The study found that the airways of a randomly chosen population of ED patients with a range of BMI and Mallampati scores can be measured by emergency sonologists with good inter-operator reliability. This study, to our knowledge, is the first that documents the feasibility of these measurements made by clinician-performed ultrasonography in the emergency department environment. Only one subject in 40 had an airway parameter that could not be obtained by ultrasound. Almost all of the measurements used including tongue base, tongue base-to-skin, epiglottic width, and pre-epiglottic space had fair to good ICCs ranging from 0.76 to 0.88. Epiglottic thickness, however, had a poor ICC at 0.57 which is related to the inherently small values of this structure, making standard errors in ultrasound measurements (often in the 1–2 mm range) mathematically much more impactful. This explanation is consistent with the finding that the ICC for epiglottic width (a larger distance) was similar to that of other parameters. Possibly using the ‘zoom’ mode on the ultrasound machine to enlarge the image might have improved the ICC for epiglottic thickness measurement; however, such maneuvers increase the complexity and time required for the task, both of which can be disadvantageous in the context of the time constraints inherent in emergency airway management [17].
Future studies comparing soft tissue measurements to metrics of airway difficulty such as the Cormack–Lehane grading system will reveal whether epiglottic thickness is necessary or even helpful in this setting. Our study also suggests that epiglottic thickness has little variation in the general population, which also makes it less likely to be a discriminatory metric. Of course such reasoning would not have any place in situations where acute epiglottic inflammation or edema are of concern, such as in cases of smoke inhalation, angioedema, or epiglottitis. Similarly, the absence of thickening might give some assurance in cases where epiglottic enlargement or obstruction is a concern.
There are several limitations in this study. Experienced airway sonologists are aware that differences in probe pressure applied while scanning the neck can significantly alter measurements of these superficial neck structures. It was thus emphasized during the ultrasound airway training that the sonologist place the probe against the neck with the least pressure needed to maintain skin contact. The goal of minimal probe pressure also coheres with the principle of avoiding anything that might narrow the upper airway in a dyspneic patient receiving preoxygenation.
The tongue is a dynamic muscle and its position within the mouth can significantly change its size and shape, and also possibly the thickness of the soft tissues of the hypopharynx [18, 19]. Patients were therefore asked to rest their tongue with the anterior tip touching the lower incisors. In unconscious, hypoxic, or altered patients, this would not be possible limiting the generalizability of the findings of the current study to the real clinical environment. We also chose to position patients’ supine with the neck extended, to ensure rigorous standardization. Since the “sniffing position” (neck flexed with the external auditory meatus level with the sternum and some head extension) is currently recommended for laryngoscopy, this also limits the generalizability of our study in the clinical environment. We believe that future studies might want to use the “sniffing position” for purposes of standardization, with the additional advantage that it would allow the ultrasound to be performed while the patient is being set up for intubation. Another limitation of the study is its failure to obtain data regarding the time needed to perform the ultrasound exam. In the context of emergent airway management, any ultrasound requiring much more than 30–60 s would be unlikely to see widespread acceptance by clinicians unless the information it provided was of critical importance. The time taken to perform ultrasound assessment will need to be the subject of future inquiry, but in the current feasibility paper the time required for ultrasound assessment was not recorded because it was felt that any information obtained would not be valid in a real clinical context. Along these lines, it would be ideal if a single measurement was found to be predictive of difficult laryngoscopy, thereby obviating the need for the multiple measurements made in the current study. The study tried to correlate the ultrasound findings with established metrics used in pre-intubation airway assessment, but for reasons of patient comfort and practicability, some metrics were not included, such as hyoid-mental distance. This metric was not included because it was thought that it would likely be used in a pre-intubation airway evaluation regardless of whether or not an ultrasound was performed because it can be obtained without an ultrasound machine. Future studies might investigate this issue.
Finally, the ultrasound exams performed in the current study were done by advanced emergency sonologists receiving fellowship training. This will limit the generalizability of the study among emergency physicians, although it should be generalizable among anesthesiologists and critical care physicians with ultrasound experience and interest in the airway.
The tongue base and tongue base-to-skin measurements were found to correlate with increasing Mallampati score. This finding is not surprising given that Mallampati score is based on oropharyngeal anatomy, and it makes these ultrasound measurements less valuable, since the Mallampati can be obtained rapidly by visual inspection. Furthermore, the wide range of measured mean soft tissue thickness within each Mallampati grade makes it unlikely that ultrasound will be a useful substitute for the Mallampati score. In fact, one study found that combining Mallampati score with sonographic assessment of the skin to epiglottis distance was a stronger predictor of difficult airway compared to individual parameters [20]. Tongue base-to-skin thickness was the only measurement to loosely correlate with BMI [3, 5]. We expected this measurement to have a stronger relationship with BMI. Regardless, with respect to airway management previous research has demonstrated that large neck circumference is a more reliable predictor of difficult laryngoscopy than Mallampati, although also not without limitations [2, 21,20,23]. One study demonstrated that ratio of tongue thickness to thyromental distance was an independent predictor of difficult airway in patients undergoing anesthesia [24]. Since Cormack–Lehane grading can only be performed after intubation is under way, it would be of interest in future studies to determine whether pre-intubation ultrasound measurements of the pre-epiglottic space are predictive of CLG. Future studies would also need to assess whether the allocation of time and resources for ultrasonography during the fraught period prior to an emergency intubation are warranted by the additional information it generates. For purposes of expediency a single parameter would be ideal. In this context, pre-epiglottic distance appeared to be a potentially useful metric since it was measured with high ICC. It also has the advantage that the normal range of values that we measured was relatively wide: a standard deviation of 2.4 mm from the mean value of 11.4 mm gives an SD of 21% of the mean value. This means that standard errors of measurement are likely to have less impact and that real differences are likely to be detected. Additional investigation using a defined and standardized scanning method in pre-operative patients undergoing intubation is needed to determine whether ultrasound can serve as a non-invasive real-time predictor of difficult intubation.
