Introduction:
Airway management in surgical procedures and during trauma is very important. This helps in the management of the proper ventilation of the patient. Failure in maintaining airway patency may lead to breathing difficulties and may cause death also. Over the years different techniques like identification of anatomical landmarks, and palpations are being used to determine airway patency. But, pathological problems, anatomical variations, and successful intubation are difficult to achieve through these procedures. Ultrasound techniques can be very useful in determining a successful intubation procedure. Also, visualization of the airway can be done through this procedure.
What Is Ultrasonography of Airway?
Ultrasonography is a non-invasive technique. In this procedure, ultrasound sound waves are projected toward the organs through a transducer. These soundwaves bounce off the organs like an echo and are captured by the transducer, which then processes these data from an image of the object. The frequency of the ultrasound waves is below 20 kilohertz. These waves are produced by lead‑zirconate‑titanate‑based small piezo‑crystals present on the transducer. In the airway ultrasonography procedure, linear high‑frequency sound waves (5 to 14 megahertz) are used in analyzing superficial anatomical structures, like the cricoid cartilage, cricothyroid membrane, epiglottis, and vocal cords. For the assessment of the deep anatomical structures, the frequency of the ultrasound should be around 8 to 10 megahertz. In such cases, convex (curvilinear) probes are often used.
The amount of echo produced by anatomical structures is known as ultrasound impedance. Some structures produce very reflective images and show a bright white appearance. These structures are known as hyperechoic images. Structures like cartilage and muscles produce a relatively less bright image in the ultrasound procedure. Such images are called hypoechoic images.
What Are the Appearance of the Upper Airway?
Different upper airway tract anatomical structures appear differently in the ultrasonographical examination.
1. Tongue and Floor of the Mouth
The tongue and floor of the mouth both are strong muscular structures. As a result, both bounce back many sound waves. The dorsal surface of the tongue produces a curvilinear structure due to the interference of the air and mucosa. The intrinsic muscle of the tongue produces a striated appearance. Among the extrinsic muscles of the tongue only the geniohyoid, genioglossus, and hyoglossus are detectable in the ultrasonographic examination. Other extrinsic muscles are obstructed by the mandibular ramus and the mastoid bone. In the sagittal bone mylohyoid and geniohyoid appear as a linear band. On the other hand genioglossus and hyoglossus muscles appear as fan-like structures.
2. Hyoid Bone:
It can be visualized both in the sagittal and transverse views. In the transverse section hyoid bone appear as an inverted U-shaped structure and in the sagittal section it appears as a narrow curved structure.
3. Vocal Cords:
The thyroid cartilage is used for the visualization of vocal cords. It appears as an isosceles triangle with a central tracheal shadow. The false vocal folds or the vestibular vocal folds appear as a more echogenic structure in the ultrasound structure. During phonation, true vocal cords oscillate, whereas false vocal folds remain immobile.
4. Cricoid Cartilage and Cricothyroid Membrane
In the sagittal view, the cricoid cartilage appears as an oval hyperechogenic structure. In the transverse view, this structure appears as a hump. The cricothyroid membrane is observed as a hyperechoic band-like structure lining the hypoechoic thyroid cartilage. The tracheal ring will appear as a hyperechogenic structure and look like a string of beads in the sagittal view. In the transverse view, these structures appear as inverted U-shaped shaped structures.
5. Thyroid Gland:
The thyroid gland can be observed through ultrasound scanning at the suprasternal position at the transverse view. Two lobes and isthmus can be identified in the anterolateral position of the trachea. The thyroid parenchyma appears as a homogeneous hyperechoic structure.
6. Esophagus:
An oblique transverse view can be used to identify Esophagus. Swallowing is responsible for the peristalsis movement of the esophagus. This can be used for the identification of the esophagus.
What Are the Application of Airway Ultrasonography?
The clinical application of airway ultrasonography are:
1. Prediction of Airway Size:
Assessment of the airway size is necessary to determine the size of the endotracheal tube (ETT). The transverse diameter of the subglottic airway is measured in this way. These measurements are done at the lower edge of the cricoid cartilage. These measurements are very helpful for the intubation of the newborn and in patients with airway stenosis.
2. Confirmation of Endotracheal Tube Placement:
Ultrasound can be used to locate the positioning of the endotracheal tube. In situations like cardiovascular arrest, and bronchoconstrictions such techniques can be helpful. This is also, a more accurate technique than capnography (a non-invasive method of measuring the partial pressure of carbon dioxide). Also, the depth of the insertion of the endotracheal tube insertion is determined by this technique.
3. Laryngeal Mask Airway (LMA) Positioning:
The laryngeal mask airway is a medical device that is used during anesthesia to maintain the patency of the airway. Malpositioning of such devices during anesthesia can lead to breathing problems. Airway ultrasonography can be used as a useful measure in the detection of laryngeal mask airway malpositioning.
4. Assessment of Vocal Cords:
The morphological assessment of the vocal cord can easily be done with this method. The positioning and the involvement of the laryngeal nerves in cases of thyroid cancer can easily be detected. Post-extubation stridor is the inspiratory noise present during the respiratory process. This is caused by the narrowing of the respiratory tract. Ultrasound helps to measure air column width at the level of the vocal cord. This can be useful in the prediction of post-extubation stridor.
5. Utilization in Surgical procedure:
Cricothyroidotomy is a life-saving procedure that is done in patients who are unable to ventilate or intubate. Real-time ultrasound guidance techniques are used to detect cricothyroid membranes. This increases the efficiency of the surgical procedure.
Tracheostomy is also a surgical airway maintenance procedure. Injuries of the avoid anterior neck structures and posterior tracheal wall injuries are the complications of this procedure. The real-time in-plane ultrasound-guided percutaneous dilational tracheostomy (PDT) can be helpful in avoiding such injuries.
Conclusion:
Maintenance of airway patency is an integral part of the surgical process. Several pathological conditions may affect this patency. Such conditions can be easily diagnosed using ultrasound methods. Also, the positioning of the endotracheal tubes can also be assessed through this method.
