ABSTRACT
Airway management is an essential life-saving skill that has to be mastered by every anesthesiologist. Complications arising from the unanticipated difficult airway or failed tracheal intubation remain a leading cause of anesthetic morbidity and mortality despite recent developments in airway management strategies. Its incidence can vary between 1.5% and 20% depending upon the patient, healthcare personnel, and environmental factors. Identification and management of difficult airways are of paramount importance to improve the outcomes in such situations. Many novel techniques, concepts, and devices have been introduced into clinical practice to reduce the morbidity and mortality associated with difficult airway. This chapter summarizes the role of these technologies in securing the airway.
KEY POINTS
- Airway management is an essential skill for anesthesiologist.
- Novel techniques, concepts, and devices have been introduced into clinical practice to reduce the morbidity and mortality associated with difficult airway.
- Imaging technologies, such as X-ray, computed tomography, magnetic resonance imaging, and ultrasonography, can assist in the difficult airway assessment and prediction.
- Artificial Intelligence-based technologies have been introduced in airway management.
- Physiological factors of difficult airway have taken precedence over anatomical factors.
- Apneic oxygenation, delayed-sequence intubation, cannot-intubate cannot-oxygenate, and complete ventilation failure have important implications in patients of anatomical as well as physiological difficult airway.
- Several new videolaryngoscopes, endotracheal tubes, supraglottic devices, fiberless videoendoscope, and devices for special scenarios have come into clinical practice.
INTRODUCTION
Airway management is an essential skill that an anesthesiologist has to acquire and exercise every day. No wonder it has been the subject of much research and innovation.2
The term difficult airway is a concept well appreciated by airway managers around the world. In its broadest sense, it can be defined as challenges associated with any of the four components of airway management, i.e., face mask ventilation, supraglottic airway insertion, tracheal intubation, and emergency front-of-neck airway (FONA) access.1 It is notable that difficulty with any of these management methods is primarily related to anatomic features that make these techniques procedurally challenging. Thus, as presently understood, the term difficult airway essentially denotes an anatomically difficult airway. Technological advances, such as flexible intubation scopes and rigid videolaryngoscopes (VLs), have substantially improved our ability to manage such cases safely. Recent advances in technology have made laryngoscopy less dependent upon a direct line of sight to achieve tracheal intubation. However, the success and safety of these devices depend on the way they are used.2 In addition, improvements in methods of pre-oxygenation, such as high-flow nasal oxygen and noninvasive positive-pressure ventilation (NIPPV) have allowed a longer duration of apnea without desaturation, allowing more time to manage the airway safely.3 This chapter summarizes some of the recent trends and technologies in airway management.
ASSESSMENT METHODS AND TOOLS
Many airway assessment methods such as history, physiopathological factors, individual and group indices, and bedside tests are routinely used, but they have very low sensitivity. So, it is necessary to optimize the prediction process and accuracy. Recently, imaging, artificial intelligence (AI), and face recognition technology as an aid to the assessment of difficult airways are described and are significantly better than the traditional assessment methods.4
Imaging Technology in Airway
X-ray, computed tomography (CT), magnetic resonance imaging (MRI), ultrasonography (USG), and other imaging techniques can assist in the assessment and prediction of the difficult airway. However, they also come with many drawbacks, such as radiation exposure, high cost, sophisticated equipment, and medical staff burden, which preclude them from being widely used in clinical practice.
Neck X-ray
Skull lateral X-ray shows the bony changes in the airway anatomy, such as linear distance from the mandibular alveolar line to the hyoid bone, from the inner edge of the mandible to the hyoid bone, and from the chin vertex to the attachment of the apex of the lower incisor and the hyoid bone. These imaging indices reflect the tongue enlargement and have been better predictors than the Mallampati classification, with sensitivity and specificity of 0.78 and 0.88, respectively.5 Although X-rays can delineate bony and structural abnormalities in the airway, they cannot be used clinically as a routine tool to evaluate difficult airways, considering its adverse hazards, such as radiation.3
Computed Tomography Scan
Airway CT scan with three-dimensional (3D) reconstruction better shows the airway anatomy and its physiological and pathological changes. The airway length is expressed as the vertical distance from the hard palate to the hyoid bone in the median sagittal plane, and the volume is stated as the cross-sectional area from the hard palate to the hyoid bone. It is found that the airway length, but not the volume, is an important variable in the prediction of apnea and hypopnea indices in patients with severe obstructive sleep apnea (OSA). Besides these, age and tongue area are also independent risk factors for difficult laryngoscopy, and when the best-predicted cutoff point of the tongue area is taken as 2,600 mm2, PPV is 37% and NPV is 89%.4 Accuracy of the tongue area to predict difficult laryngoscopy is 65%.
