Airway management is common in the ICU, yet reported complication rates of this procedure in the critically ill remain unacceptably high [1–4]. Current guidelines and advanced airway tools for difficult airway management have primarily been designed for the operating room environment. The unpredictable and emergent nature of ICU intubations has limited systematic study of airway algorithms and tools in the critical care environment, resulting in significant variation in airway equipment and approach. Recent data, however, have provided better insight into the incidence of difficult airways outside of the operating room, the importance of interdisciplinary critical care team preparation, training, and teamwork, and the application of various advanced airway adjuncts to maximize intubation success and minimize complications in this environment.
THE DIFFICULT AIRWAY
The American Society of Anesthesiologists defines a difficult airway as the existence of clinical factors that complicate ventilation by facemask or intubation by experienced and skilled clinicians . The difficult airway has been blamed as the primary source of airway complications in the ICU, as hypoxemia from inadequate ventilation, difficult tracheal or inadvertent esophageal intubation remains the most common cause of injury lawsuits from anesthesia care.
Difficult airway incidence outside the operating room
Studies have documented an associated mortality of 3% with intubation of a critically ill patient, and a complication rate more than 40 times higher than airway management in the operating room [2–4]. In the largest ICU study to date, Jaber et al.  described complications in 50% of 253 ICU intubations, including severe complications (serious hypoxemia, hemodynamic instability, cardiac arrest or death) in 28%. These data, while reporting remarkably similar rates across institutions and continents, have been challenging to analyze further due to the wide variety of patient populations, acuity, and the diversity of practitioners, experience, and equipment involved in these studies.
In 2011 Martin et al. [6▪▪] reported the incidence of difficult airways and complications during 3423 emergent intubations performed outside the operating room over 7 years. All intubations were performed by senior anesthesiology residents with at least 24 months of intubating experience. The incidence of difficult intubation in this population was 10.3%, with a composite complication rate of 4.2% (Table 1). Although 44% of these complications occurred in the ICU, the proportional rate of complications was significantly less than that associated with intubations performed on a ward (P < 0.001).
DIFFICULT AIRWAY MANAGEMENT IN THE ICU
The Royal College of Anesthetists fourth National Audit Project (NAP4) collected data on advanced airway management and its complications over a 12-month period in National Health Service hospitals throughout the UK [7▪]. Twenty percent (36/184) of airway incidents occurred in the ICU, and 61% of these episodes resulted in death or significant neurologic injury. The NAP4 project separated the contributing factors to ICU airway management complications into patient, staffing, training, equipment, and environmental considerations.
Intubation of critically ill patients presents a number of unique challenges that are not present in the operating room. Most airway management situations in the ICU are emergent, leaving providers with limited time to perform a systematic airway assessment. Physiologic derangements frequently found in ICU patients can make preoxygenation difficult, and hemodynamic instability can limit the choice and dose of commonly used induction and paralytic agents. In NAP4 47% of critically ill patients who suffered an airway complication were obese, a clear risk factor for difficult and failed airway management [7▪,8]. Loss of muscle tone, secretions, and upper airway edema are common, decreasing glottic visualization and successful procedure performance. Blood pressure changes and hypoxemia during intubation can also worsen associated conditions, such as intracranial pathology and myocardial ischemia.
Equipment, staffing, training, and environmental factors
The definition of a difficult airway depends on the skill of the operators involved, and multiple studies have demonstrated significant variability in personnel preparedness and equipment to perform airway management both among anesthesiologists and in the ICU [9–11]. In a national survey of 180 American ICU and anesthesiology directors, only 70% had a difficult airway cart in their ICU. Further, an astonishing 60% of the respondents reported that they had not been trained in the use of its equipment [12▪]. A similar survey of adult ICUs in Australia and New Zealand demonstrated that only 1% of units staffed with a dedicated night physician required that individual to possess prior airway training [13▪]. Lack of standardization of advanced airway equipment was common; in the American study only 50% of carts included a gum elastic bougie and 48% incorporated videolaryngoscopy equipment. A laryngeal mask airway (LMA) was the most common alternate ventilation device in 80% of carts, but surgical airway equipment was present less than 40% of the time. A difficult airway algorithm was present in 51% of American ICUs, and 13% of ICUs in Australia and New Zealand.
TECHNIQUES TO MANAGE THE DIFFICULT AIRWAY
Given the diversity of clinical situations, airway equipment, operator skill, and experience present in ICUs today, a systematic algorithm that is universally applicable to difficult airway management in the critically ill is an impossible task. There are, however, global considerations that can be extracted from current guidelines and available evidence that can focus a general approach to this problem . The astute ICU director should incorporate these common concepts into their clinical setting using accepted performance improvement methods, taking into consideration the characteristics of the at-risk patient population in their ICU and refining this general approach to take into consideration the equipment, skills, and training both present and required within their current practice environment [15▪].
