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 Table of Contents  
REVIEW ARTICLE
Year : 2018  |  Volume : 1  |  Issue : 1  |  Page : 17-24

Ultrasound imaging of the airway and its applications


Department of Anaesthesiology and Critical Care, Jawaharlal Institute of Postgraduate Medical Education and Research, Institute of National Importance, Puducherry, India

Date of Web Publication11-Jan-2019

Correspondence Address:
Prof. Pankaj Kundra
CII/09, Jawaharlal Institute of Postgraduate Medical Education and Research Campus, Puducherry - 605 006
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ARWY.ARWY_12_18

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  Abstract 


Airway ultrasound (US) provides a dynamic assessment of the airway. In conjunction with other imaging techniques such as computerised tomographic (CT) scan and magnetic resonance imaging (MRI), ultrasonography of the airway can influence airway management decisions. There is some evidence to support its use in the prediction of difficult airway by measuring pre-tracheal soft-tissue thickness and pre-epiglottis space thickness. However, the dynamic scan of vocal cord movements may replace direct visualisation techniques in the future. In addition, measurements of the subglottic diameter by US have been shown to predict endotracheal tube size as accurately as MRI or CT scan. US-assisted or US-guided front of the neck procedures such as percutaneous dilatational tracheostomy is safer and more accurate with decreased rate of complications. Localisation of cricothyroid membrane with US in difficult necks may become a life-saving procedure when complete ventilation failure is encountered. The use of US to overcome airway challenges is growing at a rapid pace adding newer dimensions to its use.

Keywords: Airway, clinical applications, ultrasound imaging


How to cite this article:
Cherian A, Kundra P. Ultrasound imaging of the airway and its applications. Airway 2018;1:17-24

How to cite this URL:
Cherian A, Kundra P. Ultrasound imaging of the airway and its applications. Airway [serial online] 2018 [cited 2019 Jan 22];1:17-24. Available from: http://www.arwy.org/text.asp?2018/1/1/17/250025




  Introduction Top


Historically, encountering air was considered to be a challenge while performing sonographic studies. However, it is now known that studying and understanding the artefacts produced by air can provide useful information for diagnosis and treatment. Most of the airway structures for assessment are superficial and are found within 5 cm from the skin. Therefore, a linear probe with a frequency of 5–15 MHz is most often adequate. A low-frequency (1–5 MHz) curvilinear probe is ideal for investigating areas of the tongue, submandibular and supraglottic regions.

Conventional ultrasound (US) can identify structures from below the chin up to mid-trachea, as well as into the pleural spaces, areas which until recently could be studied in detail only by computerised tomographic (CT) scans. Thus, ultrasonography provides a point-of-care, simple and radiation-free method of airway assessment.


  Current Role of Airway Ultrasound in Anaesthesia Practice Top


Sonographic assessment of the airway has grown a long way and is now able to provide useful information to the anaesthesiologists. This review focuses on the evidence available for the use of US in airway assessment and for performing procedures. The assessment of vocal cords and laryngeal structures is a dynamic study that takes place in two phases: during quiet respiration and deglutition.


  Prediction of Difficult Laryngoscopy in Surgical Patients Top


Ultrasonography of the airway was recognised as a useful repertoire in the difficult airway armamentarium for over a decade now. The relationships of various structures, their distances and the angles they form with each other could be a potential marker for predicting difficult airway. These could also be used to correlate with Mallampati or Cormack–Lehane (CL) scores. However, the results of several studies are largely controversial.

Pre-tracheal soft-tissue thickness

US was found to be equivalent to magnetic resonance imaging (MRI) assessment of pre-tracheal fat thickness with the added advantage of being cheaper, faster and easily available. Ezri et al. investigated the role of US in predicting difficult airway in obese Israeli population by measuring the distance from skin to trachea at various levels on the anterior neck. They found that among the predictors studied by them, quantification of soft-tissue mass in front of the vocal cord and suprasternal areas could differentiate patients who could be easily intubated from those who could not.[1] On the other hand, Komatsu et al. did not find pre-tracheal soft-tissue thickness as being predictive of difficulty in intubation in American population.[2] The ability to visualise hyoid bone in the sublingual view is considered predictive of lower CL scores and easier intubation.[3] In addition, anterior neck soft-tissue thickness at the level of anterior commissure, thyrohyoid membrane and hyoid bone are also considered to be independent predictors of difficult airway[4] [Figure 1], [Figure 2], [Figure 3], [Figure 4].
Figure 1: Distances between skin and underlying structures used for prediction of difficult airway. TW: Tongue width, MH: Mentohyoid distance, SH: Soft-tissue thickness at hyoid, SE: Soft-tissue thickness at epiglottis, SV: Soft-tissue thickness at anterior commissure, ST: Soft-tissue thickness at thyroid isthmus, SS: Soft-tissue thickness at suprasternal notch

