Point-of-Care Ultrasound: A Review of Ultrasound Parameters for Predicting Difficult Airways

The ultrasound image acquisition can guide the standard ultrasound assessment of the airway. Ultrasound can provide the clinician with a dynamic anatomical airway assessment, which is not possible with clinical examination alone. The technique is simple and one can practice it on oneself and colleagues.

It is possible to achieve competency after about 20 exams. Begin by asking the patient to lie down in a supine position without a pillow. To ensure standardization and best measurements, ask the patient to maintain the head and neck in a neutral position.

Ask the patient to rest their tongue on the lower incisors. For consistency, the tongue should remain in the same position throughout the ultrasound examination. Once the patient is positioned correctly, apply a gel medium on the transducer to ensure there is no air between the skin and the transducer.

Place the transducer transversely on the anterior neck of the patient with minimal pressure, while preserving skin contact. Then place the transducer midline on the central axis in the transverse position. And beginning from the submandibular space with slow fine movements, move the transducer cordally.

For a submandibular image, place the transducer longitudinally in the submental space along the body's central axis. Click on freeze, followed by clicking on measure, then measure the distance from the outer border of the mentum to the hyoid bone. As the value of the distance in centimeters pops up on the screen, click on acquire.

Rotate the transducer in the transverse position and place it over the central axis of the neck. Manipulate the transducer cordally with fine slow movements for visualizing the structures of interest. To visualize the thyroid hyoid membrane or THM, palpate the thyroid cartilage and the hyoid bone, and place the transducer in between the transverse position, while ensuring it stays in the central axis of the neck.

Sequentially, click on freeze and measure before measuring the distance from the skin to the anterior border of the epiglottis in the center. As the distance in centimeters pops up on the screen, click on acquire. Then move the transducer one centimeter to the right and as demonstrated previously, repeat the measurement.

Once the distance value appears on the screen, acquire the data. Next, move the transducer one centimeter to the left of the center and repeat the process of acquiring the measurement. Average the three measurements to obtain the THM.

Keep the transducer in the same position and stay in the next central axis to view the hypoechoic curvilinear epiglottis as a dark space. Click on freeze, followed by clicking on measure. Then measure the distance from the skin to the center of the bright white line.

As the value of the distance in centimeters pops up on the screen, click on acquire. Move the probe one centimeter left of the midline and acquire the measurement again. Then move the transducer one centimeter to the right of the midline to acquire the measurement for the third time.

Average the three measurements to obtain the distance from the skin to the epiglottis or DSC. Angle the transducer tail about 20 degrees down. Palpate the hyoid bone and place the transducer directly over the hyoid bone while keeping the probe in the central axis of the neck.

Sequentially, click on freeze and measure. Measure the distance from the skin to the center of the hyoid bone, seen as a bright echogenic line, curved upside down above the hypoechoic shadow and acquire the data. Move the probe one centimeter lateral to the midline on the left to acquire one more measurement.

For the third measurement, move the transducer one centimeter to the right of the midline and average the three measurements to obtain the distance from the skin to the hyoid bone or SHB. To measure the distance from the skin to the vocal cords or SVC, place the transducer transversely over the thyroid cartilage while staying in the central axis of the neck. Click on freeze followed by clicking on measure.

Then measure the distance from the skin to the upper border of the right vocal cord and acquire the data. Repeat the same measurement on the left vocal cord and average the two measurements to obtain the SVC. In 30 studies, several different ultrasound parameters predictive of a difficult laryngoscopy were analyzed.

Two meta-analyses identified the five most studied parameters that significantly differed between easy and difficult direct laryngoscopy views. In the neutral position, an HMD value of 3.4 to 4.5 centimeters was associated with difficult direct laryngoscopy. The parameter had a sensitivity and a specificity of 100%and 71.4%respectively, and was predictive in general and obese populations.

A TMH value higher than 2.8 centimeters predicted a difficult laryngoscopy. This parameter had a sensitivity of 64.7%and a specificity of 77.1%A meta-analysis found DSE as the parameter correlating the most with difficult direct laryngoscopy. The average DSE value greater than 2 to 2.5 centimeters had a positive predictive value of about 30%to 50%while the negative predictive value of the parameter ranged from 95%to 97%An SHB greater than 1.28 centimeters correlated with difficult direct laryngoscopy.

The parameter had a sensitivity and specificity of 85.7%and 85.1%respectively. In difficult direct laryngoscopies, SVC measurements from about 0.9 to 1.3 centimeters were reported. Maintaining the head in the neutral position and resting the tongue on the lower incisors is important.

The transducer must rest on the central axis of the neck and should be moved slowly. Ultrasound can be incorporated into daily practice as an adjunct to the clinical examination of the airway.