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Will ultrasound ever become an essential part of the toolkit for airway evaluation, just as waveform capnography has become standard for AW confirmation over the past 10 years?
Recent papers have proposed several airway-related uses for point-of-care ultrasound (POCUS):
- Confirming ET tube placement (2)
- Allowing identification of the cricothyroid membrane for emergency cricothyrotomy in patients without palpable landmarks.
- Facilitation of nerve blocks for awake intubation (3)
- Predicting post-extubation stridor (4)
Don’t tube the goose!
The use of ultrasound to confirm ET tube placement has received particular attention and the technique is now included in ACLS guidelines. ET tube placement is assessed by scanning the anterior neck to look for increased shadowing and artifact in the trachea during intubation and by evaluating lung sliding bilaterally while ventilating. A systematic review and meta-analysis, which evaluated 12 studies with a total of 1656 intubation attempts, found that the pooled sensitivity of ultrasound to detect esophageal intubation was 0.93 with a pooled specificity of 0.97 (5). Waveform capnography is considered the best test to confirm ET tube placement, but it may not always be available and may not be reliable in cardiac arrest/low flow states. POCUS can be performed in real time, thereby avoiding the potential complication of gastric insufflation (6).
So it’s amazing, right?
Although research on airway POCUS is in its infancy, this tool provides several unique advantages that can make it especially useful in airway assessment. It is fast, safe, cheap, non-invasive, portable, widely available, and can be used at bedside for unstable patients. Further, it is the only modality that allows for indirect dynamic visualization of airway structures (such as the vocal cords). Airway visualization by ultrasound had trouble getting off the ground because of the difficulties inherent to imaging air filled structures. Air has a very high acoustic impedance and so only a small fraction of ultrasound waves pass through the interface. This leads to poor image quality. Fortunately, technological improvements such as higher frequency probes have mitigated some of these issues (7). A recent study demonstrating that ultrasound based measurements of AW structures correlated well with the same measurements made on neck CT (8) shows just how far we’ve come. Ultrasound, however, is still somewhat limited. A study evaluating vocal cords among 109 female and 120 male participants found it difficult to consistently visualize the cords among older men, likely due to age- and sex-related calcification of the thyroid cartilage. While the true and false cords were seen 100% of the time among female participants and men aged 20-39, they were seen just 38.1% among males 60 years of age or older (7). Finally, a lack of research limits the application of ultrasound in airway assessment. For example, evaluating the larynx for airway concerns (e.g. vocal cord dysfunction, laryngeal stenosis, supraglottic hemangioma) is difficult without a reference standard: average measurements for the population at large have not yet been made, so it is challenging to make inferences about abnormal anatomy. One study attempted to expand our knowledge base by measuring the vocal cords with ultrasound in healthy volunteers. The authors collected measurements from 38 patients and created a list of averages and standard deviations. They found that men tended to have longer vocal cords than women but that there was no association between vocal cord length and BMI. Intended as a pilot study, this paper underlines the need for further research with larger sample sizes before such information can be applied in clinical practice (9).
Future directions
Much of ultrasound’s success in emergency medicine is based on its focused application at the bedside to answer very specific clinical questions. The FAST exam is standardized, easily acquired, and answers two simple but critical questions: whether the patient has intraperitoneal free fluid and/or a pericardial effusion. Similarly, skille doperators can confirm ET tube placement using ultrasound. To make POCUS similarly effective for general airway assessment and for the uses described at the beginning of this post, we need to first define quick and reliable exams that provide answers with the potential to change patient management. To do so, we must first understand what is “normal” (i.e. means and standard deviations of various useful airway measurements), and then expand our research to include patients with anatomically difficult airways. With further research and continuing advancements in ultrasound technology, we can overcome many of ultrasound’s current limitations. Although it may still be a few years away, ultrasound promises to be a valuable additional tool to assess the critical airway.
