Point-of-Care Lung Ultrasound in Adults: Image Acquisition

Summary

Point-of-care ultrasound (POCUS) of the lungs provides quick answers in rapidly changing clinical scenarios. We present an efficient and informative protocol for image acquisition for use in acute care settings.

Abstract

Consultative ultrasound performed by radiologists has traditionally not been used for imaging the lungs, as the lungs' air-filled nature normally prevents direct visualization of the lung parenchyma. When showing the lung parenchyma, ultrasound typically generates a number of non-anatomic artifacts. However, over the past several decades, these artifacts have been studied by diagnostic point-of-care ultrasound (POCUS) practitioners, who have identified findings that have value in narrowing the differential diagnoses of cardiopulmonary dysfunction. For instance, in patients presenting with dyspnea, lung POCUS is superior to chest radiography (CXR) for the diagnosis of pneumothorax, pulmonary edema, lung consolidations, and pleural effusions. Despite its known diagnostic value, the utilization of lung POCUS in clinical medicine remains variable, in part because training in this modality across hospitals remains inconsistent. To address this educational gap, this narrative review describes lung POCUS image acquisition in adults, including patient positioning, transducer selection, probe placement, acquisition sequence, and image optimization.

Introduction

Over the past several decades, bedside decision-making and treatment have increasingly been augmented by point-of-care ultrasound (POCUS). POCUS is the use of ultrasound for diagnostic or procedural guidance by a patient's primary treatment provider. This is in contrast to consultative ultrasound, where the ultrasound exam is requested by a patient's primary treatment provider but is performed by a separate specialist team¹.

This narrative review focuses on diagnostic POCUS of a specific organ system: the lungs. Diagnostic POCUS of the lungs has proven useful in the acute care setting, allowing the diagnosis of potentially life-threatening conditions in scenarios of respiratory failure, shock, trauma, chest pain, and other situations². Further, procedural lung POCUS is being used to guide needle placement in percutaneous thoracentesis³ and lung recruitment maneuvers⁴. However, despite its clinical significance, lung POCUS proficiency among physicians is variable⁵, limiting the appropriate use of this modality. The purpose of this review is to describe a time-efficient yet thorough image acquisition protocol for diagnostic lung POCUS in adults and to illustrate abnormal findings commonly found in clinical practice. The method described herein is not suitable for newborns and small infants. For information regarding lung POCUS imaging techniques and interpretation in this particular age group, the reader is invited to refer to specific literature^6,7.

There are multiple imaging protocols described in the literature, varying from four-point to 28-point exams depending on how much time is available and what questions the exam is seeking to answer⁸. While the diagnostic accuracy for certain pathologies might be higher when more points are scanned, a focused six-point protocol offers a reasonable trade-off between efficiency and diagnostic accuracy^2,9,10,11,12.

Protocol

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

1. Instrument settings and probe selection

NOTE: Lung POCUS can be performed with a multitude of transducers depending on what question needs to be answered.

1. Superficial lung investigation
   1. For the evaluation of abnormalities that have manifestations superficially (e.g., pneumothorax or pleural line abnormalities), perform lung POCUS using a linear high-frequency (5-10 MHz) probe, with the focal zone set at the pleural line. If a linear high-frequency probe is unavailable, perform superficial lung ultrasound using a low-frequency probe (see section 1.2), although the spatial resolution will be lower, which increases the chances of ambiguous or difficult-to-interpret findings.
2. Deep lung investigation
   1. Use a low-frequency (≤5 MHz) ultrasound probe for the evaluation of anything deeper than the interface of the visceral and parietal pleurae. Ensure that the low-frequency probe has a footprint small enough to fit in between the rib spaces (e.g., a convex array, a micro-convex array, or a linear phased-array sector arc probe).
      NOTE: The linear phased-array sector arc probe is often colloquially referred to as a "phased-array probe". However, this term is misleading, because all modern ultrasound transducers (including linear high-frequency probes) use phasing to steer the ultrasound beam^13,14. For the sake of brevity, the linear phased-array sector arc probe is referred to as a "sector probe."
   2. Preset the machine as follows: abdomen (or lung if there is no abdomen option), varying depth (6-20 cm, depending on the object of interest), harmonic imaging disabled, and indicator to the left of the screen. Perform most of the study in a two-dimensional (2D), grayscale mode called brightness mode (B-mode).
      ​NOTE: Other ultrasound modes such as motion mode (M-mode) and color doppler (CD) can occasionally provide additional information and may be used when screening for certain pathologic states.

2. Patient positioning

1. Supine versus sitting
   1. Perform the studies with the patient sitting up or supine.
2. Delimitation of the imaging regions
   1. Divide each hemi-thorax into three regions, reflecting the anatomic segmentation of the lungs (Figure 1¹⁵). In the left chest, treat the lingula as the left-sided analog of the right middle lobe.

