Current trends in emergency airway management: a clinical review

Introduction to the current guidelines of difficult airway management

Three difficult airway guidelines have been published in the past 5 years [1–4]. First, the Difficult Airway Society published the “overview of difficult intubation” guidelines in 2015 (Fig. 1) [5]. Separately, the American Society of Anesthesiologists published difficult airway guidelines in 2022, which provide recommendations on topics such as airway evaluation, preparation for difficult airway management, anticipated or unanticipated difficult airway management, endotracheal tube confirmation, extubation of the difficult airway, and follow-up care [4]. Additionally, these guidelines include algorithms for difficult airway management in both pediatric and adult patients, algorithms for awake tracheal intubation (ATI) and anesthesia, and updated standard and advanced airway management equipment. Finally, these guidelines emphasize awareness of the passage of time and limit the number of attempts made using different devices and techniques during difficult airway management [4].

Fig. 1.

The overview of the 2015 Difficult Airway Society difficult intubation guidelines. Adapted from Frerk et al. [5], with permission from Difficult Airway Society.

Meanwhile, the Canadian Airway Focus Group is updating its 2013 guidelines [2,3]. At this time, two papers have been partially published, and their foci include difficult airway management encountered in an unconscious patient and planning and implementing safe management of the patient with an anticipated difficult airway. ATI may provide an extra margin of safety when an impossible video or direct laryngoscopy procedure is predicted, when difficulty is predicted with more than one mode of airway management (e.g., endotracheal intubation [ETI] and facemask ventilation), or when the predicted difficulty coincides with significant physiological or contextual issues [3].

The ATI guidelines published by the Difficult Airway Society emphasize the importance of ATI, which is not often used by clinicians owing to reluctance despite its relatively high success rate and safety in difficult airway management [1].

Anatomically difficult airways and the emerging role of ultrasound in airway management

According to the 2022 American Society of Anesthesiologists difficult airway guidelines, “measurement of facial and jaw features, anatomical measurements and landmarks, imaging with ultrasound or virtual laryngoscopy/bronchoscopy, three-dimensional printing, and bedside transnasal endoscopy” are adopted to evaluate a difficult airway [4]. As can be seen from the methods for predicting difficult airways, the traditional focus is on the anatomical and pathological structures of the airway. Bedside screening tests commonly used to predict difficult airways include the modified Mallampati test, measurement of thyromental distance, upper lip bite test, interincisor gap, and sternomental distance [6]. None of the existing bedside screening tests have high diagnostic value when performed alone.

Instead of bedside screening tests, which do not have high diagnostic value, difficult airway prediction methods using point-of-care ultrasound, which are already widely used in the field of emergency medicine (EM) and considered essential in the field of resuscitation, are also being studied [7,8]. Airway ultrasound examinations to predict a difficult airway can be divided into anterior tissue thickness domain (TTD), anatomic positioning, and oral space protocols. Here, the tissue thickness is determined using the pre-epiglottal tissue thickness, the anatomic positioning is determined using the hyomental distance, and the oral space is determined using tongue thickness (Figs. 2–4) [9,10]. Specifically, a difficult airway is predicted if the depth from the skin to the epiglottis (TTD) is greater than 2 to 2.5 cm, the hyomental distance is less than 5.29 cm, or the tongue thickness is less than 6.1 cm [11–13].

Fig. 2.

Ultrasonographic image of the suprahyoid view. Suprahyoid view of the anterior neck with a curvilinear probe in the sagittal direction. Deep to the hypoechoic geniohyoid and mylohyoid muscles (star) lies the tongue (arrowhead). The hyomental distance is indicated by the double-headed arrow.

Fig. 3.

Ultrasonographic image of the thyrohyoid view. The thyrohyoid view of the anterior neck with a linear probe in transverse orientation. The pre-epiglottic space (double-headed solid arrow) appears between the epiglottis (single-headed solid arrow) and the thyrohyoid membrane (single-headed dotted arrow). The distance from the skin to the epiglottis is indicated by a double-headed dotted arrow. The strap muscles (arrowheads) are visible superficially to the thyrohyoid membrane.

Fig. 4.

