Introduction
Suboptimal airway management still continues to result in considerable morbidity and mortality in anaesthesia, critical care and emergency department.[1]  Ultrasound (US) imaging of the airway has been introduced in the last decade as an innovative and effective tool with a potential to improve outcome in conjunction with conventional airway practice. The concept of medical US was experimental. It is based on doctrine of sonar applied in world war-I.[2] The positive predictive value of US guided assessment and intervention is considerably high and thus may be accepted as an important weapon in the armory of an airway manager. This produces many direct and indirect dynamic clues to address airway related complications that can endanger life. In this article, an effort is made to highlight and review published data and also author’s experience related to airway assessment and management. At the end of the activity the participant will be able to

• Enumerate airway structures identifiable by US
• Describe the role of ultrasound in the management of upper and lower airways
• Select transducer and window to view particular anatomical structure

Common Clinical Uses of Airway Ultrasound

1. Airway Assessment   

1. Anticipation of difficult airway situations:

Patients who require emergency intubation in the intensive care and in emergency room are often too sick to co-operate for the clinical tests. Moreover, preprocedural clinical tests to predict difficult airway have limited validity and reliability. Preprocedural US of the upper airway have potential to capture detail anatomical data,[1,3]  The hyomental distance ratio, in the neutral and head extended positions, clubbed with tongue volume and muscles of the floor of the mouth offers the highest predictive value (sensitivity 88% and specificity 60%) for anticipating challenging laryngoscopy which may end up in difficult intubation. The US during preanaesthetic airway assessment can be useful to forecast Cormack Lehane grading during direct laryngoscopy and with moderate precision as an add-on to the available noninvasive airway assessment tools, based on the ratio of the pre-epiglottic space depth and the distance between epiglottis to vocal cords.[4] Patients having anterior soft tissue thickness or more at the level of the vocal cords and neck circumference more than 50 cm are likely to experience difficulty in laryngoscopy.[5] Sublingual US has the privilege to contribute more information about complex oropharyngeal and laryngeal architecture.

2. Recognition of factors that can influence plan of airway management:

High frequency transverse linear probe can identify and evaluate pathologies and their possible influence on airway management. Inflammatory conditions like epiglotitis, mucosal swelling and other pathologies can be imaged by preintubation US. Laryngeal US can assess the spread inside and outside of most of the laryngeal tumours. [6] Evaluation of vocal cord function is possible reliably even in children.[7] Precise location and aberration of trachea is at times challenging, particularly in patients with morbid obesity, neck masses and US imaging is of immense help to get rid of this situation safely.[8]

3. Precise marking of the cricothyroid membrane:

Sagittal and parasagittal views on US can clearly capture this membrane as a hyperechoic band linking two hypoechoic structures, thyroid and cricoid cartilages. The cricothyroid membrane is an essential landmark in difficult airway setting where transcricoid access is required for oxygenation to rescue a life. Accuracy of surface landmark for precise recognition is stated no more than 30% but US allows quick, reliable bedside identification of marking cricothyroid membrane to avoid airway management related adverse outcome. The cricoid cartilage looks like oval hypoechoic area on the parasagittal view and an arch-like presentation on the transverse view.[9]

4. Identification and quantification of gastric contents:

Both experimental and clinical evidence advocate that US can reliably identify full stomach state in right lateral decubitus in between peristaltic contractions. Prior to emergency endotracheal intubation, the quantification of the stomach contents is particularly useful to plan the airway management by measuring the cross-sectional area of the gastric antrum.[10,11] The stomach and its contents can be seen by changing the probe sagitally in the epigastrium.