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Figure 2: Transverse view at the level of thyroid cartilage showing soft-tissue thickness between skin and anterior commissure. A: Anterior commissure, S: Skin, VC: False vocal cords, TC: Thyroid cartilage

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Figure 3: Transverse view of the trachea at the level of suprasternal notch showing soft-tissue thickness between skin and thyroid isthmus. Is: Thyroid isthmus, S: Skin, TG: Thyroid gland, Tr: Trachea, TR: Tracheal ring

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Figure 4: Transverse view at the level of hyoid showing soft-tissue thickness between skin and hyoid. S: Skin

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Pre-epiglottic space thickness

The pre-epiglottic space (pre-E) thickness measured anterior to the thyrohyoid membrane was recently found to be a better predictor of difficult laryngoscopy than that measured at the level of the vocal cords. However, the cut-off values derived were different in each of these studies. A value of 2.54 cm was predictive of a laryngoscopic view of at least CL 2b by Falcetta et al.[5] This was supported by another recent study which found the thickness of 18 mm at the level of thyrohyoid membrane among other measurements to be more predictive of difficult airway.[6] The cut-off values for this thickness of anterior neck at the level of the thyrohyoid membrane vary between 2.75 and 2.8 cm. The ratio of pre-E and E-VC (distance from epiglottis to vocal cords) may also have a role in predicting higher CL grading [Figure 5].
Figure 5: Transverse view at the level of thyrohyoid membrane showing soft-tissue thickness between skin and epiglottis. E: Epiglottis, S: Skin, SCM: Sternocleidomastoid muscle

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Several sonographic parameters, such as soft-tissue thickness anterior to the trachea, tongue base width, volume and cross-sectional area of the tongue, thickness of the lateral pharyngeal wall, hyomental distance ratio (HMDR) and pre-epiglottic space have been considered useful for prediction of difficult airway[6] [Figure 6] and [Figure 7]. A combination of soft-tissue thickness on the anterior aspect of the trachea (at three levels) and the neck circumference (>50 cm) and HMDR <1.1 on US (ratio of hyomental distance in the neutral and hyperextended neck) correlated with difficult laryngoscopy in morbidly obese patients.[5] The ratio of tongue base width and pharyngeal wall thickness was found to be associated with obstructive sleep apnoea (OSA). In patients with sleep apnoea, US visualisation of the approximation of tongue base posterior and inferiorly toward the hypopharynx correlated with airway obstruction. Intraoral sublingual approach using visibility of the hyoid bone has been suggested as a surrogate to predict difficult airway. Airway evaluation to predict OSA revealed that the distance between lingual arteries, lateral pharyngeal wall thickness and geniohyoid muscle thickness (GMT) correlated well with severe OSA, with GMT showing the strongest correlation.[7]
Figure 6: Transverse submandibular view showing the tongue width, intrinsic tongue muscles and palate. MH: Mylohyoid muscle, GH: Geniohyoid muscle, HG: Hyoglossus, GG: Genioglossus

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Figure 7: Extended submental sagittal view with curvilinear probe to measure the mentohyoid distance (distance between mentum and hyoid). M: Mentum, H: Hyoid, E: Epiglottis, T: Tongue, P: Palate

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In a review of ten studies with the outcome variable of poor CL grade, 114 patients among 681 patients were found to correlate with difficult laryngoscopic views. Significance for sonographic prediction of difficult laryngoscopy occurred at three locations: mentohyoid distance (52.6 ± 5.8 mm [P < 0.01]), anterior tissue at the hyoid bone (16.9 mm [95% confidence interval (CI): 11.9–21.9] and 15.9 ± 2.7 mm [P < 0.0001]) and the thyrohyoid membrane (34.7 mm [95% CI: 28.8–40.7] and 23.9 ± 3.4 mm [P < 0.0001] and 28.25 ± 4.43 mm [P < 0.001]). However, the vocal cord and sternal notch levels had conflicting significance. Limitations of the studies included heterogeneous populations and lack of standard scanning protocols.[8] The sonographic HMDR of 1–1.05 was found to be predictive of difficult airway in a study of 12 obese and morbidly obese patients.[9]

However, not all studies have supported the usefulness of this form of US assessment for predicting difficult laryngoscopy. There is no concurrence in the correlation between scoring systems and pre-epiglottic space or E-VC or their ratios.