Ultrasonographic image of the cricothyroid view. Cricothyroid view of the anterior neck with a linear probe in transverse orientation. The cricothyroid membrane (solid arrow) overlies the trachea with a noticeable reverberation artifact (dotted arrow) in the tracheal lumen.

In addition to helping to predict difficult airways, ultrasound can be used for airway management in various ways; for example, the presence of vocal cord palsy can be confirmed using ultrasound in patients with stridor by checking for vocal cord movement [14]. If a “double-ring” sign is visible in the esophagus after intubation, it indicates esophageal intubation (Fig. 5) [15]. Ultrasound can also be used to examine the cricothyroid membrane when securing a surgical airway [7]; specifically, confirming the cricothyroid membrane using ultrasound can help identify anatomical landmarks. In a randomized controlled cadaver study by Siddiqui et al. [16], damage to the larynx and trachea as well as complications of incorrect insertion were reduced when using ultrasound compared to the classic digital palpation method, but the time required for final insertion was significantly longer. Considering that cricothyroidotomy is performed urgently, especially in “can’t intubate, can’t oxygenate (CICO)” situations, further research is needed to determine whether cricothyroidotomy using ultrasound is an appropriate modality, even in patients with anatomical structures that are not difficult to navigate in emergency airway management situations [4].

Fig. 5.

Ultrasonographic images of intubation. (A) Endotracheal intubation. The image shows the trachea and the comet tail sign (arrow). (B) Esophageal intubation. Two comet tail artifacts are observed (arrows). The “double-ring” sign is visible.

Physiologically difficult airways

While an anatomically difficult airway is one in which obtaining a glottic view or passing an endotracheal tube is challenging, a physiologically difficult airway is one in which physiological derangements place the patient at greater risk of cardiopulmonary collapse with ETI and conversion to positive pressure ventilation [17]. Physiologically difficult airways include those of critically ill patients with hypoxia, hypotension, metabolic acidosis, right heart failure, or neurological damage; however, healthy patients, pregnant women, obese people, and children can also present with these airways [17–21].

The INTUBE (International Observational Study to Understand the Impact and Best Practices of Airway Management in Critically Ill Patients) study, involving 2,964 critically ill patients from 29 countries undergoing ETI, recorded at least one major adverse peri-intubation event in 45.2% of patients [22]. The predominant complication was cardiovascular instability (42.6%), followed by severe hypoxemia (9.3%) and cardiac arrest (3.1%). Risk factors for major adverse events included lower systolic arterial pressure, administration of a fluid bolus before ETI, a higher heart rate, and cardiovascular instability as a reason for ETI.

Therefore, critically ill patients may require the following treatments according to their physiological characteristics: sufficient preoxygenation and apneic oxygenation before and after ETI, use of vasopressors (including bolus administration), use of drugs that have minimal effects on blood pressure, and administration of fluids to correct hypovolemia and increase venous return during positive pressure ventilation [17,19,22]. Existing guidelines include bolus administration of 500 mL of crystalloid before or during peri-intubation in critically ill patients without heart failure to prevent hypotension that may occur after ETI [23–25]. Although there is insufficient evidence for the administration of a fluid bolus before ETI, an alternative strategy, such as the use of low-dose vasopressors or a combination of fluid and vasopressors, might be beneficial in preventing cardiopulmonary collapse in these patients and needs to be evaluated [26]. In hypoxemia, preoxygenation and apneic oxygenation are important. Also, in patients with shunt physiology due to pulmonary edema or acute respiratory distress syndrome, noninvasive positive pressure ventilation can improve oxygenation and alveolar recruitment [17]. If noninvasive positive pressure ventilation is not applicable, delayed sequence intubation may be considered. In patients with severe metabolic acidosis, it is important to maintain spontaneous ventilation during intubation or with mechanical ventilation; meanwhile, rapid sequence intubation (RSI) should be avoided if possible, but, if deemed necessary, a short-acting neuromuscular blocker such as succinylcholine may be used [17]. Additionally, in order to maintain respiratory compensation after intubation, it is recommended to use the ventilator mode, which allows the patient to maintain their own minute ventilation. Using the pressure-targeted ventilator mode will allow the patient to set the tidal volume and rate received [17]. In right ventricular failure, continuous norepinephrine infusion should be adopted prior to induction to maintain a higher mean arterial pressure than the pulmonary artery pressure in hypotensive patients [17]. The goals of mechanical ventilation include maintaining a low mean airway pressure and preventing hypercapnia, hypoxemia, and atelectasis, which increase right ventricular afterload [27].

