1. Kerrey BT, Rinderknecht AS, Geis GL, Nigrovic LE, Mittiga MR. Rapid sequence intubation for pediatric emergency patients: higher frequency of failed attempts and adverse effects found by video review. Ann Emerg Med 2012;60(3):251-9. DOI: 10.1016/j.annemergmed.2012.02.013.
2. Takeda T, Tanigawa K, Tanaka H, Hayashi Y, Goto E, Tanaka K. The assessment of three methods to verify tracheal tube placement in the emergency setting. Resuscitation 2003;56(2):153-7. DOI: 10.1016/s0300-9572(02)00345-3.
3. Kelly JJ, Eynon CA, Kaplan JL, de Garavilla L, Dalsey WC. Use of tube condensation as an indicator of endotracheal tube placement. Ann Emerg Med 1998;31(5):575-8. DOI: 10.1016/s0196-0644(98)70204-5.
4. Panchal AR, Berg KM, Hirsch KG, et al. 2019 American Heart Association Focused Update on Advanced Cardiovascular Life Support: Use of Advanced Airways, Vasopressors, and Extracorporeal Cardiopulmonary Resuscitation During Cardiac Arrest: An Update to the American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 2019;140(24):e881-e894. (In eng). DOI: 10.1161/cir.0000000000000732.
5. Kleinman ME, de Caen AR, Chameides L, et al. Part 10: Pediatric basic and advanced life support: 2010 International Consensus on Cardiopulmonary Resuscitation and Emergency Cardiovascular Care Science With Treatment Recommendations. Circulation 2010;122(16 Suppl 2):S466-515. DOI: 10.1161/CIRCULATIONAHA.110.971093.
6. Li J. Capnography alone is imperfect for endotracheal tube placement confirmation during emergency intubation. J Emerg Med 2001;20(3):223-9. DOI: 10.1016/s0736-4679(00)00318-8.
7. Brown CA, 3rd, Bair AE, Pallin DJ, Walls RM, Investigators NI. Techniques, success, and adverse events of emergency department adult intubations. Ann Emerg Med 2015;65(4):363-370 e1. DOI: 10.1016/j.annemergmed.2014.10.036.
8. Chou HC, Chong KM, Sim SS, et al. Real-time tracheal ultrasonography for confirmation of endotracheal tube placement during cardiopulmonary resuscitation. Resuscitation 2013;84(12):1708-12. DOI: 10.1016/j.resuscitation.2013.06.018.
9. Ariff S, Ali KQ, Tessaro MO, et al. Diagnostic accuracy of point-of-care ultrasound compared to standard-of-care methods for endotracheal tube placement in neonates. Pediatr Pulmonol 2022;57(7):1744-1750. (In eng). DOI: 10.1002/ppul.25955.
10. Salem MR. Verification of endotracheal tube position. Anesthesiol Clin North Am 2001;19(4):813-39. DOI: 10.1016/s0889-8537(01)80012-2.
11. Li X, Zhang J, Karunakaran M, Hariharan VS. Diagnostic accuracy of ultrasonography for the confirmation of endotracheal tube intubation: a systematic review and meta-analysis. Med Ultrason 2023;25(1):72-81. DOI: 10.11152/mu-3594.
12. Gottlieb M, Bailitz JM, Christian E, et al. Accuracy of a novel ultrasound technique for confirmation of endotracheal intubation by expert and novice emergency physicians. West J Emerg Med 2014;15(7):834-9. DOI: 10.5811/westjem.22550.9.22550.
13. Chou HC, Tseng WP, Wang CH, et al. Tracheal rapid ultrasound exam (T.R.U.E.) for confirming endotracheal tube placement during emergency intubation. Resuscitation 2011;82(10):1279-84. DOI: 10.1016/j.resuscitation.2011.05.016.
14. Chenkin J, McCartney CJ, Jelic T, Romano M, Heslop C, Bandiera G. Defining the learning curve of point-of-care ultrasound for confirming endotracheal tube placement by emergency physicians. Crit Ultrasound J 2015;7(1):14. DOI: 10.1186/s13089-015-0031-7.
15. Adi O, Chuan TW, Rishya M. A feasibility study on bedside upper airway ultrasonography compared to waveform capnography for verifying endotracheal tube location after intubation. Crit Ultrasound J 2013;5(1):7. DOI: 10.1186/2036-7902-5-7.
16. Abbasi S, Farsi D, Zare MA, Hajimohammadi M, Rezai M, Hafezimoghadam P. Direct ultrasound methods: a confirmatory technique for proper endotracheal intubation in the emergency department. Eur J Emerg Med 2015;22(1):10-6. DOI: 10.1097/MEJ.0000000000000108.
17. Hemmerling TM, Donati F. Neuromuscular blockade at the larynx, the diaphragm and the corrugator supercilii muscle: a review. Can J Anaesth 2003;50(8):779-94. DOI: 10.1007/BF03019373.
18. Weaver B, Lyon M, Blaivas M. Confirmation of endotracheal tube placement after intubation using the ultrasound sliding lung sign. Acad Emerg Med 2006;13(3):239-44. DOI: 10.1197/j.aem.2005.08.014.
19. Tessaro MO, Salant EP, Arroyo AC, Haines LE, Dickman E. Tracheal rapid ultrasound saline test (T.R.U.S.T.) for confirming correct endotracheal tube depth in children. Resuscitation 2015;89:8-12. (In eng). DOI: 10.1016/j.resuscitation.2014.08.033.
20. Wani TM, John J, Rehman S, et al. Point-of-care ultrasound to confirm endotracheal tube cuff position in relationship to the cricoid in the pediatric population. Paediatr Anaesth 2021;31(12):1310-1315. DOI: 10.1111/pan.14303.
21. Walsh B, Fennessy P, Ni Mhuircheartaigh R, Snow A, McCarthy KF, McCaul CL. Accuracy of ultrasound in measurement of the pediatric cricothyroid membrane. Paediatr Anaesth 2019;29(7):744-752. DOI: 10.1111/pan.13658.
22. Fennessy P, Walsh B, Laffey JG, McCarthy KF, McCaul CL. Accuracy of pediatric cricothyroid membrane identification by digital palpation and implications for emergency front of neck access. Paediatr Anaesth 2020;30(1):69-77. DOI: 10.1111/pan.13773.
23. Siddiqui N, Arzola C, Friedman Z, Guerina L, You-Ten KE. Ultrasound Improves Cricothyrotomy Success in Cadavers with Poorly Defined Neck Anatomy: A Randomized Control Trial. Anesthesiology 2015;123(5):1033-41. DOI: 10.1097/ALN.0000000000000848.
24. Lin J, Bellinger R, Shedd A, et al. Point-of-Care Ultrasound in Airway Evaluation and Management: A Comprehensive Review. Diagnostics (Basel) 2023;13(9). DOI: 10.3390/diagnostics13091541.
25. Yildiz G, Goksu E, Senfer A, Kaplan A. Comparison of ultrasonography and surface landmarks in detecting the localization for cricothyroidotomy. Am J Emerg Med 2016;34(2):254-6. DOI: 10.1016/j.ajem.2015.10.054.
26. Uya A, Gautam NK, Rafique MB, et al. Point-of-Care Ultrasound in Sternal Notch Confirms Depth of Endotracheal Tube in Children. Pediatr Crit Care Med 2020;21(7):e393-e398. DOI: 10.1097/PCC.0000000000002311.