3. Scanning technique

1. Apply ultrasound gel to the transducer.
2. Scanning the right hemithorax
   1. R1: right upper lobe (anterior lung zone) (Figure 2¹⁵)
      1. Place the probe in the mid-clavicular line in the 1st-3rd intercostal spaces (ICSs). Position the probe in the parasagittal orientation, with the indicator mark pointing cranially.
      2. Axis: Center on the pleural line so that the cranial and caudal rib shadows are visible on the edges of the images.
      3. Depth: If the dominant pattern is A-lines (see "Normal lung ultrasound findings" in the representative results section) with ≤ two B-lines (see "Pathologic lung POCUS findings" in the representative results section), decrease the depth so that only a single A-line is visible. If there are >three B-lines, increase the depth until at least three A-lines are visible.
         NOTE: B-lines are vertical hyperechoic artifacts that arise from the pleural line, become wider from superficial to deep, reach the deepest visible portion of the ultrasound screen, and efface the A-lines where the two intersect.
      4. Overall gain: Adjust the gain until the pleural line and A-lines are visible as distinctly echogenic (bright) lines and the spaces between the pleural line and A-lines are hypoechoic (dark).
      5. Click on acquire.
   2. R2: right middle lobe (antero-lateral lung zone) (Figure 3¹⁵)
      1. Place the probe in the anterior axillary line in the 4th-5th ICSs. Position the probe midway between the parasagittal and coronal orientations, with the indicator mark pointing cranially.
      2. Axis: See step 3.2.1.2.
      3. Depth: See step 3.2.1.3.
      4. Overall gain: See step 3.2.1.4.
      5. Click on acquire.
   3. R3: right lower lobe (posterior-lateral lung zone) (Figure 4¹⁵)
      1. Place the probe in the mid-to-posterior axillary line in the 5th-7th ICSs. Position the probe in the coronal plane with the indicator mark pointing cranially.
      2. Axis: Center on the diaphragm such that both the sub-diaphragmatic and supra-diaphragmatic structures are visible at the same time.
      3. Depth: Increase the depth until the sub-diaphragmatic spine is visible.
      4. Overall gain: Increase the gain until the liver/spleen appears slightly hyperechoic.
      5. Click on acquire.
3. Scanning the left hemithorax
   1. L1: left upper lobe (anterior lung zone)
      1. Probe positioning: See step 3.2.1.1.
      2. Axis: See step 3.2.1.2.
      3. Depth: See step 3.2.1.3.
      4. Overall gain: See step 3.2.1.4.
      5. Click on acquire.
   2. L2: lingula of the left upper lobe (lateral lung zone)
      1. Probe positioning: See step 3.2.2.1.
      2. Axis: See step 3.2.1.2.
      3. Depth: See step 3.2.1.3.
      4. Overall gain: See step 3.2.1.4.
      5. Click on acquire.
   3. L3: left lower lobe (postero-lateral lung zone)
      1. Probe positioning: See step 3.2.3.1.
      2. Axis: See step 3.2.3.2.
      3. Depth: See step 3.2.3.3.
      4. Overall gain: See step 3.2.3.4.
      5. Click on acquire.

Representative Results

Normal lung ultrasound findings (Video 1, Video 2, Video 3, Video 4, Video 5, Video 6, and Supplementary File 1)
Due to the marked discrepancy in acoustic impedance between the air in the lungs and the superficial tissues, normally all the ultrasound energy that reaches the interface of the parietal and visceral pleurae is immediately reflected back to the ultrasound transducer. As a result, at the depth of the lung parenchyma, the image seen on the screen of an ultrasound machine normally shows non-anatomic artifacts: artifacts with locations on the ultrasound screen that do not correspond to anatomic structures at that level in the body¹⁶.

Further, the normal lung exam differs depending on whether one is evaluating anterior/antero-lateral (AAL) views (i.e., L1/R1 and L2/R2) or postero-lateral views (i.e., L3/R3). For AAL views, the ultrasound probe is placed overlying the interspace between two ribs in the sagittal plane. This normally generates an image colloquially called the "batwing sign", consisting of the following: a cranial and a caudal rib and their associated shadows, and an echogenic (bright) horizontal line between them that has been termed by sonographers as the "pleural line" (Figure 5¹⁵). Usually, this pleural line is the sonographic representation of two structures in direct contact: the static parietal pleura and the mobile visceral pleura. The movement of the visceral pleura during respiration is what superimposes a dynamic visual finding on the pleural line called "lung sliding": a large-amplitude, horizontal movement in the pleural line synchronous with the patient's respiratory rate. Further, the pleural line also normally shows a dynamic finding called the "lung pulse": a small-amplitude, vertical movement synchronous with the patient's heart rate. The presence of either a lung pulse or lung sliding indicates that, at the examined interspace, the visceral and parietal pleurae are directly opposed to one another, with no intervening air between them (i.e., no pneumothorax). Additionally, lung sliding (when present) indicates that the examined portion of the lung is being ventilated, whereas a lung pulse provides no information about whether or not an examined area of the lung is being ventilated^2,16.

