Advances in airway management in recent 10 years from 2013 to 2023

Difficult airway management has been the focus in the field of anesthesiology. Clinical research in difficult airway management has made some progress in the last 10 years. We searched the relevant literature and summarized the latest clinical research in the field of difficult airway management in this review to provide some practice strategies for difficult airway management for anesthesiologists as well as a range of professionals.


Graphical Abstract 1 Introduction
Airway management is one of the most important tasks of clinical anesthesia, intensive care unit, and emergency rescue.Difficult airway is associated with a higher rate and severity of complications, which is a cause of death during anesthesia.Airway management has been the focus in the field of anesthesiology.In recent 10 years, the definition of difficult airway has changed.Several institutions and organizations have updated the guidelines for the management of difficult airway for different patients, making progress in the evaluation of difficult airway, ventilation methods for application in difficult airway, use of devices, monitoring, training, and artificial intelligence for application in difficult airway.
We searched the relevant literature published from inception to February 10, 2023, with the following databases: MEDLINE (PubMed), EMBASE, CENTRAL (the Cochrane Library), and Web of Science, and summarized the latest clinical research in difficult airway management in this review.We aimed to provide some references about difficult airway management for anesthesiologists as well as a range of professionals.It may facilitate the practice of difficult airway management, improve the success rate of difficult airway management, and reduce the occurrence of difficult airwayrelated complications.

Progress in the definition of difficult airway
Difficult airway is defined as where "an experienced provider anticipates or encounters difficulty with any or all of face mask ventilation, direct or indirect laryngoscopy, tracheal intubation, supraglottic device use, or surgical airway [1]".Difficult airway is also divided into the nonemergency airway and emergency airway according to the absence or presence of difficult facemask ventilation.
Non-emergency airway allows anesthesiologists to have sufficient time to consider other airway control methods.The emergency airway does not only manifest difficult endotracheal intubation but also difficult facemask ventilation.Patients are prone to suffer from hypoxia and even some patients encounter "cannot intubate, cannot ventilate (CICV)", which can lead to serious consequences such as brain injury or even death.However, there is no standard definition for difficult airway in the existing literature [2].Heidegger defined difficult airway as a condition of difficult facemask ventilation, difficult endotracheal intubation, difficult supraglottic airway device (SAD) placement, and recognition of the need for an emergency surgical airway anticipated or encountered by an experienced physician [3].
"Difficult or Failed Tracheal Extubation" was added to the definition of difficult airway in the 2022 version of the Practice Guidelines for Management of the Difficult Airway issued by the American Sociological Association (ASA) [4].Difficult or failed tracheal extubation means the loss of airway patency and adequate ventilation after the removal of an endotracheal tube or SAD from a patient with a known or suspected difficult airway.In addition, the guidelines added "inadequate ventilation" in the category of difficult airway.Indicators of inadequate ventilation include absent or inadequate exhaled carbon dioxide, absent or inadequate chest movement, absent or inadequate breath sounds, auscultatory signs of severe obstruction, cyanosis, gastric air entry or dilatation, decreasing or inadequate oxygen saturation, absent or inadequate exhaled gas flow as measured by spirometry, anatomic lung abnormalities as detected by lung ultrasound, and hemodynamic changes associated with hypoxemia or hypercarbia (e.g., hypertension, tachycardia, bradycardia, arrhythmia).Additional clinical symptoms may include changed mental status or somnolence.This is the "physiologically difficult airway" proposed recently.In addition to anatomical factors, there is also a "physiologically difficult airway" caused by physiological dysfunction in critically ill patients.These patients are still accompanied by more complications and higher mortality in the endotracheal intubation process.The physiologically difficult airway mainly involved endotracheal intubation of critically ill patients in intensive care units, emergency departments, and general wards, with major adverse events including circulatory instability (42.6%), followed by severe hypoxemia (9.3%) and cardiac arrest (3.1%) [5].Therefore, we should not only pay attention to anatomically difficult airway but also physiologically difficult airway in critically ill patients.The American Society for Airway Management's Special Projects Committee issued consensus recommendations for the assessment and management of a physiologically difficult airway [6].The recommendations stated that critically ill patients with reduced functional residual volume should receive pre-oxygenation with high flow oxygen for at least 3 min or 8 tidal volume breaths before endotracheal intubation, and oxygenation during apnea could be performed with a 15 L/min nasal cannula or 40-70 L/ min high-flow nasal cannula.The positive end-expiratory pressure model should be added in pre-oxygenation for patients with pregnancy, obesity, and acute respiratory distress syndrome.For patients with refractory hypoxemia, it was recommended to maintain spontaneous breathing and awake intubation.In patients with hemodynamic instability, vasoactive drugs should be administrated before intubation, and in patients with severe cardiogenic shock, extracorporeal membrane oxygenation (ECMO) may be considered before intubation.
In recent years, the numbers of airway tumors, airway stenosis, airway reconstruction surgeries involving subglottic airway management are more than before.These kinds of surgeries require both surgeons and anesthesiologists to control the airway, so these kinds of complicated surgeries are not limited to simple endotracheal intubation but need some special airway devices and methods.The difficulty and risk degree are much higher compared with other types of surgeries.Based on the above reasons, some experts put forward the concept of a "subglottic difficult airway".The causes of the subglottic difficult airway may include compression of the trachea and bronchus by extratracheal tumor; primary tracheal tumor and congenital tracheal dysplasia and tracheomalacia; tracheal edema caused by long-term indwelling endotracheal tube or peritracheal surgical operation, scar stenosis caused by intubation trauma and ischemic necrosis induced by high cuff pressure; airway foreign body, regurgitation, and aspiration [7].With the progress of anesthesia technology, recent reports of new methods of subglottic surgical airway management have emerged.However, these new methods remain controversial.Given the quantity and quality of the available shreds of evidence, the safety in subglottic surgical airway management remains unclear and further studies are needed [8].
There is a potential for preexisting surgical airway, especially tracheostomy, to create a difficult airway situation.Tracheostomy-related adverse events which may arise from tube obstruction and displacement, accidental decannulation, or hemorrhage account for half of all airway related hypoxic brain damage and deaths in critical care units [9].Confusion regarding airway anatomy (e.g., laryngectomy and tracheostomy) can also create a difficult airway situation.Whereas patients with tracheostomy can be orally intubated, for patients with laryngectomy, oral intubation is never appropriate, and a life-threatening difficult airway situation can arise when oral intubation is attempted [10].