  Evaluation of Airway Pathology Top


US is useful in localising and evaluating masses and/or obstruction in the airway. This can be helpful in preoperative planning of airway management. The potential of US for this use was recognised as early as three decades ago.[10] US can also be used to evaluate the epiglottis, planning airway management by evaluating the airway from the tongue to the thyroid cartilage, the vallecula and the pyriform fossa and in assessing the mobility of vocal cords.

Vocal cord pathology

The conventional method of evaluating the vocal cords and its movement is by indirect laryngoscopy or telelaryngoscopy. This may not be feasible in many situations as it requires trained personnel. US can be used to evaluate the cords before surgery on thyroid or other neck structures is performed. Kundra et al. reported a post-thyroidectomy case wherein nerve injury could be diagnosed in the immediate postoperative period by assessing the vocal cord movement on bedside US, thus concluding that superior and recurrent laryngeal nerve palsy can be detected early.[11] Similarly, correlation between vocal cord asymmetry (as identified on US in patients who had undergone thyroid surgery) and the development of voice changes in the postoperative period has also been observed. The rate of vocal fold visualisation and accuracy of US diagnosis of vocal fold palsy are better in females when compared to males.[12] US can reliably diagnose vocal cord paralysis postoperatively in patients undergoing thyroid and parathyroid surgery and can be used as a screening tool with further laryngoscopic evaluation limited to those with asymmetrical cord movement. There is a high correlation between US-based and laryngoscopy-based evaluation of vocal cord function [Figure 8].
Figure 8: Transverse view at the level of thyroid cartilage to observe the movements of vocal cord during abduction and adduction. SCM: Sternocleidomastoid muscle, TC: Thyroid cartilage, AC: Anterior commissure, Vc: Vocal cords, A: Arytenoids

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Pharyngolaryngeal pathology

US can also differentiate solid and cystic lesions on the vocal fold. Similarly, airway abscesses may also be detected to aid in the decision to perform awake fibreoptic-aided intubation. Airway of patients with severe trismus can be evaluated noninvasively before performing a nasal intubation. Furthermore, in patients planned for retromolar intubation, the retromolar space can be examined to identify contraindications for nasal intubation. Benign vocal cord lesions such as cysts, polyps and papillomatosis can be identified with high accuracy in paediatric patients with translaryngeal US.[13] Furthermore, ultrasonographic appearance of subglottic haemangioma, laryngeal stenosis and cysts, papillomas and pharyngeal pouch are well described.


  Prediction of Endotracheal Tube Size Top


Subglottic diameter

US-based subglottic diameter measurements can be used to predict endotracheal tube (ETT) size, especially in paediatric patients. Several studies have demonstrated that US-based measurements of the subglottic diameter correlate well with that obtained by MRI (the gold standard) as well as CT scan.[14],[15]

Studies have also demonstrated a better correlation between subglottic diameter measurement and correct ETT size than age-based formulae.[14],[15],[16],[17] This is true for both cuffed and uncuffed tubes. Further, the technique is easy to learn and the procedure time is short. The impact of this advantage is significant in the paediatric age group wherein complications such as airway trauma and tracheal stenosis are more common with bigger tubes or repeated attempts at intubation. An undersized tube may result in poor ventilation owing to air leaks. Kim et al. derived and validated a formula from US-based measurement for ETT selection. Their data inform that the correlation was very good in children 12 months of age and older and not very good in those <12 months.[18] This was supported by another study by Bae et al. who concluded that though US-based measurement for ETT selection resulted in better correlation, the success rate of selecting a correct size tube did not exceed 60%[16] [Figure 9].
Figure 9: Transverse view just at the lower edge of cricoid cartilage to measure the subglottic diameter for predicting size of the endotracheal tube. CC: Cricoid cartilage, SGD: Subglottic diameter