In the AAL lung views, another normal finding is the presence of A-lines. A-lines are echogenic (bright) horizontal lines on the ultrasound screen and are reverberation artifacts of the pleural line. Reverberation artifacts appear as a series of equally spaced horizontal lines and are produced when ultrasound energy repeatedly bounces back and forth between two strong acoustic reflectors (in this case between the ultrasound transducer and the pleural line). A-lines, similar to other reverberation artifacts, are non-anatomic artifacts; there is no structure in the body corresponding to the A-lines at the depth where the A-lines appear on the ultrasound screen. A-lines by themselves have no diagnostic value and must be interpreted in the context of whether the pleural line is active or static. In the presence of an active pleural line (i.e., in the presence of lung sliding and/or a lung pulse), the presence of A-lines and no B-lines (see "Pathologic lung POCUS findings") indicates that the lung parenchyma at the examined location is free of fluid or fibrosis^2,16. Thus, the normal AAL lung exam shows the following constellation of findings: i) cranial and caudal ribs with their associated rib shadows; ii) an active pleural line with lung sliding and a lung pulse between the ribs; iii) the presence of A-lines with no B-lines deep to the pleural line (see "Pathologic lung POCUS findings").

In AAL views, M-mode can potentially be used to increase the temporal resolution of the scan. However, as per existing lung ultrasound guidelines, M-mode is not a required part of the lung POCUS exam sequence⁸. Further, M-mode can often be more challenging to interpret than conventional 2D ultrasound. This is because M-mode's temporal resolution is so high that any slight motion of the transducer or the patient's body in relation to one another can convert the "barcode"-like image expected in a pneumothorax into a "seashore"-like image seen in the normal lung (Video 7; Supplementary File 1). Nevertheless, M-mode may be useful in some situations, such as when rapid shallow breathing is difficult to evaluate with 2D ultrasound alone.

Compared to the AAL lung POCUS views, the expected normal findings are different in postero-lateral lung (PL) views (R3/L3). First, in contrast to the sagittal AAL views, the PL views are obtained in the coronal plane. Second, the target anatomy is different; whereas the AAL views focus on relatively superficial structures (i.e., the pleural line and what is immediately deep to that line), the PL views are intended to screen for pathology deeper in the body (e.g., pleural effusions and lung consolidations) and, thus, require the visualization of deeper landmarks. The deep landmarks that should be seen in the PL views are the following: (1) the diaphragm; (2) the supra-diaphragmatic space; and (3) the sub-diaphragmatic spine. Normally, the structures above have the following behavior: (1) bilateral hemi-diaphragms move caudally during inspiration and cranially during exhalation; (2) the supradiaphragmatic space contains a combination of rib shadows and A-lines; and (3) the sub-diaphragmatic spine is visible, but the supra-diaphragmatic spine is not. The violation of any of these patterns is abnormal, as explained below (see "Pathologic lung POCUS findings").

Pathologic lung POCUS findings
Absence of lung sliding
The absence of lung sliding at a given interspace can be caused by any of the following: i) a lack of airflow to the examined lung segment during the exam (e.g., bradypnea, mucous plug, contralateral mainstem intubation, or poorly ventilated emphysematous bleb); ii) adhesions between the parietal and visceral pleurae, preventing normal visceral pleural movement; or iii) a pneumothorax.

Pneumothorax
A pneumothorax is, by definition, the presence of air between the parietal and visceral pleurae. Since air reflects essentially all ultrasound energy back to the transducer, a pneumothorax blocks the visualization of structures that lie deep to it (e.g., the visceral pleura and lung parenchyma). However, structures superficial to the pneumothorax are visible, such as the parietal pleura. Since the parietal pleura does not move during the respiratory cycle, this means that a pneumothorax appears on ultrasound simply as a static pleural line. Specifically, pneumothorax is suspected at a given rib interspace when one is able to visualize a pleural line and there is an absence of all of the following: (1) lung sliding, (2) a lung pulse, and (3) lung parenchymal pathology (e.g., B-lines or consolidation/effusion; see next sections)⁸. A pleural line with no lung sliding, no lung pulse, and no signs of deeper lung pathology is highly suggestive of a pneumothorax (Video 8; Supplementary File 1), especially when the examined area is documented to have had lung sliding recently. However, the absence of the latter signs may also happen in a number of conditions aside from pneumothorax¹⁷. For instance, the false-positive diagnosis of pneumothorax with lung POCUS has been reported in severe chronic obstructive pulmonary disease, emphysematous bullae, and pleural adhesions¹⁸. Notably, the presence of any of the three findings (i.e., lung sliding, B-lines, or a lung pulse) effectively rules out pneumothorax in the lung zone studied^17,19.