5. Early detection of Pneumothorax:

Lung US has a superior sensitivity and specificity in detecting pneumothorax (91% sensitivity and 98% specificity), compared to conventional chest radiography (sensitivity of 50.2 % and specificity 99.4%.)[12]  Diagnostic dilemma to detect occult pneumothorax is prevalent in 30-40% patients in supine posture in the intensive care or operative room, because chest radiography, often fails to detect the pathology at the bedside and is later confirmed by computed tomography.[13] The plus points of bedside lung US are its rapidity, simplicity, feasibility, acceptable sensitivity, excellent specificity, and the ability to detect the expansion of the pneumothorax which altogether lessen the transportation of ailing patients for computer tomography. Existence of lung sliding or lung pulse excludes the possibility of pneumothorax because visceral and parietal pleurae are required to be in direct contact with each other to produce such effect under the probe.

The lung point, best viewed in real-time US, is pathognomonic of pneumothorax with a sensitivity of 79% and specificity of 100%.[9] There are four specific unmistakable lung US clues that prevail for the diagnosis of pneumothorax. [14]
Lung point: The lung point means the area under the transducer where the sliding lung is alternating with reverberation artifacts from pleural lines, called A lines, synchronous with ventilation. The same image on M-mode constitutes only parallel lines covering whole depth, known as ‘Stratosphere sign’.
Lung sliding: The presence of lung sliding or B lines acknowledges that the two pleural surfaces are in close proximity and rules out pneumothorax. The absence of lung sliding may occur in variety of situations like lung pathology, apnoea etc and so, can be a useful hint or suggestion for the clinician to consider the possibility of pneumothorax.
B lines: These are perpendicular narrow based hyperechogenic lines emanating only from the pleural line and continue up to the edge of the US screen. They are also called ‘comet tail artifacts’.
Lung pulse: this is transmitted cardiac pulsation through non-ventilated (immobile) lungs.
Besides this, lung US is also helpful to diagnose other lung pathologies like lung consolidation, pneumonia, interstitial syndrome, etc.
2. Airway Management   
A. Early Recognition of oesophageal intubation in real time without initiating ventilation:
Tracheal passage of tube is experienced as a succinct flicker deep to the thyroid cartilage, whereas the oesophageal intubation which produces a hyperechoic curved line with a distal shadow on one side deep to the trachea.[15] Confirmation of endotracheal intubation by US in obese patients is observed quicker than the conventional practice of auscultation and capnography.[16]  Inadvertant oesophageal intubation constitutes an unique ‘double trachea’ sign and visualization of an empty oesophagus precludes oesophageal intubation by a process of elimination..[17] US permits the early recognition of oesophageal intubation specially when there is low or no circulation, and where estimation of CO2 may not be a dependable guide. Earlier detection before initiating ventilation also reduces the risk of stomach inflation and its subsequent complications.

US is superior to auscultation, end tidal CO2 and oesophageal suction devices in having increased precision in detecting the anatomical documentation of ETT.  The ability to achieve successful tracheal intubation in paediatric patients is challenging to many airway managers and demands considerable competence, US of the paediatric airway can differentiate between oesophageal and tracheal intubation in real time with considerable precision.

B. Calculating proper sized endotracheal, endobronchial and tracheostomy tubes:
For years, age-based formulae are applied to determine the appropriate size of the ETT for best fitting endotracheal tube size in children. One of the limitations of these formulae is variation of growth of airway among young children. Moreover, the estimation of the outer diameter of the ETT seems more relevant than using the inner diameter. The US measured transverse subglottic diameter is promising towards selection of the most appropriate size of ETT in children as both over- and under-sized tubes are troublesome.[18]

Selecting the appropriate size of the double lumen tube (DLT) is crucial for lung isolation technique. If the tube is bigger in size, it is likely to cause injury and if the DLT is smaller, the chance of misplacement is more. The chest radiograph is the conventional guide to select the correct diameter of DLT.US can aid to predict the correct size of the endobronchial tube prior to administration of anaesthesia. The outer diameter of trachea can be calculated by US with precision just above the sternoclavicular joint. The ratio between CT scan derived left main stem bronchus and US determined outer tracheal diameter is observed to be 0.68.[3]
Brodsky[19] has formulated a rule for detecting proper size of left DLT based on tracheal diameter.

