Continuing Education Activity

A difficult airway is a clinical situation wherein a healthcare provider who is skilled at airway management experiences difficulty with one or more recognised techniques. Various algorithms have been outlined by expert international bodies to provide a basic pathway for managing difficult airways. It is vital that all practitioners who attempt advanced airway management be familiar with the equipment and techniques necessary for successful airway management. This activity reviews how to identify a difficult airway and highlights the role of the inter-professional team in the management of these patients.

Objectives:

• Recall the criteria used in the Mallampati classification to assess an airway.
• Describe the presentation of a patient in respiratory distress.
• Summarize the technique of managing a difficult airway
• Discuss interprofessional team strategies for improving care of patients with a difficult airway and improve outcomes.

Introduction

A difficult airway is a clinical situation in which a healthcare provider who is skilled at airway management encounters difficulty with one or more standard methods of airway management. The definition is not standardized in the literature and there are some variations between national expert guidelines. The American Society of Anesthesiologists defines a difficult airway as existing when "a conventionally trained anesthesiologist experiences difficulty with facemask ventilation of the upper airway, difficulty with tracheal intubation, or both."[1] Canadian guidelines are broader, defining it as where "an experienced provider anticipates or encounters difficulty with any or all of face mask ventilation, direct or indirect (e.g., video) laryngoscopy, tracheal intubation, SGD [supraglottic device] use, or surgical airway."[2] Another potential factor is the expected status of the airway at the time of extubation.

The difficulty of airway management is highly variable and will depend on several factors including patient characteristics, medical and surgical history, airway examination, the clinical context for which airway management is required (including the nature of any planned surgical procedure), and the patient's current status and vital signs.[3] Appropriate training, experience, risk assessment, and clinical judgment are seen as essential to reliably predict the difficulty of managing a particular patient's airway. Where an airway is judged to be basic or straightforward, it is expected that a healthcare professional who is well-trained in airway management would normally be able to proceed without complications. More advanced airways lack these reassuring factors and are potentially complex; they may require infrequently-used techniques and access to a team with special skills or devices. However, real-world airway assessment is often highly subjective and even specialist clinicians will have significant difficulty in accurately predicting which cases will be challenging. Some studies suggest that more than 90% of difficult airways are unanticipated.[4]

Algorithms for managing the difficult airway have been outlined by national and international bodies such as the American Society of Anesthesiology and the Difficult Airway Society. These provide a basic pathway for difficult intubation. It is vital that all clinicians who attempt intubation be familiar with the equipment and techniques necessary for successful intubation. In the Closed Claims Analysis performed by the American Society of Anesthesiology, failed intubation remains a major cause of morbidity and mortality.[5][6][7]

Anatomy and Physiology

Evaluation of the airway for signs of potential difficulty should be performed as soon as possible on any patient in respiratory distress. A quick assessment of congenital or acquired anatomic defects should be performed initially. Particular attention should be paid to patients who have suffered facial, head, or neck trauma, or who have oral bleeding, regurgitated gastric contents, or frothing of the mouth.

In patients who are cooperative, a Mallampati evaluation should be performed. The practitioner asks the patient to open their mouth and views the oral cavity. This examination reveals the size of the tongue, adequacy of mouth opening, and condition and presence of the teeth and uvula. The more uvula that is visible behind the tongue, the easier the intubation should be. Externally, one should examine the thyromental distance. The closer the chin is to the chest wall with the head in mid-position, the more difficult the intubation may be. Similarly, a drastic overbite can cause difficulty with laryngoscope positioning. Neck trauma may result in the inability to move the head safely, and difficult intubation may be secondary to non-optimized positioning. In all patients with neck instability, in-line stabilization should be performed during every intubation attempt. Obesity can also contribute to a difficult airway. Redundant skin folds in the buccal cavity, and increased fatty tissue around the posterior pharynx can make visualization of the vocal cords difficult. 

Indications

The indications for intubation are unprotected airways, trauma to the airway, a Glascow Coma Scale (GCS) less than eight, a rapid decline or loss of consciousness, and procedural sedation and paralysis[8].