Progress in guidelines for the management of difficult airway
Many countries around the world have published their guidelines for the management of difficult airway since ASA published the first guideline in 1993.ASA guidelines have been updated every decade to improve the management of difficult airway [11].
The results of the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society showed that aspiration was the leading cause of patient death in airway adverse events [12].The 2022 ASA guidelines have newly recommended the assessment of aspiration risk [4].For the anticipated difficult airway, the 2022 ASA guidelines clarified the indication for awake intubation: (1) difficult ventilation (face mask/ supraglottic airway), (2) increased risk of aspiration, (3) the patient is likely incapable of tolerating a brief apneic episode, (4) there is expected difficulty with emergency invasive airway rescue.In addition, the UK published awake intubation guidelines for adults in 2019 [13], which stated that awake intubation is the golden standard for anticipated difficult airway management.
The 2022 ASA guidelines emphasized limiting the number of attempts (four times) of different devices and techniques to avoid potential injuries and complications, which prompted us to completely change the thinking of "intubation first and intubation repeated".For an unanticipated difficult airway, if the intubation failure is encountered, ventilation should be maintained while asking for help.SAD should be used to maintain ventilation as soon as possible and practitioners should try to make the patient return to spontaneous breathing or even wake up the patient.Repeated intubation should be avoided.It has been shown that if the first intubation fails, the probability of success is greatly reduced for each subsequent attempt, especially repeated attempts with the same device and technique [14].Repeated attempts can cause airway damage and laryngeal edema, and edema tissue can cause difficult facemask ventilation, which may lead to "CICV".
During the epidemic of COVID-19, Several guidelines for difficult airway management for patients with COVID-19 have also been published [15,16].These guidelines include the following elements: emergency tracheal intubation, anticipated or unanticipated difficult tracheal intubation, and tracheal extubation.
At present, anesthesiologist associations in many countries have issued guidelines for the management of difficult airway, which were detailed but slightly cumbersome, and were used less frequently actually [17,18].Therefore, Professor Ma Wuhua designed the "ABS algorithm for management of difficult airway" [19] in 2008: "A" means "asking for help", "B" is short for "breathing", "S 1 " is "spontaneous breathing", "S 2 " is "stab cricothyroid membrane", "S 3 " is "surgical airway".
(1) For an anticipated difficult airway, one should ask for help first, and then complete the endotracheal intubation while maintaining the patient's spontaneous breathing, (2) for an unanticipated difficult airway, one should ask for help first, and then use the laryngeal mask and other SAD to maintain ventilation.If ventilation is adequate, the safest method for patients is to restore the patient's spontaneous breathing.The intubation can be tried again, but the number of attempts should be strictly limited to less than 3 times; if the ventilation is insufficient or fails, the surgical airway should be performed decisively.The "ABS algorithm" is simple, safe, effective, and easy to be trained and promoted.However, its validity has yet to be tested and further evidence is necessary to define its clinical efficacy.
Whether the difficult airway management guidelines and procedures can effectively improve the success rate of difficult airway management should be supported by a higher level of evidence-based medical evidence.However, we cannot conduct randomized controlled trials to test the clinical efficacy and efficiency of these guidelines in most emergency airway management cases.Based on the ethical requirements, patient safety is the first consideration, which is the limitation of verifying the effectiveness of the guidelines and procedures.According to a prospective study [20], if the department referred to the guidelines for difficult airway management and developed an airway management strategy, it would reduce the incidence of airway-related adverse events from 15.4% to 11.4%.