Three-dimensional CT scan: 3D airway CT assists in the construction of a 3D printed model of the patient's airway for evaluation and planning. A good correlation has been shown between predicted nasal intubation difficulty on CT (nasal diameter ≤6.3 mm indicates nasal intubation difficulty) and the actual nasal intubation difficulty.6 The negative predictive value and positive predictive value of CT predicting in nasal intubation difficulty were 90.7% and 71.4%, respectively. 3D CT can be used as an additional evaluation tool for patients with poor airway conditions. However, there are limitations to the routine use of CT for difficult airway assessment, such as radiation risk, medical burden, equipment requirements, human and material resources, and the long operation time and large space requirements.
Neck MRI
Magnetic resonance imaging avoids the radiation hazards associated with other imaging technologies, such as CT and X-ray, and has advanced 2D and 3D features to detect structural changes in the soft tissues of the airway. Position of the vocal cords predicts difficult laryngoscopy. The vocal cord in the difficult airway is close to the cranial pyramid, while those in the easy group are located in the fifth cervical spine.7
Ultrasonography
With the increasing availability of portable ultrasound, role of USG imaging in airway mangement has been greatly enhanced. It helps in rapid assessment of the airway anatomy in the operation theater, intensive care unit (ICU), 4emergency department, and remote environment. The thickness of the anterior epiglottis, the thickness of the anterior hyoid bone, the thickness of the anterior vocal cord, the thickness of the lateral pharyngeal wall, and the distance from the base of the tongue to the skin have been described in the literature as predictors of the difficult airway. Other indicators include tongue thickness and volume, oral exposure ratio, and hyomental distance ratio (HMDR).
Anterior neck soft tissue: It can be measured at the level of the hyoid bone, epiglottis, cricothyroid membrane, thyroid isthmus, suprasternal notch, and vocal cords. To standardize the measurements, the patient must lie supine and put the head in a neutral position. Generally, a 6–13 MHz high-frequency linear transducer is used for scanning. Neck circumference >50 cm and a soft-tissue thickness >28 mm at the level of vocal cord indicate a difficult laryngoscopy. Other parameters, such as median distance from skin to the epiglottis 25.4–27.5 mm (sensitivity 82%; specificity 91%), epiglottic area 5.04 cm2 (sensitivity 85%; specificity 88%), and thickness of the anterior hyoid bone soft tissue at a cut-off value of 1.28 cm (sensitivity 85.7%, specificity 85.1%), are also found to be significantly related to the difficult airway. Inferior positioning of hyoid bone and the thick pharyngeal wall is found to be associated with an increased risk of OSA. Even though there is plenty of variation in the cutoff value of the anterior cervical soft-tissue thickness at different levels and even at the same level, most studies believe that it is an independent risk factor.
Tongue measurements: Tongue thickness, cross-sectional area, tongue volume, and its ratio to oral cavity volume are found to be predictors of the difficult airway. To standardize the assessment, measurements are taken in supine, mid-sagittal plane, closed mouth, relaxed tongue, and tip gently touching the incisors and makes no sound. Tongue volume is found to have the greatest diagnostic power for difficult airways.8 Tongue thickness of >6.1 cm is labeled as an independent risk factor for predicting difficult intubation, with sensitivity and specificity of 0.75 and 0.72, respectively.
Hyomental distance with derived ratio: HMDR refers to the ratio of the HMD measured in the extended position to the distance measured in the neutral position. An increase of ≤ 20% in HMD during hyperextended position or a ratio of sniffing to neutral position ≤1.06 predicts difficult intubation.
