DIFFICULT AIRWAY

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      The fundamental responsibility of an anesthesiologist is to maintain adequate gas exchange in the patient. For this to be done, the patient’s airway must be managed so that it is almost continuously patent. Failure to maintain a patent airway for more than a few minutes results in brain damage or death. Techniques and practices in airway management have long been an important concern of the American Society of Anesthesiologists (ASA), as illustrated by the publication of original and revised difficult airway guidelines.

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    There are three common ways to maintain airway patency and gas exchange:
Mask ventilation: Inspired gas is delivered to the face

through a mask that is sealed to the patient’s face, while the natural airway from the face to the vocal cords is kept patent with or without external jaw thrust maneuvers or internal upper airway devices.

Supraglottic airway: Inspired gas is delivered through a supraglottic airway, such as the laryngeal mask airway. 

Endotracheal intubation: The airway is kept open to inspired gas by a tube passed from the environment to

some point below the vocal cords.

        Management of the airway is paramount to safe periope- rative care, and the following steps become necessary to favorably affect outcome:

  • A thorough airway history and physical examination 

  • Consideration of the ease of rapid tracheal intubation, by direct or indirect laryngoscopy

  • Formation of management plans for use of a supraglottic means of ventilation (e.g. face mask, supraglottic airways)

  • Weighing the risk of aspiration of gastric contents 

  • Estimating the relative risk to the patient of failed airway maneuvers 

 
 

Review of Airway Anatomy

        The term airway refers to the upper airway—consisting of the nasal and oral cavities, pharynx, larynx, trachea, and principal bronchi. The airway in humans is primarily a conducting pathway. Because the oroesophageal and nasotracheal passages cross each other, anatomic and functional complexities have evolved for protection of the sublaryngeal airway against aspiration of food that passes through the pharynx. The anatomically complex airway undergoes growth and development and significant changes in its size, shape, and relation to the cervical spine between infancy and childhood. As are other bodily systems, the airway is not immune from the influence of genetic, nutritional, and hormonal factors.

      Nine cartilages provide the framework of the larynx. These are the unpaired thyroid, cricoid and epiglottis and the paired arytenoids, corniculates, and cuneiforms. They are connected and supported by membranes, synovial joints, and ligaments. The ligaments, when covered by mucous membranes, are called folds. The shield-shaped thyroid cartilage acts as the anterior “protective housing” of the vocal mechanism.

      Movements of the laryngeal structures are controlled by two groups of muscles: the extrinsic muscles, which move the larynx as a whole, and the intrinsic muscles, which move the various cartilages in relation to one another.

      The cricothyroid membrane provides coverage to the cricothyroid space. The membrane, which in the adult is typically 9 mm in height and 3 cm in width, is composed of a yellow elastic tissue that lies directly beneath the skin anda thin facial layer. It is located in the anterior neck between the thyroid cartilage superiorly and the cricoid cartilage inferiorly. It can be identified 1 to 1.5 fingerbreadths below the laryngeal prominence (thyroid notch). It is often crossed horizontally in its upper third by the anastomosis of the left and right superior cricothyroid arteries. The membrane has a central portion known as the conus elasticus and two lateral, thinner portions. Directly beneath the membrane is the laryngeal mucosa. Because of anatomic variability in the course of veins and arteries and the membrane’s proximity to the vocal folds (which may be 0.9 cm above the ligaments’ upper border), it is suggested that any incisions or needle punctures to the cricothyroid membrane be made in its inferior third and be directed posteriorly (a posterior probing needle will strike the backside of the ring-shaped cricoid cartilage).

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        At the base of the larynx, suspended by the underside of the cricothyroid membrane, is the cricoid cartilage. The name cricoid is derived from the Greek words krikos and eidos, meaning shaped like a ring, hence its frequent reference to a signet ring-shape. The cricoid cartilage represents the anatomic lower limit of the larynx and helps support it. It is thicker and stronger than the thyroid cartilage and represents the only complete cartilaginous ring in the airway. Thus, cautious downward pressure on the cricoid cartilage to prevent passive regurgitation is possible without subsequent airway obstruction. This cartilage is approximately 1 cm in height anteriorly, but almost 2 cm in height in its posterior aspect as it extends in a cephalad direction creating the dorsal wall of the larynx at the level of the cricothyroid membrane and the thyroid cartilage. The trachea is suspended from the cricoid cartilage by the cricotracheal ligament. The trachea measures approximately 15 cm in adults and is circumferentially supported by 17 to 18 C-shaped cartilages, with a membranous posterior aspect overlying the esophagus.