Progress in imaging examination for evaluation of difficult airway
In addition to facial features and anatomical measures such as mouth opening, mandibular mobility, head, and neck mobility, Mallampati and modified Mallampati scores, thyromental distance, sternomental distance, interincisor distance, and neck circumference, the 2022 ASA guidelines recommended the ratio of neck circumference to thyromental distance, the ratio of height to thyromental distance, hyomental distance, and use of ultrasound for airway assessment.Measurements obtained from ultrasound included skin-to-hyoid distance, tongue volume, and distance from skin to the epiglottis.The specificity and sensitivity of ultrasound for the diagnosis of difficult airway are similar to that of computerized tomography (CT) and X-rays, and the method is simple, easily available, low cost, and no radiation [21].In recent years, CT and X-rays is gradually replaced by ultrasound measurements for the diagnosis of difficult airway [21].However, the study by Kasinath reached the opposite conclusion [22].The prospective observational study by Alessandri [23] confirmed that ultrasound measurement of the minimum distance of the skin surface to the hyoid bone could be used not only for the assessment of difficult laryngoscopy but also for difficult facemask ventilation.The meta-analysis by Carsetti [24] indicated that the skin-to-epiglottis distance was the most studied predictor of difficult laryngoscopy in clinical trials.The meta-analysis by Sotoodehnia [25] suggested that the anterior soft tissue thickness of the anterior neck (the distance from the skin to the epiglottis) at the level of the vocal cord and hyoid bone was more accurate for predicting difficult intubation.
In addition to the prediction of difficult airway, ultrasound, and other imaging examinations are also used to assess the risk of aspiration [26], locate the position of the endotracheal tube, and predict the postoperative indications for extubation by diaphragm ultrasound [27].In the process of establishing the cervical emergency airway, ultrasound is superior to palpation in identifying the cricothyroid membrane [28].Therefore, the location of the cricothyroid membrane by ultrasound measurement has become a new standard for anterior cervical airway access [29], but ultrasound assessment takes more time.It is recommended that routine ultrasonic positioning should be performed before anesthesia induction in patients with anticipated difficult airway in case of emergency [30].

