USE OF ULTRASOUND IN AIRWAY ASSESSMENT

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Use of Ultrasound in Airway Assessment

Ultrasound has evolved into a valuable point-of-care tool for pre-intubation assessment, procedural guidance, and post-intubation confirmation. Its real-time, non-ionizing, and bedside nature makes it particularly useful in emergency, critical care, and anesthetic settings. The applications can be organized into five major domains.

1. Sonographic Anatomy of the Airway

A high-frequency linear probe (7-15 MHz) is used for most anterior neck assessments. Key structures visible on ultrasound include:

Tracheal rings - C-shaped hypoechoic cartilages with characteristic posterior acoustic shadowing due to air attenuation
Thyroid gland - hyperechoic, flanking the trachea laterally
Cricothyroid membrane (CTM) - the soft tissue gap between the thyroid and cricoid cartilages
Esophagus - visualized posterior and slightly lateral to the trachea in transverse views

The transverse (axial) view gives a cross-section of the trachea and is the most useful for ETT confirmation, while the midline sagittal view is preferred for CTM identification.

Transverse trachea ultrasound showing skin, subcutaneous tissue, trachea, thyroid gland, and posterior acoustic shadowing

Fig. 1 - Transverse view of the trachea. The anechoic area posterior to the tracheal rings represents acoustic shadowing from dense cartilage. The thyroid gland is visible bilaterally. (Morgan & Mikhail's Clinical Anesthesiology, 7e)

2. Pre-Intubation Assessment / Difficult Airway Prediction

Ultrasound can supplement traditional bedside assessment (Mallampati, thyromental distance, mouth opening) for predicting difficult intubation:

Parameter	Ultrasound Finding	Clinical Relevance
Pretracheal soft tissue thickness	Measured in sagittal/transverse plane at thyroid cartilage level	Increased in obese patients; predicts difficult mask ventilation and intubation
Tongue base thickness	Measured submentally	Larger tongue correlates with higher Mallampati score
Hyoid-to-skin distance	Distance from skin surface to hyoid bone	Increased distance associated with difficult laryngoscopy
Epiglottis visibility	May be seen in midline sagittal plane	Helps assess anatomy prior to intubation

According to Barash's Clinical Anesthesia (9e): "Ultrasound can be used in trauma care to predict difficult airways, to identify the cricothyroid membrane, to facilitate rapid front-of-neck access in expected difficult airways, and to confirm endotracheal (vs. esophageal) tube position, especially in low cardiac output state where capnography can be less reliable."

Morbidly obese patients (BMI ≥30) deserve particular attention - ultrasound can quantify redundant pharyngeal tissue and neck circumference that may predict both difficult intubation and difficult bag-mask ventilation.

3. Cricothyroid Membrane (CTM) Identification

This is one of the highest-yield applications of airway ultrasound, especially in the "can't intubate, can't oxygenate" (CICO) scenario.

Why it matters

Traditional palpation-based CTM identification fails in up to 30-60% of obese or anatomically challenging patients. Ultrasound localizes the CTM accurately even in difficult necks.

Technique

Place a linear probe midline sagittal on the anterior neck
The thyroid cartilage appears as a hyperechoic structure superiorly
Below it, the CTM appears as the echogenic soft tissue gap between thyroid and cricoid cartilages
The air-mucosal interface appears as a bright hyperechoic line with posterior reverberation artifacts

Ultrasound CTM identification - sagittal view showing thyroid cartilage, cricoid cartilage, and air-mucosal border

Fig. 2 - Midline sagittal ultrasound view of the CTM. Thyroid cartilage (left), cricoid cartilage (right), and the air-mucosal border are clearly labeled. This view guides emergency cricothyrotomy needle/scalpel placement. (Barash Clinical Anesthesia, 9e)

Clinical application

Roberts & Hedges' Clinical Procedures in Emergency Medicine describes the use of ultrasound to localize the CTM before simulated failed intubation, with studies showing ultrasound-guided marking significantly increases accuracy compared to digital palpation alone. The 4-step cricothyrotomy technique can be modified to use ultrasound pre-marking of the CTM to reduce laryngeal/tracheal injury.

POCUS improves success rates for:

Emergency cricothyrotomy
Percutaneous tracheostomy (identification of tracheal rings and avoidance of blood vessels)

4. Endotracheal Tube (ETT) Placement Confirmation

Confirming tracheal vs. esophageal intubation is a critical application, particularly when capnography is unreliable (low cardiac output states, cardiac arrest, CPR).

Tracheal intubation

The probe is placed transversely on the anterior neck at the level of the suprasternal notch or just above. During intubation:

The ETT passing through the trachea appears as a hyperechoic structure with a "double-wall" sign within the tracheal air shadow
Tube motion can be visualized in real time as the ETT passes beneath the probe

Real-time tracheal intubation - ETT motion visualized in transverse trachea view

Fig. 3 - Transverse view during tracheal intubation. The arrow points to increased echogenicity and visible motion of the ETT passing just distal to the tracheal cartilage. (Morgan & Mikhail's, 7e)

Esophageal intubation

Immediately recognizable: a second tubular structure appears posterior and lateral to the trachea - the esophagus - with two parallel echogenic lines representing the inner and outer walls of the ETT passing through it.