Patient positioning

Patient positioning before ETI is important to ensure FPS and to prevent and reduce complications that may occur during ETI. The “sniffing” position helps facilitate successful ETI by keeping the oropharyngeal–laryngeal axis as straight as possible and visualizing the vocal cords. The “ramped” position benefits preoxygenation by maintaining the functional residual capacity and reduces the risk of aspiration. However, there are conflicting results on the “ramped” position. Turner et al. [45] reported that “ramped” position increased FPS rate, while Semler et al. [46] said that the "ramped" position actually increased the procedural difficulty of ETI and consequently decreased FPS rate. Meanwhile, a large retrospective study reported that a combination of both positions reduced the rates of complications, including desaturation [47].

Preoxygenation and apneic oxygenation

Adequate preoxygenation is essential for patient safety during emergency airway management. If sufficient preoxygenation is achieved, it prolongs the safe apnea time and prevents complications, such as hypoxemia, in susceptible patients. According to the previous study, preoxygenation was performed for three groups categorized by oxygen saturation by pulse oximetry (SpO₂) level, as follows: a low-risk group (SpO₂, 96%–100%), who received non-rebreather masks with maximal oxygen flow rate; a high-risk group (SpO₂, 91%–95%), who received non-rebreather masks or continuous positive airway pressure or bag valve mask (BVM) treatment with positive end-expiratory pressure; and a hypoxemic group (SpO₂, <90%), who received continuous positive airway pressure or BVM with positive end-expiratory pressure therapy [48]. In general, supplemental oxygen delivered via a nasal cannula or mask is used for preoxygenation; however, this approach is not sufficient for critically ill patients, such as those with hypoxemic respiratory failure. The FLORALI-2 study compared noninvasive ventilation and high-flow nasal cannula oxygen for preoxygenation in patients with hypoxemic respiratory failure [49]. There was no significant difference in the incidence of severe hypoxemia during ETI between the two groups; however, subgroup analysis suggested that patients with a PaO₂ to fraction of inspired oxygen ratio of <200 gleaned a potential benefit from the use of noninvasive ventilation [49].

Apneic oxygenation is the continuous supply of oxygen via a nasal cannula during apnea following the administration of an NMBA, extending the safe apnea time. One study found that there was no benefit to apneic oxygenation after adequate preoxygenation before ETI in the intensive care unit [50]. However, a review paper on apneic oxygenation during intubation in the ED reported that apneic oxygenation lowered the occurrence of desaturation and increased the FPS rate [51]. This may be because the ED is an uncontrolled environment in which the patient is unstable and preoxygenation or preparation time is limited, and, in this situation, apneic oxygenation may be helpful.

Induction agents for sedation

The choice and dosage of drugs for pretreatment, induction, and paralysis in RSI are critical factors. This decision must be made individually, taking into account not only the drug's pharmacokinetic profiles and side effects but also patient factors like the indication of ETI, hemodynamics, and underlying disease [52]. Drugs commonly used for RSI are listed in Table 1.

Table 1.

Commonly used drugs during rapid sequence intubation

Although atropine and lidocaine have also been used as pretreatments before the induction in RSI, there is currently no obvious benefit associated with either. However, when it is necessary to prevent an increase in intracerebral blood pressure or cardiovascular hyperactivity before ETI, low doses of fast-acting opioids can be administered as pretreatment. Fentanyl is a short-acting synthetic opioid that can be used to suppress the increase in sympathetic tone 3 to 5 minutes before induction [53].