Progress in ventilation methods for application in difficult airway
Jet ventilation has been shown to be a technique to maintain oxygenation effectively, and the ASA guidelines for difficult airway management have repeatedly listed it as an effective treatment in the emergency difficult airway [2].Supraglottic jet oxygenation and ventilation (SJOV) is a new non-invasive ventilation method.SJOV can provide adequate oxygenation and ventilation during difficult intubation rescue.Liang [31] reported a case of successful rescue after the use of SJOV.Wu [32] stated that the use of SJOV significantly improved the success rate and minimized the occurrence of hypoxemia during fiberoptic bronchoscopy in the treatment of difficult intubation.
High-flow nasal oxygenation (HFNO) is a technique that delivers humidified, heated gas from the nasal cavity to the airway through nasal congestion, producing 21% to 100% stable FiO 2 .As a non-invasive respiratory support technique, HFNO is now widely used in patients of all ages [33].HFNO can prolong the apnea time in anesthetized patients and has been successfully used in various perioperative occasions in recent years, such as preoxygenation, awake intubation, apnea oxygenation technology, sedative gastroenteroscopy, esophageal tracheal fistula surgery, and airway stenosis surgery, etc. [34].However, HFNO caused a significant increase in PaCO 2 during apnea oxygenation as shown by Riva [35].
ECMO is an effective way to maintain oxygen supply in patients with severely difficult airway.The 2022 ASA guidelines for difficult airway management recommended ECMO when an emergency airway occurred.With the help of ECMO, patients with emergency difficult airway can be treated reliably.However, related complications such as bleeding may also occur in the use of ECMO.

Progress in the development of airway devices
Visualization techniques for the management of difficult airway are rapidly developed [36].

Visual endotracheal tube
The anesthesiologists can observe the airway and the endotracheal tube position in real-time through imaging devices [37].Gawlowski [38] found that airway management with ETView (ETView Medical Ltd., Misgav, Israel) was performed better compared with direct laryngoscopy in patients requiring cervical spine stabilization.Liu [39] reported that continuous monitoring of ETView could be achieved via a portable display screen, which avoided complications caused by the movement of the endotracheal tube during lobectomy.

Video laryngoscope
Video laryngoscope is gradually becoming the preferred device for the management of difficult airway [40].Compared with direct laryngoscopy, a video laryngoscope can provide a clearer glottal view and reduce the airway damage [41][42][43].Evidence suggests that video laryngoscope has significant advantages for obese patients [44], critically ill patients [45,46], anticipated or unanticipated difficult airway patients [47] and pediatric patients [48].There is growing evidence that the video laryngoscope should be available in instances of anticipated difficult airway management [49], and when a direct laryngoscopy is unsuccessful, avoiding repeated attempted direct laryngoscopy and instead swiftly transitioning to video laryngoscope is most likely to achieve success and to do so with the least delay.

Visual supraglottic airway devices
Researchers have developed visual SAD by combining video laryngoscopy technology with SAD [50].Visual SAD can provide continuous visual supraglottic ventilation and endotracheal intubation in patients with difficult airway and can effectively avoid overdeep placement [51].However, compared with direct laryngoscopy, visual SAD requires a longer time [52] and often obstructs the field of view [53].Video laryngeal mask achieves the integration of multiple functions such as laryngeal mask placement, endotracheal intubation and visualization [54,55].

