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].

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

Pitfalls

Lung ultrasound relies on artefacts generated by the pleural line, so the operator must ensure the probe is positioned perpendicular to the chest and pleural line to maximize visualization of these artefacts. An obliquely oriented probe can obscure the visualization of normal lung artefact.

Small, vertical projections from the pleural line that do not cross A-lines or project to the end of the screen are often seen in normal lung due movement at the visceral-parietal pleural interface. These artefacts are known as z-lines or comet tail artefacts and should not be confused with B-lines (figure 6).

Figure 6:  Z-line or comet tail artefact—small vertical line arising from the pleura that fades quickly and does not cross A-lines

It is also important to note, some areas of lung are difficult to access and image with ultrasound including the supraclavicular, scapular, axillary and peri bronchial areas.

What is NOT normal?

Interstitial syndromes of the lung include a variety of pathologic conditions that involve either localized or diffuse involvement of the lung. The thickened interlobular septa from fibrosis, edema or excess extravascular lung water result in a characteristic appearance on ultrasound.  In these conditions the air-filled alveoli and water-filled interstitium interact to create a reverberation artifact called B-lines. B-lines are characterized as (figure 4):

• Sharp, vertical lines
• Arising from the pleura and extending to the edge of the screen
• Move with respiration
• Erase A-lines

Figure 4:  Sonographic B-line

 

While one or two B-lines can sometimes be seen normally in dependent portions of lung or areas of interlobar fissures, when more than three are seen in one field of view they are considered pathological. In addition, when B-lines are seen diffusely in the chest in multiple fields of view, particularly in non-dependent areas they indicate pathology.

 

Quantifying B-lines

In the acute setting, a qualitative assessment of LUS findings is usually adequate. The number of B-lines on ultrasound correlates well with disease severity as well as response to therapy. In less severe diseases B-lines appear multiple and discrete, as disease progresses, they can become confluent giving a “white out appearance of the lung (figure 5). Studies have shown that B-lines rapidly resolve in response to treatment for heart failure and through the course of dialysis for those with ESRD [11]. In addition, B-lines and other abnormalities found in patients with viral pneumonia, ARDS and bronchiolitis resolve along with their clinical course [1,3].

 

Figure 5:  In severe disease B-lines coalesce and form confluent B-lines resulting in a “white out” appearance of the lung.

 

In non-critical patients a more careful assessment with quantification of B-lines can be useful for assessment and monitoring response to therapy. While beyond the scope of this module there are several techniques described to quantify B-lines.

 

Interpreting B-lines

Sonographic B lines have multiple causes including pulmonary edema, infection, ARDS, contusion, and fibrosis; US interpretation must occur within the clinical context. Focal B-lines with associated pleural line abnormalities or consolidation indicate pneumonia or contusion (video 2).

Video 2: Focal B-lines adjected to pleural irregularity with consolidation in bacterial pneumonia

 

Multifocal but patchy B-lines with spared areas can be seen in bronchiolitis, viral pneumonia, pneumonitis, contusion, ARDS and fibrosis—often along with pleural line abnormalities and/or small subpleural consolidations. Finally diffuse and homogenous B-lines with a regular pleural line is a pattern expected pulmonary edema or fluid overload (video 3) [2]. Generally, the patient’s history and clinical information can help guide the diagnosis improving the sensitivity and specificity of LUS in diagnosing specific etiologies.

 

Video 3: Diffuse and symmetric B-lines with a normal pleural line in a patient with congestive heart failure.

What is normal?

In normal lung, the pleura is easily visualized as ultrasound waves are deflected by aerated lung. Reverberation artifacts from the pleural line (secondary to the significant change in acoustic impedance at the pleural-lung interface) generate horizontally arranged artifacts called A-lines (figure 2). Other normal structures visualized are the rib and corresponding rib shadowing below, and at the base of the lung the double line of the diaphragm muscle (figure 3).

 

Figure 2: Normal lung with a-line reverberation artefact

Figure 3:  Normal lung base with the beginning of the diaphgram noted on screen right

What am I looking at?

It is first important to note the following normal anatomic structures and their sonographic appearance (video 1).

Chest wall:

• Most superficial structure
• Hypoechoic with irregular fascial lines

Ribs:

• Oval, hyperechoic periosteum
• Dark shadow behind

Pleural line:

• Hyperechoic horizontal line
• Runs between and deep to the ribs

Lung:

• Deep to the pleural line
• Uniform grey haze

 

Video 1: Normal lung anatomy

Indications

• Dyspnea
• Cough
• Monitoring for volume overload

 

Equipment

• Ultrasound machine
• High frequency linear probe (6-12 MHz) is most used, although a curvilinear may be better especially in older patients. A phased array probe may also be used.
• Gel

 

Technique

1. Position the patient: It is easiest to completely expose the thorax. LUS can be conducted with the patient in any number of positions; in the parent’s arms (helpful for posterior exam), seated on or lying in a stretcher (lateral decubitus can be used to examine the posterior chest).
2. Warm the gel if possible: Younger or sleeping patients may respond better if the gel is warmed. Consider warming the gel between your gloved hands and applying a layer of warmer gel to the chest.
3. Scan the patient:
   1. Set the depth to between 5 – 10 cm, choosing the shallowest depth that gives an appropriate image to maximize resolution.
   2. Orient the probe marker to the patients’ head and identify the pleural line deep to the ribs in the longitudinal orientation. Ensure the probe is perpendicular to the chest wall to give the clearest image of the pleural line possible.
   3. Scan the patient’s chest systematically: this can be done by dividing the chest into anterior and lateral areas and by investigating each zone superiorly and inferiorly (figure 1). Alternatively, you can choose to investigate each rib space by sliding the probe from cranial to caudal until the diaphragm is reached in each zone as you would in pneumonia, including the posterior zone (figure 2).
   4. For any abnormalities detected, the area should be investigated fully by sliding the probe along the rib space or changing the probe to a transverse view to scan in-between the ribs.

 

Figure 1: 8 zone technique for interstitial disease

 

Figure 2: Alternative technique similar to pneumonia

 

Tips:

• It may be easiest to position the child seated in a parents’ arms
• For lateral and posterior views lifting the arm or crossing arms in front of the body can allow greater access to the chest superiorly
• Consider warming the gel
• Consider the size of the patient when selecting a probe
• Consider using the “lung preset” if available on your US machine