        In the adult, the first tracheal ring is anterior to the sixth cervical (C6) vertebra. The tracheal cartilages are interconnected by fibroelastic tissue, which allows for expansion of the trachea both in length and diameter with inspiration/expiration and flexion/extension of the thoracocervical spine. The trachea ends at the carina, where it bifurcates into the principal bronchi. The right principal bronchus is larger in diameter than the left, and deviates from the plane of the trachea at a less acute angle. Aspirated materials, as well as a deeply inserted endotracheal tube (ETT), tend to gain entry into the right principal bronchus, although leftsided positioning shouldbe excluded. Cartilaginous rings support the first seven generations of the bronchi.

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 Muscles of the Larynx

         The function and innervation of extrinsic and intrinsic muscles of larynx are summarized in Tables 1.1 and 1.2 respectively.

Table 0101 Extrinsic muscles of the lary
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 Innervation of the Larynx

        The larynx is innervated by two branches of each vagus nerve: the superior laryngeal and recurrent laryngeal nerves.

         Because the recurrent laryngeal nerves supply all of the intrinsic muscles of the larynx (with the exception of cricothyroid), trauma to these nerves can result in vocal cord dysfunction. As a result of unilateral nerve injury, airway function is usually unimpaired, although the protective role of larynx in preventing aspiration may be compromised.

 Vocal Cord Palsies

        The recurrent laryngeal nerve may be traumatized during surgery on the thyroid and parathyroid glands. Malignancy or benign processes of the neck, trauma, pressure from an endotracheal tube or a laryngeal mask airway, and stretching the neck may also affect the nerve. The left recurrent laryngeal nerve may be compressed by neoplasms in the thorax, aneurysm of the aortic arch, or an enlarged left atrium (mitral stenosis). It may be occasionally injured during ligation of a patent ductus arteriosus. The left nerve is likely to be paralyzed twice as frequently as the right one because of its close relationship to many intrathoracic structures. Damage to the superior laryngeal nerve (external branch) during thyroidectomy is the commonest cause of voice change.

       Under normal circumstances, the vocal cords meet in the midline during phonation (Fig. 1.4 ). On inspiration they move away from each other. They return toward the midline on expiration, leaving a small opening between them.

      When laryngeal spasm occurs, both true and false vocal cords lie tightly in the midline opposite each other. To arrive at a clinical diagnosis, the position of the cords must be examined laryngoscopically during phonation and inspiration.

      The recurrent laryngeal nerve carries both abductor and adductor fibers to the vocal cords. The abductor fibers are more vulnerable, and moderate trauma causes a pure abductor paralysis (Semon’s law). Severe trauma causes both abductor and adductor fibers to be affected. Pure adductor paralysis does not occur as a clinical entity.

Difficult Airway

 
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Difficult Facemask Ventilation

         The ASA Task Force defined difficult mask ventilation as occurring when it is not possible for the unassisted anesthesiologist to maintain oxygen saturation more than 90% using 100% oxygen and positive pressure mask ventilation in a patient whose oxygen saturation was more than 90% before anesthetic intervention; and/or, it is not possible for the unassisted anesthesiologist to prevent or reverse signs of inadequate ventilation during positive pressure mask ventilation.

          Common reasons for difficult mask ventilation are:

      inadequate mask seal, excessive gas leak, or excessive resistance to the ingress or egress of gas.

         Signs of inadequate face mask ventilation include (but are not limited to:)

  • absent or inadequate chest movement,

  • absent or inadequate breath sounds,

  • auscultatory signs of severe obstruction,

  • cyanosis,

  • gastric air entry or dilatation,

  • decreasing or inadequate oxygen saturation (SpO2),

  • absent or inadequate exhaled carbon dioxide,

  • absent or inadequate spirometric measures of exhaled gas flow, and

  • hemodynamic changes associated with hypoxemia or hypercarbia (e.g. hypertension, tachycardia, arrhythmia).

             Important risk factors associated with difficult mask ventilation:

  • Obesity

  • Edentulous

  • History of snoring

  • Beard

  • Retro/micrognathia

  • Short neck with full teeth.

        Alternate techniques/adjuncts to difficult mask ventilation: 

  • Laryngeal mask airway

  • Oral/nasal airway

  • Esophageal Tracheal combitube

  • Rigid ventilating bronchoscope 

  • Invasive airway access (e.g. tracheostomy) 

  • Transtracheal jet ventilation.

