Unanticipated Difficult Intubation in an Adult Patient

Anatomy and Physiology

Some anatomic predictors of difficult intubation include: small mouth opening, short thyromental distance, full set of teeth with prominent incisors, reduced mandibular protrusion, reduced submandibular compliance, short neck, large neck circumference, limited neck extension, Mallampati 3 or 4, obesity, surgery or radiation-induced changes. These are not often critical but do additively contribute to a higher likelihood of difficult direct laryngoscopy but may not for video laryngoscopy.

In addition to the anatomical airway factors, the physiologically difficult airway is a newer concept in airway assessment.  This assessment encompasses that airway management is difficult due to hypoxia, hypotension, severe metabolic acidosis, or right ventricular failure.[7][8]

There can also be contextual issues that contribute to difficult or failed airway management. Examples may be the skill level of the clinician, available help, proper equipment, or patient factors such as intoxication or disability.[9]

Indications

Pregnant patients requiring emergency general anesthesia, trauma patients with cervical spine injury with or without concurrent closed head injury, morbidly obese patients, and patients with previous head and neck cancer are some subsets of patients at high risk for difficult or failed intubation.  

Failed intubation occurs in about 1 in 390 patients, and maternal mortality is four times higher than in other populations.[2] Anatomical and physiological changes of pregnancy contribute to more rapid oxygen desaturation and a high risk of aspiration. The Obstetric Anesthetists Association (OAA) and Difficult Airway Society (DAS) have created guidelines for managing difficult or failed intubation.[10][11] The best approach to airway management in pregnant patients is to avoid it, when possible, by early regional anesthetic placement in high-risk patients.[7] This requires collaboration with obstetricians and nursing.  

In trauma patients with a cervical spine injury, there should be a heightened suspicion for difficult intubation because of a rigid cervical collar for immobilization, higher Cormack-Lehane views of the larynx, and high aspiration risk. Intubation is often required for airway protection, mechanical ventilation, and/or emergency surgery. There may also be a concurrent closed head injury. Avoiding neck movement during intubation attempts and avoiding hypotension, hypoxia, and hypercarbia are all paramount.[12] Removing the anterior panel of the rigid cervical collar is acceptable. There is little evidence that the rigid cervical collar immobilizes the cervical spine but does make bag-mask ventilation and laryngoscopy more difficult. Manual in-line stabilization (MILS) and cricoid pressure should be maintained with minimal neck movement during endotracheal tube placement. Jaw thrust, mask ventilation, intubation attempts of any method, or supraglottic airway insertion all cause some movement of the neck, so securing the airway must be done cautiously.

Obesity and especially morbid obesity present challenges in all aspects of airway management, from bag-mask ventilation to laryngoscopy to emergency cricothyrotomy.[13][14] Two-thirds of the patients with adverse outcomes reported in NAP 4 were obese.[2] Safe apneic time is also much shorter with rapid desaturation in these patients. The approach for airway management may need to be altered in this population.

History of previous head and neck cancer with surgical resection and/or radiation[2] and presenting pathology is important to note in the airway examination. This should raise the anesthesiologist's index of suspicion of a difficult airway and that awake intubation may be the safest choice. Even when the anesthesiologist recognizes that it is the best choice, not choosing to do awake intubation might be because of time pressure, lack of confidence, or loss of required skills. Using a fiberoptic scope is the only tool that allows multi-axial movements and thus allows navigation when anatomy has been distorted by surgical resection or radiation or other presenting pathology.

Equipment

• Have a difficult airway cart fully stocked and readily available
• All staff know what is on the cart and where it is stored
• Have video laryngoscopy readily available. Ideally, in every operating room
• Stock more than one type of supraglottic airway on the cart
• Scalpel, bougie, and small endotracheal tube for emergency surgical airway
• Cognitive aids like the vortex and algorithms attached to the cart

Personnel

This article is directed to anesthesiologists, emergency medicine, pulmonary and critical care physicians, plus any other clinicians responsible for emergency airway management and intubation.

Preparation

The following are some tools and “pearls” to help prepare for high-risk patients and when faced with unanticipated difficult intubation.