The only finding thought to be pathognomonic for pneumothorax is the "lung point", when lung sliding is seen entering into and then completely retreating from an otherwise completely static pleural line (Video 9; Supplementary File 1)⁸. Lung points can be visualized at the edges of a pneumothorax, where the static pleural line identifies the portion of the rib interspace occupied by the pneumothorax, and the lung sliding identifies the normal lung temporarily displacing the pneumothorax during inhalation. Notably, a lung point may not be seen in at least two types of pneumothorax: (1) loculated pneumothorax, and (2) severe tension pneumothorax. In the former case, the fixed location of the pneumothorax may result in the pneumothorax being missed entirely by a focused lung POCUS exam covering only three zones per hemithorax. In the latter case, a lung point may not be seen if the intra-luminal pressure of the pneumothorax is higher than the alveolar peak pressure, preventing the lung from expanding into the pneumothorax space even briefly.

Pneumothorax should be initially sought in the uppermost non-dependent lung zones: the anterior zones in a supine patient-as air is less dense than lung tissue. In terms of transducer selection, screening for a pneumothorax can be performed with different transducers ranging from low to high frequency. However, if low-frequency transducers provide ambiguous data about the presence/absence of pneumothorax, switching to a high-frequency transducer can improve the image quality by offering better spatial resolution of the superficially located pleural line.

To our knowledge, there is no published evidence that adding M-mode to 2D ultrasound measurably improves the ability to diagnose pneumothorax. Further, the only available guidelines on lung ultrasound merely acknowledge that M-mode can be used in lung ultrasound but do not provide a recommendation that it should be used at all⁸. Based on the published literature and our own experiences performing lung POCUS, the authors of this manuscript have differing views as to whether M-mode has value when screening for pneumothorax. Some authors have found that M-mode's high temporal resolution is helpful in the setting of severe tachypnea, where shallow breathing makes it difficult to screen for lung sliding using 2D ultrasound alone. Conversely, other authors have found M-mode to be problematic because of its tendency to generate ambiguous data. Specifically, if M-mode is to be used, the classical teaching is that applying M-mode to a lung interspace free of pneumothorax should generate a "seashore sign": either a continuous seashore sign when M-mode is obtained during lung sliding, or an intermittent seashore sign when M-mode is obtained during lung pulse². Further, the classical lung POCUS teaching is that, when M-mode is applied to an interspace containing a pneumothorax, the M-mode tracing should generate an uninterrupted "barcode sign"². However, M-mode's high temporal resolution means that any slight movement of the ultrasound transducer and the patient's tissues relative to one another often creates an M-mode pattern of an intermittent seashore sign, which interrupts the barcode in cases of true pneumothorax (Video 7; Supplementary File 1). For users who find M-mode problematic and wish to avoid using it when screening for pneumothorax, the following two steps can help resolve ambiguous 2D findings: (1) switching from a low-frequency to a high-frequency transducer, and (2) scanning additional adjacent lung interspaces to ensure that a pattern suggestive of pneumothorax is present beyond a single interspace.

In summary, the diagnosis of pneumothorax with POCUS is (1) suspected by the simultaneous loss of lung sliding, B-lines, and a lung pulse (indirect evidence) and (2) confirmed by the demonstration of the lung point (direct evidence with 100% specificity)⁸.

Interstitial syndrome
"Interstitial syndrome" is a term unique to lung sonography that refers to a pathologic state in which POCUS reveals the presence of at least one rib interspace harboring pathologic B-lines⁸. B-lines are vertical ring-down (reverberation) artifacts. In contrast to other types of vertical ring-down artifacts that may be seen with lung POCUS, B-lines also have the following distinct features: (1) they begin superficially at the pleural line; (2) they descend to the deepest portion of the ultrasound screen; (3) they efface the A-lines where the two artifacts intersect; and (4) they widen from superficial to deep on the ultrasound screen (Figure 6¹⁵). One to two thin B-lines per rib interspace are considered within the range of normal. However, B-lines are considered pathologic when a rib interspace contains either of the following: (1) three or more B-lines (Video 10; Supplementary File 1) or (2) a large confluent B-line occupying the majority of an interspace (Video 11; Supplementary File 1)²⁰.

Physically, the sonographic artifact of B-lines is formed when the normally thin interstitium of the lung is filled in with some sort of density, such as fluid or fibrosis. As the lung density increases in a given rib interspace, the number of B-lines increases until, ultimately, the B-lines become confluent (e.g., when interstitial edema evolves into alveolar edema)²⁰.

The presence of pathologic B-lines in any rib interspace indicates the presence of "interstitial syndrome". Interstitial syndrome (sometimes called interstitial-alveolar syndrome) can be unilateral or bilateral. The finding of unilateral interstitial syndrome narrows the differential diagnosis to any of the following⁸: early atelectasis, early pneumonia, pneumonitis, pulmonary contusion, pulmonary infarction, pleural disease, or lung malignancy.

