Background and Indications for Exam
Endotracheal intubation can be one of the more challenging tasks in emergency medicine and critical care. Misplacement of the tube, most often into the right main stem bronchus or esophagus, has been reported in up to 8% of cases in the literature. Secondary confirmation of tube placement also presents various challenges to the practitioner. Traditionally, direct visualization of the endotracheal tube followed by at least one or more secondary confirmations is required to confirm tube placement. End-tidal colorimetric devices are most frequently used in the acute care setting, as well as auscultation of the thorax and epigastrium. Esophageal bulb detectors, visualization of mist in the endotracheal tube, and bilateral chest rise are also employed. Each of these techniques and devices has specific limitations and pitfalls. Bedside ultrasound may also be used to dynamically observe tube passage into the trachea or esophagus, providing an additional method of confirmation. Following successful intubation, bedside ultrasound may also be used to visualize the presence of bilateral lung sliding and comet-tail artifact, additional indicators of correct placement, and lung expansion post-intubation.
Bedside ultrasound is a rapid and reliable way to confirm proper endotracheal intubation. Sensitivity and specificity are high, especially when compared to end-tidal colorimetry in the acute care setting.
Direct ultrasound visualization of the endotracheal tube passing through the trachea may be particularly helpful for physicians who are supervising a trainee performing an intubation, allowing real-time confirmation of correct placement. Bedside ultrasound gives the senior physician the ability to instantly determine the location of the endotracheal tube, prior to bagging one that is incorrectly placed into the esophagus.
Probe Selection and Technical Considerations
To visualize the trachea and thorax, a linear probe with a high frequency such as 6.0–12.0 MHz should be utilized. A lower frequency, microconvex or curvilinear probe may be helpful in more obese patients.
The focal zone should be placed at the depth of the trachea or pleural line in order to optimize the lateral resolution.
The far gain or time-gain compensation may need to be adjusted in order to enhance and brighten signals returning to the machine monitor. This will help the sonographer observe the shadow created by the tracheal cartilage and endotracheal tube in the far field.
An image that is either too deep or too shallow can be disorienting. Generally, it is better to start deeper than necessary, and once the appropriate anatomy is identified, the depth should be adjusted so that the pertinent structures are taking up most of the screen.
The trachea originates at the vocal cords and traverses distally. In the neck, the trachea lies posterior to the thyroid gland, which has a homogenous appearance on ultrasound, similar to the liver or spleen. The cartilaginous rings of the trachea create a bright white hyperechoic line just deep to the thyroid gland. In a transverse view of the anterior neck, with the probe indicator pointed toward the patient’s right, the ultrasound beam will encounter superficial structures first starting with the skin and subcutaneous tissue, then thyroid tissue, and lastly trachea (Fig. 22-1). The longitudinal or sagittal view may also be viewed, but less experienced sonographers will more easily interpret the transverse view. Due to the cartilaginous composition of the tracheal rings and the inability of the ultrasound beam to pass through them, a shadowing effect will be seen posteriorly (see Fig. 22-1). The esophagus is typically not visualized due to its compressibility with the ultrasound probe unless an endotracheal tube is present within its lumen (Fig. 22-3).
This image illustrates a transverse view of the trachea with landmarks. The anechoic area posterior to the trachea represents shadowing resulting from an attenuation of the ultrasound beam through the dense cartilage of the rings.