Contraindications

A patient with a difficult airway does not preclude any procedures which are medically necessary. However, the absolute contraindication to direct laryngoscopy is facial trauma [8]. Patients in respiratory distress must have a protected airway to ensure adequate oxygenation and ventilation. It is prudent to allow the most experienced personnel present to manage the suspected difficult airway.

Equipment

The standard instruments required for any intubation should be available. For intubation, the provider will need a laryngoscope handle and blade with proper lighting, a properly-sized endotracheal tube, an air-filled syringe to inflate the cuff, a stylet, and a source of oxygen. The gold standard for monitoring endotracheal intubation is end-tidal carbon dioxide monitoring [9]. Suction should always be prepared if available. Non-paralyzing or depolarizing induction agents such as propofol can be used to determine if the patient can be ventilated before long-acting paralytic agents are administered. Non-depolarizing induction agents can be used to perform rapid sequence intubation also; however, these should not be used until it is determined that a patient can be properly ventilated via a bag-valve-mask. 

Additional equipment necessary for intubating a difficult airway should be located at the bedside as well. This equipment will be dictated by the clinician who is performing the procedure and what is available. It is prudent to have a 'difficult airway cart' in any unit where intubations are frequently performed such as the emergency department (ED), intensive care unit (ICU), operating room (OR). Options include a video laryngoscope, Combitube, bougies, intubating laryngeal mask airway, intubating stylet, fiber-optic bronchoscope, and an articulating laryngoscope.

Personnel

The personnel necessary include the healthcare provider performing the intubation. While not completely necessary, it is helpful to have an assistant available. In the case of a suspected difficult airway, it is vital that an anesthesiologist is consulted early and that the patient is transported to an area where mechanical ventilators are present.

Preparation

As for any procedure, the patient should be prepared adequately. The first step is to have the patient positioned ideally on the bed. The body should be located just at the head of the bed so that the occiput is supported on the edge. The head is placed in neutral alignment with the vertebral axis, and if the cervical spine is stable, the head is slightly extended to achieve the 'sniffing' position. The bed is then placed at the level of the xiphoid process of the provider.[10][11][12]

If time allows, denitrogenation of the lungs should be performed. This is accomplished with 100% oxygen via a facemask for eight vital capacity breaths or three minutes before administration of an induction dose of either propofol or etomidate [13]. In a healthy patient, this will allow for 8-10 minutes of safe apnea which is ample time for rapid sequence intubation and for potentially prolonged intubations. Pre-oxygenation should always be performed with the optimal end-tidal oxygen being 90% prior to intubation unless there is an absolute contraindication [13]. Patients who are obtunded should be mask ventilated with 100% oxygen before any intubation attempts to achieve the same denitrogenation. In the obtunded patient, it should be confirmed before mask ventilation that no obstruction is present, and the mouth and posterior pharynx can be examined and suctioned to accomplish this.

All required equipment should be immediately available at the bedside, and all necessary medications should be drawn up in syringes. Reliable intravenous access should be confirmed prior to intubation.

Technique or Treatment

Distinguishing difficult intubation from difficult ventilation is important. If the patient is difficult to intubate, adequacy of bag-mask ventilation becomes paramount especially if the patient is already paralyzed or not breathing on their own. This technique requires a mask attached to a self-inflating bag with an oxygen source. The bag is placed over the nose and mouth, and the non-dominant hand is used to create a seal with the face and provide chin lift. The thumb and forefinger are used to seal the mask to the face. The other three fingers should be positioned on the bone, with the fifth finger under the angle of the mandible providing lift. It is vital that the pressure is applied to the mandibular bone as soft tissue manipulation can easily occlude the airway. The right hand is used to squeeze the bag and provide positive pressure ventilation. In some cases two person ventilation may be required which require one person to use both hands must be used to provide an adequate seal on the mask and to provide enough chin lift while another person provides tidal breaths. Two-person ventilation is required more often in patients who are obese and in those whom it is difficult to provide optimal seal. It is vital that a call for help should be made immediately if a difficult airway is identified. A skilled practitioner can support a patient for prolonged periods of time with bag-mask ventilation alone. If the unanticipated difficult airway arises, bag-mask ventilation can provide adequate oxygenation while equipment is retrieved or more skilled providers arrive for intubation [14][15][16]. If the patient cannot be intubated, practitioners should attempt to place an LMA for ventilation support. 