Difficult Laryngeal Mask Ventilation

Inability within three insertions to place the laryngeal mask airway (LMA) in a satisfactory position to allow clinically adequate ventilation and to maintain airway patency (tidal volume >7ml/kg).

Difficult Laryngoscopy

It is not possible to visualize any portion of the vocal cords after multiple attempts at conventional laryngoscopy. This usually corresponds to Cormack and Lehane’s Grade IV laryngoscopic view.

Difficult Tracheal Intubation

Tracheal intubation requires multiple attempts, in the presence or absence of tracheal pathology.

The ASA Task Force defined difficult tracheal intubation as occurring when proper insertion of the tracheal tube with conventional laryngoscopy requires more than 3 attempts or more than 10 minutes.

Failed Intubation

Placement of the endotracheal tube fails after multiple intubation attempts.

Intubation Attempt

Intubation activities occurring during a single continuous laryngoscopy maneuver.

Optimal Best Attempt at Mask Ventilation

The first component of an optimal-best attempt at conventional mask ventilation is that it should be a two- person effort because far better mask seal, jaw thrust, and therefore tidal volume can be achieved with two persons versus one person. This may be achieved either by a proper two-person mask ventilation effort when the second person knows how to perform a jaw thrust, or when the second person is capable only of squeezing the reservoir bag with the first person giving jaw thrust and trying for a tight mask fit. The second component of an optimal, best attempt at conventional mask ventilation is to use large oropharyngeal or nasopharyngeal airways, or both. If mask ventilation is very poor or nonexistent with a vigorous two-person effort in the presence of large artificial airways, it is time to move on to a potentially organ-life saving plan B.

Optimal-Best Attempt at Laryngoscopy

Defined as involving

i. A reasonably experienced (at least 3 full recent years)laryngoscopist,

ii. Use of optimal sniffing position,

iii. No significant muscle tone,

iv. Use of optimal laryngeal external manipulation—OLEM or backward, upward, rightward pressure-BURP

v. Change of length or type of blade one time.

With this definition, and with no other confounding considerations, an optimal-best attempt at laryngoscopy may be achieved on the first attempt and should not take more than amaximum of three attempts.

Optimal laryngeal external manipulation (OLEM)

It should be an inherent part of laryngoscopy and is performed when laryngoscopic view is poor. 90% of the time, the best view is obtained by pressing over the thyroid cartilage or the cricoid cartilage. Pressing over the hyoid bone may also be effective. OLEM frequently can improve the laryngoscopic view by at least one whole grade.

Evaluation of the Airway

 

      Three basic decisions needed before induction of anesthesia in every patient are whether:

          a. To maintain spontaneous ventilation

          b. To use a percutaneous technique for invasive airway access

          c. To use awake endotracheal intubation

       These three strategies are safer than the use of an intravenous anesthetic with neuromuscular blocking drugs (NMBDs) in patients with potential airway difficulty, but they require more time and effort, and the anesthesiologist needs evidence on which to base these decisions. The purpose of airway assessment is to identify possible difficulty with direct laryngoscopy (and hence tracheal intubation), mask ventilation, or creation of a surgical (percutaneous) airway.

       Potential difficulty may be obvious in patients with anatomic or pathologic abnormalities, and further tests are not needed. Conditions requiring particular caution include lesions at the base of the tongue, recent onset of hoarseness, upper airway obstruction, and obstructive sleep apnea. However, the challenge is to detect potential difficulty in apparently normal patients.

       Airway assessment includes taking a history and performing a physical examination. Imaging is valuable in assessing a pathologic airway but is not practical for routine assessment.

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A. History

A thorough airway-relevant history must be obtained during the preoperative evaluation. The history includes review of available previous anesthesia records, direct questioning of the patient, and in those with reduced consciousness, a search for communication about previous airway difficulty. A history of previous airway difficulty has a higher positive predictive value and lower negative predictive value than any tests. However, a history of previous easy laryngoscopy does not guarantee straightforward intubation in as much as increased age or pathology may result in increased difficulty. Many congenital and acquired conditions are associated with difficult airway management (Table 1.3).