Visual stylets
After the placement of the optical stylets, a light spot on the sternum is the sign of successful glottic placement [56].Combining the visibility of video laryngoscopes with the plasticity of visual branchofiberoscope, the visual stylets have emerged in the last decade and have become effective devices for the management of the difficult airway.Direct laryngoscopy has an adverse effect on endotracheal intubation for trauma patients with cervical injury and cervical instability [57].One of the characteristics of the visual stylet is that it can adapt to the physiological curve of the airway, and it is safe and effective in patients with difficult airway with limited cervical extension.When performing awake intubation in patients with cervical instability, visual stylets can significantly reduce intubation time compared with visual branchofiberoscope [58], and it has a higher success rate and fewer complications [59].A recent network meta-analysis showed that the flexible branchofiberoscope, visual stylet, and video laryngoscope are interchangeable airway devices in the setting of awake intubation in adults.Visual stylets cost the shortest intubation time and the flexible branchofiberoscope had the longest intubation time [60].However, it still has some disadvantages for its function could be limited by blind probing and uncertainty of the tip position [61].

Visual flexible intubation scope
The role of visual flexible intubation scope in the management of difficult airway is dominant [13] and is considered the best choice for awake intubation [4,62].Law [63] reported that the first attempt success and final success rate of visual flexible intubation scope were significantly higher than that of the video laryngoscope in awake intubation.Moreover, the visual flexible intubation scope is particularly suitable for patients with oropharyngeal deformities such as temporomandibular joint sclerosis and patients undergoing oral and maxillofacial surgery [64,65].In addition, another advantage of the visual flexible intubation scope is that it is not limited by the patient's position and can be used in the lateral position or even the prone position [66].However, mastering visual flexible intubation scope requires a longer learning time.

Monitoring of the endotracheal tube cuff pressure
Monitoring of endotracheal tube cuff pressure may be often overlooked by anesthesiologists.The ideal cuff pressure is generally between 20 and 30 cmH 2 O to ensure an effective seal while minimizing pressure damage to the trachea wall [67].Excessive inflation of the cuff may cause complications such as tracheal stenosis [68].At present, the conventional measurement method in the clinic is still manual palpation of the pilot balloon, which is mostly based on the experience of the anesthesiologists to judge the amount of cuff inflation, and the result is inaccurate and easy to be affected by subjective feeling.Since then, minimal occlusive volume and minimum leak technique have been recommended, but both methods have measurement errors [69].The emergence of the automated cuff manometer (e.g., Mallinckrodt electronic cuff pressure controller ® , VBM Medizintechnik GmbH, Sulz am Neckar, Germany) has largely addressed the problem of cuff pressure measurement [67].Wang [70] developed an application for a mobile device to perform cuff pressure measurement.Continuous monitoring of endotracheal tube cuff pressure during the perioperative period will play an important role in preventing pressurerelated complications.

Endotracheal tube position monitored by ultrasound devices
The most widely used method to confirm the endotracheal tube position is to monitor the end-tidal carbon dioxide (ETCO 2 ) waveform, and direct positioning with the visualization devices is regarded as the golden standard for confirmation of tube position [71].However, when pulmonary blood flow is insufficient, such as pulmonary embolism or cardiac arrest, the waveform of ETCO 2 cannot be displayed.In recent years, the application of ultrasound for the positioning of the endotracheal tube has attracted attention [72].Ultrasonography has a unique diagnostic value in monitoring the tube location, with a sensitivity and specificity of up to 0.98 and 0.94 [73,74].Rajan [75] compared the speed and effectiveness of confirming the position of the endotracheal tube between the ultrasound group and auscultation group in normal and obese surgery patients and found that the duration of the ultrasound group did not differ between normal and obese patients and that the accuracy of ultrasound group was better than auscultation group.Color Doppler imaging improves its localization by monitoring tube movement [76].

Diaphragmatic function monitored by ultrasound devices
Mechanical ventilation is one of the most important causes of acute diaphragmatic atrophy and dysfunction, which is closely related to poor clinical prognosis [77].Therefore, it is essential to monitor the diaphragm function during mechanical ventilation.Traditional methods assessing diaphragm function have limitations: low specificity and sensitivity, and invasive manipulation [78].Diaphragm ultrasound is widely accepted in clinical practice for its rapid, accurate and non-invasive characteristics.Diaphragm excursion, diaphragm thickness, and other related indicators are generally used to evaluate its function [79].By monitoring diaphragm function, the metabolism of muscle relaxants can be evaluated to predict whether extubation can be performed safely.

