Abstract
Point-of-care ultrasound (PoCUS) of the lung and diaphragm has moved from an area of interest to a significant component of perioperative and critical care practice. Lung PoCUS can rapidly assist in the identification of numerous pathologies including pneumothorax, increased interstitial fluid, pleural effusion, and consolidation. Diaphragm PoCUS complements these findings using either qualitative or quantitative assessment to provide a bedside analysis of diaphragmatic function. PoCUS has been shown to improve patient-centred outcomes in many areas, but its widespread use varies by institution and speciality.
This review combines current concepts in lung and diaphragm PoCUS into a practical, anesthesiology-focused framework for image acquisition, interpretation, and bedside application. We aim to simplify practice by outlining nomenclature, scanning windows, key sonographic signs, and recognizing treatable conditions, while integrating the best available evidence to support perioperative decision-making.
While lung and diaphragm PoCUS may enable rapid, repeatable, and actionable bedside assessment; realizing their full impact requires the development of consistent training and integration into decision pathways.
1
Introduction
In recent years, point-of-care ultrasound (PoCUS) has emerged as an essential bedside modality in both perioperative and critical care environments ,. Once considered the domain of radiologists, lung and diaphragm ultrasound are now frequently performed by clinicians to guide real-time decision-making, which may have a significant impact on patient outcomes ,. Rather than providing detailed anatomical assessments, PoCUS helps determine whether key findings, such as pneumothorax, excess interstitial fluid or diaphragmatic dysfunction, are present or absent. This targeted approach facilitates immediate, at the bedside decision-making . The examination performed is intentionally limited to reduce complexity and improve operator consistency. With its portability, non-invasiveness, and a lack of ionizing radiation, PoCUS can be implemented anywhere in both hospital and out of hospital settings.
Despite its increasing popularity, the implementation of lung and diaphragm PoCUS into perioperative care remains heterogenous. Operator training, image acquisition technique, and interpretive thresholds vary widely across clinical practice. This paper aims to provide a structured overview of lung and diaphragm PoCUS tailored for anesthesiologists.
1.1
History
The clinical integration of point-of-care ultrasound into the management of pulmonary disease began gaining traction in the early 2000s, most notably with the work of Dr Daniel Lichtenstein and the development of the BLUE protocol (Bedside Lung Ultrasound in Emergency), which offered a simplified, reproducible algorithm for diagnosing causes of acute respiratory failure . By clearly defining specific sonographic profiles and their correlation with clinical syndromes such as pneumothorax, pulmonary oedema, and pneumonia, the BLUE protocol laid the foundation for structured lung PoCUS in acute care. Several other standardized protocols have been developed to expand and refine lung ultrasound applications in various settings, including ETUDES , RADiUS , and FLUID . Since then, lung PoCUS has undergone substantial evolution, including a lung-specific I-AIM implementation that standardizes the pathway from clinical question to bedside decision . The lung PoCUS is now widely integrated into many clinical settings– most notably critical care, emergency, and respiratory medicine. It plays a key role in the detection of pneumothorax, pleural effusion, pulmonary oedema, consolidation, atelectasis, and bronchial intubation, serving as a core tool for respiratory evaluation, optimization, and complication management .
The use of ultrasound to visualize diaphragmatic motion was first described in 1969, primarily to assess diaphragmatic paralysis . However, diaphragm PoCUS transitioned from a niche technique to a routine bedside assessment tool, particularly in the ICU, where it supports evaluation of diaphragmatic excursion and thickening as indicators of ventilatory competence . It can also be used in the delivery of regional anaesthesia to assess diaphragm function either before or after the administration of a brachial plexus block .
1.2
Image acquisition
The successful application of lung and diaphragm PoCUS requires a basic understanding of probe selection, patient positioning, and standardized scanning windows.
1.2.1
Probe selection
For most lung PoCUS applications, a low-frequency curvilinear (2 – 5 MHz) or phased-array probe is preferred due to its ability to penetrate deeper structures and provide a wide field of view, ideal for visualizing pleural sliding, B-lines, and consolidations. A high-frequency linear probe (5 – 13 MHz) may be used to enhance near-field resolution, especially when assessing the pleural line or subtle pneumothorax signs in thin patients or children.