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Double lumen tubes

The correct size of double-lumen tubes (DLT) can also be selected based on the outer diameter of trachea just above the sternoclavicular joint using US.[19] The measurements obtained by US correlated well with CT scan measurements of the trachea. Three-dimensional US can be used to reconstruct the subglottic space with the added advantage that unlike in conventional US, the posterior wall can also be assessed.[20]


  Confirmation of Endotracheal Tube Placement Top


Real-time visualisation of ETT passing the trachea in suprasternal view reliably detects proper tracheal intubation while a hyperechoic line deep to the trachea, called the double-tract sign, indicates oesophageal intubation.[21] The oesophagus can be seen to the left of the trachea [Figure 10].
Figure 10: Transverse view at suprasternal notch to view oesophagus and confirm tracheal intubation. Oe: Oesophagus, Tr: Trachea, TG: Thyroid gland

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Chou et al. described a method for this called the tracheal rapid US examination for assessing successful tracheal intubation by placing the US probe transversely on the trachea in the suprasternal area.[22] Bilateral diaphragmatic or pleural slide is also taken as surrogate markers of correct tube placement.[23],[24] Using US for confirming the success of intubation has several advantages. Unlike ETCO2 which is the gold standard, there is no need to wait for 4–5 breaths until the appearance of the capnographic waveform. Similarly, it does not rely on cardiac output and so can be used in patients with pulmonary embolism or during cardiac arrest.

Several studies have shown high specificity and sensitivity for this method.[25],[26],[27],[28] A meta-analysis that pooled data for 969 patients identified sensitivity and specificity of 0.98 (CI: 0.97–0.99) and 0.98 (CI: 0.95–0.99), respectively.[29] The authors demonstrated that US was able to detect ETT placement earlier than either ETCO2 or routine clinical methods. Another meta-analysis of 12 studies on the use of US for the detection of correct placement of ETT concluded that oesophageal intubation could be detected with a high degree of accuracy.[30] The use of US to detect the correct placement of DLT with lung isolation has been tried.[31] DLT placement was correctly detected even in the presence of pneumothorax.[32] This method may have useful implications in emergency situations. However, the disadvantage with US-based evaluation is that visualisation may be hampered in the presence of subcutaneous emphysema, oedema or excessive fat. Interestingly, confirmation of laryngeal mask airway (LMA) placement has also been tried by obtaining a transverse tracheal view at the suprasternal level and has shown to correlate well with fibreoptic view. In addition, displacement of LMA and cause for interference in the placement of LMA can also be identified.[33]


  Procedures on the Trachea Top


Several procedures on the trachea can be performed using US guidance. It provides a fast, easy and reliable method of evaluation. It is especially useful in patients with altered neck anatomy and obesity.

Tracheostomy

US is useful in localising the trachea and the site for performing tracheostomy. This prevents several potential complications: pleura can be visualised by US to avoid pneumothorax; Doppler flow can detect vessels, thus preventing vessel injury or haemothorax and tracheal rings can be accurately localised, thereby avoiding cricoid/first tracheal ring incision and the resulting stenosis, especially in children.[34]

Percutaneous dilatational tracheostomy

US guidance increases the safety and ease of performing percutaneous dilatational tracheostomies (PDTs). US enables visualisation and selection of intercartilaginous membrane, tracheal midline, thyroid isthmus and vascular structures. In a review article published by Rudas,[35] three studies have reported a 23%–50% rate of change in the site of insertion for PDT after US assessment of the anterior neck due either to the presence of thyroid isthmus, a vessel or tracheal deviation. Hatfield et al. reported the presence of anterior jugular vein in about half the patients, and four patients had abnormal carotid artery courses which rendered PDT unsafe.[10] Surgical tracheostomy was performed subsequently on these patients. The blinking frog sign has been described recently which refers to the presence of vasculature in the pre-tracheal area and if seen in the transverse view is best avoided.[36]

In conditions with altered neck anatomy and cervical spine immobiliser and in the obese, US can be of immense help in decreasing complications and morbidity. Bronchoscopic transillumination is unreliable in obese patients. Sustec et al. in their autopsy study of 26 cadaver tracheas found that those who had a PDT inserted blindly had more cranially placed tubes (between the cricoid and the first tracheal ring); there were no misplaced tubes in the trachea of patients who underwent PDT under US guidance.[37] US also estimates the distance from the skin to the trachea and therefore can help in preventing perforation of the posterior tracheal wall.[38] The incidence of developing hypercarbia is significantly higher in patients undergoing PDT under continuous bronchoscopic guidance when compared to PDT done under US guidance.[39]