Summary of Steps

1. Obtain baseline view of the sonoanatomy of the patient’s airway prior to intubation 
   • ​​Place the high frequency linear probe above the patient’s suprasternal notch in the transverse plane​ 
   • Identify the trachea, thyroid cartilage, esophagus
2. Choose between static ​and​​ dynamic technique for Airway POCUS ​
   • Static: intubate and repeat the ultrasound​.​​ Identify ​the double tract sign if present
   • Dynamic: keep the probe ​​position​ed in the suprasternal notch​​.​​  Watch ​for the snowstorm​ (motion artifact), bullet (spreading of the vocal cords)​ and double trachea ​signs​.
3. Assess for the depth of the ETT 
   • Evaluate lung sliding
   • Evaluate ​saline filled ​cuff placement 
4. If urgent cricothyroidotomy is required, consider ultrasound identification of the cricothyroid membrane

 

Summary

• POCUS has shown to be a fast, effective and safe adjunct to other ETT position confirmatory methods  

• To confirm ETT placement, the technique can be performed dynamically or statically using a high frequency linear probe placed in the transverse plane at the level of the suprasternal notch 

• With endotracheal intubation, the esophagus should be collapsed​.​​  Only​​ one air filled structure should be seen (trachea) and the snowstorm sign can be seen dynamically. 