Practitioners who are not regularly performing intubations should not attempt a difficult airway unless it is an emergency. Failed intubation attempts lead to increased gastric insufflation, trauma to the posterior pharynx, increased blood and secretions in the airway, and edema to the subglottic structures which can make subsequent intubation more difficult or complete airway obstruction. In this instance, a surgical airway may be obtained immediately.

After a difficult airway is identified, one needs to determine what equipment is available to make informed decisions about how to proceed. Preferably, this is done before there is an immediate threat of desaturation. For each practitioner, the difficult airway tools used will be those which they are most familiar with and have achieved the best results with anecdotally. The options include the intubating laryngeal mask airway, fiberoptic bronchoscope, intubating stylet, articulating laryngoscope, video laryngoscope, and cricothyroidotomy.

If the patient is breathing spontaneously, consideration should be made to postpone the administration of neuromuscular blocking drugs. Often, in traumatic airways, the only way to identify the tracheal opening is to watch the subglottic area for spontaneous respiration. In elective intubations, the patient with a history of difficult intubation can be intubated while awake. The airway is prepped with nebulized viscous lidocaine 10-15 mL for 15 minutes to achieve proper anesthetic of the buccal, subglottic, and tracheal anatomy. A fiber-optic bronchoscope with a preloaded endotracheal tube is introduced through the oropharynx. The vocal cords are identified, and the scope is passed through the vocal cords. After visualization of the tracheal rings, an endotracheal tube is passed over the bronchoscope, and position is confirmed through direct visualization. The bronchoscope is removed, and the patient is sedated immediately and supplemental ventilation is provided. Fiber-optic bronchoscopy is disadvantageous as it requires considerable experience to be performed successfully.  It can also be difficult to visualize laryngeal structures when the airway is bloody or full of secretions since the aspiration port on the scope is small, and it can be difficult to suction large volumes or thick mucus.

In an emergency situation, the patient should have the buccal cavity suctioned completely, and bag-mask ventilation should be attempted. If bag-mask ventilation is successful, direct laryngoscopy can be performed. If direct laryngoscopy fails because the vocal cords cannot be visualized due to an anterior larynx, an articulating laryngoscope blade may provide a better view. This blade has a lever attached to the tip which allows the tip a greater range of anterior motion thus elevating the epiglottis and allowing visualization of the vocal cords. If direct laryngoscopy fails, a lighted scope may make the visualization of the cords easier. The endotracheal tube can then be placed under direct video guidance using the specialized, rigid stylet that is placed inside the endotracheal tube. If a lighted scope is not available, placement of an intubating LMA may be done. This device is placed in the posterior pharynx, and the patient can be ventilated through the device. It should be noted that LMAs do not provide a protected airway, and the patient is at risk of aspiration. The iLMA is designed to accept a specific endotracheal tube that should be kept in the same package which passes through the LMA and may blindly enter the trachea based on the appropriate positioning of the tip of the device in the esophagus. The Combitube functions similarly to this in that it is placed blindly. The tip is placed into the esophagus, and the distal balloon inflated, sealing the esophagus. A proximal balloon is then inflated, and the result is the lumen of the ventilatory circuit isolated around the larynx. Positive pressure can then be applied to ventilate the patient. This device also is easily displaced, and other difficulties with cuff pressures can disrupt the mucosal membranes in which they come into contact. A Combitube should not be considered a long-term solution; it does not protect from aspiration.

Another option is the intubating stylet. This small-diameter plastic tube is hollow and has an adapter that allows ventilation. The benefit to this device is that it is more flexible than an ETT and can be passed through small openings when tracheal intubation is impossible with a regular ETT. It can also accept a pre-applied curvature to allow access to an anterior vocal cord position. A standard ETT can be passed over the intubating stylet if long-term ventilation is necessary.

