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REVIEW ARTICLE
Year : 2022 | Volume
: 23
| Issue : 2 | Page : 83-90
Use of ultrasound in pediatric airway: Revisiting the past, reviewing the present, and recommending for the future
MV Eeshwar, Rajasekhar Metta, Ghansham Biyani
Department of Anesthesiology, All India Institute of Medical Sciences, Guntur, Andhra Pradesh, India
Correspondence Address:
Dr. Rajasekhar Metta
Department of Anesthesiology, All India Institute of Medical Sciences, 425, 4th Floor, OPD Building, Mangalagiri, Guntur - 522 503, Andhra Pradesh
India
Source of Support: None, Conflict of Interest: None
CheckDOI: 10.4103/TheIAForum.TheIAForum_93_22
Despite few inherent issues, ultrasound (US) has emerged as a new frontier in the assessment and management of pediatric airway due to easy availability of US machines in operation theaters, noninvasive and radiation-free properties, and reproducibility. In this narrative review, we discussed the ergonomics and equipment needed to perform the airway scan, types of scans performed and their clinical applications, normal appearance of airway structures, and important clinical applications of US in the management of pediatric airway. The greatest advantage of US lies in accurate location of cricothyroid membrane and tracheal rings, in measuring the narrowest part of the airway (cricoid) to determine the size of the endotracheal tube, to rule out esophageal intubation, and in assessing the movements of the vocal cords, among others. In our view, upper airway US is a convenient, cost-effective, noninvasive, first-line airway assessment tool which is dynamic in nature. It can be used preoperatively, intraoperatively for real-time guidance in performing interventional procedures, and in the postoperative period.
Keywords: Airway assessment, pediatric airway, point-of-care ultrasound, ultrasound
Sir C. V. Raman once said, “Water is the elixir of life.” Similarly, in modern-day anesthesia practice, ultrasonography has become one of the most reliable sources of imaging and a point-of-care tool for both diagnostic and therapeutic purposes. The important diagnostic roles include point-of-care ultrasound (POCUS) examination in trauma and critical care, echocardiography, preprocedural scanning for neuraxial anatomy, etc. The therapeutic interventions include performing peripheral nerve blocks, interventional pain procedures, and securing vascular access.
However, when it comes to using ultrasound (US) for airway assessment and management, there are a few inherent issues. It is usually said that air and bone are the enemies of US waves as the sound waves either get absorbed or get reflected to a large extent, making visualization of deeper structures very difficult (acoustic shadowing), and while examining the airway structures, we expect air to be present everywhere. Moreover, airway US needs technical proficiency and competency in the interpretation of the acquired sonographic images. In spite of all this, there has been significant research undertaken in the last two decades trying to understand the normal airway anatomy and to apply this knowledge in predicting a difficult airway and performing life-saving therapeutic interventions under real-time US guidance. There has been refinement in technology over the years that has resulted in better image acquisition. In addition, the easy availability of US machines in operation theaters, non-invasive and radiation-free properties, and reproducibility have made US to become a new frontier in airway assessment.
The role of US in the airway has been described as early as the 1980s when it was used to assess the function of vocal cords and to perform a percutaneous tracheostomy.[1],[2],[3] However, after the 2000s, researchers started looking into various parameters that could help assess airway and predict difficult airway in the adult population.[4],[5],[6] However, when it comes to the role of US in the management of pediatric airway, there is only a limited amount of data available in the form of small-scale randomized controlled trials (RCTs), editorials, and case reports.
The airway sonoanatomy of a child differs from that of an adult in being:
Anatomically different (interpretation of sonoanatomy)Lack of cooperation (movements of the neck)Scanning is cumbersome due to small built (technically challenging)There is a need for small footprint US transducer (hockey stick probe)Long learning curve (in comparison to adult airway).Based upon the currently available literature, in this nonsystematic review, we have discussed the ergonomics and equipment needed to perform the airway scan, types of scans performed and their clinical applications, normal appearances of airway structures including the identification of cricothyroid membrane (CTM), important applications of POCUS in theaters, and the likely future of US in the management of the pediatric airway. We looked into the literature using the MeSH terms ultrasound, paediatric airway, POCUS in airway, and cricothyroid membrane in Cochrane library, Embase, Medline, PubMed, Scopus, and Web of Science in writing this review.
Ergonomics and EquipmentSonoanatomy of the airway in an adult or a child is usually performed in supine position with a pillow under the occiput (mimicking the sniffing position) [Figure 1]. To achieve optimal exposure, while scanning the suprahyoid structures, the neck is flexed. It is extended for scanning the infrahyoid structures. The examiner can stand on one side of the child and US machine be positioned on the other side.