Preparation, planning, and teamwork reduce complications
The American Society of Anesthesiologists Task Force Practice Guidelines for Management of the Difficult Airway recommend careful preparation – including airway assessment, preoxygenation, and a preplanned management strategy – to maximize intubation success and minimize risk of complications . In a two-phase, multicenter prospective study of 244 intubations, Jaber et al. [16▪▪] demonstrated that implementation of the following protocolized ICU intubation bundle reduced the incidence of both life threatening and other complications by 13 and 12%, respectively:
- Presence of two operators
- Fluid loading (isotonic saline 500 ml or hetastarch 250 ml) in absence of cardiogenic pulmonary edema
- Preparation of long-term sedation
- Preoxygenation for 3 min with noninvasive positive pressure ventilation (NIPPV) in case of acute respiratory failure [inspired oxygen fraction (FiO2) 100%, pressure support ventilation level between 5 and 15 cm H2O to obtain an expiratory tidal volume between 6 and 8 ml/kg and positive end-expiratory pressure (PEEP) of 5 cmH2O].
- During intubation
- Rapid sequence induction: etomidate 0.2–0.3 mg/kg or ketamine 1.5–3 mg/kg combined with succinylcholine 1–1.5 mg/kg in absence of allergy, hyperkalemia, severe acidosis, acute or chronic neuromuscular disease, burn patient for more than 48 h and major crush injury
- Sellick maneuver
- Immediate confirmation of tube placement by capnography
- Norepinephrine if diastolic blood pressure remains < 35 mmHg
- Initiate long-term sedation
- Initial ‘protective ventilation’: tidal volume 6–8 ml/kg of ideal body weight for a plateau pressure below 30 cmH2O.
Implementation of a standardized difficult airway management program has also been shown to significantly reduce the need for emergent surgical airway procedures . In a more detailed report that described a carefully designed iterative performance improvement program in airway management at a single institution, Mayo et al. [18▪▪] demonstrated that a combined team approach that included intensive simulation-based airway skills and scenario training for pulmonary/critical care fellows, crew resource management training, use of a mandatory checklist during each intubation, and postevent debriefing to drive interactive change produced significant improvements in patient safety during airway management events.
Oxygenation, not intubation, matters most
Adequate preoxygenation provides the intubator more time to maneuver through a difficult airway. Increasing the duration of preoxygenation improves intubation success in many critically ill patients, although it may be associated with deterioration in others . Use of a PEEP valve and NIPPV can delay critical desaturation during intubation attempts, even after standard measures improve oxygen saturations above 90% . Preoxygenation in a 20 degree upright position can also increase the duration of apneic normoxia in obese patients, and is likely also helpful in patients with atelectasis and reduced lung compliance .
Inability to perform adequate facemask ventilation or preoxygenate despite the measures above is an airway emergency. As attempts at tracheal intubation in this condition place patients at risk for prolonged hypoxemia, some experts have advocated early use of an extraglottic airway (EGA) in this setting instead [22▪]. In a large prospective validation study of a novel airway algorithm in 12 225 operating room patients, an EGA was used as a primary rescue strategy at the first indication of inadequate ventilation after induction [23▪▪]. Patients had no adverse outcomes and a very low incidence of hypoxemia; the incidence of inadequate ventilation was low, limiting its generalization to the critically ill. However, in the absence of data to the contrary, we strongly support early adoption of EGA when bag-mask ventilation is inadequate.
A detailed review of currently available EGA devices has been recently published [24▪▪]. It is our strong bias that EGA selection in the ICU should provide clinicians with the option of one-step intubation through the device. Intubation through EGAs has proven successful following failed direct laryngoscopy and as a more rapid primary approach in patients with a predicted difficult airway [25,26].
Maximizing intubation success, safety: role of advanced airway tools
If initial stabilization and preoxygenation is possible, a rapid and focused airway assessment can be helpful both to identify high-risk patients and to plan a management strategy. Common features associated with reduced glottic visualization or ventilation difficulties after induction are listed in Table 2 . It is important to recognize that although helpful, these findings have limited positive and negative predictive values for difficult airway identification or exclusion. Multivariate models provide better test performance, but are less practical in an emergent clinical situation .
Cessation of respiratory efforts after paralysis can result in rapid desaturation in critically ill patients. Awake intubation provides an important opportunity to preserve spontaneous respiration in cooperative patients with a predicted difficult airway, provided they can be stabilized for the time required for adequate airway and equipment preparation for this technique [29,30]. Upright fiberoptic-assisted intubation through a Williams or Ovassapian intubating airway requires skills familiar to many pulmonary/critical care physicians, and is a common and effective approach. Patients typically require a combination of nebulized or atomized lidocaine directed at the base of the tongue and tonsillar pillars. Antisialogues are frequently employed to aid visualization, and low-dose narcotics such as remifentanyl appear to be superior to dexmedetomidine when combined with low-dose midazolam for sedation [31▪].