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B. Physical examination

Over the last 2 decades, several physical evaluation measures have become popularized, although their reproducibility

and predictability are disputed. The difficulty in developing the perfect airway evaluation tool lies in two interrelated areas: simplicity and interdependency. Simple bedside evaluation tools are useful, but adequate evaluation may require endoscopic, radiologic, or other currently uncommon examinations. Interdependency refers to the predictive value of one airway examination measure based on the findings of another. The common examination findings suggestive of difficult airway are shown in Table 1.4.

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On physical examination, concentrate on the following features:

  • Look for evidence of maxillary growth, hypoplasia, absence (postmaxillectomy)

  • Micro/macrognathia

  • Microstomia

  • Buck teeth/loose teeth/absent teeth/malocclusion of teeth

  • Swelling/scarring of the neck

  • Previous tracheostomy scar

  • Deviation of trachea

  • Evidence of a compromised airway, e.g. sternal retraction

  • Use of accessory muscles of breathing

  • Stridor-produced by turbulent airflow through narrowed air passage. In mild to moderate narrowing, stridor may not be present during quite breathing but can be appreciated when patient takes deep and rapid breaths. Inspiratory in supraglottic obstruction; Expiratory or biphasic in infraglottic lesion as in tracheal foreign body

  • Identify the location of the cricothyroid membrane for possible use in unexpected airway loss.

  • Determine if the patient is able to assume the sniffing position in the awake state.

 

The individual physical examination indices may be classified and described as given below:

i. Assessment of the adequacy of the oropharynx for laryngoscopy and intubation

Mallampati Classification:

         The Mallampati classification is elicited with the patient seated and the head in neutral position. The patient is asked to open the mouth as widely as possible and to protrude the tongue out as far as possible. The patient should not phonate during this evaluation. The observer then relates the size of the base of the tongue to the uvula, faucial pillars, and soft palate.

          Mallampati’s original supposition reasoned that if the base of the tongue was proportionate to the size of the oropharynx, then exposure of the laryngeal inlet would not be difficult during direct laryngoscopy. This statement presumed the absence of other factors predictive of the presence of a difficult airway.

This method of assessment originally was published by Mallampati as three distinct classes (Fig. 1.9):

          I. Uvula, faucial pillars, soft palate visible

        II. Faucial pillars and soft palate visible

        III. Only soft palate visible.

Samsoon and Young3 modified the Mallampati classification to include a fourth class:

         IV. Only hard palate visible.

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       The correlation between Mallampati classification and the laryngoscpoy view is shown in Table 1.5. Class I and II are associated with easy laryngoscopic view of the glottis. Class III and IV offer difficult or impossible viewing of the glottis by conventional laryngoscopy. Mallampati classification has a sensitivity of 0.4–0.67 and specificity of 0.52–0.84.

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        The class zero pharyngoscopic view, wherein the tip and posterior aspect of the epiglottis can be visualized on mouth opening and protrusion of the tongue has been added recently. First described by Ezri et al. The ventilation may be difficult in these patients due to large floppy epiglottis obstructing laryngeal inlet.

ii. Assessment of the mandibular space

The space anterior to the larynx can be expressed as thyromental or hyomental distance. Laryngoscopy pushes the tongue into this space and if reduced or narrowed, exposure to the glottis may be inadequate and the alignment of the pharyngeal and laryngeal axis may be compromised.

i. Thyromental distance (TMD): This is the distance between the thyroid notch and mental symphysis when the neck is fully extended (Patil’s test)TMD> 6.5 cm is reassuring; but TMD<6 indicates difficult laryngoscopy.

ii. Hyomental distance: This is the distance between the mentum and hyoid bone. Less than 4 cm hyomental distance is usually associated with difficult or impossible laryngoscopy and intubation.

iii. Sterno-mental distance: This is the distance from the suprasternal notch to the mentum. It is measured with the head fully extended on the neck with the mouth closed. A value of less than 12 cm is found to predict a difficult intubation.

iii. Assessment of tempero-mandibular joint (TMJ)

       The two functions of the TMJ are rotation of condyle in the synovial cavity and forward displacement of the condyle. The former is responsible for 2–3cm mouth opening and the latter for a further 2–3cm mouth opening. First, ask the patient to open his mouth wide and place his 3 fingers (index, middle and ring) in the mouth opening. If done, it indicates adequate mouth opening for laryngoscopy. Next, place the finger in front of the tragus and thumb in front of the lower part of the mastoid process behind the ear. Ask the patient to open his mouth wide. As the condyle of the madible slides forward, the index finger can be indented in its place and the thumb can feel the sliding of condyle. This suggests good subluxation of the lower jaw.