Different types of probes are used for airway scanning based upon the depth of area of interest [Figure 2]. High-frequency (4–15 MHz) linear transducer provides better resolution required for scanning the superficially located structures (thyroid and cricoid cartilages, hyoid bone, trachea, CTM, movement of the vocal cords, etc.). However, in younger children aged 8 years or less, linear transducer with small footprint (hockey stick) is required to get a better surface contact. On the other hand, the low-frequency transducer (2–5 MHz) is used to scan deeper structures (tongue, tonsils, palate, etc.) as the penetration is higher, but it comes at the cost of poor resolution. A micro-convex probe (8 MHz) can also be used to easily access the neck of small children and is considered to be useful in visualizing both the superficial and deeper structures. The smaller surface area of this probe makes it convenient to have a firm skin contact.[6],[7]
Figure 2: Ultrasound probes used for airway sonongraphy (left to right – Hockey Stick - High frequency, Linear–High frequency and Curvilinear–Low frequency probes)There are three different planes normally used while studying the sonoanatomy of the airway and the transducer needs to be placed in one of the three ways, as shown in [Figure 3].
Figure 3: Transducer positions (left to right – transverse, sagittal, parasagittal)Transverse plane (transverse across the midline)Sagittal plane (parallel to the midline)Parasagittal plane (parallel but lateral to the midline).It is equally essential to adjust the contrast (gain) and depth on the US machine, thereby keeping the area of interest in the middle of the screen and being able to identify distinct sonoanatomy of the relevant structures. A marker pen and blunt needle are required for marking the landmarks such as the tracheal rings, CTM, cartilages, and hyoid bone.
Basics of Airway SonoanatomyAs the US waves travel deeper to the skin, they get reflected back to a varying extent based on the acoustic impedance properties of the underlying structures. Different body tissues have different echogenicity, thereby they vary in their appearance ranging from bright white (hyperechoic) to dark (hypoechoic or anechoic). In the airway, the air–mucosal interface (A-M) appears as a bright hyperechoic line beyond which the sound waves do not pass to a greater extent, as air is a poor medium for sound waves to travel. As a result, visualization of structures located at a greater depth and posterior to A-M interface becomes difficult. The echogenicity of some of the common structures that will be encountered during the scanning of the airway is listed in [Table 1].
Sonoanatomy of the submandibular area (tongue and floor of the mouth)
Upon placing the US probe transversely in the submental region, the tongue along with its muscles is visualized [Figure 4]a. The intrinsic muscles appear striated whereas the extrinsic muscles are distinctly visualized. In the sagittal plane, the mentum of the mandible anteriorly and the hyoid bone posteriorly cast an acoustic shadow.[8] The muscles of the floor of the mouth together with the tongue and hard palate are well appreciated [Figure 4]b.
Figure 4: (a) Submandibular transverse view (low-frequency probe) (b) Submandibular sagittal view (low-frequency probe)Sonoanatomy at the level of hyoid bone
Hyoid bone is normally visualized in the transverse view as a superficial hyperechoic inverted U-shaped structure with greater cornua on either end, which serve as a landmark for the blockade of superior laryngeal nerve. The hyoid bone casts an acoustic shadow below it [Figure 5]a.
Figure 5: Transverse scans, (a) Over the hyoid bone, (b) At thyrohyoid membrane showing epiglottis with hyperechoic preepiglottic space and air–mucosal interface, (c) At the level of thyroid cartilage and false vocal cords, (d) At the level of cricoid cartilage seen as horseshoe-shaped hypoechoic structure (inverted-C) with a hyperechoic A-M interface beneath it, (e) At the suprasternal notch showing hypoechoic tracheal rings with a hyperechoic A-M interface and comet tail appearance beneath it. AM: air-mucosal interface, CC: cricoid cartilage, CTA: comet tail appearance, IsthT: thyroid isthmus, TC: thyroid cartilage, TG: thyroid gland, TrC: tracheal cartilageSonoanatomy at the level of thyrohyoid membrane
The epiglottis and the preepiglottic space (PES) are seen with the linear probe over the thyrohyoid membrane (THM). The epiglottis appears as a linear hypoechoic structure with the hyperechoic PES anteriorly and A-M interface posteriorly [Figure 5]b.