For diaphragm imaging, both subcostal and intercostal views are commonly employed.
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Curvilinear or phased-array probes are used for excursion measurements via a subcostal approach.
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High-frequency linear probes are optimal for thickening fraction evaluation at the zone of apposition (ZOA).
1.2.2
Patient positioning
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In the supine or semi-recumbent position, anterior and lateral lung fields can be evaluated for pneumothorax, interstitial syndrome, and pleural effusion. In patients with respiratory symptoms, the semi-recumbent position is often the most comfortable. Scanning of the posterior thorax may be helpful to assess localized pathology but may not be possible in anesthetized patients.
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For diaphragm scanning, supine positioning with the probe placed below the costal margin (for excursion) or in the midaxillary line (for thickening) is ideal.
1.2.3
Scanning
In clinical practice, lung PoCUS is frequently structured around four core diagnostic targets: pneumothorax, interstitial abnormalities, pleural effusion, and diaphragmatic function. This compartmentalized approach enhances clarity and reproducibility at the bedside.
Lung PoCUS is performed by scanning anatomically defined thoracic zones using a systematic approach. The chest wall is typically divided into anterior, lateral, and posterior regions, each further split into three interspaces ( Fig. 1 ). In most perioperative applications, the anterior and lateral zones are prioritized due to accessibility in the supine or semi-recumbent position.
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Anterior chest : from parasternal line to anterior axillary line.
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Lateral chest : from anterior axillary line to posterior axillary line.
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Posterior chest : best accessed in seated or lateral decubitus position if feasible.
Anterior, lateral, and posterior areas for lung PoCUS scanning.
Scanning is conducted in both longitudinal (sagittal) and transverse planes. The probe is typically placed in an intercostal window , perpendicular to the ribs, using minimal appropriate pressure.
Diaphragmatic assessment can involve either qualitative or quantitative evaluation, depending on clinical need. Dome of diaphragm (DOD) excursion is measured via a subcostal approach (usually right-sided), using M-mode with a curvilinear or phased-array probe. The liver acts as an acoustic window on the right side, and diaphragmatic motion is tracked during quiet and deep breathing. The spleen is the acoustic window for the left side.
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Thickening fraction is assessed in the ZOA using a high-frequency linear probe. The diaphragm is visualized as a three-layer structure (two echogenic lines and a hypoechoic muscle layer in between). Measurements can be taken at end-expiration and end-inspiration, allowing calculation of thickening fraction (TF). Standard scanning is performed in the 8th to 10th intercostal spaces , between the anterior and midaxillary lines, ideally on the right hemithorax. The ABCDE approach (A: mid-Axillary position; B: confirm Breathing with lung sliding; C: scan Caudally; D: observe Diaphragm thickening; E: Examine TF) standardizes the identification of the transition zone and measurement points, making it fast and reproducible .
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Measure thickness at end-expiration (TEE) and end-inspiration (TEI) without changing probe angle; compute the thickening fraction
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This can also be performed in a more qualitative fashion by observing diaphragm thickening without measurement of TF%, which may be more convenient for use in rapid perioperative assessment.
Probe positioning for various indications is demonstrated in Fig. 2 .
Probe positions for; A: Lung ultrasound in general, B: Lung ultrasound with suspected pneumothorax, C: Diaphragm ultrasound for excursion, D: Diaphragm ultrasound for ZOA thickening.
1.3
image interpretation
1.3.1
Pleural line and pleural space
The assessment of the pleural line and adjacent pleural space serve as crucial landmarks for identifying both normal physiology and pathological conditions. There are a number of specific findings that should be assessed when performing lung PoCUS as their systematic assessment allows for rapid clinical judgement of the presence or absence of relevant pathology.