Other measurements: A few more indices assess difficult airway, such as condyle translation and epiglottic distances ratio. Condyle translation displacement of ≤10 mm during the opening of the mouth indicates difficulty in laryngoscopy. The ratio of pre-epiglottic soft-tissue space to the epiglottis to the midpoint of vocal cord (PE/E-VC) may be an independent risk factor for predicting difficult laryngoscopy with a sensitivity of 67–68%. However, some studies have shown that the sensitivity and specificity of this parameter vary significantly in different studies, and it is not recommended as an indicator.95
Artificial Intelligence
The Difficult Airway Management Guidelines of all societies emphasize the importance of early identification and planning of difficult airways.4 Artificial intelligence (AI) predicting difficult airways is a field worthy of further researches. Recent literature shows that AI facial recognition technology is expected to replace experienced doctors in the fully automatic intelligent assessment of difficult airways and optimize the clinical work to reduce the risk of misdiagnosis of difficult airways.
AI Combined with Imaging-assisted Modeling in Difficult Airway Assessment
It has been found that the predictive value of the computer-aided model constructed using the random forest algorithm was significantly better than the currently latest clinical predictive scale model and the predictive model constructed by traditional logistic regression analysis. By analyzing and processing facial images through machine learning algorithms, critical points of the face are analyzed, considering the discriminative ability of each parameter through logistic regression.
AI Combined with Facial Images in Difficult Airway Assessment
This is a fully automated system to collect a patient's facial photo in different scenarios, such as mouth open, tongue stretched out, and head stretched vertically and laterally rotated. Then machine learning algorithms were used to analyze and process the facial photos to predict difficult airways.
Airway Assessment in Intensive Care Unit
Full airway assessment in a critically ill patient is often impractical; however, a basic airway assessment, such as previous records, body habitus, external predictors, and USG can be performed safely. In a multicentric study, De Jong et al. developed and validated a score (MACOCHA) to predict the difficult airway in critically ill patients (Table 1). The score included seven parameters, out of which four were patient-related, two were pathology-associated, and one was operator-related. Each parameter has been given one point except for Mallampati and OSA, with five points and two points each. The difficulty of intubation increases as the score increases from 0 to 12, but a cutoff value of ≥3 is taken as a predictor of difficult intubation in critically ill patients. This test has a sensitivity of 73% and has not been validated for VLs.106
RECENT CONCEPTS IN AIRWAY
Physiologically Difficult Airway
It is defined as the airway in which reduced physiological reserve pertaining to a disease process places the patient in a potentially life-threatening situation during intubation and transition to mechanical ventilation. The distinction between anatomical and physiological difficult airways is crucial because awake intubation is the gold standard in predicted an anatomically difficult airway. In contrast, awake intubation can worsen the already deranged physiology of a critically ill patient in case of inadequate blunting of airway reflexes, such as sudden rise in intracranial pressure or cardiac ischemia in the predisposed individuals. Various types of physiologically difficult airways have been described in the literature, but the presence of hypoxemia and hypotension with a shock index of ≥0.9 increases the risk of cardiac arrest by almost fourfold (Table 2).10
Preoxygenation
In certain patients with a physiologically difficult airway, such as having an increased alveolar-arterial (A-a) gradient, the traditional method of preoxygenation is ineffective. Therefore, these patients should be preoxygenated using NIPPV [inspiratory pressure 5–15 cm, positive end-expiratory pressure (PEEP) 5 cm, tidal volume 6–8 mL/kg] in a head-up position or with a high-flow nasal cannula with oxygen flow at 70 L/min in patients of moderate hypoxemia. It decreases shunt fraction by recruitment of collapsed alveoli.107
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Apneic Oxygenation