Sonoanatomy at the level of thyroid cartilage
The thyroid cartilage appears as an inverted V-shape structure [Figure 5]c. It provides the best window to visualize the vocal cords and their movement. The false vocal cords appear as hyperechoic structures cephalad to the true vocal cords. The false vocal cords are relatively immobile as compared to true vocal cords during phonation and respiration. The vocal cord function can be assessed preoperatively, postsurgery like thyroidectomy, following trauma or in cases of malignancy when the child presents with hoarseness of voice or with stridor, and in any other clinical conditions where an abnormality in the superior and recurrent laryngeal nerves or in the movement of vocal cords is suspected.
Sonoanatomy at the level of cricoid cartilage
Cricoid cartilage appears as a horseshoe-shaped hypoechoic structure (inverted-C) with a hyperechoic strip line of A-M interface directly beneath it [Figure 5]d. It is the bulkiest cartilage of larynx and is placed more anteriorly compared to the tracheal rings. The transverse diameter of the subglottic space at the level of cricoid cartilage best assesses the size of endotracheal tube (ETT) as the subglottic space is the narrowest part of airway in children.
Sonoanatomy at the level of trachea
Place a linear probe in transverse plane just above the suprasternal notch and scan cephalad to identify the tracheal cartilages. The A-M interface and comet tail appearance, which are formed due to the reverberation artifact, are seen posteriorly [Figure 5]e. By rotating the probe 90°, “string of beads” appearance of tracheal cartilages comes into the view.
Identification of the cricothyroid membrane
Identification of CTM requires identification of the thyroid cartilage cranially and cricoid cartilage caudally. There are two techniques commonly described for its identification, one in the sagittal plane and the other in the transverse plane.
Sagittal plane: Cephalad to the longitudinal “string of pearls” appearance of tracheal ringsTransverse plane: Thyroid cartilage-airline-cricoid cartilage-airline.The sagittal plane is preferred for the identification and location of the CTM in adults, as the entire length of the membrane is visible, but due to limited neck space for the longitudinal placement of the US probe in children, the transverse plane is frequently used.
Transverse thyroid-airline-cricoid-airline technique
Place a linear probe transversely over the front of the neck to identify the thyroid cartilage, and then move the probe caudally until the CTM is identified as a hyperechoic line (airline) due to the A-M interface. The probe is then moved further caudally and the cricoid cartilage is identified. The probe is slightly moved back cranially to identify the CTM again. The skin is then marked at both sides of the US probe at the midpoint. The four points are connected and they intersect at the midpoint of the CTM.
Longitudinal “string of pearls” appearance
A linear probe is placed over the suprasternal notch in the transverse orientation and the tracheal cartilage is identified. The transducer is then rotated by 90° to place it along the midline of the trachea. Dark hypoechoic rings that resemble a string of pearls appear anteriorly (tracheal rings) and a hyperechoic line is seen posteriorly (air–tissue interface). As the transducer is slide cranially, a large elongated hypoechoic structure (cricoid cartilage) appears which is placed more anterior to the tracheal rings. Upon sliding further upward, the inferior border of the thyroid cartilage appears in view. The CTM is identified sandwiched between the two cartilages and skin is marked at the midpoint of the probe by passing a needle [Figure 6] under the transducer and is fixed at the midpoint of the CTM where it casts its shadow. The transducer is then removed and the midpoint of the CTM is marked.
Figure 6: Identification of the cricothyroid membrane (longitudinal scan using high-frequency probe). CC: Cricoid cartilage, CTM: Cricothyroid membrane, TC: Thyroid cartilage, TrC: Tracheal cartilage Brief Review of the Existing Literature As discussed earlier, there is a paucity of data published on the use of US in the management of pediatric airway. Some of the applications found in the literature include the following:
To measure the transverse diameter at the level of cricoid to estimate the size of ETTTo confirm the position of ETT in tracheaTo rule out endobronchial intubationTo rule out inadvertent esophageal intubationTo confirm laryngeal mask airway (LMA) positionTo identify and mark the CTMTo identify and mark the tracheal ringsTo assess the movement of the vocal cordsOthers: To assess the risk of aspiration and size of palatine tonsils, to predict difficult airway and postextubation stridor, to diagnose epiglottitis and pneumothorax, to perform airway blocks, etc.To measure the transverse diameter at the level of cricoid to estimate the size of endotracheal tube
Formulas based upon age and height in calculating the size of ETT in pediatric populations have found to be largely inaccurate, resulting in selection of over- or under-sized ETT. The oversized tube results in multiple attempts at intubation and excessive pressure on the tracheal mucosa with the potential for airway damage, while the under-sized ETT results in cuff leak, poor ventilation, inaccurate monitoring of end-tidal gases, increased gas flow resistance, and increased risk of aspiration.[9] Moreover, the variable outer diameter of the ETT despite the same internal diameter makes the selection of a proper-sized ETT even more difficult. There is increasing evidence to show that US helps in accurate estimation of size of the ETT in children.[10],[11],[12],[13]
In a RCT by Shibasaki et al.,[14] the authors enrolled 192 children aged between 1 month and 6 years, and measured the subglottic diameter using US and used regression equation model to predict the optimal size of the ETT. They found that subglottic upper airway diameter was highly correlating with the outer diameter of both the cuffed and uncuffed ETT with high accuracy (98% and 96%, respectively) compared to the age-based formula (35% and 60%, respectively). In another study by Schramm et al,[15] the authors compared the correct size of an uncuffed ETT with the minimal transverse diameter of the subglottic airway in pediatric population and found that US facilitates selection of the appropriate size of uncuffed ETT resulting in reduced number of reintubations.
