Acromegaly
Thick mandible, large tongue and epiglottis, overgrowth of mucosa and soft tissues of the pharynx, larynx and vocal cords, as well as arthritis at the temporomandibular joint may make mask ventilation and laryngoscopy difficult. Glottic and subglottic narrowing may require a smaller endotracheal tube size. Nasal intubation or placement of a nasal airway may be impossible due to nasal turbinate enlargement
Angioedema
Progressive swelling of the tongue and pharyngeal mucosa may make mask ventilation and laryngoscopy difficult or impossible
Ankylosing spondylitis
Flexion deformity of cervical spine may make direct laryngoscopy extremely difficult, if at all possible, and involvement of the temporomandibular joint (TMJ) will compound the problem further
Burns of the head and neck
Massive mucosal edema within 2–24 h from thermal damage to the upper airway may cause severe airway compromise and difficult laryngoscopy and intubation. Scars developing, as the burns heal, may limit mouth opening and neck mobility
Cervical spine limitations
Osteoarthritis, degenerative changes, fusion, etc. Limitations of cervical spine mobility (both extension and flexion) may render mask ventilation, laryngoscopy, and intubation difficult
Diabetes mellitus
Long-term diabetes may reduce atlanto-occipital joint mobility and make laryngoscopy difficult
Hypothyroidism
Development of myxedema and macroglossia make mask ventilation and laryngoscopy difficult
Infections
Epiglottitis, retropharyngeal and submandibular abscess, Ludwig’s angina. Airway may be severely distorted making mask ventilation and laryngoscopy and intubation extremely difficult
Irradiation
To the head and neck (fibrosis) may make mask ventilation and laryngoscopy difficult to impossible
Obstructive sleep apnea
Anatomical and physiological features of obstructive sleep apnea (OSA) reduce the skeletal confines of the tongue, change the shape of the airway, and predispose to both difficult mask ventilation (DMV) and difficult intubation (DI). DI is related to the severity of OSA: patients with apnea-hypopnea index > 40 have a higher incidence of difficult intubation
Pregnancy
DI is reported to be 1.3–16.3 % in parturients, with an incidence of failed intubation around 1:300 to 1:800, which is higher than the general population. Difficulties in airway management are attributed to generalized soft tissue swelling, which may cause macroglossia, supraglottic edema, and increased tissue friability. Laryngeal edema worsens during labor and pushing. Weight gain with deposition of fat around the neck, breast engorgement, positioning requirements, cricoid pressure may all interfere with laryngoscopy
Rheumatoid arthritis
TMJ involvement leads to limited mouth opening, cervical spine arthritis, impaired neck mobility with subsequent DMV and DI. Atlantoaxial subluxation compounds the problem and increases the risk of spinal cord injury
Scleroderma
Small mouth with decreased opening and tight facial skin, hardening of the submandibular tissues make laryngoscopy difficult
Trauma
Maxillary or mandibular injury, cervical spine injury, neck trauma or surgery with edema, hematoma, airway disruption
Tumors
Maxillofacial region, oropharyngeal, laryngeal, or neck malignancies distort the anatomy
Miscellaneous
Lingual tonsil hypertrophy, laryngeal papillomatosis, laryngeal sarcoidosis, foreign bodies may lead to airway obstruction and difficult mask ventilation and intubation
Table 4.2
Congenital syndromes associated with a difficult airway
Down’s | Obstructive sleep apnea, small mouth opening, large tongue, subglottic stenosis, atlantoaxial instability |
Goldenhar | Hemifacial microsomia, cervical vertebral anomalies, scoliosis |
Klippel-Feil | Congenital synostosis of some or all of cervical vertebrae resulting in neck rigidity |
Pierre Robin | Micrognathia, cleft palate, glossoptosis, small mouth |
Treacher Collins | Maxillary, zygomatic, and mandibular dysplasia |
Turner | Short neck with limited mobility, contracture of the temporomandibular joint, maxillary and mandibular hypoplasia |
A prior history of airway management should be carefully reviewed for any difficulties with mask ventilation, laryngoscopy, intubation, or supraglottic airway placement. It has been reported that a history of difficult or failed intubation by direct laryngoscopy, as a stand-alone test, has a likelihood ratio of approximately 6 and 22, respectively, for the prediction of subsequent difficult or failed intubation. For the test to be regarded as a powerful discriminator, a likelihood ratio over 10 should be present, which means that a history of failure is a better predictor of subsequent problem with intubation than a history of difficulty. Nonetheless, any prior difficulties should be taken very seriously, and an anesthesia provider should formulate a plan for airway management. It is also important to document any encountered airway problem and notify the patient.