Other indices to assess TMJ are:

Interincisor distance: It is the distance between the upper and lower incisors.

Normal is 4.6 cm or more, while < 3.8 cm predicts difficult airway.

Subluxation (sLux): Maximum forward protrusion of the lower incisor beyond the upper incisor.

       Grade I—Lower incisor anterior to upper.

       Grade II—Lower incisor equal to upper

       Grade III—Lower incisor fail to reach upper and remained posterior

Upper lip bite test: Performed according to the following criteria:

       Class I—Lower incisors can bite the upper lip above the vermilion line;

       Class II—Lower incisors can bite the upper lip below the vermilion line;

       Class III—Lower incisors cannot bite the upper lip.

iv. Assessment of cervical and Atlanta-occipital joint

        These functions can be assessed directly and also indirectly, especially in patients with stiff-joint syndrome.

Direct assessment: First, ask the patient to touch his manubrium sterni with his chin. If he is able to do so, this assures neck flexion of 25–30°. Next, ask the patient to look at the ceiling without raising the eyebrows to check the extension (85°) at atlantooccipital joint. Two-third or complete reduction of extension at atlantooccipital joint is

an indicator of difficult laryngoscopy.

Indirect assessment: In patients with long-standing diabetes or stiffjoint syndrome, laryngoscopy may be difficult. They will have difficulty in approximating their palms and cannot properly extend their fingers (prayer sign). If present, it should alert regarding difficult laryngoscopy and intubation.

Another test is palm print test where the patient is asked to give his palm prints on a white paper kept over a firm surface. If phalangeal areas are hardly visible or only fingertips are printed, the laryngoscopy is likely to be difficult. This is based on the hypothesis that joint rigidity seen in diabetic patients due to tissue glycosylation may also involve the laryngeal and cervical joints leading to difficult laryngoscopy.

Palm print test may be categorized as:

      Grade 0: All phalangeal areas are visible.

      Grade 1: Deficiency in the interphalangeal areas of the 4th and 5th digits.

      Grade 2: Deficiency in the interphalangeal areas of 2nd to 5th digits.

      Grade 3: Only the tips of digits are seen.

v. Assessment of the quality of glottic view during laryngoscopy

     This may be assessed either by indirect mirror laryngoscopy or using “awake look” direct laryngoscopy with topical anesthesia and appropriate sedation. Cormack and Lehane5 published a landmark study classifying the view obtained during direct laryngoscopy into four grades (Fig. 1.10).

     Grade 1: visualization of entire laryngeal aperture.

     Grade 2: visualization of only the posterior portion of the laryngeal aperture.

     Grade 3: visualization of only the epiglottis.

     Grade 4: visualization of only the soft palate (no glottic structures seen).

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        Cook further subdivided the grades 2 and 3 as:

2a—Visualization of posterior part of the vocal cords

2b—Only arytenoids and epiglottis seen

3a—Epiglottis lifftable

3b—Epiglottis adherent or only tip visible

Group indices to predict difficult laryngoscopy and intubation

Wilson’s risk sum score:

Total risk score ranges from 0 to 10. The five parameters are given in Table 1.6.

A score more than 2 predicts 75% of difficult intubation.

A score of 4 or more predicts 90% difficult intubation.

However the sensitivity of Wilson’s scoring is less, comparable to that of Mallampati.

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Rapid airway assessment (1-2-3):

In emergency situation with severe time constraint, 1-2-3 finger assessment test may be performed rapidly to assess:

(a) Temporomandibular joint (TMJ) function (1 finger insinuated in TMJ—adequate subluxation of the lower jaw)

(b) Mouth opening (2 finger breadth; > 3 cm—adequate for 2 cm flange of direct laryngoscope) and

(c) Mandibular space (place 3 average sized index, middle and ring fingers in the submandibular space). This can be performed in less than 15 seconds.

LEMON Law

L = Look externally (facial trauma, large incisors, beard or moustache, large tongue)

E = Evaluate the 3-3-2 rule (incisor distance—3 finger breadths, hyoid-mental distance—3 finger breadths, thyroid-to-mouth distance— 2 finger breadths)

M = Mallampati (Mallampati score > 3).

O = Obstruction (presence of any condition like epiglottitis, peritonsillar abscess, trauma).

N = Neck mobility (limited neck mobility)

Patients in the difficult intubation group have higher LEMON scores.