1.3.1.1
Normal findings
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“ Bat Sign ”: The initial landmark for orientation. In the longitudinal plane, the rib shadows form the “wings,” and the pleural line between them represents the “body” of the bat. This sign confirms correct intercostal placement of the probe and ensures the pleural line is visualized. It is important to identify this image, particularly for novice users, as facial layers can be mistaken for a static pleural line.
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Pleural Line : A hyperechoic, smooth horizontal line just beneath the ribs. It marks the visceral-parietal pleural interface ( Fig. 3 ).
Fig. 3 A-lines and the pleural line in a normal aerated lung.
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Lung Sliding : The shimmering, back-and-forth motion of the pleural line during respiration, confirming apposition of pleural layers and ruling out pneumothorax in that zone. Absent in the presence of a pneumothorax
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A-lines : Horizontal reverberation artifacts parallel to the pleural line, repeating at regular intervals equal to the distance between the ultrasound probe and the pleura. They indicate air in the thoracic cavity but may be visible in the presence of a pneumothorax. Tend to be obliterated with increased interstitial fluid ( Fig. 3 ).
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Lung Pulse : Subtle transmission of cardiac pulsation through aerated lung, causing pleural line to move in synchronicity with heartbeat. Seen in the absence of ventilation (e.g., during apnea or mainstem intubation), the widespread presence of the lung pulse rules out pneumothorax.
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Curtain sign : Dynamic artifact at the base of the lung showing the cranio-caudad movement of normal aerated lung in front of the probe. Reduced or completely absent in lower lung pathology including pleural effusion, hemothorax, atelectasis and consolidation.
1.3.1.2
Abnormal findings
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Absent Lung Sliding with lung pulse suggests apnea or bronchial intubation (not pneumothorax). No lung sliding without lung pulse and/or with A-lines is highly suspicious for pneumothorax.
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Lung Point : Transition zone where sliding lung meets static pneumothorax area indicating the point at which parietal and visceral pleura separate. Pathognomonic for pneumothorax.
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Thickened, fragmented, or irregular pleural line: Indicates pleural inflammation, malignancy, fibrosis, or pleural adhesions.
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Pleural Effusion: Anechoic or hypoechoic space above the diaphragm, often collecting in the dependent thorax (e.g., the costophrenic recess) ( Fig. 4 ).
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Spine sign: Visualization of the thoracic vertebra due to the presence of pleural fluid in the thoracic cavity. Normally not visible due to impedance from aerated lung.
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Sinusoid sign (M-mode): Respiratory oscillation of the visceral pleura toward/away from the parietal pleura within fluid.
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Jellyfish or Lung monster sign (lung flapping) sign: Wedge-shaped mass of atelectatic lung moving freely within a large effusion with a unique appearance on ultrasound.
Fig. 4 Pleural effusion with pathological signs: atelectasis of compressed lung, air bronchograms, lung monster sign, and a positive spine sign.
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1.3.1.3
Dynamic signs in M-mode
M-mode ultrasound, or “motion-mode”, is an ultrasound technique that allows the user to visualize movement over time expressed as a one-dimensional line. It provides characteristic patterns that help interpret pleural dynamics. In a healthy lung with normal sliding, the M-mode image demonstrates the “seashore sign” , where static chest wall structures above the pleura appear as horizontal lines, and the movement of the lung creates a grainy, sandy appearance below the pleural line, resembling a seashore. In contrast, when lung sliding is absent, as in pneumothorax, the same horizontal line pattern persists above and below the pleura, creating the “barcode” or “stratosphere” sign , which reflects no pleural movement ( Fig. 5 ).
Lung point on M-mode ultrasound: transition from normal lung sliding with a seashore sign to absent sliding with a barcode/stratosphere sign at the pleural line, identifying the pneumothorax boundary (lung point).
While these signs may be useful in the review of stable patients, in perioperative contexts, especially during or after airway manipulation, assessment of the anterior thorax, confirming the presence of lung sliding and pulse, are more sensitive than auscultation for detecting bronchial intubation or pneumothorax . This can be helpful in challenging airway situations such as the placement of a double lumen tube, pediatric airway management, or in low resource settings where bronchoscopy may not be readily available.