• With ​esophageal intubation, the double tract (double trachea sign) will be seen 

• To confirm ETT placement, ultrasound can be used to assess lung sliding, ​and ​cuff placement​.​ ​     ​ 

• Ultrasound can be used to identify the location of the cricothyroid membrane​ to facilitate surgical airway management.​ 

Identification of the cricothyroid membrane​ to facilitate surgical cricothyrotomy​​

​​​​In the context of surgical airways, ​​​POCUS can also be used to identify the cricothyroid membrane in children [21]. Ultrasound outperforms digital palpation of the cricothyroid membrane in children [22]​​​.​​​​​ ​Furthermore, it’s application ​has been ​linked to​​​ improve​d​ success ​rates​​​ ​in ​correct cricothyroid tube placement in adult​ patients [​​​23]. However, it ​is important to note that using ultrasound in this context can be more time consuming. Ultrasound use will typically take ​​​​​17s​,​ ​​compared to traditional palpation which takes about 8s [25]​.​​​​ This time cost ​may be ​worthwhile​​​ in patients with higher BMI​ where palpation may be more difficult or in the anticipated difficult airway when there is sufficient time prior to intubation​.  

Airway POCUS has also been proposed as a risk prediction tool for difficult laryngoscopy in adults. Pediatric literature on prediction of ​airway difficulty​​​ ​​is scant and beyond the scope of this module.  

The cricothyroid membrane can be visualized by ultrasound using the linear probe ​placed​​ in the longitudinal plane (Figure 3B). Figure 2 C-D ​demonstrates​ the sonoanatomy. The probe should first be ​placed​ at the suprasternal notch ​where the​ tracheal rings (the string of pearls) ​may be​​     ​ identified. The probe is moved cephalad until a larger pearl is seen: the cricoid with its posterior acoustic shadowing. Above it, the other hyperechoic structure with hypoechoic shadow is the thyroid cartilage. In between lies the cricothyroid membrane. Following its identification, the clinician can mark the skin and use this information ​as a static ​​​landmark for surgical airway.  

In general, you will see the same overall landmarks—cricoid and thyroid cartilages with the cricothyroid membrane in between—throughout the lifespan. However, the echogenic appearance and degree of posterior acoustic shadowing can vary with patient age. This is largely because pediatric laryngeal cartilages are more pliable and less calcified than those of older adolescents or adults. As patients get older and the cartilage begins to calcify, you will see a more prominent hyperechoic line with a stronger acoustic shadow. 

In younger children, the cartilage tends to appear less bright (less calcified) on ultrasound, and the shadows they cast are often more subtle. Nevertheless, the positional relationships—thyroid cartilage, cricothyroid membrane, cricoid cartilage—remain the same, so you can still use the same “string of pearls” approach and carefully identify the membrane for procedural marking. The “pearls” may be a smaller and dimmer in younger pediatric scans compared to older patients. 

Cricothyrotomy is an advanced skill that is rarely performed by physicians. Use of ultrasound should never delay definitive airway establishment. Expert surgical consultation should be sought early. 

Evaluation of  Lung Sliding

​​​The depth of ETT insertion can also be evaluated using lung POCUS. ​Normal lung sliding indicates aeration of the lung. The movement caused by inflation of the lung can be seen as the visceral and parietal pleura move and is called lung sliding or shimmering. As such, presence​​​​​ of bilateral lung sliding indicates endotracheal intubation with an ETT positioned above the carina. ​The absence of lung sliding suggests the lung is not being ventilated and raises suspicion for esophageal intubation. If unilateral lung sliding is seen, it can suggest mainstem intubation [18]. ​​​ 

​​​Lung ​u​​​ltrasound can be performed with the patient in ​the ​supine position, using a linear or curvilinear​ probe placed in the longitudinal plane on the anterior chest of the patient at the midclavicular line​.​ ​​​​​​​​​This is performed on both the left and right anterior chest walls. ​The ​relevant​​​ anatomic structures for this view are the pleural line, chest wall musculature and the ribs and their ​acoustic ​shadows (Video 3) 

​​​ ​Important limitations to using lung sliding for confirmation of ETT depth include diseases and conditions that affect the pleura and therefore lung sliding on POCUS. This includes pneumothorax, pleural disease and contralateral lung sliding despite mainstem intubation because of retrograde air movement. ​​ 

Video 3. Lung ultrasound video performed using a linear probe placed in the longitudinal plan on the anterior chest of the patient at the midclavicular line demonstrating pleural line with normal sliding.