The finding of bilateral interstitial syndrome narrows the differential diagnosis to three general categories^8,21: i) hydrostatic pulmonary edema (e.g., congestive heart failure, negative-pressure pulmonary edema, transfusion-associated circulatory overload); ii) non-hydrostatic pulmonary edema (e.g., acute respiratory distress syndrome, transfusion-associated lung injury, and bilateral pneumonia); and iii) pulmonary fibrosis.

Lung POCUS alone is generally unable to differentiate between hydrostatic and non-hydrostatic pulmonary edema with certainty, but there are some sonographic clues that make one more likely than the other^8,21. Sonographic findings that support hydrostatic pulmonary edema include the following: (1) homogenous bilateral B-lines that start in dependent zones and continue cranially, and (2) a smooth pleural surface with globally preserved lung sliding. Findings that support non-hydrostatic pulmonary edema include the following: (1) a bilateral heterogeneous distribution of B-lines interposed with healthy-looking parenchymal areas, (2) rough pleural surfaces with subpleural consolidations and/or areas with a loss of lung sliding, and (3) parenchymal consolidations and air bronchograms²¹ (see "Lung consolidation" below). Additionally, when trying to determine whether pulmonary edema is hydrostatic or non-hydrostatic, adding cardiac POCUS to the lung ultrasound findings can be useful^22,23. However, a full discussion of cardiac POCUS in pulmonary edema is beyond the scope of this lung POCUS image acquisition review and has already been presented in other published papers^22,23. Finally, lung POCUS is capable not just of screening for the presence of interstitial syndrome but also of monitoring disease progression and response to therapy²⁴.

Pleural effusion/consolidation pattern
On ultrasound, pleural effusions and lung consolidations typically co-occur because the pleural cavity is constrained in size and normally fully occupied by air-filled lungs. When the lung aeration decreases, a lung consolidation forms, which typically occupies less volume than the air-filled lungs. The remaining space is typically filled in by some degree of reactive pleural fluid formation. The causal sequence also works in the other direction; an accumulation of pleural fluid mechanically compresses the normal aerated lung, creating a lung consolidation. Hence, it is useful in sonography to treat pleural effusions and lung consolidations as related phenomena.

Pleural effusions
On ultrasounds, an anechoic or hypoechoic space between the parietal and visceral pleurae indicates the presence of a pleural effusion (Figure 7; Video 12)^2,15. Pleural effusions facilitate the propagation of ultrasound in the chest, and result in better definition of the deep thoracic structures, such as the deeper lung parenchyma and vertebral bodies. As opposed to pneumothorax, pleural effusions tend to accumulate in the most gravity-dependent thoracic zones, as fluid is denser than the lung parenchyma. The posterior-lateral zone is the most representative in a supine patient². The sonographic appearance of the fluid varies somewhat depending on the nature of the fluid. Whereas transudative fluid is thought to always be anechoic, exudative fluid can be anechoic or hypoechoic. Bloody fluid (i.e., hemothorax) has a variable appearance depending on the acuity of the bleeding. Fresh blood is typically homogenously hyperechoic (Video 13; Supplementary File 1), whereas blood that has had at least a few hours to settle appears hyperechoic in gravity-dependent locations, and hypoechoic or anechoic in less gravity-dependent locations. Empyema typically appears as heterogeneous fluid, often with debris ("plankton sign"), in the setting of ipsilateral pneumonia (Video 14; Supplementary File 1).

A typical image of a pleural effusion reveals a wedge of atelectatic lung "floating" in the fluid-filled chest cavity (sometimes referred to as a "jellyfish" sign), caudally bound by the diaphragm and liver/spleen (Video 7; Supplementary File 1). Small effusions may "disappear" during inspiration due to lung expansion and the downward motion of the diaphragm and reappear during expiration. M-mode imaging of pleural effusion produces the "sinusoid" sign, which consists of respiratory variation of the diameter of the fluid-filled pleural space⁸. The volume of a free-flowing effusion can be estimated by multiple formulas. A formula that is relatively simple and easy to use at the bedside is Balik's; a supine patient is scanned in the posterior axillary line to obtain a transverse section of the lung base with visible pleural separation (see Figure 8¹⁵). The maximum diameter (in millimeters) of separation (SEP in the formula below) between the parietal and visceral pleura at end-expiration is multiplied by 20, giving an estimate of the volume of the effusion (in milliliters)²⁴.

Lung consolidation
In the context of sonography, the term "lung consolidation" refers to a broad range of conditions that cause a section of the lung to appear like a solid organ on the ultrasound: an appearance that has been termed "sonographic hepatization". Lung consolidations vary in size from small subpleural ones to large lobar ones. Subpleural consolidations appear on ultrasounds as focal areas of sonographic hepatization surrounded in a single lung interspace by normal lung parenchyma (Figure 9¹⁵). The boundary between the normal lung parenchyma and the subpleural consolidation has been termed the "shred sign" (Video 15; Supplementary File 1): an irregular hyperechoic line ("fractal line") from which vertical ring-down artifacts² propagate. The shred sign's vertical ring-down artifacts resemble B-lines, except that B-lines emanate down from the pleural line, whereas the shred sign's vertical artifacts emanate down from the deepest portion of the subpleural consolidation. While B-lines can be caused by anything that increases the lung density, the vertical ring-down artifacts of "shred signs" indicate that the increase in lung density is specifically due to the presence of a lung consolidation.