If there are additional studies available, such as chest X-ray, CT scan, or flexible laryngoscopy, the results should be carefully reviewed to identify possible problems: deviation and compression of the trachea, degree of airway compression and its localization, evidence of distorted laryngeal anatomy, etc.
Physical Examination
An anesthesia provider should be aware and look for signs and symptoms of airway obstruction: marked respiratory distress, intolerance of supine position, altered voice, dysphagia, odynophagia, and the hand-to-throat choking sign. Stridor is a sign of imminent airway obstruction and indicates that the airway diameter has been reduced to 4 mm or less.
Physical examination should start with the basics: consciousness level, presence of any intoxication, and language barrier. This piece of information may profoundly influence the choice for airway management from the beginning. Any facial abnormalities, presence of facial trauma, beard, and the body habitus should be noted. A focused airway examination should be part of the evaluation of any patient presenting for anesthesia. The LEMON criteria can be used for simple airway assessment (Table 4.3).
Table 4.3
LEMON score for airway assessment
L = Look externally | Facial trauma, narrow mouth, short thick neck, large incisors, presence of a beard, protruding jaw, large tongue |
E = Evaluate | The 3-3-2 rule Inter-incisor distance (mouth opening)—normal >3 fingerbreadths Hyoid-mental distance—normal >3 fingerbreadths Thyroid cartilage-mouth floor distance—normal >2 fingerbreadths |
M = Mallampati score | Class I–IV |
O = Obstruction | Presence of any condition that could cause an obstructed airway (abscess, hematoma, epiglottitis, tumor) |
N = Neck mobility | Check for neck flexion, extension, and limited neck mobility (avoid in patients with neck injury) |
1.
Mallampati score
The patient should be in sitting position (if possible), with the neck in neutral position for proper assessment. The mouth should be opened maximally and the tongue protruded without phonation. An observer grades the view depending on oropharyngeal structures seen (Fig. 4.1).
Fig. 4.1
Mallampati airway classification
Class I—soft palate, fauces, uvula, and tonsillar pillars (anterior and posterior) visible
Class II—soft palate, fauces, and uvula visible
Class III—soft palate and base of the uvula visible
Class IV—soft palate is not visible at all
Although Mallampati classes III and IV correlate with almost sixfold increase of difficult intubation, only about 35 % of the patients with difficult intubation are correctly identified using the score.
2.
Jaw protrusion test or its modification—upper lip bite test (ULBT). ULBT evaluates the presence of mandibular subluxation and buckteeth at once. Additionally, one should look for a recessed mandible or protruding jaw.
Class I—lower incisors can bite above the vermilion border of the upper lip
Class II—lower incisors cannot reach vermillion border
Class III—lower incisor cannot bite upper lip
3.
Dentition should be assessed and findings documented: prominent upper incisors (protruding teeth), loose or missing teeth, dentures.
4.
Neck range of motion: both flexion and extension are checked, and any neurological changes with the movement of the cervical spine noted. Normal neck extension at atlanto-occipital joint is 35°.
5.
Mouth opening: normal inter-incisor distance is 4–6 cm (>3 finger breadths).
6.
Thyromental distance: mentum to upper border of thyroid cartilage is measured (normal >3 ordinary finger breadths, corresponds to 6 cm).
7.
Compliance of submandibular space should be checked: it is the space where the tongue is displaced during direct laryngoscopy.
8.
Miscellaneous: large tongue, short and thick neck, deviated trachea.
9.
Presence of any airway pathology (tumor, abscess).