Place a low-flow nasal cannula or high-flow oxygen if available on patients. It will extend safe apnea time, especially in obese patients.[15][16][17]

NO DESAT is an acronym coined by Dr. Richard Levitan.[18]

• N - nasal
• O - oxygen
• D - during
• E - efforts
• S - securing
• A - a
• T - tube

The Vortex cognitive aid is a simple, practical but effective visual tool.[19] Options for intervention include effective mask ventilation, placement of supraglottic airway, or endotracheal placement. If one of these is successful, there is an opportunity to move into the green zone and regroup. If all three are unsuccessful, there is a circling downward toward the center of the Vortex.  This is a "cannot intubate and cannot oxygenate" (CICO) situation and requires performing an emergency surgical airway to move back into the green zone. "Cannot intubate and cannot oxygenate" (CICO) has replaced the term "cannot intubate and cannot ventilate" (CICV).  Avoiding or reversing hypoxia is the highest priority rather than intubation.

There are also reminders of interventions that might help.  Some of these include re-positioning the head or neck, applying external laryngeal manipulation (ELM), changing laryngoscope blade type, or attempting video laryngoscopy. Adjuncts such as a bougie or stylet may be helpful. Suctioning may aid.  Additionally, administering a drug for paralysis likely will improve the situation.[20][21] This is contrary to traditional teaching that required demonstrating the ability to successfully mask ventilate prior to administering a paralytic drug.[20]

The Difficult Airway Society (DAS) has published several algorithms. One especially salient to this article is for the management of unanticipated difficult intubation in adult patients.[22] An overview is shown on page one. Page two provides more details for optimizing each attempt. Page three provides instructions for performing open cricothyrotomy. 

• Plan A: Effective mask ventilation and successful intubation. If unsuccessful, then there are suggestions to improve mask ventilation and successfully intubate. These include repositioning the patient's head or neck, administering a paralytic drug, applying external laryngeal manipulation, removing cricoid pressure, or using an adjunct such as a bougie.  If still unsuccessful, then the algorithm moves to Plan B. 
• Plan B: Maintaining oxygenation with a supraglottic airway.  If successful, there is an opportunity to regroup. These options include aborting the procedure, using the supraglottic airway as is, and proceeding or using it as a conduit to intubation. If still unsuccessful, then the algorithm moves to Plan C.
• Plan C: Reattempt mask ventilation with paralysis and using a two-person technique. If still unsuccessful, the algorithm moves to Plan D.
• Plan D: Emergency front of neck access (eFONA) - open scalpel cricothyrotomy

A maximum of 3 + 1 laryngoscopy attempts (direct or video) is emphasized. It is unlikely that more than three attempts by the same clinician will be successful and can result in airway trauma.[23] Continuing to attempt intubation without making a change in approach is not advisable. The plus one attempt is reserved for situations when an expert arrives to help.

Use of a second-generation supraglottic airway is recommended. This separates the respiratory from the gastrointestinal tracts, has a cuff with higher sealing pressures, and is more protective against aspiration compared to earlier supraglottic airways.

Open cricothyrotomy, also called the emergency front of neck access (eFONA), is now preferred. The NAP 4 executive summary findings found that an open approach achieved higher rescue success with fewer complications than the needle cricothyroidotomy when an emergency surgical airway was required.[2]

As previously mentioned, there is also an algorithm for the failed intubation in obstetric patients requiring general anesthesia.[11] Much of the format is like that of the unanticipated difficult intubation. Additionally, there is a section to guide decisions weighing the risks to both mother and baby and the urgency of delivery. Often it is prudent to proceed with a supraglottic airway provided oxygenation and ventilation are adequate.

Other guidelines address airway management in the intensive care unit (ICU), awake intubation, and extubation. All are based on an extensive review of the literature, excellent resources, and free. Readers are encouraged to review all these guidelines on the DAS website.

Technique or Treatment

Video laryngoscopy has revolutionized airway management. The premise is that of looking around the corner, which is the tongue, instead of aligning the oral, pharyngeal, and laryngeal axes. The Cormack-Lehane views of the larynx are often one to two grades improved with higher first-pass success.[24] It is now recommended to be the first-choice technique by many experts.[25] and been included in various society's guidelines. High first-pass success is especially important in patients at high risk for aspiration. Achalasia is one such example. Video laryngoscopy was also recommended as the first go-to device during the COVID pandemic.[26]

When attempting intubation with video laryngoscopy, it is not often due to the inability to see the glottic opening if difficulty is encountered; it is in the delivery of the endotracheal tube. The patient’s head should be in a neutral position with a “limited” glottic view. The classic sniffing position may make intubation more difficult.