To confirm the position of endotracheal tube in trachea
The incidence of malposition of ETT is higher in children compared to adults due to short length of the trachea, resulting in inadvertent endobronchial intubation or accidental extubation. US is found to be a useful tool to verify the correct placement of ETT within the tracheal lumen above the level of carina [Video 1].
In a study by Marciniak et al.,[16] the researchers investigated the characteristic real-time ultrasonographic findings of the normal pediatric airway in 30 healthy children during tracheal intubation using four different criteria such as identification of the trachea and tracheal rings, visualization of vocal cords, widening of glottis as the ETT passes through, and position of ETT above the carina.
Tessaro et al.[17] used tracheal rapid US saline test for confirming correct ETT depth in children at the level of suprasternal notch and confirmed the same by fiber-optic bronchoscope. This study showed that US is 98.8% sensitive and 96.4% specific in excluding the endobronchial intubation. In a recent systematic review and meta-analysis by Congedi et al.,[18] the authors analyzed 14 eligible studies and found that US was 96.8% sensitive in accurately diagnosing the ETT position, compared to 80% with chest X-ray.
To rule out endobronchial intubation
Endobronchial intubation can be diagnosed using the US by looking at two different parameters: the movement of the diaphragm and the presence or absence of lung sliding sign. Bilateral movements of the diaphragm with the presence of bilateral lung sliding sign indicate correct position of the ETT within the tracheal lumen. Inadvertent endobronchial intubation results in the presence of diaphragmatic movements with lung sliding sign on the ventilated side, and the absence of both on the nonventilated side.
To observe or rule out inadvertent esophageal intubation
While scanning for the tracheal rings in transverse plane, look for the esophagus on the left-hand side, the identification of which can be made easy by asking the child to swallow, resulting in visible peristaltic movement. The sonoanatomy at this place helps in ruling out esophageal intubation, as the entry of the ETT into the esophagus appears as a classical “double track sign” or “double trachea sign [Figure 7].”[19],[20]
Figure 7: Transverse scan at the suprasternal level showing “double track sign” suggestive of inadvertent esophageal intubationTo confirm laryngeal mask airway position
LMA is easier to place compared to the ETT, but the risk of misplacement is higher. This may result in hypoxemia, interrupted ventilation, necessity for LMA replacement, and endotracheal intubation. Several clinical tests have been advocated to assess the placement of LMA, but none of them can be used as a confirmatory test to determine acceptable positioning.[21]
Arican et al. conducted a study in 82 children aged between 3 and 15 years, and found that US is an excellent noninvasive technique to verify the LMA placement.[22] In another observational study by Kim et al.,[23] the authors compared US with fiber-optic bronchoscopy to evaluate the incidence of LMA malposition. Based upon graded sonographic arytenoid cartilage elevation on transverse plan, they found that US could detect LMA malrotation easily in pediatric patients with high sensitivity (93%) and specificity (82%).
To identify and mark the cricothyroid membrane
The CTM, a relatively avascular structure, is incised when a 'cannot intubate cannot ventilate' scenario is encountered, as a part of emergency front of the neck access (eFONA), a rescue strategy in children aged older than one year. However, accurate identification and location of the CTM by palpatory technique be it in infants or in children is found to be difficult and largely inaccurate.[24] US helps in this aspect increasing the sensitivity to nearly 100%. Bill Walsh and his team used US in localizing and measuring the dimensions of CTM and compared it with magnetic resonance imaging as the reference standard, and found a correlation coefficient of 0.98 (P < 0.0001) with higher precision.[25]
Identification of the CTM using US before the administration of general anesthesia is found to increase the success rate of performing cricothyroidotomy if the need arises, and it is now highly recommended to mark it prior to induction of anesthesia in all cases of anticipated difficult airway.