 

Arne’s risk index

simplified score (Table 1.7)

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C. Radilogical examination

1. From skeletal X-ray films:

Lateral cervical X-ray film of the patients with head in neutral position closed is required for the following measurements:

   i. Mandibulohyoid distance:

An increase in the mandibulohyoid distance resulted in an increase in difficult laryngoscopy.
   ii. Atlantooccipital (AO) gap:

AO gap is the major factor which limits the extension of head on neck. Longer the AO gap, more space is available for mobility of head at that joint with good axis for laryngoscopy and intubation. Radiologically, there is reduced
space between C1 and occiput.

   iii. Relation of mandibular angle and hyoid bone with cervical vertebra and laryngoscopy grading:

A definite increase in difficult laryngoscopy was observed when the mandibular angle tended to be more rostral and hyoid bone to be more caudal, position of mandibular angle being more important.
    iv. Anterior/posterior depth of the mandible:
The posterior depth of the mandible (PDM) i.e. the distance between the bony alveolus immediately behind the 3rd molar tooth and the lower border of the mandible is an important measure in determining the ease or difficulty of laryngoscopy. PDM expressed as a ratio of the effective mandibular length (EML), is another useful predictor. EML is the distance between the tip of lower incisors to the temporomandibular joint (Fig 1.11). If the EML is less than 3.6
times the PDM, direct laryngoscopy will be difficult. The converse is also true.

   v. C1-C2 gap: Calcified stylohyoid ligaments are manifested by crease over hyoid bones on radiological examination. Laryngoscopy is difficult because of inability to lift the epiglottis from posterior pharyngeal wall as it is firmly attached to the hyoid bone by the hyoepiglottic ligament.

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2. Fluoroscopy for dynamic imaging

cord mobility, airway malacia, and emphysema.

3. Esophagogram

inflammation, foreign body, extensive mass or vascular ring.

4. Ultrasonography

assessment of anterior mediastinal mass, lymphadenopathy, differentiates cyst from mass and cellulitis from abscess.

5. Computed tomography/Magnetic Resonance Imaging—MRI

congenital anomalies, vascular airway compression.

6. Videooptical intubation stylets

combines viewing capability with the familiar handling of intubation devices.

Assessment of Pediatric Airway

 

      Assessment of difficult airway in pediatric patients, as in adults begins with a comprehensive history and physicalexamination.

History

     Questions regarding complaints of snoring, apnea, daytime somnolence, stridor, hoarse voice and prior surgery or radiation treatment to face or neck should be made. This information may indicate hypoxemia and pulmonary hypertension. History should also consist of a review of previous anesthetic records with attention being paid to history of oropharyngeal injury, damage to teeth, awake tracheal intubation or postponement of surgery following an anesthetic.

Physical examination

It should focus on the anomalies of face, head, neck and spine.

  • Evaluate size and shape of head, gross features of the face; size and symmetry of the mandible, presence of submandibular pathology, size of tongue, shape of palate, prominence of upper incisors, range of motion of jaw, head and neck.

  • The presence of retractions (suprasternal/sternal/infrasternal/intercostal) should be sought for they usually are signs of airway obstruction.

  • Breath sounds—Crowing on inspiration is indicative of extrathoracic airway obstruction whereas, noise on exhalation is usually due to intrathoracic lesions. Noise on inspiration and expiration usually is due to a lesionat thoracic inlet.

     Many investigators have attempted to develop methods that predict a difficult laryngoscopy in this age group. These methods have been primarily studied in adults and have variable sensitivity in children.

Predictive Tests for Pediatric Difficult Airway

Mallampati classification does not accurately predict a poor view of glottis during direct laryngoscopy in pediatric patients. Moreover, appropriate classification by Mallampati method is often hampered by a lack of cooperation in infants and young children. The mandibular space assessment is mainly suitable for older children as a predictor of difficult airway. Thus values for thyromental, hyomental and horizontal mandibular lengths do not exist for the pediatric population.

This places the pediatric anesthesiologist at a disadvantage and increases the likelihood of being confronted with an unexpected difficult airway. It underscores the need for a good history and importance for always being prepared for difficult airway.

Several tests may be done to predict a difficult airway in children.

a. Plain radiography—For evaluation of nasopharynx, pharynx, subglottic lesion and trachea.

b. CT scan and MRI can detect choanal atresia, lymphatic malformation of neck, mediastinal masses, etc.

c. Direct or indirect endoscopy of the upper and lower airway for functional assessment and diagnosis of a pathology in nasopharynx, supraglottic, glottic and subglottic areas.

d. Fluoroscopy—For assessment of dynamic pathology, e.g. airway malacia specially when stridor, cough and

dysphagia are present.

e. USG studies—To assist in evaluation of functional and organic airway disorders, assess the dynamic state of certain pathologies.