It is defined as providing a continuous supply of oxygen from the beginning of apnea until the initiation of positive-pressure ventilation. The constant uptake of oxygen with just 8–20 mL/min of CO2 diffusion in alveoli creates a pressure gradient between the upper airway and alveoli, leading to the bulk flow of oxygen-rich gases. However, patency of the airway is mandatory for this phenomenon to occur. Dead-space ventilation and cardiogenic oscillations further augment apneic ventilation. With effective preoxygenation, continuous high-flow oxygen, and patent airway, PaO2 9can be maintained for >100 mm Hg for >100 minutes, but with severe hypercapnia and acidosis (PaCO2 increases 5 mm Hg in the first minute followed by 3 mm Hg per minute). Complications of prolonged apneic oxygenation include hypercarbia, acidosis, hyperkalemia, raised intracranial pressure, and pulmonary hypertension. Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) creates a continuous positive airway pressure (CPAP) of around 7 cm H2O, causing the opening of small airways and consequent CO2 clearance. The rise of CO2 is approximately one third of the expected values.10
Delayed-sequence Intubation
Modified rapid-sequence intubation (RSI) with cricoid pressure (can be released in case of inadequate visualization) and adequate denitrogenation are the techniques of choice in emergencies. However, altered, agitated, and noncooperative patients cannot be adequately preoxygenated, leading to desaturation during the period of apnea. To overcome this situation, Weingart11,12 came with the concept of “delayed-sequence intubation (DSI)”, where the induction agent is temporally separated from the administration of the muscle relaxant to allow adequate preintubation preparation. A small dose of ketamine is used to sedate the patient to achieve adequate preoxygenation before paralysis. This technique of DSI is shown to have drastically improved the safe apnea time in critically hypoxemic uncooperative patients. Ketamine sequence intubation (KSI), also called graded sequence intubation (GSI), is another technique proposed by Reuben Strayer. It is similar to DSI, except that neuromuscular blockade is not used.10
Cannot-intubate Cannot-oxygenate
Traditional Can't-intubate Can't-ventilate (CICV) has been now changed to “cannot-intubate cannot-oxygenate (CICO)” situations when all efforts to oxygenate the patient using a facemask, supraglottic airway device (SAD), and tracheal intubation have failed, and the patient is consuming oxygen faster than it can be delivered and is at risk of imminent hypoxic brain injury, cardiac arrest, and death. The quickest method of oxygenating a patient in this situation is to recognize the CICO emergency and perform an emergency FONA, usually in the form of a cricothyrotomy. CICO emergencies are rare and are associated with significant mortality and morbidity, especially if there is a delay in recognizing the situation or performing cricothyrotomy.13
Complete Ventilation Failure
In a situation when the airway manager is unable to face mask ventilate, intubate a patient, and insert a SAD, the Difficult Airway Society (DAS) had 10labeled the condition as CICV in 2008, which was subsequently coined as CICO in 2015. However, the All India Difficult Airway Association guidelines define complete ventilation failure (CVF) as a trigger for surgical airway. This is a situation where intubation, ventilation using SAD, and face mask have failed after giving the best attempt, even if oxygenation may be maintained. They recommend proceeding to emergency cricothyroidotomy when there is CVF even if oxygenation could be maintained, not only when hypoxemia sets in. During these events, nasal oxygen insufflation should continue. Before declaring CVF, a final attempt at mask ventilation should be made after ensuring complete muscle relaxation. This will give the best chance for optimizing mask ventilation and also create good operating conditions for cricothyroidotomy.14
NEWER DEVICES IN AIRWAY MANAGEMENT
Videolaryngoscope
Since the development of GlideScope in 2001, several VLs have flooded the market. Despite few limitations associated with VLs, these devices have been shown to improve the laryngeal view and success rate of tracheal intubation. They have now become the first backup technique after failed intubation attempts. Most of the difficult airway guidelines emphasize the role of VL in the management of both anticipated and unanticipated difficult airways and even as the first choice in COVID patients.15–17
Of the various VLs available, each is unique in design. They can be categorized into three main types—one with the standard Macintosh-shaped blade, one with the angulated blade, and one with a channel for tube passage. The channeled VL has a tube slot to deliver the endotracheal tube (ETT). In channeled VL, the tube cannot be manipulated independently and hence, the whole of the scope blade must be directed toward the glottis. The larynx may not be directly visible in VL having blades angled at 60° upward (e.g., GlideScope, McGrath). A stylet is required while intubating with these blades. On the contrary, Storz DCI and C-MAC blades are similar to the conventional Macintosh, which does not require stylet during intubation (Flowchart 1).
