Endotracheal tubes (ETT) are wide-bore plastic tubes that are inserted into the trachea to allow artificial ventilation. Tubes come in a variety of sizes and have a balloon at the tip to ensure that gastric contents are not aspirated into the lungs. Adult tubes are usually approximately 1 cm in diameter. Tubes have a radiopaque strip within them so that they are visible on radiographs.
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Radiographic features
Plain radiograph
ETT position is usually assessed on a frontal chest radiograph.
The position of the ETT is dependent on the position of the head. If the neck is flexed, the tip of the tube descends in the trachea.
If included in the film, the mandible can be used for assessment of whether the neck is in a neutral position. In a neutral position, the lower border of the mandible should be projected over C5/C6. When flexed, the mandible projects around T1 and in extension, over C3/C4.
The carina is usually projected over T5-T7 (it descends with increasing age).
The desired position of an ETT is 5 ± 2 cm above the carina, but markedly varies with neck position and rotation and hence, the inclusion of the mandible is a helpful indicator:
- flexed: 3 cm (± 2 cm) above carina
- neutral: 5 cm (± 2 cm) above carina
- extended: 7 cm (± 2 cm) above carina
In children, the trachea is shorter, and the optimum position for the tip of the ETT is 1.5 cm above the carina.
When the carina cannot be visualized (usually due to technical factors) the ideal position of ETT is in the middle third of the trachea at T2 to T4 level (with the neck in neutral position) 2.
An overinflated balloon can result in tracheal mucosal ischemia, with potentially ensuing tracheopleural or tracheo-esophageal fistula and tracheal stenosis. Overinflation by definition is a balloon diameter >1.5 times that of the trachea 5.
Ultrasound
Using a linear array transducer placed in the suprasternal notch combined with lung ultrasonography, ultrasound has been used at the point of care to confirm endotracheal intubation before a definitive chest film can be taken. Sonographic features of tracheal intubation include 3:
- one hyperechoic air-mucosal interface with comet-tail artifacts and posterior acoustic shadowing
- the normal esophagus is typically not visualized
- esophageal intubation will demonstrate a second hyperechoic, shadowing interface which appears similar to the trachea
- during dynamic visualization, one may see manipulation of the trachea
- one study also demonstrated high specificity for tracheal position using a saline-filled endotracheal tube cuff
- bilateral, symmetric lung sliding
- the abovementioned findings may also occur in right mainstem intubation
- right mainstem intubation will result in normal sliding lung on the right with a loss of lung sliding on the left
- the tube may then be withdrawn until sliding lung is visible bilaterally
Complications
The main issue with the malposition of an ETT is that it is inserted too far, resulting in intubation of the right main bronchus. This results in overinflation of the right lung and collapse of the left lung. In some cases, this can lead to a right-sided tension pneumothorax. A grossly dilated stomach may result from esophageal intubation. Vocal cord damage and aspiration may occur if the tip of the tube is positioned in the larynx or pharynx 4.
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