There are two different types of video laryngoscopes - non-channeled and channeled video laryngoscopes. Non-channeled ones are those more frequently available. Channeled scopes have a more acute angle and a conduit that guides the endotracheal tube towards the glottic opening. This results in the least neck movement and is especially useful in a patient with an unstable cervical spine injury. In reviewing the literature, the term video laryngoscopy is used generically without always delineating which type.[27] It is important to know the type that is being reviewed or compared. There can, however, rarely be situations where video laryngoscopy may be difficult or fail.  It is important to consider what some of those predictors are so contingency plans can be made.

Additionally, there are several techniques for using supraglottic airways as a conduit for intubation successfully.  Using an intubating laryngeal mask airway with a specially designed endotracheal tube that is passed blindly through the supraglottic airway is one way.  This type of supraglottic airway is especially useful in obese patients.[28] This is also useful when lingual tonsillar hyperplasia/hypertrophy is encountered, which is a common cause of unanticipated difficult or failed intubation.[29] It is below what can be seen in doing the usual airway examination. The supraglottic airway will push aside the extra tissue in the airway in an obese patient or the hyperplastic tonsillar tissue and aid intubation. Findings should be documented in the patient's chart for future procedures as they may have had previous uneventful intubations.  Because this is reactive lymphoid tissue, findings may be different than previously recorded.

Another tactic for securing the airway through a supraglottic airway device is by loading an endotracheal tube or airway exchange catheter over a fiberoptic scope and then driving the flexible scope through the supraglottic airway device so that the endotracheal tube or exchange catheter can be placed in the trachea.

Complications

Complications of difficult or failed airway management include hypoxia, aspiration, esophageal intubation, emergency surgical airway, cardiovascular instability, cardiac arrhythmias, ischemic encephalopathy, and death.

Clinical Significance

There is no perfect way to predict when difficult intubations will occur.[30][31] That leaves clinicians with the need for rescue tactics and knowledge of predictors of failure for each of the rescue tactics.  As previously discussed, when the oxygen saturation is failing and the fine motor skills of the provider are deteriorating, there are only four options - effective mask ventilation, placement of a supraglottic airway or endotracheal tube, or emergency surgical airway.  Understanding what may contribute to failure for each rescue device may help determine which one will be successful when needed.

Predictors of failed mask ventilation have been reviewed in the Airway Assessment StatPearls article.[32]

Predictors of difficult or failed video laryngoscopy include:[9]

• Obesity
• Clinician inexperience with video laryngoscopy
• Blood, secretions, or vomit in the oropharynx
• Higher Cormack-Lehane grades with direct laryngoscopy
• Limited mouth opening

Predictors of difficult or failed supraglottic airway placement include:[9]

• Obesity
• A small mouth opening
• Upper airway pathology
• Limited neck extension
• Cricoid pressure

Predictors of difficult or failed emergency front-of-neck airway access (eFONA) include:[9]

• Obesity
• Indurated skin changes due to surgery and/or radiation
• Flexed neck deformity
• Displaced trachea
• Female sex

Obesity and especially morbid obesity may make airway management more difficult and contribute to an unsuccessful rescue.  This may mandate that the best approach might be awake intubation in some cases.

Management strategies for the physiologically difficult airway include:[7][8]

• Providing supplemental oxygen (low or high flow) or non-invasive positive pressure ventilation (NIPPV)
• Treating hypotension with intravenous fluids or pressors
• Delaying intubation until the severe metabolic acidosis is improved
• Transthoracic echocardiography (TTE) to assess the severity of right ventricular dysfunction/failure
• Choice of drugs based on the patient's hemodynamics
• Possibly securing the airway with the patient awake and breathing spontaneously

Enhancing Healthcare Team Outcomes

Human factors play a role in decisions and especially in situations of failed airway management. Avoiding loss of situational awareness, avoiding perseveration on the same task, or delaying performing a surgical cricothyrotomy is essential.[33][34] There is a need for communication, teamwork, and care collaboration with other clinicians, nursing, and respiratory therapists as interprofessional healthcare team members. Exercising interprofessional teamwork and sharing information about the patient's status will result in better outcomes. [Level 5] Simulation training may be helpful to build these skills in advanced airway management.

Attending difficult airway workshops to practice techniques, networking with experts, and looking for opportunities to practice them in elective procedures are excellent ways to build skillsets.