To identify and mark the tracheal rings
US helps in accurately identifying and counting the tracheal rings for choosing the correct site for percutaneous and surgical tracheostomies, to rule out the presence of any vascular structure in the path of insertion, correct placement of the needle below the level of the thyroid isthmus within the tracheal lumen, to estimate the size and depth of tube insertion, and to prevent injury to the posterior tracheal wall. This is of particular importance in obese children, and in children with short neck, limited neck movements, distorted anatomy, etc.[9],[26]
To assess the movement of the vocal cords
The determination of cause of stridor or vocal cord palsy in infants and children is a demanding task as performing flexible laryngoscopy is invasive and requires cooperation. The use of US is found to be a highly accurate technique to document vocal cord paralysis, with a high degree of inter-rater reliability [Video 2]. In addition, it is noninvasive, painless, and does not require any sedation or anesthesia. The results can be recorded and printed for hard copy storage.[2],[27],[28],[29]
To assess the risk of aspiration
Nil-by-mouth practices currently used across the globe often exceed the standard recommendation and may result in inadvertent stressful event in children under anesthesia, hypotension, dehydration, etc.
Estimation of gastric volume using US in the immediate preoperative period helps us to a larger extent in deciding the anesthetic plan of management in terms of airwayzg the surgery, etc. The use of US for calculating the residual gastric volume is of particular importance when there is barrier for communication leading to unclear fasting history, in cases that come for emergency surgery, in cases with pyloric stenosis or post-tonsillectomy bleed etc.
To assess the size of palatine tonsils
Tonsils in children can be affected by a myriad of pathologies, resulting in their enlargement, leading to upper respiratory tract infection, acute respiratory obstruction and obstructive sleep apnea, unanticipated difficult airway, etc.
Asimakopoulos et al.[30] first described the transcervical sonography method for assessing the size of the tonsillar volume. Recently, in a prospective cohort study, Kay-Rivest et al.[31] confirmed accurate measurement of tonsillar volume in all 3 dimensions in 75 pediatric subjects and correlated with ex vivo specimens on pathology.
To predict/assess difficult airway
The possibility of a difficult airway can be predicted using multiple parameters measures using USG. These include thickness of the tongue, PES, pretracheal soft-tissue, distance from skin to hyoid bone. But the problem lies in the fact that none of these parameters are found to be highly sensitive and specific, and most of the data is coming from adult population.[32] There is no quality evidence available on this subject in pediatric population, making it difficult for us to make any comments on this aspect.
To diagnose epiglottitis
A linear transducer is placed in longitudinal plane over the THM. A hyperechoic appearance of thickened epiglottis in relation to the acoustic shadowing of the hyoid bone appears as a “P-sign,” indicative of epiglottitis.[33]
To detect or diagnose pneumothorax
A great advantage of US lies in its greater sensitivity and specificity in detecting pneumothorax intraoperatively. The presence of bilateral lung sliding, a dynamic sign identified on US as horizontal movement of the lung tissue (visceral pleura) over the parietal pleural, excludes the possibility of significant pneumothorax.
Future PerspectivesUpper airway US is a potential tool which can be incorporated into the future standard of care for airway assessment. However, more number of well-designed RCTs and systematic reviews are needed for the validation of various parameters measured using USG and used for predicting difficult airway. More studies are needed for validating the use of USG in real time monitoring of ETT placement and confirmation of its position, confirming the position of tracheostomy tubes and LMAs, and to assess the educational learning curve when compared with the current scenarios.
ConclusionIn our view, upper airway US is a convenient, cost-effective, noninvasive, first-line airway assessment tool which is dynamic in nature. It can be used preoperatively for airway assessment, intraoperatively for real-time guidance while performing interventional procedures, and also in the postoperative period. POCUS plays a significant role in accurately identifying the CTM as a step for readiness toward the eFONA, detecting inadvertent esophageal intubation, and diagnosing life-threatening conditions like pneumothorax.
Airway gadgets like video laryngoscope, fibreoptic bronchoscope improve the visualisation of the glottis and warn the anaesthesiologist about any unanticipated issues before securing the airway. Similarly, US has the potential to warn the anesthesiologist beforehand of what to expect, thereby increasing preparedness that helps preventing a catastrophic outcome.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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