  C. Prognosis after withdrawal of mechanical ventilation- predicting Post-extubation Stridor:
The width of the square shaped air column with cuff inflated, and the trapezoid shaped air column with deflated balloon cuff can be measured and the mean difference can be calculated precisely to predict possible episodes of stridor after extubation. Mean cuff leak volume and mean air column difference measured with ultrasound vary considerably between who develop stridor and those who do not.[20]

   D. Extubation Outcome                                                                               
The cranio-caudal displacement of the liver and spleen acts as a surrogate marker of the diaphragmatic movement and the calculated value reflects overall performance of respiratory muscles and assists in predicting extubation outcome.[21]The trachea is portrayed on US screen as alternating hypo- and hyper-echoic bands (beads) representing the cartilaginous rings and annular ligaments, respectively.[22]

5. Nerve block for preparation of the airway

Superior laryngeal nerve block is often required to prepare patient for airway maneuver. The superior laryngeal nerve space, delineated by hyoid bone, thyroid cartilage, pre-epiglottic space, thyrohyoid muscle and thyrohyoid membrane can be observed with US in most of the patients to institute a safe and precise nerve block.[23] Measurement of the tongue base (width) along with the thickness of the lateral parapharyngeal wall correspond well with diagnosis and severity of obstructed sleep apnoea.[24]

Accuracy of tracheostomy and percutaneous dilatational tracheostomy
Despite widespread use, bronchoscope-guided percutaneous dilatational tracheosomy did not decrease the complication rate but could result in hypercarbia and raised intracranial tension.
Two dimensional linear array ultrasound probe readily identifies the position and anatomical relation of the thyroid and cricoid cartilage, the tracheal rings, the thyroid gland and the carotid and jugular vessels. Aberrant vascular structures crossing the midline can be further assessed by colour or spectral Doppler. Real-time imaging can identify the desired level of puncture in a sagittal plane in the midline over the front of neck and a 90-degree rotation if the probe allows for an out-of-plane approach to guiding the needle in desired level of puncture. The short-axis approach has the advantage of visualizing the midline of the trachea, but the course of the needle and the precise tracheal ring spaces are difficult to visualize. Conversely the long-axis approach allows for direct visualization of the needle and tracheal rings but has a disadvantage in its difficulty with assessing the tracheal midline.[25]

The mean number of introducer needle punctures was significantly lower in the sonography group compared to the landmark group (1.4±0.7 versus 2.6 ± 0.9; P<0.001) and puncture accuracy (72.7% in the sonography group compared to 8.3% in the landmark group, P <0.001).  The total tracheosotomy time was significantly shorter. In obese, ultrasound guide often modifies the puncture site with respect to those chosen solely on anatomical position.[25]  Bronchoscopy transillumination cannot identify the puncture site due to short , thick neck in obese which can be easily overcome by ultrasound. It has been reported that there is a 5 times increase in serious adverse events in obese patients than in non-obese due to bronchoscopy-guided Percutaneous cricothyrotomy. Nevertheless, most of the complications could be anticipated using US examination of neck. In obese, the required sonoanatomical knowledge of the neck and identification of the balloon of the endotracheal tube cuff were the two principal limitations.

Miscellaneous
The current evidence suggests that US can reliably identify and to some extent quantify gastric contents in patients who need emergent endotracheal intubations.[9] The stomach and its contents can be seen by changing the probe in sagittal plane in the epigastrium. Measurement of the tongue base (width) along with the thickness of the lateral parapharyngeal wall corresponds well with the diagnosis and severity of obstructed sleep apnoea.[24] Bedside US for precise placement of nasogastric tube in stomach by nonradiologists is a reliable approach with 97% sensitivity. Radiographic endorsement can be reserved for patients in whom US results are unconvincing. Similarly, gastric balloon of Sengstaken- Blakemore tube can be inflated slowly under US vision which presents as an expanding echogenic circle in the stomach and thus can avert inadvertent bloating/inflation of gastric balloon in the oesophagus.[26,27]