Fig. 4 - Transverse view during esophageal intubation. The trachea and esophagus are visible. The ET tube is seen posterior-lateral to the trachea within the esophagus lumen (two parallel echogenic lines). (Morgan & Mikhail's, 7e)

Intratracheal vs. esophageal confirmation (Tintinalli view)

Side-by-side tracheal (A) vs. esophageal (B) ETT placement on ultrasound

Fig. 5 - Intratracheal (A) vs. esophageal (B) ETT placement. In tracheal placement, the tube is seen within the single midline trachea. In esophageal placement, the tube creates a second circular structure lateral to the trachea. (Tintinalli's Emergency Medicine)

Additional ETT confirmation methods

Lung sliding - bilateral lung sliding on pleural ultrasound after intubation confirms bilateral ventilation and excludes right mainstem bronchus placement
Diaphragmatic movement - bilateral symmetric diaphragm excursion
Absence of lung sliding on one side - suggests mainstem intubation

5. Tracheal Sizing and ETT Selection

Miller's Anesthesia (10e) notes: "POCUS can help determine size of trachea and endotracheal tube, accurately locate cricothyroid membrane, and improves success of procedures like cricothyrotomy, tracheal rings for percutaneous tracheostomy."

Ultrasound measurement of the subglottic transverse diameter allows appropriate ETT size selection, particularly important in:

Pediatric patients - where cuffed vs. uncuffed tube selection and tube size is critical
Post-intubation cuff pressure assessment - the cuff can be visualized as a fluid-filled structure distending within the tracheal lumen

6. Gastric Content Assessment (Aspiration Risk)

Though not airway anatomy per se, gastric ultrasound is an important pre-intubation tool to stratify aspiration risk - especially relevant in emergency cases where fasting history is unclear.

The gastric antrum is scanned in the right lateral decubitus position using a curvilinear probe. The antrum appears as a target-like structure adjacent to the liver:

Empty antrum (flat, collapsed walls) - low aspiration risk
Clear fluid (anechoic contents) - moderate risk; volume can be estimated
Solid contents (heterogeneous, hyperechoic) - high aspiration risk; RSI and precautions indicated

Barash's Clinical Anesthesia notes that point-of-care gastric ultrasound has been used to evaluate gastric emptying times, with recent pediatric studies showing clear fluid emptying in under 1 hour, informing institutional fasting guideline modifications.

7. Supplementary Uses - Lung/Thoracic Ultrasound

In the post-intubation period and for airway decision-making:

Finding	Significance
Lung sliding present, bilateral	Tracheal intubation, bilateral ventilation confirmed
Absent lung sliding, unilateral	Mainstem intubation or pneumothorax
B-lines (comet-tail artifacts)	Pulmonary edema, may influence airway/ventilation strategy
"Barcode sign" (M-mode)	Pneumothorax - guides need for decompression before or after intubation
Absent lung sliding + lung point	Pneumothorax - locates the transition zone

These findings from E-FAST (Extended Focused Assessment with Sonography in Trauma) are now part of standard critical care airway evaluation.

Summary Table - Ultrasound Applications in Airway Management

Application	Probe	View	Key Finding
Anatomical landmarks	Linear (7-15 MHz)	Transverse/sagittal	Trachea, thyroid, esophagus
CTM identification	Linear	Sagittal midline	Gap between thyroid/cricoid
Difficult airway prediction	Linear	Sagittal at thyroid	Pretracheal fat thickness
ETT confirmation (tracheal)	Linear	Transverse suprasternal	Tube motion in trachea
Esophageal intubation exclusion	Linear	Transverse	Double tube sign lateral to trachea
Right mainstem exclusion	Linear	Bilateral anterior chest	Bilateral lung sliding
Pneumothorax	Linear	Anterior chest	Absent sliding, lung point
Gastric content/aspiration risk	Curvilinear	Right lateral decubitus	Antrum contents
ETT/trachea sizing	Linear	Transverse subglottic	Transverse diameter measurement
Percutaneous tracheostomy	Linear	Sagittal/transverse	Tracheal ring identification

Key Clinical Points

Ultrasound is best used as an adjunct to - not a replacement for - clinical assessment and capnography
Real-time ultrasound during laryngoscopy allows the operator to watch the ETT pass into the trachea via a transverse neck view, providing near-instantaneous confirmation
In cardiac arrest, capnography is unreliable due to low pulmonary blood flow; ultrasound is especially valuable here
CTM identification by ultrasound should be performed pre-emptively in anticipated difficult airways (obese, female, short neck) before the airway is instrumented
The entire airway ultrasound examination can be performed in under 2 minutes with practice, making it suitable for emergency scenarios

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