Optical devices and videolaryngoscopes
Patients who require more emergent intubation or are uncooperative commonly require sedation with or without paralysis to optimize intubating conditions. This situation is especially high risk in patients with a predicted difficult laryngoscopy or tracheal intubation. Recent years have seen the development of a wide array of commercially available indirect optical devices and videolaryngoscopes, the characteristics and technical use of which have been recently and extensively reviewed [32▪▪,33▪▪,34▪]. These devices have gained significant popularity despite limited data supporting their utility over standard direct laryngoscopy.
Videolaryngoscopes provide better glottic visualization than direct laryngoscopy in both low-risk and difficult airways. Current literature suggests that videolaryngoscopy offers little advantage in low-risk patients, although it can serve a valuable role in faculty supervision and feedback of trainees during intubation attempts [32▪▪,35,36]. In patients with difficult airway risk factors or failed airway attempts using direct laryngoscopy, however, several studies have now demonstrated high rates of intubation success with several videolaryngoscopes [37▪▪,38▪▪,39▪]. This may be particularly valuable in morbidly obese patients, a risk factor clearly relevant in the critical care environment . The Airtraq optical laryngoscope has performed less favorably in difficult airways in the prehospital setting, but the generalizability of these results to the ICU remains unclear .
Gum elastic bougie
It is important to recognize that inadequate glottic visualization is not the only cause of a failed airway; inability to intubate the glottis or to advance the endotracheal tube is also a common reason for failure. In the emergent intubation series published by Martin et al. [6▪▪], the addition of a gum elastic bougie to direct laryngoscopy was the most frequently used strategy to successfully address difficult airway situations. It has also been associated with significant success when combined with the Airtraq laryngoscope [23▪▪].
Surgical airway management
Surgical cricothyroidotomy is an uncommon procedure generally reserved for the emergency airway situation when an EGA cannot be effectively employed due to upper airway abnormalities, blood, or secretions that obviate proper placement and function. It is also an important rescue strategy in the setting of the failed airway, especially when ventilation and oxygenation are inadequate. Of the four techniques described, surgical cricothyroidotomy with a cuffed tube or a commercially available kit that allows placement of a wide bore cannula over a needle or wire provides a more secure airway in a critically ill patient than a needle cricothyroidotomy with low pressure or jet ventilation. Cricothyroidotomy can be performed most rapidly with minimal surgical skills using a rapid four-step technique (RFST) . A modified RFST, performed by inserting a bougie into the trachea through an incision to serve as a guide for a standard endotracheal tube, has also been shown to be successful in the hands of the novice nonsurgeon . A cricothyroidotomy kit that utilizes a Seldinger approach is also commercially available, but has been associated with longer time to placement .
Major complications of the procedure include esophageal perforation, subcutaneous emphysema, and bleeding.
A systematic approach to intubation management that emphasizes planning, preparation, and teamwork can significantly reduce intubation complications. Given the wide variability of equipment and training currently present in many ICUs, ICU directors must apply the general principles available from current medical evidence to their specific practice environment and patient population using accepted performance improvement methodology. An EGA with the option of one-step intubation is essential both as a rescue tool in emergency airway situations and as a back-up plan to manage the failed airway; it may also offer a useful primary approach when difficulties are predicted. Although awake intubation is the safest approach in the difficult airway patient, the time, preparation, and patient cooperation required makes this technique less practical in many ICU airway scenarios. In patients who can be adequately preoxygenated but present intubation difficulties after induction, use of a gum elastic bougie and videolaryngoscopy have both been associated with a high rate of intubation success. Cricothyroidotomy is an uncommonly used but important rescue procedure in the setting of the emergency or failed airway in which an EGA cannot be used. Although these guidelines are reasonable for use in the critically ill, there is clearly the need for high-quality research studies to validate and further refine this approach to the difficult airway in the ICU.
A.S.N. and K.C.D. are co-chairs of the American College of Chest Physicians (ACCP) Airway Management Training Program, and have previously received faculty honoraria for participating in this program. The general guidelines for the management of the difficult airway in the critically ill patient outlined in this article are heavily influenced by this course, and are the product of the collaborative efforts of the core faculty of the ACCP Airway Management Training Team.
Conflicts of interest
Disclosure of funding: None.
The views expressed in this review are those of the authors and do not reflect the official policy or position of the Department of the Army, Department of Defense, or the US Government.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 66–68).
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