Difficult Airway Management

 

If a difficult airway is known or suspected, the anesthesiologist should:

      i. Inform the patient (or responsible person) of the special risks and procedures pertaining to management of the difficult airway.

      ii. Ascertain that there is at least one additional individual who is immediately available to serve as an assistant in difficult airway management.

      iii. Administer face mask preoxygenation before initiating management of the difficult airway. The uncooperative or pediatric patient may impede opportunities for preoxygenation.

      iv. Actively pursue opportunities to deliver supplemental oxygen throughout the process of difficult airway management. Opportunities for supplemental oxygen administration include (but are not limited to) oxygen delivery by nasal cannulae, face mask, laryngeal mask airway (LMA), insufflation, or jet ventilation during intubation attempts and oxygen delivery by face mask, blow-by, or nasal cannulae after extubation of the trachea.

Strategy for Intubation of the Difficult Airway

      The literature suggests that the use of specific strategies facilitates the intubation of the difficult airway. Although the degree of benefit for any specific strategy cannot be determined from the literature, there is strong agreement among consultants that a preplanned strategy may lead to improved outcome. The anesthesiologist should have a preformulated strategy for intubation of the difficult airway. The algorithm shown in Figure 1.12 is a strategy recommended by the Task force, the ASA difficult airway algorithm (ASA-DAA). This strategy will depend, in part, on the anticipated surgery, the condition of the patient, and the skills and preferences of the anesthesiologist.

       The strategy for intubation of the difficult airway should include:

1. An assessment of the likelihood and anticipated clinical impact of four basic problems that may occur alone or in combination:

a. Difficulty with patient cooperation or consent

b. Difficult mask ventilation

c. Difficult supraglottic airway placement

d. Difficult laryngoscopy

e. Difficult intubation

f. Difficult surgical airway access

2. A consideration of the relative clinical merits and feasibility of three basic management choices:

a. Awake intubation versus intubation after induction of general anesthesia

b. Use of noninvasive techniques for the initial approach to intubation versus the use of invasive techniques (i.e. surgical or percutaneous tracheostomy or cricothyrotomy)

c. Preservation of spontaneous ventilation during intubation attempts versus ablation of spontaneous ventilation during intubation attempts

3. The identification of a primary or preferred approach to:

a. Awake intubation

b. The patient who can be adequately ventilated but is difficult to intubate

c. The life-threatening situation in which the patient cannot be ventilated or intubated

4. The identification of alternative approaches that can be employed if the primary approach fails or is not feasible:

a. Table 1–8 displays common options for difficult airway management.

b. The uncooperative or pediatric patient may restrict the options for difficult airway management, particularly options that involve awake intubation. Airway management in the uncooperative or pediatric patient may require an approach (e.g. intubation attempts after induction of general anesthesia) that might not be regarded as a primary approach in a cooperative patient.

c. The conduct of surgery using local anesthetic infiltration or regional nerve blockade may provide an alternative to the direct management of the difficult airway, but this approach does not represent a definitive solution to the presence of a difficult airway, nor does it obviate the need for a preformulated strategy for intubation of the difficult airway.

5. The use of exhaled carbon dioxide to confirm tracheal intubation.

           The ASA-DAA truly becomes useful in the unanticipated difficult airway (Box B in Fig. 1.12, unable to intubate by direct laryngoscopy (DL) after the induction of anesthesia).

          When induction agents (with or without muscle relaxants) have been administered and the airway cannot be controlled, vital management decisions must be made rapidly. Typically, the clinician has attempted direct or video laryngoscopy and tracheal intubation after successful or failed anesthesia mask ventilation. Even if the patient’s oxygen saturation remains adequate throughout these efforts, the number of laryngoscopy attempts should be limited to three. As discussed earlier, significant soft tissue trauma can result from multiple laryngoscopies, thereby worsening the situation. First, mask ventilation should be instituted. If face mask ventilation is adequate, the ASA-DAA nonemergency pathway is entered. The clinician may, then turn to the most convenient and/or appropriate technique for establishing tracheal intubation, if needed. This might include, but is not limited to video laryngoscopy, intubation facilitated by a fiberoptic bronchoscope, LMA, LMA Fastrach, bougie, lighted stylet, or a retrograde wire. A surgical airway will sometimes be the most appropriate approach. When mask ventilation fails, the algorithm suggests supraglottic ventilation, via any LMA. If successful, the nonemergency pathway of the ASA-DAA has again been entered and alternative techniques of tracheal intubation may be used, if needed (e.g. perhaps LMA ventilation is adequate for the remainder of the surgical procedure).