Endotracheal Tubes
VivaSight
VivaSight are new-generation single- and double-lumen tubes with an integrated high-resolution camera. The integrated camera technology makes the placement of the tube fast and effective and provides continuous visual monitoring of ETT and endobronchial blocker placement throughout the procedure. VivaSight-SL is indicated for use during routine and difficult intubation procedures. Together with the single-use endobronchial blocker and VivaSight-EB, lung isolation can be fast and effective, ensuring that dislocations are easily detected and corrected.1811
TaperGuard™
Endotracheal tube designs to decrease the risk of ventilator-associated pneumonia (VAP) include supraglottic suctioning or modifications of the cuff shape. The TaperGuard™ (Covidien, Boulder, CO) ETT has a tapered, polyvinyl chloride cuff designed to reduce microaspiration around channels that form with a standard barrel-shaped cuff. Multiple independent and manufacturer-sponsored laboratory and clinical trials have demonstrated the decreased passage of fluid or dye around the ETT compared with conventional barrel-shaped cuffs. But recently, clinical evaluations in relatively small studies of unselected patient populations have failed to demonstrate a decrease in the incidence of postoperative pneumonia.19
Suction above Cuff Endotracheal Tube
Peritubal leak and aspiration of the oropharyngeal secretions are primarily responsible for the occurrence of VAP.20 Tracheal tubes with a suction port above the cuff can assist in reducing the rate of VAP. As the Suction Above Cuff Endotracheal Tube (SACETT) facilitates the suctioning of excessive secretions in the subglottic area, it is preferred in ICUs as a modality to decrease VAP.12
AI-assisted ETT Placement
Misplaced ETT is a common finding in nonoperating room-intubated patients. To overcome this issue, GE Healthcare has developed AI-installed X-ray machines that can detect and alert the physician about any ETT malposition along with other radiological abnormalities, such as pneumothorax and collapse, immediately.21
Automated Cuff Pressure Regulator
Tracoe Smart Cuff ManagerTM monitors and automatically regulates the internal pressure of high-volume low-pressure cuffs of tracheostomy tubes and ETTs. It is associated with a reduced incidence of cuff pressure underinflation or overinflation than routine manual intermittent correction.22
Supraglottic Airway Devices
Supraglottic airway devices with a conduit for blind tracheal intubation are gaining popularity as a bridge connecting ventilation and intubation in all genres of patients. Laryngeal mask airways (LMAs) with intubation conduit are useful and are also recommended by “All India Difficult Airway Association” guidelines 2016.14
LMA Gastro
This second-generation, silicone-based, cuffed LMA offers an additional and separate channel for the passage of instruments (such as an endoscope) of up to a width of 16 mm in diameter. While the majority of gastrointestinal endoscopies are performed under conscious sedation by nonanesthesia personnel, there is a shift toward deep sedation or general anesthesia for advanced procedures and interventions, especially for patients with higher American Society of Anesthesiology (ASA) physical status of ≥3, high BMI, OSA, and severe comorbidities that require the presence of an anesthesiologist. New LMA Gastro™ Airway demonstrates good efficacy in adults as well as pediatrics without any detrimental or harmful side effects.23–25
LMA Protector
The LMA Protector™ (Teleflex Medical, Co. Westmeath, Ireland) is a single-use, second-generation SAD designed with a large volume conduit with gastric access and a fixed curved structure to facilitate insertion. The medical-grade silicone inflatable cuff has been built for a primary oropharyngeal seal and distal esophageal seal, potentially improving the ability of the LMA Protector™ in providing positive-pressure ventilation and preventing aspiration. It has a dual gastric channel, silicone cuff pilot technology, and second seal with an intubation facility.2613
Endoscope
Flexible Intubation Video-endoscope
Flexible Intubation Video Endoscope (FIVE) from KARL STORZ offers a single-use solution that is remarkable in every respect. It is suitable for airway inspection, suction of bronchial mucus, and foreign body removal and biopsies. It is compatible with the multifunctional C-MAC® monitor.