The differential diagnosis of lung consolidations is broad and includes all the following: late infiltrative processes (e.g., late pneumonia or late neoplasia), late atelectasis, pulmonary infarct (including infarcts due to pulmonary embolism), and lung contusion, among others⁸. Although the ultrasound appearance of all these conditions overlaps significantly, the integration of the ultrasound findings with other clinical data points can help to narrow the differential diagnosis further^8,17. Additionally, there is one sonographic finding thought to be highly specific for infiltrative processes: dynamic air bronchograms (DABs). DABs are point-like, round echogenic areas within a consolidation that move during the respiratory cycle (Video 16; Supplementary File 1). DABs indicate that the bronchi are permitting some airflow, which strongly suggests that a consolidation is being caused by an infiltrative process such as pneumonia and not by atelectasis, where one would expect a complete abolition of airflow⁹. Color Doppler, demonstrating blood flow in the examined area, rules out pulmonary infarction.

Figure 1: External correlates of each of the five lobes of the lung. Note that pathologic states (i.e., volume loss from ipsilateral mucous plugging and/or atelectasis) and variability in body habitus can cause substantial differences in the relationship between usual surface landmarks and the underlying viscera. Such considerations are especially imperative for the safe performance of thoracic procedures and highlight the importance of a thorough and skilled ultrasound evaluation. This image was reprinted with the author's permission¹⁵. Abbreviations: RUL = right upper lobe; RML = right middle lobe; RLL = right lower lobe; LUL = left upper lobe; LLL = left lower lobe. Please click here to view a larger version of this figure.

Figure 2: Scanning orientation and anatomic location for the R1 view evaluating the right upper lobe. Shown as a schematic illustration (left panel) and a demonstration on a standardized patient (right panel). The left panel was reprinted with the author's permission¹⁵. Abbreviations: RUL = right upper lobe; RML = right middle lobe; RLL = right lower lobe; LUL = left upper lobe; LLL = left lower lobe. Please click here to view a larger version of this figure.

Figure 3: Scanning orientation and anatomic location for the R2 view evaluating the right middle lobe. Shown as a schematic illustration (left panel) and a demonstration on a standardized patient (right panel). The left panel was reprinted with the author's permission¹⁵. Abbreviations: RUL = right upper lobe; RML = right middle lobe; RLL = right lower lobe; LUL = left upper lobe; LLL = left lower lobe. Please click here to view a larger version of this figure.

Figure 4: Scanning orientation and anatomic location for the R3 view evaluating the right lower lobe. Shown as a schematic illustration (left panel) and a demonstration on a standardized patient (right panel). The left panel was reprinted with the author's permission¹⁵. Abbreviations: RUL = right upper lobe; RML = right middle lobe; RLL = right lower lobe; LUL = left upper lobe; LLL = left lower lobe. Please click here to view a larger version of this figure.

Figure 5: Expected normal sonographic findings when examining the anterior (L1/R1) and antero-lateral (L2/R2) lung zones. This figure was reprinted with the author's permission¹⁵. Please click here to view a larger version of this figure.

Figure 6: Lung ultrasound with B-lines. In contrast to other vertical ring-down artifacts seen in lung ultrasound (e.g., the "shred sign"), B-lines have the following sonographic features: (1) they begin superficially at the pleural line; (2) they descend to the deepest portion of the ultrasound screen; (3) they efface the A-lines where the two artifacts intersect; and (4) they widen from superficial to deep on the ultrasound screen. This image was reprinted with the author's permission¹⁵. Please click here to view a larger version of this figure.

Figure 7: A large pleural effusion. Schematic of a large pleural effusion (left panel) and a still image of an R3 view containing a lung consolidation within a large pleural effusion (right panel). The right panel is a still image obtained from Video 12. The left panel was reprinted with the author's permission¹⁵. Please click here to view a larger version of this figure.

Figure 8: Representative schematic showing how to use Balik's formula²³ to estimate the pleural effusion volume. The image is obtained by starting with either an L3 or R3 (depending on the location of the pleural effusion) and then rotating the ultrasound probe until the indicator mark is pointing anteriorly. This requires a 90° rotation, clockwise from the R3 view and counterclockwise from the L3 view. This rotates the probe from the coronal plane of the body (L3/R3 view) to the body's transverse plane. When the patient reaches end-expiration, a still image should be obtained. In the resulting still image, the caliper function of the ultrasound machine (white dotted line in the image) can then be used to measure the parietal-to-visceral pleural separation distance in centimeters. This separation distance can then be entered into the Balik formula as the SEP term to estimate the pleural effusion volume in milliliters. This image was reprinted with the author's permission¹⁵. Please click here to view a larger version of this figure.