Video 4. Lung ultrasound video performed using a linear probe placed in the longitudinal plane on the anterior chest of the patient at the midclavicular line demonstrating absent lung sliding 

M-Mode can also be used to confirm the presence of lung sliding. In a normal lung where lung sliding is present, the “sand on the beach” or “seashore” sign can be observed (Figure 6A). Conversely, absence of lung sliding will create the “stratosphere” or “barcode” sign (6B). 

Figure 6: Use of M-mode to identify A) presence (​seashore​​ sign) or B) absence (stratosphere sign) of lung sliding  

 

**For more information on lung sliding and lung PoCUS, please revisit the KidSONO Pneumothorax Module**

Tracheal Widening with ETT Cuff Inflation

If the ETT cuff is appropriately placed in the suprasternal notch, distention of the cuff with air can be visualized as widening of the air-filled column on ultrasound (Video 6). 

Video 6: Transverse view of widening of the air-filled column due to ETT distention with air

 

Visualization of a Saline-filled Cuff within the Trachea

Depth can also be evaluated ​if​​​ the ETT cuff is filled with saline, making it directly visible ​within the airway​​​. Visualization of the saline filled cuff at the suprasternal notch confirms endotracheal intubation whereas non visualization of the cuff signifies endobronchial intubation with a sensitivity of 98.8% specificity 96.4% when compared with fiberoptic bronchoscopy [19]. Position of the ETT cuff above the suprasternal notch in relation to cricoid, subglottic area and tracheal ring can have important ​implications​​​ for tissue damage ​of the glottic​​​ and subglottic ​structures​​​. Airway POCUS can be used rapidly and accurately in children to determine the depth of the ETT using saline filled cuff [20] (Figure 7). 

It is ESSENTIAL to remove the saline once the ETT position is confirmed and reinflate the cuff with air. This limits the possible complication of pressure necrosis with saline filled cuff.  

Figure 7. A) Endotracheal vs B) endobronchial vs C) appropriate depth endotracheal intubation intubation using a saline-inflated ETT cuff allowing visualisation of tissues that are posterior to the trachea (*) with saline acting as an acoustic window. Air within the tube results in acoustic shadowing as (white arrowhead), reproduced from Tessaro et al, Resuscitation, 2015 [19] 

The airway can be evaluated by ultrasound from the suprahyoid area to the suprasternal notch in both the transverse and longitudinal planes (Figure 3). The transverse view is most commonly used to confirm the location of the ETT, but the longitudinal view can aid in landmarking when planning a surgical airway.  

Figure 3. Airway POCUS using the linear probe in a supine patient in the A) transverse and B) longitudinal planes with corresponding sonographic appearance

Confirmation of ETT Position

Dynamic Technique

The dynamic evaluation is best performed with the probe in the transverse plane at the level of the suprasternal notch (Figure 3A). Identification of normal sonoanatomy including thyroid cartilage, air filled trachea and collapse esophagus (Figure 2 E-F) should be done before the intubation is started. In the majority of individuals, the esophagus will be left sided. However, it can also be ​positioned​ to the right or posterior to the trachea. Sonographers could adjust their scanning planes by slightly sliding and tilting the probe left and right to identify the position of the esophagus. 

During endotracheal intubation, motion artefact (the snowstorm sign) will be visualized as the ETT is advanced in the trachea (Video 1). If the tube is inadvertently placed in the esophagus, a second air filled structure with comet tail artifact will appear, this is also called double trachea or double tract sign (Video 2).  

Video 1: Transverse view of ETT advancement into the trachea with snowstorm sign.