Surgical
Front-of-neck Airway Access
Various new devices have been introduced in the market for easy surgical access. A brief description of the recent advances in the surgical airway is shown in Table 3.27
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DEVICES FOR SPECIAL AIRWAY SITUATIONS
Intubation Box
The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus continues to spread, infecting millions worldwide. To minimize peri-intubation healthcare worker infection risk from COVID-19, an additional protection barrier known as an aerosol box or intubation box was introduced. Although hospitals worldwide have used various prototypes of aerosol boxes, their effect on intubation remains unclear. It has been observed that the time to intubation was significantly longer when an aerosol box was used than VL alone.28–3015
MADgic® (Teleflex)
The MADgic® Laryngo-Tracheal Mucosal Atomization Device is used to administer medications across the entire upper airway and beyond to prepare the airway for awake intubation. The MADgic® device is not prefilled so the clinician can select the type of medication and exact dosing desired. The malleable stylet enables precise, targeted delivery specific to the patient's anatomy. Various modifications of this device are popular in markets, such as MADgic Airway, combining atomized topical anesthetic and oxygen delivery in a flexible-scope compatible airway. MAD nasal is another device for delivering medications to the nose and throat. The typical particle size delivered is 30–100 µ.
Optiflow THRIVE (Fisher & Paykel)
It is a humidified oxygenation system with heated inspiratory tubing and anatomically designed high-flow nasal cannula. They are used chiefly to provide high-flow oxygen therapy.
SuperNO2VA Nasal PAP Ventilation (Vyaire Medical)
This nasal mask delivers NIPPV when connected with the ventilatory circuit or resuscitation bag. It is ideal for patients with OSA, morbid obesity, cardiopulmonary disease, and patients requiring endoscopy under mild sedation.
O2-MAXTM Trio (Pulmodyne)
It is a disposable CPAP device with an integrated nebulization facility. It is provided with three FiO2 (30, 60, 90) and PEEP (2.5–20 cm H2O) settings.
Ventrain Device (Ventinova Medical)
It is a manually operated ventilation device through a transtracheal catheter during CICO situations. Ventilation is based on bidirectional flow. It supplies not only oxygen during inspiration but also suction air during the expiratory phase. It requires a high-pressure oxygen source with a pressure compensated regulator.
Chin-UP® Airway Support Device (Dupaco)
It is a hands-free airway support device used to lift and hold the patient's chin during monitored anesthesia care (MAC) and total intravenous anesthesia.16
Troop Elevation Pillow (CR Enterprises-Mercury Medical)
It is a foam-based positioning device that quickly achieves laryngoscopic position. It is beneficial for patients with difficult airway and obesity. It also offers an added advantage of infection control with its barrier covers.
Robotic Intubation
It is a short step technologically to replacing the human with sensors to map the airway and guide the intubation stylet into the trachea. Engineers have built such intubating robots that use electromagnetic guidance (Ohio State) and infrared light (Hebrew University).31 In emergencies, the necessity to perform tracheal intubation may occur unexpectedly, infrequently, and under unfavorable conditions. Robotic Endoscope Automated via Laryngeal Imaging for Tracheal Intubation (REALITI) has been developed to enable automated tracheal intubation.32 By using the capacity of REALITI for real-time image recognition and automated distal tip orientation, comparable results have been obtained in anesthetists and lay participants with no medical training in manikin-based study.32 Apart from this remote robot-assisted intubation system (RRAIS), Intu-bot and Kepler intubation systems have also been used.33–35 Despite this, further research and time are required to authenticate and incorporate these in clinical practice.
CONCLUSION
Although many technologies are being introduced every day, we must excel in basic techniques and skills of airway management. Availability of advanced devices and recent innovations ranging from AI to robotic intubation assist anesthesiologists, currently it is difficult to predict if they can replace humans in the near future. However, the pace with which these are being developed, it is very likely that they may surpass human competence in due course of time.
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