Limitations
US have shortcomings like it is observer dependant, less sensitive to bone and air and areas like posterior pharyngeal wall and posterior tracheal wall are not approachable. Age-dependant calcification of cartilages may interfere with image quality. Resolution is still a major problem and radiography is often preferred because of higher resolution. There are some technical concerns like holding the probe which becomes troublesome due to prominent Adam’s apple. Infra-hyoid US appears more reliable than supra-hyoid compartment possibly due to movement of head and neck. Comprehensive evidence available till date is inadequate. Many good quality studies and randomized controlled trials are necessary to build a robust evidence to recommend its use in routine practice. We need to compare US with other accepted standard practices at present for evaluation and management of airway.
The anatomical data and orientation accessed with three dimension (3D) ultrasound is more comprehensive than 2D than images and comparable with MRI findings. Traditionally laryngoscopy, CT and MRI are regarded as the accepted standard for appraisal of the upper airway of which the laryngoscopy is an invasive procedure and not feasible to perform routinely in awake patients.

Conclusion
US is coming up as a visual stethoscope for anaesthesiologists working in operation room, intensive care and emergency department. Airway imaging is noninvasive, safe, easy to learn, rapid, reliable, repeatable technique, mostly available at point-of-care service. This should be used dynamically before, during and after the airway intervention to get optimal benefit. A robust competence of the sonoanatomy of the upper airway can support anaesthesiologist to plan airway intervention in many airway related critical conditions safely. This can also recognize factors predictive of troublesome airway intervention even in uncooperative small children.US can establish precise placement of ETT without commencing ventilation or noting circulatory status. Pinpointing the midpoint of cricothyroid membrane is feasible before airway management while dealing with a patient with anticipated difficult airway. Bilateral lung sliding presents an indirect witness of ventilation and correct location of the ETT. To detect pneumothorax early US is the first choice of diagnostic investigation with accuracy even to pick up localized pneumothorax. Appropriate diameter of endotracheal, endobronchial and tracheostomy tubes for both paediatric and adult patients can be anticipated with high fidelity. Quantification and nature of stomach contents in emergency nonfasted patients requiring airway intervention can be determined with authenticity. US guidance escalates safety margin in many airway related procedures like percutaneous dilatational tracheostomy by determining intended tracheal ring interspace, calculating the depth from skin to tracheal ring and bypassing the blood vessels, prior to the procedure. Besides critical care setting, US is expanding its horizon and there is increasing evidence of utility of US in airway management. Considering available literature, this can be assumed that US will be incorporated in everyday practice as an adjunct in airway assessment and management particularly challenging cases.
                              

References:
                            

• Cook TM, Woodall N, Harper J, Benger J. Major complications of airway management in the UK: results of the fourth national audit project of the royal college of anaesthetists and the difficult airway society. Part 2: intensive care and emergency departments. Br J Anaesth 2011; 106: 632-42.
• Moore CL, Copel JA. Point-of-care ultrasonography. N Engl J Med 2011; 364:749-57.
• Sustic A. Role of ultrasound in the airway management of critically ill patients. Crit Care Med 2007; 35: S173-7.
• Gupta D, Srirajakalidindi,  Ittiara B, Apple L, Toshniwal G, Haber H. Ultrasonographic modification of Cormack Lehane classification for Pre-Anaesthetic airway assessment. MEJ Anesth 2012;21:835-42.
• Kundra P, Mishra SK, Ramesh A. Ultrasound of the airway. Indian J Anaesth 2011; 55: 456-62.

6. Xia C, Zhu Q, Zhao H,  Yan ,Li S, Zhang S. Usefulness of ultrasonography in assessment of laryngeal carcinoma.Br J Radiol 2013; 86: 20130343.
7. Friedman EM Role of ultrasound in the assessment of vocal cord in infants and children. Ann Otol Rhinol Laryngol 1997; 106:199-209.
8. Munir N, Hughes D, Sadera G, Sherman IW. Ultrasound-guided localization of trachea for surgical tracheostomy.  Eur Arch Otorhinolaryngol 2010; 267: 477-9.