Video 2: Transverse view of ETT placement within the esophagus (double trachea sign)

Advanced Technique

​A more advanced technique is described in the literature:  dynamic visualisation of ETT placement which can also be done at the level of the cricothyroid membrane in the transverse plane (Figure 1, level 3). In this position, the vocal cords are visualized and form an ‘A.’ This is called the triangle sign. When the ETT is passed in the trachea, brief movement is seen in this location, referred to as ‘snowstorm’ and then the vocal cord​s​ appear more spread out. This is called the bullet sign [16] (Figure 4 A-B). 

Figure 4: Dynamic movement of the vocal cords at the level of the cricothyroid membrane A) before intubation and B) after intubation 

Static Technique

Static evaluation of tube placement is performed by placing the ultrasound probe in the suprasternal notch before the intubation to identify the sonoanatomy. Once the intubation is completed, the ultrasound is repeated. The image interpretation will be the same as the dynamic technique. With esophageal intubations, a second air filled structure with comet tail ​artifact​ will appear revealing ​the ​double tract ​sign (Figure 5)​.  

Figure 5: Transverse view with ultrasound position above the ​suprasternal​​ ​notch showing comet tail artifact (c); and ​double-tract​ sign (d)

Anatomy Review

The airway can be evaluated by ultrasound from the suprahyoid area to the suprasternal notch (Figure 1) in both the transverse and longitudinal planes. The transverse view is most commonly used to confirm the location of the ETT, but the longitudinal view can aid in landmarking when planning a surgical airway. 

 

Figure 1: Reproduced from Lin et al, Diagnostic 2023 [24]

 

Ultrasound Anatomy Review

The sonoanatomy of the airway includes important landmarks including the vocal cords (Figure 2 A-B), cricothyroid membrane (Figure 2 C-D), trachea, esophagus, thyroid cartilage and air-mucosa interface (Figure 2 E-F).  

Figure 2. Ultrasound image of A-B) transverse plane at the thyroid level showing the vocal cords (blue stars), C-D)  longitudinal plane over the cricothyroid membrane (yellow line), thyroid cartilage (blue circle), cricoid cartilage (green circle) and tracheal rings string of pearls (purple circles), E-F) transverse plane over the suprasternal notch showing the trachea (purple circle), esophagus (yellow circle) and thyroid (blue area) with the air-mucosa interface (green line). 

Indications

• Confirmation of ETT position 

• Evaluation of ETT depth 

• Identification of the cricothyroid membrane 

In the context of airway management, PoCUS provides a rapid and reliable method for confirming endotracheal tube position, assessing depth, and identifying key anatomical landmarks like the cricothyroid membrane. Whether in the emergency setting or during elective intubation, point-of-care ultrasound can improve clinical decision-making, reducing reliance on traditional confirmation methods alone.

 

Equipment

• Ultrasound machine  
• High frequency linear probe or curvilinear probe for obese patients 
• Ultrasound Gel  
• Endotracheal tube  
• Saline water 
• ​​Syringe​​

​Introduction

Endotracheal intubation is an essential procedure in​​ the care of critically ill children. ​Immediate and accurate​ confirmation of ETT (endotracheal tube) position and depth is essential ​for ensuring ​adequate ventilation and oxygenation. Misplaced endotracheal tube insertions may lead to​ potentially life-threatening complications including​ inadequate ventilation, mainstem intubation, lung collapse, pneumothorax, hypoxia and cardiorespiratory arrest [1].

Traditional methods to confirm ETT placement ​such as​ auscultation and visualization of condensation in the ETT are not consistently reliable [2, 3]. ​According to the American Heart Association and Pediatric Advanced Life Support guidelines, e​nd-tidal and colorimetric capnography ​are​ the current gold standard for assessment of endotracheal intubation [4, 5].  ​Unfortunately, ​ capnography ​may be​ limited in cardiac arrest due to poor ventilation and poor lung perfusion ​which limits the​ delivery of carbon dioxide [6]. ​While d​irect visualization of the endotracheal tube passing through the vocal cords is helpful to confirm ETT placement​,​ ​it​ is not always possible in ​complex​ airway situation​s​.

Despite ​the application of​ all ​of ​the aforementioned confirmatory methods, esophageal ​intubation still​​ ​occurs​ in up to 4% of adult intubations [7] and is more common during cardiopulmonary resuscitation ​with a reported rate of ​10% [8, 9].  The failure rate at first attempt endotracheal intubation in​​ children is even higher (41%) [1].  