Figure 9: Schematic demonstrating the typical sonographic appearance of a subpleural consolidation. This figure was reprinted with the author's permission¹⁵. Please click here to view a larger version of this figure.

Video 1: The expected normal findings when the following zone of the lung is investigated with lung ultrasound: R1. Please click here to download this Video.

Video 2: The expected normal findings when the following zone of the lung is investigated with lung ultrasound: R2. Please click here to download this Video.

Video 3: The expected normal findings when the following zone of the lung is investigated with lung ultrasound: R3. Please click here to download this Video.

Video 4: The expected normal findings when the following zone of the lung is investigated with lung ultrasound: L1. Please click here to download this Video.

Video 5: The expected normal findings when the following zone of the lung is investigated with lung ultrasound: L2. Please click here to download this Video.

Video 6: The expected normal findings when the following zone of the lung is investigated with lung ultrasound: L3. Please click here to download this Video.

Video 7: Concurrent brightness mode (B-mode) and motion mode (M-mode) clips of a pneumothorax demonstrating the lack of diagnostic value provided by the M-mode tracing. The B-mode clip (top) shows a completely static pleural line, which is consistent with a pneumothorax. When using M-mode, classically a pneumothorax is supposed to appear as a continuous "barcode" sign uninterrupted by any "seashore" pattern. In contrast, when using M-mode, the finding of an intermittent "seashore" pattern would indicate the presence of a "lung pulse," a finding that rules out pneumothorax at the examined interspace. However, the M-mode tracing here (bottom) shows a "barcode" intermittently interrupted by a "seashore" pattern. This is because M-mode's extremely high temporal resolution captures brief and clinically insignificant movement of the pleural line and ultrasound probe relative to one another, interrupting the "barcode" pattern of the pneumothorax with an intermittent "seashore" pattern. As a result, the M-mode here actually turns an unambiguous 2D finding of a static pleural line into an ambiguous M-mode tracing that is indeterminate for pneumothorax.Please click here to download this Video.

Video 8: Paired clips obtained from the left and right hemothorax of the same patient. A linear high-frequency transducer showing the following: (i) L1 with normal lung sliding and likely B-lines (i.e., pneumothorax NOT possible at the examined location) and (ii) R2 with the absence of lung sliding, a lung pulse, and B-lines (i.e., pneumothorax possible at the examined location).Please click here to download this Video.

Video 9: L2 view showing a lung point. The presence of lung sliding entering into and then completely retreating from an otherwise static pleural line. In this clip, the lung sliding is seen entering from the left side of the screen (cranial side of the clip) and represents a normal aerated lung expanding into the space of the pneumothorax during inhalation. The static pleural line indicates the location of the pneumothorax. A lung point is thought to be pathognomonic for pneumothorax and is seen at the edges of the pneumothorax.Please click here to download this Video.

Video 10: An example of a rib interspace containing pathologic B-lines: R2 view showing more than three B-lines. Please click here to download this Video.

Video 11: A second example of a rib interspace containing pathologic B-lines: R2 view showing large confluent B-lines occupying the majority of the interspace. Please click here to download this Video.

Video 12: R3 view containing a lung consolidation floating inside a large pleural effusion. Please click here to download this Video.

Video 13: R3 view obtained peri-cardiac arrest in a patient found to have acute bleeding into a chronic right pleural effusion, creating a right-sided hemothorax. This acute blood appears homogenously hyperechoic (bright) because it has not yet had time to layer into separate plasma (hypoechoic) and cellular (hyperechoic) layers. Note that this clip was obtained in non-standard fashion (in cardiac mode with the indicator on the right of the screen). Please click here to download this Video.

Video 14: L3 view demonstrating a heterogeneous pleural effusion with free-floating debris ("plankton sign"). Pleural fluid that appears heterogeneous on ultrasound is nearly always exudative on chemical testing. Please click here to download this Video.

Video 15: L3 view demonstrating a "shred sign": an irregular hyperechoic line ("fractal line") in the middle of the lung parenchyma from which vertical ring-down artifacts propagate. Please click here to download this Video.

Video 16: L3 view demonstrating dynamic air bronchograms (DABs)-point-like, round echogenic areas within a consolidation that move during the respiratory cycle. DABs indicate that the bronchi are permitting some airflow, which strongly suggests that a consolidation is being caused by an infiltrative process such as pneumonia and not by atelectasis, where one would expect a complete abolition of airflow. Please click here to download this Video.

Video 17: L1 view demonstrating subcutaneous emphysema. The finding during lung ultrasound of an irregular horizontal line that prevents the visualization of the ribs. Please click here to download this Video.

Supplementary File 1: Still images of all the videos. Please click here to download this File.