Among the induction agents, ketamine acts on various receptors to maintain the patient's respiratory effort and induces hypnosis in addition to its sedative and analgesic effects. For this reason, it is used for “delayed sequence intubation or medication-assisted preoxygenation,” which is a method of administering an NMBA with a significant delay after induction. When performing ETI in uncooperative or agitated patients, there is an advantage to patient safety by ensuring sufficient preoxygenation time after ketamine administration [54]. However, the use of ketamine has some controversies and adverse effects; for example, there is controversy about the use of ketamine in patients with head trauma or traumatic brain injury due to the risk of increased intracranial pressure [55,56]. However, several clinical studies have not recorded significant increases in intracranial pressure when using ketamine, and it is widely considered safe for use in this patient population [57,58]. Additionally, ketamine is suggested as an induction agent for patients with shock and those at risk of postintubation hypotension due to its effect of increasing blood pressure, but there are also claims that it actually increases the incidence of postintubation hypotension [59,60]. Ketamine also maintains the patient's respiratory effort; however, it should be noted that high doses can cause respiratory depression or apnea [61]. Other adverse effects that may occur include nystagmus, sialorrhea, and emergence phenomena [52].

Etomidate has a rapid onset and a short duration of action, making it an ideal RSI induction agent. Additionally, it does not cause hypotension or cardiovascular instability; therefore, it is frequently used during ETI in critically ill patients [62]. One side effect is that it may cause temporary adrenal insufficiency, as etomidate acts as a selective adrenocortical 11 β-hydroxylase inhibitor. For this reason, some have raised the issue that etomidate should be avoided in patients with sepsis, but whether avoiding its use is clinically meaningful remains controversial [63]. In addition, there is also evidence that hydrocortisone supplementation reduces the risk of adrenal insufficiency when etomidate is administered for ETI to patients with septic shock [64].

Propofol causes vasodilatation and cardiac depression and may provoke hemodynamic instability when used as an induction agent. This trend is particularly evident in patients with impaired cardiac function and hypovolemia. If propofol needs to be used in these patients, titrating the dose and/or ensuring hemodynamic optimization using vasopressor agents and judicious fluid administration may prevent significant hemodynamic perturbations [65].

NMBAs for paralysis

The choice of NMBAs in RSI is determined by a variety of factors, and, except in some special situations, their use is recommended. NMBAs reduce upper airway muscle tone and facilitate facemask ventilation and supraglottic airway (SGA) insertion. They also optimize the intubation conditions by reducing the number of ETI attempts [66].

Succinylcholine, a depolarizing neuromuscular blocker, boasts a rapid onset and short duration of effect and is ideal for RSI. However, despite its advantages, succinylcholine is associated with several life-threatening conditions. Succinylcholine causes depolarization of upregulated nicotinic acetylcholine receptors, resulting in a shift of intracellular potassium into the plasma. This process can lead to acute hyperkalemia in susceptible patients [67]. Patients with reduced plasma cholinesterase activity due to burns or trauma within the last 24 to 72 hours and myopathy should avoid the use of succinylcholine for paralysis [67]. Succinylcholine has also been associated with malignant hyperthermia, a rare yet fatal condition [68].

Rocuronium and vecuronium, both nondepolarizing neuromuscular blockers, are not associated with life-threatening complications but have a longer duration of action, and their effects are dose dependent. Sugammadex, a modified γ-cyclodextrin, binds and encapsulates steroidal NMBAs in the plasma, thus reducing their concentration and rapidly terminating their blocking effect [69]. When an RSI dose of rocuronium was used, the effects of the neuromuscular block were reversed 3 minutes after the administration of sugammadex (16 mg/kg).

There are few papers comparing the efficacy of NMBA. According to a 2015 Cochrane review, succinylcholine provides better intubating conditions compared to rocuronium [70]. However, this study only involved intubated patients in the operating room or intensive care unit and did not include ED patients. In a later study evaluating FPS by using the National Emergency Airway Registry database, there was no difference in FPS rates between succinylcholine and rocuronium (87% vs. 87.5%) [71]. Elsewhere, although there was no difference in mortality among patients with low-severity head injuries, succinylcholine led to increased mortality compared to rocuronium in patients with high-severity head injuries [72].