 The depth o​f​ ETT insertion is often evaluated using chest radiographs [10]. This exposes patients to radiation and may delay patient care if access to radiography is limited.

 Point-of Care Ultrasound (POCUS) of the airway can also be a useful adjunct to help clinicians confirm ETT position and depth and to evaluate the anatomy ​prior to performing a ​surgical airway. 

 

Why POCUS?

Airway POCUS allows clinicians to visualize the position of the ETT in real time. ​This technique​ can be performed ​both ​during (dynamic phase) ​and​ following (static phase) endotracheal intubation​.​​ ​Further, r​ecent meta-analyses have ​shown ​POCUS to ​have high diagnostic accuracy with a sensitivity of 98% and a specificity of 95% ​when​ used for ETT confirmation in the adult population.    

Airway POCUS for confirmation of ETT position is ​rapid. It can​ ​typically ​be performed within 9 seconds by expert sonographers and 36s by novice sonographers [12]. ​On average​​,​ ​the ​time to confirm ETT position using ultrasound is less than 10 ​seconds​​ [8​, 13].  Moreover, the learning curve for distinguishing between esophageal and endotracheal intubation on imaging is steep and rapid. Emergency physicians ​have demonstrated the ability​​ to quickly (average 4s) and accurately (90%) identify the correct placement of the ETT on ultrasound videos and images [14]. 

Airway POCUS correlates with capnography in patients who are not in cardiac arrest [15] and can be performed non-invasively during cardiopulmonary resuscitation ​in arrest scenarios ​when capnography results are not reliable [​8].

References

1. Volpicelli et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med (2012) 38:577–591 doi: 10.1007/s00134-012-2513-4
2. Dietrich et al. Lung B-line atrefacts and their use. J Thorac Dis (2016) doi: 10.21037/jtd.2016.04.55
3. Mohamed et al. Frequency of Abnormalities Detected by Point-of-Care Lung Ultrasound in Symptomatic COVID-19 Patients: Systematic Review and Meta-Analysis. Am. J. Trop. Med. Hyg., 103(2), 2020, pp. 815–821 doi:10.4269/ajtmh.20-0371
4. Al Deeb et al. Point of Care Ultrasonography for Diagnosis of Acute Cardiogenic Pulmonary Edema in Patiehs Presenting With Acute Dyspnea: A Systematic Review and Meta-analysis. Acad Em Med (2014) doi: 10.1111/acem.12435
5. Rihal CS, Davis KB. Ward Kennedy J, Gersh BJ. The utility of clinical, electrocardiographic, and roentgenographic variables in the prediction of left ventricularfunction. Am J Cardiol. 1995;75:220–3.
6. Fonseca C, Mota T, Morais H, et al. The value of the electrocardiogram and chest X-ray for confirming or refuting a suspected diagnosis of heart failure in the community. Eur J Heart Fail 2004;6:807–12.
7. Knudsen CW, Omland T, Clopton P, et al. Diagnostic value of B-type natriuretic peptide and chest radiographic findings in patients with acute dyspnea. Am J Med 2004;116:363–8.
8. Martingale, Noble and Liteplo. Diagnosing Pulmonary Edema: lung ultrasound versus chest radiography. EJEM (2013) DOI: 10.1097/MEJ.0b013e32835c2b88
9. Volpicelli et al. Bedside lung ultrasound in the assessment of the alveolar-interstitial syndrome. AJEM (2016) doi:10.1016/j.ajem.2006.02.013
10. Jones BP, Tay ET, Elikashvili I, Sanders JE, Paul AZ, Nelson BP, Spina LA, Tsung JW, Feasibility and Safety of Substituting Lung Ultrasound for Chest X-ray When Diagnosing Pneumonia in Children: A Randomized Controlled Trial, CHEST (2016), doi: 10.1016/j.chest.2016.02.643
11. Volpicelli et al. Lung ultrasound in diagnosing and monitoring pulmonary interstitial fluid. Radiol Med (2011) DOI 10.1007/s11547-012-0852-4

Summary 

• PoCUS is sensitive, specific, and reliable for the detection of interstitial disease
• PoCUS performs better than CXR in the identification of interstitial disease
• LUS findings of interstitial disease are characterized by B-lines which are sharp vertical projections arising from the pleural line, move with respiration and cross A-lines.
• The etiology of B-lines must be interpreted within the clinical context