Citations
(1) Singh S, Chin KJ, Chan VWS, et al. Use of sonography for airway assessment. J Ultrasound Med. 2010; 29: 79-85.
(2) Ma G, Davis DP, Schmitt J, et al. The sensitivity and specificity of transcricothyroid ultrasonography to confirm endotracheal tube placement in a cadaver model. J Emerg Med. 2007; 32(4): 405-7.
(3) Kristensen MS, Teoh WH, Graumann O, et al. Ultrasonography for clinical decision-making and intervention in airway management: from the mouth to the lungs and pleurae. Insights Imaging. 2014; 5: 253-79.
(4) Ding LW, Wang HC, Wu HD, et al. Laryngeal ultrasound: a useful method in predicting post-extubation stridor. A pilot study. Eur Respir J. 2006; 27: 384-9.
(5) Chou EH, Dickman E, Tsou PY, et al. Ultrasonography for confirmation of endotracheal tube placement: a systematic review and meta-analysis. Resuscitation. 2015; 90: 97-103.
(6) Gottlieb M and Bailitz J. Can transtracheal ultrasonography be used to verify endotracheal tube placement? Ann Emerg Med. 2015; 66(4): 394-5.
(7) Hu Q, Zhu SY, Luo F, et al. High-frequency sonographic measurements of true and false vocal cords. J Ultrasound Med. 2010; 29: 1023-30.
(8) Prasad A, Yu E, Wong DT, et al. Comparison of sonography and computed tomography as imaging tools for assessment of airway structures. J Ultrasound Med. 2011; 30: 965-72.
(9) Bright L, Secko M, Mehta N, et al. Is there a correlation of sonographic measurements of true vocal cords with gender or body mass indices in normal healthy volunteers? J Emerg Trauma Shock. 2014; 7(2): 112-5.
[/et_pb_text][/et_pb_column][/et_pb_row][et_pb_row make_fullwidth=”off” use_custom_width=”off” width_unit=”off” custom_width_px=”1080px” custom_width_percent=”63%” use_custom_gutter=”off” gutter_width=”3″ padding_mobile=”off” allow_player_pause=”off” parallax=”off” parallax_method=”on” make_equal=”on” column_padding_mobile=”on” parallax_1=”off” parallax_method_1=”on” parallax_2=”off” parallax_method_2=”on” parallax_3=”off” parallax_method_3=”on” parallax_4=”off” parallax_method_4=”on” disabled=”off”][et_pb_column type=”1_2″ disabled=”off” parallax=”off” parallax_method=”on” column_padding_mobile=”on”][et_pb_team_member name=”Armin Gollogly, MD” position=”Author” image_url=”http://theempulse.org/wp-content/uploads/2018/09/IMG_28036.jpg” animation=”off” background_layout=”light” admin_label=”Person” use_border_color=”off” border_color=”#ffffff” border_width=”1px” border_style=”solid” custom_margin=”|40px||40px” custom_padding=”|20px||20px” custom_padding_last_edited=”off|desktop” disabled=”off”]
Dr. Gollogly is a categorical Emergency Medicine resident at LIJ Medical Center.
[/et_pb_team_member][/et_pb_column][et_pb_column type=”1_2″ disabled=”off” parallax=”off” parallax_method=”on” column_padding_mobile=”on”][et_pb_team_member name=”Dr. Josh Guttman, MD, FRCPC” position=”Reviewer and Ultrasound Editor – theEMPulse.org ” image_url=”http://theempulse.org/wp-content/uploads/2015/10/Guttman-e1453253661766.jpg” animation=”off” background_layout=”light” admin_label=”Person” use_border_color=”off” border_color=”#ffffff” border_width=”1px” border_style=”solid” disabled=”off” saved_tabs=”all”] Dr. Guttman graduated from the at Mt. Sinai Medical Center Emergency Ultrasound Fellowship and is now an Attending Physician at the LIJ Division of Emergency Ultrasound. [/et_pb_team_member][/et_pb_column][/et_pb_row][/et_pb_section]