1.3.2
Interstitial space
In normal lungs, the interlobular septa and pulmonary interstitium are too thin to generate visible ultrasound signals. Furthermore, air is a poor conductor of ultrasound; it reflects and scatters ultrasound waves, making it nearly impossible for sonographic beams to traverse aerated lung parenchyma. As a result, lung ultrasound in healthy individuals shows a clear pleural line with minimal vertical reverberations. However, when interstitial or alveolar structures are affected, such as in pulmonary oedema, fibrosis, or inflammation, the thickened or fluid-filled interlobular septa become capable of reflecting ultrasound beams characteristically.
1.3.2.1
Normal findings
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A-lines : Horizontal, equidistant reverberation artifacts beneath the pleural line; signify normally aerated lung.
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Comet-tail artifacts : Short, tapering vertical reverberations arising from the pleura that fade with depth and do not erase A-lines, in contrast to B-lines which extend to the far-field of the image. They may appear in healthy individuals, especially in dependent regions, and are not, by themselves, pathological.
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Mirror artifacts : A strong, smooth reflector (typically the diaphragm and liver/spleen interface) creates a duplicated, inverted “mirror” image that appears to project above the diaphragm into the thorax. It mimics consolidation but copies abdominal echotexture and vanishes or shifts with small angle changes and should be confirmed in two planes to avoid error.
1.3.2.2
Pathological findings
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B-lines : Discrete, hyperechoic vertical artifacts that originate from the pleural line, extend to the bottom of the screen without fading, move with respiration, and obliterate A-lines ( Fig. 6 ). Occasional isolated B-lines in dependent zones can be seen in normal states and should be interpreted in context. ≥3 B-lines in a single intercostal space suggest interstitial syndrome.
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Diffuse, symmetric B-lines across multiple zones often indicate cardiogenic pulmonary oedema (typically with a smooth pleural line and preserved sliding).
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Focal or patchy B-lines indicate nonnon-cardiogenic processes such as pneumonia, contusion, or early acute respiratory distress syndrome (ARDS).
Fig. 6 Increased B-line count compatible with interstitial syndrome.
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Coalescent B-lines : These may present as a “white-out” of the lung field and reflect marked alveolar–interstitial involvement (e.g., severe oedema or ARDS).
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C-lines : Short, coarse vertical reverberations arising from within a subpleural consolidation, not from an intact pleural line. They typically cluster in basal or dependent areas where tissue is consolidated. Their presence supports parenchymal disease such as pneumonia, atelectasis contusion.
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Consolidation and Hepatization : Loss of aeration allows ultrasound penetration into the lung parenchyma, producing a tissue-like or hepatized appearance. Consolidations are typically subpleural, wedge-shaped or lobar, with an irregular deep margin (shred sign) at the interface with aerated lung ( Fig. 7 ).
Fig. 7 A hepatized subpleural consolidation with shred sign (irregular deep border at the aerated-consolidated interface).
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Air Bronchograms : Bright, branching or punctate echoes within a consolidated region, arising from air-filled bronchi in fluid-filled alveoli. These may be dynamic and move with respiration, or static. Dynamic patterns are commonly seen in pneumonic consolidation, whereas static patterns align with a more atelectatic picture ( Fig. 4 ).
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Shred sign : At the interface between aerated lung and consolidation, the deep border of the hepatized area appears irregular, serrated, and “torn” rather than smooth. This ragged margin is known as the “shred sign” ( Fig. 7 ). It reflects a transition from tissue-like consolidation to adjacent aerated lung, where ultrasound beam scattering creates a frayed edge. The sign supports parenchymal consolidation (e.g., pneumonia, dependent collapse) rather than simple effusion. It is critical not to confuse small pleural-line irregularities with a shred sign, as it tends to appear at the deep border of a tissue-like region, not on the superficial pleural line. Rib shadow and mirror artifacts can mimic such irregular edges, and as such, this should be confirmed in two planes.
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