• Kristensen MS. Ultrasonography in the management of the airway. Acta Anaesthesiol Scand 2011; 55: 1155-73.
• Koenig SJ, Lakticova V, Mayo PH Utility of ultrasonography for detection of gastric fluid during urgent endotracheal intubation, Intensive Care Med 2011; 37: 627-31.
• Cubillos J, Tse C,Chan VW, Perlas A. Bedside ultrasound assessment of gastric content : an observational study. Can J Anesth 2012 ; 59: 416-23.

12. Alrajhi K, Woo MY, Vaillancourt C. The characteristics of ultrasonography for the detection of pneumothorax : a systematic review and meta-analysis.Chest 2012; 141: 703-8

• Lichtenstein DA, Meziere G, Lascols N, Biderman P, Courret J, Gepner A, et al. Ultrasound diagnosis of occult pneumothorax. Crit Care Med 2005; 33: 1231-8.

14. Volpicelli G, Elbarbary M, Blaivas M,Lichenstein DA, Mathis G, Kirkpatrik AW et al. International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Med 2012; 38: 577-91.

• Milling TJ, Jones M, Khan T, et al.Transtracheal 2d ultrasound for identification of oesophageal intubation.J Emerg Med 2007; 32 :409-14.
• Pfeiffer P, Bache S, Isbye DL, Rudolf SS, Rovsing L, Borglum J. Verification of endotracheal intubation in obese patients – temporal comparison of ultrasound vs. auscultation and capnography. Acta Anaesthesiol Scand 2012; 56: 571-76.
• Tsung JW, FensterD, Kessler DD, Novik J. Dynamic anatomic relationship of the esophagus and trachea on sonography : implications for endotracheal tube confirmation in children. J Ultrasound Med 2012; 31: 1365-70.
• Kim EJ, Kim SY, Kim WO, Kim H, Kil HK. Ultrasound measurement of subglottic diameter and an empirical formula for proper endotracheal tube fitting in children. Acta Anaesthesiol Scand 2013; 57: 1124-30.

19. Brodsky JB Fiberoptic bronchoscopy need not be a routine part of double-lumen tube placement. Curr Opin Anaesth 2004; 17:7-11.

• Ding LW, Wang HC, Wu HD, Chang CJ, Yang PC. Laryngeal ultrasound: a useful method in predicting post-extubation stridor. A pilot study. Eur Respir J 2006; 27: 384-9.
• Jiang J, Tsai T, Jerng J, Yu C, Wu H,Yang P. Ultrasonographic evaluation of liver/spleen movements and extubation outcome. Chest 2004; 126: 179-85.
• Bumpous JM, Randolph GW. The expanding utility of office-based ultrasound for the head and neck surgeon. Otolaryngol Clin N Am 2010; 43:1203-8.
• Barbaret G, Henry Y, Tatu L, Berthier F, Besch G,Pili-Floury S, Samain E.Ultrasound description of a superior laryngeal nerve space as an anatomical basis for echoguided regional anaesthesia. Br J Anaesth 2012; 109: 126-8.
• Marciniak B, Faioux P, Hebrard A,Krivosic-Horber R, Engelhardt T, Bissonnettee B. Airway management in children : Ultrasonography assessment of tracheal intubation in real time. Anesth Analg 2009; 108: 461-5
• Dinh VA, Farshidpanah S, Lu S, Stokes P, Chrissian A, Shah H et al. Real-time sonographically guided percutaneous dilatational tracheostomy using a long-axis approach compared to the landmark technique. J Ultrasound Med 2014; 33:1407-15.
• Chenaitia H, Brun PM, Querellou E, et al. Ultrasound to confirm gastric tube placement in prehospital management. Resuscitation 2012; 83(4):  447-51.
• Vigneau C, Baudel JL,GuidetB, Offenstadt G, Maury E. Sonography as an alternate to radiography for nasogastric feeding tube location. Intensive Care Med 2005; 31: 1570-2.