Anesthesia and sedation for ATI

ATI is considered when an anatomically or physiologically difficult airway is expected and/or when eliminating spontaneous breathing threatens patient safety. In ATI, an NMBA is not administered, and a lower dose of the induction agent is used compared to that in RSI. Appropriate airway anesthesia and judicious sedation are important to reduce patient anxiety and discomfort during the procedure, and airway patency must be maintained. A backup plan should be prepared in case of failure.

The success of ATI depends upon the effective application of local anesthetic to the airway. Local anesthetics can be administered by topical application or injection into the airway, either separately or simultaneously. Following topical application, airway secretions may dilute the local anesthetic or wash it away from areas requiring anesthesia. Adding glycopyrrolate is useful because it reduces airway secretions; a dose of 0.1 to 0.2 mg/kg should be administered at least 3 to 5 minutes before the application of local anesthetic [52]. Appropriate application of local anesthetic allows adequate airway assessment in patients without the administration of sedatives. Lidocaine is the most commonly used local anesthetic and has low cardiovascular effects and systemic toxicity [73].

Generally, in the ATI of critically ill patients, sedatives are used after optimization with a local anesthetic. Oversedation may cause a loss of airway patency, hypoxia, aspiration, or cardiovascular instability; therefore, sedation must be adjusted with careful monitoring [1]. Remifentanil and dexmedetomidine are also associated with greater patient satisfaction and a lower risk of oversedation and airway obstruction when used in ATI [74]. When considering combination therapy rather than a single drug for sedation, remifentanil and midazolam can be used [1].

BVM and orotracheal and nasotracheal intubation in prehospital setting

In most scenarios, it is advisable to initiate basic airway and ventilatory support as a preliminary step before considering advanced airway management techniques. Generally, BVM ventilation and non-rebreather masks are suitable for patients requiring airway or ventilatory support. BVM ventilation should be promptly administered to patients with apnea or hypoventilation. Whenever feasible, BVM ventilation should be performed using a two-rescuer technique while considering the use of the oropharyngeal or nasopharyngeal airways to optimize the procedure [85]. Nasal cannulas should be avoided in patients with compromised airways [86].

Adjustments in patient or rescuer positioning aimed at optimizing the airway management process, such as raising the bed or stretcher to align with the rescuer's waist level or positioning the patient supine, can affect the quality of laryngoscopic views during airway management. Notably, well-documented techniques such as the “sniffing position” and head-elevation methods, extensively discussed in the otolaryngology, anesthesia, and EM literature, are widely recognized for their capacity to enhance glottic exposure [87,88]. In addition, elevating the backboard in cases involving trauma patients requiring immobilization and the use of prone or kneeling positions when intubating patients situated on the ground can potentially improve intubation performance [89,90].

Numerous techniques are currently available for ETI. Techniques such as the "backwards–upwards–rightwards pressure" method and external laryngeal manipulation have been documented to improve vocal cord exposure during orotracheal intubation in the prehospital setting [91,92]. In cases where orotracheal approaches are not feasible, owing to factors such as clenched teeth, tongue edema, or the need for a seated position, nasotracheal intubation can be a viable alternative [93,94]. Additionally, some small prehospital studies have reported the use of gum elastic bougie [95,96]. Prehospital intubation plays an important role in maintaining an intact airway and ventilation during transport. However, recent studies have proposed that trauma patients, especially those with a traumatic brain injury, have a poor prognosis when prehospital intubations are performed [97,98]. In traumatic brain injury patients, it is believed that ETI induces hyperventilation and worsens neurological injury compared to BVM [97]. Therefore, prehospital intubation for traumatic brain injury should be performed considering various confounding factors.

SGA in the prehospital setting

The choice of SGA as either a primary or secondary prehospital airway intervention depends upon various factors, including clinician expertise and the specific circumstances available for airway interventions. Previous studies have reported the use of SGAs as rescue airway devices in cases where ETI attempts have proven unsuccessful, with varying success rates [99,100]. However, as SGAs do not provide full protection against gastric insufflation, regurgitation, or aspiration, it is imperative to exercise caution during use and to maintain vigilant observation. To enhance safety, continuous-waveform end-tidal CO₂ monitoring should be employed. Additionally, it is advisable to limit SGA-insertion attempts to a maximum of three for optimal patient care [5].