US measurements done to predict the soft-tissue distance from the skin to the trachea, especially in the obese, can also help prevent accidental extubation. Real-time US has also been used to perform PDT successfully to significantly improve the first-pass accuracy.[40],[41] Complications such as bleeding, early tube dislodgement and granuloma formation are also reduced.[42],[43]

Localisation of cricothyroid membrane and cricothyroidotomy

Cricothyroidotomy is often performed as an emergency life-saving procedure in situations where the patient cannot be oxygenated or intubated. It is considered as one of the core skills for an anaesthetist. US assists in the accurate localisation of the cricothyroid membrane (CTM).

Three different techniques of identifying this structure by the landmark method had a success rate of 46%–66% only, probably due to equipment or procedural faults.[44] Failure to correctly detect this structure can be fatal during resuscitation. The success rate of correctly identifying the CTM by palpatory method varies from 39% to 32% when compared to the accuracy obtained by US.[45],[46] Pre-procedure US can help in locating anatomical structures and abnormal vasculature in addition to the ability to measure the distance for insertion.[43] There is little evidence to demonstrate its use in real-time scenarios of complete ventilation failure. Real-time CTM needle insertion under US guidance has also been described in 16 patients who underwent emergency cricothyroidotomy in view of impending airway obstruction after all other measures to secure airway failed. These patients either had severe maxillofacial injuries or cervical spine immobilisers in place.[47] US was also used to locate the landmark in the presence of Ludwig's angina before performing cricothyroidotomy[48] [Figure 11].
Figure 11: (a) Sagittal view over thyroid and cricoid cartilage to visualise the cricothyroid membrane. TC: Thyroid cartilage, E: Epiglottis, VC: Vocal cords, CTM: Cricothyroid membrane, CC: Cricoid cartilage. (b) Needle shadow above the cricothyroid membrane to mark the site of puncture

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Retrograde intubation

Successful US-guided retrograde intubation has also been described in a patient with haemorrhaging friable tumour in the airway presenting with stridor.[49]

Airway blocks

Anatomical structures relevant to airway block (superior laryngeal nerve) can be visualised in 81% of patients. However, the nerve itself is not visualised.[50]


  Prediction of Post-Extubation Stridor Top


Post-extubation stridor (PES) may be a result of several[48] conditions, namely laryngotracheal oedema, trauma, mucosal injury and airway granulomas. However, these are often recognised when complications such as stridor or reintubation occur following extubation.

Several studies analysed the utility of US in predicting successful extubation, especially for patients on ventilators in the intensive care unit. These studies compared the air column width (ACW) at the level of the CTM before and after ETT cuff deflation as a surrogate for the conventional leak test (CLT). Although earlier studies found low sensitivity and specificity of both CLT and US methods in predicting PES,[51],[52] a few recent studies have shown better results.[53],[54],[55],[56] In a study involving 400 children out of whom 44 developed PES, a high level of sensitivity (93%), specificity (83%) and accuracy (91%) was demonstrated if the difference between the ACW before and after cuff deflation was <0.8 mm.[55] In adults, an ACW difference of 0.9 mm was significant in predicting PES[54] [Figure 12].
Figure 12: (a) Transverse view just below the cricoid cartilage with endotracheal tube in trachea with tracheal cuff inflated (bullet-shaped air column, no change in width of air column). (b) Transverse view just below the cricoid cartilage with endotracheal tube in trachea with tracheal cuff deflated (increased width, trapezoid-shaped air column). ET: Endotracheal tube, ACW: Air column width

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  Conclusion Top


Ultrasonography has proven to be useful in managing the airway, especially when airway anatomy is distorted. Repeated dynamic assessments can be performed, and the accuracy is comparable to CT and MRI of the airway with the added benefit of being real time. It can be used immediately before induction of anaesthesia to assess airway anatomy as well as predict difficult airway. It improves the safety of performing percutaneous tracheostomies and cricothyroidotomies. However, its overambitious use cannot be recommended in situations where other gadgets have proven to be superior.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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