Discussion

Diagnostic POCUS is the use of ultrasound at the bedside by a patient's primary treatment provider to answer clinical questions. The questions most amenable to diagnostic POCUS are those that are qualitative or binary in nature and that need to be answered faster than would be possible or practical with consultative ultrasound services.

A few steps are critical for image acquisition. The first one is probe selection. The authors recommend that the initial assessment be performed using the sector probe. This type of probe can easily be found in most ultrasound machines, it is suitable for the visualization of both superficial and deep structures, and it has a small footprint, which permits optimal positioning between the ribs while minimizing rib shadowing. After the initial assessment, a different type of probe can then be chosen based on the preliminary findings. The second critical step is patient positioning. Here, the examiner must be mindful that the positioning affects the distribution of pleural contents and parenchymal infiltrates. Whereas air occupies the uppermost nondependent areas, free-flowing pleural effusion and pulmonary edema distribute preferentially to the lowermost dependent regions. Regardless of the positioning selected, subsequent studies should be performed in the same way for optimal serial evaluation of the patient. Finally, the third critical step is image storage. Although often neglected in emergency situations, image storage is crucial for the sake of documentation, comparison of the disease course and/or response to treatment, and educational purposes. Beginners should review the acquired images with experienced sonographers to develop optimal imaging techniques and diagnostic capacity. This can only be done if the acquired images have been appropriately stored.

A few words deserve to be mentioned regarding some common difficulties with image acquisition. One of them is insonating directly through the ribs instead of rib spaces, which leads to poor visualization of the lung structures due to acoustic shadowing. The solution here is to optimize the probe orientation in the cranio-caudal plane to insonate through the rib interspace rather than the rib itself. Another common issue is difficulty in the visualization of the full anatomy of the R3 or L3 zones, including the diaphragm and liver/spleen. In this case, the examiner can move the probe further posteriorly, even past the posterior axillary line, aiming slightly anteriorly toward the vertebral bodies. The examiner should start cranially (around the 5th intercostal space, or nipple level) and slowly move caudally until the diaphragm, liver, or spleen come into view. If the kidney is visualized, the examiner is imaging the abdomen and should translate (slide) the probe back toward the chest and repeat the move just suggested.

Lung POCUS is ideal for investigating the signs/symptoms of cardiorespiratory dysfunction, including the following: dyspnea, tachypnea, hypoxemia, hypercapnia, chest pain, and/or hypotension. In this regard, the diagnostic performance of lung POCUS is superior to that of supine anteroposterior chest radiography (CXR) for the diagnosis of pneumothorax, pleural effusion, interstitial lung syndromes, and alveolar consolidation^8,18,25. Lung POCUS is also a reasonable alternative to computed tomography (CT) of the chest, the diagnostic gold standard for most acute respiratory syndromes, due to the lower cost, shorter turnaround time, and the fact that it does not require patient transportation or the emission of ionizing radiation^2,25.

However, some limitations of lung POCUS must be mentioned. First, image acquisition in patients with subcutaneous emphysema (SCE) may be difficult, as the air pockets prevent sound transmission (Video 17; Supplementary File 1). Thus, patients found to have SCE on lung ultrasound require non-sonographic imaging to determine whether any pathology lies below the subcutaneous air. Second, lung pathologies outside of the examined areas can easily be missed. This is especially the case with deep/central areas of consolidation or loculated effusions or pneumothorax. Third, some patients may have complex lung pathologies (e.g., recurrent pneumothorax, bronchopleural fistulas) and require CT for a more thorough investigation. Fourth, lung ultrasound is inherently limited to lung evaluation and frequently needs to be supplemented with the diagnostic evaluation of other organ systems involved in critical illness, such as the upper airway, heart, abdomen, and kidneys, based on the patient's presentation and symptoms.

Finally, a surmountable limitation of lung POCUS is the lack of proficiency. As with any ultrasound technique, diagnostic POCUS is highly operator-dependent and, thus, prone to high inter-operator variability. To address this variability, some professional medical societies have proposed national training programs and curricula. For instance, the American Society of Anesthesiologists Ad Hoc Committee on POCUS recently made recommendations regarding a minimum educational curriculum, suggesting that trainees perform the following minimum numbers of training studies to achieve competence in lung ultrasound: 30 exams performed and interpreted and 20 exams interpreted that need not be personally performed²⁶. Other professional medical societies have recommended slightly different minimum training numbers²⁶, so the reader is invited to refer to specialty-specific POCUS curriculums and competency requirements, which are beyond the scope of this article. As these specialty-specific societies' training standards become implemented, inter-operator variability is likely to decrease. Further, we hope that this manuscript will help to standardize one aspect of diagnostic POCUS: lung ultrasound image acquisition.

Materials

Name	Company	Catalog Number	Comments
Edge 1 ultrasound machine	SonoSite	n/a	Used to obtain two of the abnormal images/clips (Figures 11 and 12)
Affiniti ultrasound machine	Philips	n/a	Used to obtain all normal and all abnormal images/clips except for Figures 11 and 12