Emergency front of neck access
For "CICV", establishing an emergency front of neck access (eFONA) is the crucial step of emergency rescue [80,81].Lockey reviewed the rescue of the difficult airway after failed intubation in 7256 traumatic patients and found that the success rate of anesthesiologists was significantly higher than that of emergency physicians, internists, and other medical staff, emphasizing the importance of training and rescue experience [82].In addition, several studies have pointed out that teaching and training can significantly improve the success rate of eFONA [83,84].Multiple guidelines for difficult airway management recommended that a "scalpel-bougie-tube" could maximize the success rates of eFONA [4,81].

The training for gastric ultrasound technology
Regurgitation and aspiration are serious perioperative complications [85].Standardized preoperative fasting cannot ensure that every patient achieves a fasting state.Therefore, real-time, accurate and rapid assessment of gastric contents before anesthesia induction is significant for preventing regurgitation and aspiration [86].Gastric ultrasound is a non-invasive and effective method to assess gastric contents.By measuring the cross-sectional area of the antrum, it can accurately provide information on the volume and nature of gastric contents and reduce the risk of regurgitation and aspiration [87].Furthermore, the gastric inflation of gas can be assessed by measuring the cross-sectional area of the gastric antrum, thus assessing the risk of regurgitation and aspiration [88].However, gastric ultrasound technology is not yet universal, and more teaching and training are needed.

The training for multidisciplinary difficult airway team
Although anesthesiologists have a pivotal role in securing the difficult airway, the importance of teamwork cannot be ignored.Anesthesiologists, otolaryngologists, intensivists, nurses and respiratory care practitioners should be the components of the difficult airway response team.Multidisciplinary difficult airway team is associated with improved clinical outcomes compared to unstructured emergency airway management.Implementation of a multidisciplinary difficult airway team is associated with a higher airway securement success rate at the first attempt and a swifter airway securement time [89].

Progress in artificial intelligence for application in difficult airway
Artificial intelligence (AI) is an emerging technology.The combination of computer science, algorithms, and data science can quickly identify the complex relationships between large-scale data.The advantage of the medical field is that it has a large amount of clinical medical data, which is an ideal scenario for using AI.AI is also an important support for promoting medical progress [90,91].At present, AI has been used in the field of difficult airway management, involving airway evaluation before intubation and assistance with intubation [92].Currently, researchers have developed multiple AI ways that can predict difficult airway such as an automated image face analysis model to predict difficult intubation [93], a model to predict difficult intubation by neck circumference and thyroid height [94], a convolutional neural networks-based model to predict difficult laryngoscopy using the features of cervical X-ray in thyroid surgery patients [95], and a model to assess difficult intubation and to classify it only by evaluating the patient's facial photographs [96].
In addition, AI can also play a role during endotracheal intubation.The researchers have developed an endoscopic robot that can guide the endotracheal tube into the trachea [97].Matava developed a procedure that allows real-time feedback on the vocal cord and airway anatomy during video laryngoscopy or bronchoscopy to improve perioperative airway management [98,99].AI is a new subject of computer science and statistics, and it represents a new direction of medicine in the future.

Conclusions
Effective airway management is the cornerstone of perioperative patient safety.Difficult airway remains a common challenge for anesthesiologists, intensivists, emergency physicians and otolaryngologists.Treating patients with difficult airway following an airway management algorithm helps to reduce the incidence of airway-related complications.Anesthesiologists as well as a range of professionals should develop appropriate strategies according to their own experience, patient status, type of surgery and available airway devices to achieve reliable preoperative evaluation and effective intraoperative monitoring to maximize the degree of patient safety.In the future, it is inevitable for anesthesia decisions assisted by AI.