       Should LMA ventilation fail to sustain the patient adequately, the emergency pathway is entered. The ASA- DAA suggests use of an esophageal-tracheal combitube, rigid bronchoscopy, transtracheal oxygenation, or a surgical airway.

     At any juncture, the decision to awaken the patient should be considered based on the adequacy of ventilation, the risk of aspiration, and the risk of proceeding with intubation attempts or the surgical procedure.

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Airway Approach Algorithm

       Rosenblatt suggested a preoperative decision tree: the airway approach algorithm (AAA), which is a simple one- pathway algorithm for entering into the ASA-DAA. Branch choice, like the previously noted statement from the ASA practice guidelines, is highly dependent on the clinician’s skill and experience (Fig. 1.13).

      1. Is airway control necessary? No matter how routine sedation or general anesthesia become, whether or not to make a patient apneic should always be considered seriously and alternatives should be contemplated.

      2. Could tracheal intubation be (at all) difficult? If there is no indication that rapid tracheal intubation by direct laryngoscopy (DL), indirect laryngoscopy (e.g. video laryngoscopy or other means familiar to the operator) will be difficult, the clinician may proceed with any technique (induction, DL, LMA, and so forth) as clinically appropriate. This is the essence of Box B of the ASA-DAA (prototypical case: rapid-sequence induction). If there is an indication, based on history or physical examination, that there may be difficulty with rapid tracheal intubation, the AAA is followed to the next question. By choosing to continue down the algorithm, the clinician is not assuming tracheal intubation difficulty, rather he or she is anticipating the viability of rescue maneuvers should difficulty occur.

      3. Can supraglottic airway (SGA) ventilation be used if needed? If the clinician thinks that there is a physical reason that SGA ventilation (by face mask, LMA, or other device) could be difficult, he or she is projecting the possibility that a juncture of “cannot intubate (question 2)—cannot ventilate (question 3)” could be reached. Because this is a preoperative algorithm, Box A of the ASA-DAA may be the preferred root entry point (Awake intubation; maintaining spontaneous ventilation).

     4. Is there an aspiration risk? As discussed earlier, the patient at risk for aspiration is not a candidate for elective SGA use. Because the AAA is a preoperative algorithm, and therefore allows the luxury of discretionary paths, the juncture of “cannot intubate/should not ventilate” can be avoided by entering the ASA-DAA at Box A.

     5. Will the patient tolerate an apneic period? Question 3 is difficult to answer and is highly dependent on the skills and experience of the clinician. Should intubation fail, and SGA ventilation is inadequate, the patient’s ability to sustain oxygen saturation will dictate the ability to tolerate an apneic period. Factors such as age, pregnancy, pulmonary status, abnormal oxygen consumption (e.g. fever), and choice of induction agents will influence this. If time to oxyhemoglobin desaturation is limited (limited time to correct hypoxemia), Box A may be prophylactically chosen.

    6. Can hypoxia be rapidly corrected through other means? The question that arises here is with access to the patient’s anterior trachea, the availability of equipment and knowledgeable personnel, and the experience of the operator. For example, if an error in judgment is made and the operator finds himself or herself in a cannot intubate/cannot ventilate scenario, will these conditions allow for using transtracheal jet ventilation (TTJV) to temporize the situation. All conditions may be right, but if the patient is morbidly obese or has had scarring or radiation over the larynx/trachea, this option may not be available.

          The exception to the AAA is the patient who is unable to cooperate owing to mental retardation, intoxication, anxiety, depressed level of consciousness, or age. This patient may still be approached by Box A, but awake intubation may need to be modified in favor of techniques that maintain spontaneous ventilation (e.g. inhalation induction).

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Regional Anesthesia in Difficult Airway

        The decision to proceed with regional anesthesia because the airway cannot be assessed or has been proven to be difficult to manage must be considered in terms of risks and benefits (Table 1.9).

        The ASA Closed Claims Database project has identified failure in regional anesthesia as a source of serious error when no airway strategy was prophylactically considered.

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