If all else fails, and the patient cannot be ventilated or intubated, a surgical airway is necessary. If a potentially difficult airway is identified, a surgeon should be made aware early. There are also kits available using a Seldinger technique, with a needle inserted through the cricothyroid membrane. On successful aspiration of air, a wire is passed into the trachea, and the needle removed. A small incision is made in the skin, and a dilator is passed over the wire. This should produce a large air leak on positive ventilation, confirming the placement. Then a cricothyrotomy tube is passed through the opening and the patient ventilated. Proper placement of an airway device should always be confirmed with an end-tidal carbon dioxide (CO2) device immediately.

Complications

The most common form of failed intubation is inadvertent esophageal intubation. This may present in a delayed fashion if adequate pre-oxygenation is performed. It can be difficult in obese patients to recognize distension of the abdomen from gastric expansion, and initial end-tidal CO2 monitors may show a color change or some quantitative end-tidal CO2. Auscultation of breath sounds is notoriously ineffective in definitively determining esophageal intubation due to the proximity of the stomach to the diaphragm. The obvious sign then becomes desaturation despite the external appearance of adequate ventilation. In this case, the endotracheal tube should be removed immediately, and mask ventilation resumed to provide oxygenation. It may be necessary to decompress the abdomen via a nasogastric or orogastric tube to allow effective mask ventilation and decrease the risk of aspiration.

Advancing the endotracheal tube (ETT) distal to the carina may lead to bronchial intubation. Usually, the right mainstem bronchus branches at a more acute angle than the left main stem bronchus thus leading to right mainstem bronchial intubation. A rise of peak pressure may be seen if bronchial intubation occurs. Auscultation of right-sided breath sounds and absent or decreased left-sided sounds can confirm this. It may also become evident when the post-intubation chest x-ray is examined. The ETT may be pulled back carefully until bilateral breath sounds are auscultated.

Displaced vocal cords are another complication, especially in difficult patients. The presence of a displaced vocal cord may not be apparent until extubation. The patient may have persistent hoarseness, stridor, or fail extubation completely due to airway obstruction. This occurs if the ETT lumen catches on an arytenoid and the tube is forced. An ETT which does not pass easily should never be forced into the trachea. Consultation with otolaryngology should be sought immediately if this is identified.

As with any intubation, there is always a risk to the teeth. Care needs to be taken that proper laryngoscope technique is used and the top of the blade does not break a tooth. The fragments may be aspirated, and bronchoscopy is then necessary to retrieve them. Extremely loose teeth should be removed before intubation to avoid this complication. Poor dentition is also a harbor for pathogenic bacteria, and significant pneumonia may arise from aspiration of bacteria or obstruction to the airways.

Other less common complications can arise, such as mediastinal intubation, tracheoesophageal fistula, and tracheal stenosis. The endotracheal tube should never be forced if resistance is felt once the tube passes below the cords. If the tube meets resistance that cannot be overcome with rotation, consideration should be made to using a smaller ETT size.

Clinical Significance

A difficult airway can become a life-threatening condition very quickly if care is not taken to identify and prepare for this eventuality properly. All practitioners responsible for intubating patients should be familiar with three or four of the above techniques. If the primary attempt at intubation fails, a clear plan that addresses the reason for failure should exist for subsequent attempts. The abilities to ventilate, oxygenate, and maintain a patent airway are crucial in patient survical.

Enhancing Healthcare Team Outcomes

There are multiple methods to difficult airway management. Proper identification is paramount to avoiding airway trauma, morbitidy, and mortality. An early call for help is essential. If the patient is able to be ventilated, then there is ample time for intubation. When a difficult airway is encountered, healthcare providers should limit direct laryngoscopic intubation attempts to two and should have the equipment available to change techniques. When a difficult airway is identified, and ventilation cannot be provided, it is essential that the most experienced provider handle subsequent attempts.