Airway management in cardiac arrest

The choice of an airway management strategy in cases of out-of-hospital cardiac arrest (OHCA) can be influenced by multiple factors, including the circumstances of the arrest, the patient's condition, the stage of resuscitation, the available equipment, and the skill level of the emergency medical service clinician [101,102]. Additionally, the timing of different airway techniques may vary, with the significance of oxygenation and ventilation increasing as resuscitation efforts continue. According to the US Agency for Healthcare Research and Quality 2023 updated guideline, BVM, SGA, and ETI are recommended with the same level of evidence for prehospital airway management [103]. While there are limited data available, it is noteworthy that patient-centered outcomes in recent studies appear to be better with ETI or SGA than with BVM among adult cardiac arrest cases [104,105]. However, because these were retrospective studies, it is still too early to generalize, so additional research is needed. Two methods are used for chest compressions and ventilation during cardiopulmonary resuscitation (CPR), including synchronous CPR, which consists of a cycle of 30 uninterrupted chest compressions with a cycle of two ventilations, and asynchronous CPR, which consists of uninterrupted ventilation with advanced airway management. Interruption of ventilation due to synchronous CPR can significantly affect the chest-compression fraction. In contrast, advanced airway management using ETI or SGA is an asynchronous CPR method that allows ventilation without interruption of chest compression and may have shown a better prognosis in cases of OHCA because it can increase the chest-compression fraction [106]. However, since this has not yet been established, additional well-designed studies will be needed for clinical application.

While advanced airway management is an important aspect of OHCA treatment, emergency medical service clinicians should prioritize interventions known to enhance patient outcomes during cardiac arrest resuscitation, irrespective of the chosen airway management approach. These include ensuring high-quality chest compressions (e.g., optimizing the rate, depth, recoil, and chest-compression fraction), prompt administration of defibrillation for shockable rhythms, and addressing reversible causes of the arrest. Maintaining high-quality CPR while managing airways is crucial. This can be achieved by carefully timing airway interventions, avoiding interruptions in chest compressions when placing advanced airways, and minimizing the potential harm associated with ventilation following the insertion of advanced airways [107].

Emergency invasive airway access

In the 2015 Difficult Airway Society guidelines overview released in the United Kingdom, the process of securing an open airway in front of the neck for rapid and appropriate oxygen supply in CICO situations is referred to as Emergency Front-of-Neck Access (eFONA) [110]. The eFONA techniques include scalpel cricothyroidotomy, cannula cricothyroidotomy, surgical tracheostomy, and percutaneous tracheostomy.

The American Society of Anesthesiologists introduced practical guidelines for difficult airway management in 2022, which describe invasive airway techniques, including surgical cricothyrotomy, needle cricothyrotomy using pressure-regulated devices, large-bore cannula cricothyrotomy, and surgical tracheostomy. Additionally, elective invasive airway techniques, including retrograde wire–guided intubation, percutaneous tracheostomy, rigid bronchoscopy, and extracorporeal membrane oxygenation, are discussed [4].

The eFONA or an emergency invasive airway creation is feasible in all CICO cases, and there are no absolute contraindications. Relative contraindications include possible or known tracheal surgery, a fractured larynx, laryngotracheal disruption, and pediatric status [111]. Cricothyrotomy is a relative contraindication in children 0 to 12 years because of the funnel shape of the pediatric airway and a theoretically increased risk of subglottic stenosis [112].

Complications of emergency invasive airways

The rate of complications varies across clinical scenarios, levels of education, procedural settings, and studies, ranging from 0% to 54% [113]. Anatomically, because the superior thyroid artery and vein cross the cricothyroid membrane, bleeding is the most common complication of this procedure. Some degree of bleeding is inevitable during the procedure; however, if severe bleeding persists, applying pressure to the area or attempting hemostasis with gauze packing may be necessary. Other potential complications include injury to the thyroid, cricoid, or tracheal cartilage; airway perforation; creation of a false tract, and the possibility of infection [114,115]. Long-term complications may include subglottic stenosis and changes in voice quality [116].