the tissue level, it indicates thickened abnormal interlobular septa. When thickened by fluid it is due to pulmonary edema, when by exudate, it is due to pneumonia, and when there is tissue thickening, it is due to fibrosis. It encompasses three components and can be caused by either or any of the three.

Pulmonary edema - It may be hemodynamic (fluid overload or cardiac failure) or permeability induced as in acute respiratory distress syndrome (ARDS). In IS due to fluid overload or cardiogenic, lung sliding and pleural line are normal, and B lines are more anteriorly, closely spaced, and more than three between rib spaces [Figure 8]. There is no evidence of consolidation. In IS due to ARDS, there may be reduced lung sliding with irregular pleural line, with consolidation and air bronchograms and B lines more in the dependant regions[2]Interstitial pneumonia – Lung sliding may be reduced with the irregular pleural line, presence of inhomogenous unilateral or bilateral B lines and air bronchogram [Figure 9][2],[3]Lung fibrosis – Lung sliding may be decreased with thickening of pleural line often >3 mm with irregularities, subpleural abnormalities, significant but spaced, nonhomogeneous B lines [Figure 10][3],[17]{Figure 8}{Figure 9}{Figure 10}

Alveolar syndrome-It consists of alveolar consolidation and atelectasis.[2]

Alveolar consolidation - there is the presence of air bronchogram which appear white. They may appear as punctate white spots if the ultrasound beam is transverse to the beam or may appear linear white and branching if longitudinal to the beam[2] [Figure 11]. The presence of dynamic air bronchogram which moves with respiration indicates a high likelihood of pneumonia.[2] The border between the consolidated and aerated lung appears irregular called the “shred sign” indicating nontranslobar consolidation [Figure 12]. Once the lung has extensive amount of fluid, it resembles the liver and is hepatized, called the “tissue–like sign.”[2],[3],[14] This happens in translobar consolidation.[14]{Figure 11}{Figure 12}

Atelectasis – the presence of air bronchograms which are static or does not move with respiration indicate atelectasis. There may be decreased lung sliding. The absence of lung sliding with the rhythmic transmission of the vibrations of the heart through a motionless lung to the pleura appreciated on ultrasound is called the “lung pulse” [Video 2]. The lung pulse is a sign of complete atelectasis or collapse and is observed before the radiological changes set in.[18]

Pleural effusion – due to gravity, the pleural effusions are best seen at the PLAPS-point in supine patients in the intensive care unit (ICU). A phased array probe can be used for locating all effusions regardless of their volumes.[2],[14] Pleural effusion is usually anechoic, except in cases of empyema and hemothorax, where they appear echoic.[14] In case of pleural effusion, the boundaries formed by the parietal pleura above, the visceral pleura below, and the rib shadows on either side for the “quad sign” [Figure 13]. In M-mode, the movement of the visceral pleura towards the now separated parietal pleura is called the “sinusoid sign” [Figure 14].[14] An inspiratory interpleural distance of 15 mm is required for safe thoracocentesis[14],[19] [Figure 15]. Sometimes in cases of large effusion and lung consolidation, the lung can be seen floating, also known as the “jellyfish sign” [Figure 16] and Video 3].[7] In cases of pleural effusions with cellular debris, when the debris move with respiration, it is known as “plankton sign.”[20] Various formulae have been proposed to estimate the quantity of pleural effusion by ultrasound.[21] Balik et al. have suggested that pleural effusion volume can be estimated by the formula:{Figure 13}{Figure 14}{Figure 15}{Figure 16}

Volume (mL) =20 × maximum interpleural distance at end-expiration (mL).[22]

Pneumothorax – here, the rib and rib shadows are visualized, along with the hyperechoic pleural line. However, there is the absence of lung sliding due to air present between the parietal and visceral pleura. M mode shows horizontal lines called the “barcode sign” or the “stratosphere sign” [Figure 17]. A lines are visualized, however there is characteristic absence of B lines. Typically, lung point is present. The lung point is the transitional area between absence of lung sliding and presence of lung sliding [Figure 18] and Video 4]. A linear probe placed anteriorly is used to evaluate lung sliding to rule out pneumothorax. Thus for the diagnosis of pneumothorax, three steps are essential-absence of lung sliding, A lines with no B lines, and the lung point.[14] It may be noted that apart from pneumothorax, there are other conditions where lung sliding can also be absent.[20] They are as follows[Table 1].[20]{Figure 17}{Figure 18}{Table 1}

Pulmonary embolism – usually, there will be the presence of A lines, with small peripheral infarcts, usually wedge-shaped with well-defined margins and a localized pleural effusion.[2],[3] The use of LUS can be done to rule out other possible causes of respiratory failure, as identification of emboli is not a competence area of LUS.[2] the presence of a thrombosed vein can be suggestive. The presence of lung sliding with A lines and a thrombosed vein provides 99% specificity for pulmonary embolism, however, sensitivity is about 81%.[15]

Asthma/chronic obstructive pulmonary disease (COPD) - there is the presence of lung sliding, which may, however, be diminished if hyperinflation is present, and A lines. LUS is useful in differentiating the cause of respiratory distress between pulmonary edema, pneumonia, and COPD exacerbation.[2]

Subcutaneous emphysema-E lines are characteristic of subcutaneous emphysema. The pleural line, as well as the classical “bat sign” is not visible as the subcutaneous air obstructs its image formation.[3]

Patients admitted in the ICU frequently present with respiratory distress. The BLUE protocol has been outlined by Lichenstein and Meziere for the evaluation of patients within 3 min for the assessment of respiratory distress in the emergency scenario.[15] An elaborative and modified protocol has been described by Saraogi, which is as follows: [Figure 19].[3]{Figure 19}

 Demerits of Lung Ultrasound



One of the biggest drawbacks of LUS is the ability to assess lung pathologies in the presence of subcutaneous emphysema since the artifacts cannot be visualized and interpreted.[14] The presence of dressing over the thorax also impede the use of LUS in any patient.[14] In the absence of proper aseptic measures, the ultrasound probe can transfer multidrug-resistant strains between patients in ICU, which may be fatal.[7] Ultrasound machines fall under the category of noncritical items with skin contact only and thus require chlorine-based or alcohol disinfectant for sterilization.[7] The user must follow strict asepsis while using the ultrasound probes. Finally, its accuracy in diagnosing conditions like pulmonary embolism is questionable, where pulmonary angiography is always preferred.

The utility of LUS compared to other imaging modalities-It has been shown that bedside.

LUS has better sensitivity in the diagnosis of pneumonia compared to chest radiograph.[23] LUS has also been described as a tool with diagnostic accuracy comparable to CT thorax.[24] It has also been concluded in a systematic review that LUS is sensitive as well as specific for revealing consolidation which can be detected by CT thorax.[25]

 Conclusion



Ultrasound assessment of lung provides prompt, accurate, radiation-free, inexpensive bedside care to the critically ill patient. It is the device for the present and the future in the management of a patient in respiratory distress anywhere in the hospital.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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