Prediction of Difficult Mask Ventilation
Standard definition of difficult mask ventilation (DMV) is lacking at present, which may be related to the very subjective and operator-dependent nature of the skill. Risk factors for DMV are listed in Table 4.4. The acronym “OBESE” can be used to remember the predictors of DMV (O-obese, B-bearded, E-elderly, S-snorers, E-edentulous). The incidence of DMV has been reported to be 1.4–2.2 %, while that of impossible mask ventilation 0.15 %. Although DMV does not necessarily mean difficult intubation, there is a relationship between the two. Patients with DMV have a fourfold increase in the incidence of difficult intubation and a 12-fold increase in the incidence of impossible intubation. ASA definition for DMV is as follows:
Table 4.4
Risk factors for difficult mask ventilation
BMI > 30 kg/m2 |
Presence of a beard |
History of snoring/obstructive sleep apnea |
Age > 55 years |
Mallampati III or IV |
Limited mandibular protrusion test |
Airway masses/tumors |
Male gender |
Edentulous state |
Neck radiation changes (strong predictor) |
It is not possible for the anesthesiologist to provide adequate ventilation because of one or more of the following problems: inadequate mask or SGA seal, excessive gas leak, or excessive resistance to the ingress or egress of gas. Signs of inadequate 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).
The use of Han’s Mask Ventilation and Description Scale may be recommended for clinical description of mask ventilation:
Grade 0—ventilation by mask not attempted
Grade 1—ventilated by mask
Grade 2—ventilated by mask with oral airway or other adjuvants
Grade 3—difficult mask ventilation (inadequate, unstable, or requiring two practitioners)
Grade 4—unable to mask ventilate
Prediction of Difficult Intubation
To date there is no international agreement on the definition of “difficult intubation.” The American Society of Anesthesiologists defines difficult intubation as tracheal intubation requiring multiple attempts, in the presence or absence of tracheal pathology. Often the terms “difficult intubation” and “difficult laryngoscopy” are used interchangeably, though difficult laryngoscopy does not always lead to difficult intubation. With difficult laryngoscopy, it is not possible to visualize any portion of the vocal cords after multiple attempts at conventional laryngoscopy (Cormack-Lehane Grade 3 and Grade 4 view of glottic opening). The reported incidence of DI varies and may be as high as 10.3 % for emergent intubation, with the incidence of failed intubation from 0.05 to 0.35 %. Generally accepted predictors of difficult intubation are listed in Table 4.5.
Table 4.5
Predictors of difficult intubation
History of prior difficult intubation |
Long, protruding upper incisors |
Prominent “overbite” (maxillary incisors override mandibular incisors) |
High ULB test scores (failed TMJ translation) |
Inter-incisor distance less than 3 cm |
Mallampati Class III or IV |
Noncompliant submandibular space |
Thyromental distance less than 6 cm (three ordinary finger breadths) |
Highly arched or very narrow hard palate |
Short thick neck |
Limited cervical spine range of motion (flexion or extension) |
BMI > 35 kg/m2 |
Conventional teaching requires establishing mask ventilation after induction of anesthesia before giving muscle relaxants in fear of not returning to spontaneous ventilation and the ability to wake up a patient in case of difficulties with airway management. Some data suggests that avoidance of neuromuscular blocking agents may actually increase the risk of difficult tracheal intubation. That may especially be the case with high-dose opioids sometimes producing vocal cord adduction. The continuing practice of mandatory conformation of ventilation before administration of muscle relaxants contradicts the widely accepted practice of rapid sequence induction, where total muscle paralysis is achieved without any such conformation.
Since none of the current tests can reliably predict difficult airway in patients whose airway looks “normal,” it is imperative for the anesthesia provider to be prepared to deal with unforeseen difficulties at any time.
Prediction of Difficult Insertion of Supraglottic Airway Devices
In spite of the worldwide use of numerous supraglottic airway devices (LMA Classic used in about 200 million anesthetics), data on predictors of difficult insertion and predictors of failure of such airway devices are lacking. Supraglottic airway devices are incorporated in the ASA Difficult Airway Algorithm as rescue devices in the “cannot intubate, cannot ventilate” situation and have been shown to be effective in such scenarios on multiple occasions.
Limited mouth opening and restricted atlanto-occipital joint range of motion insertion (especially fixed flexion deformity of the neck) may present difficulties during laryngeal mask airway (LMA) insertion. LMA is also not recommended for use in patients with oropharyngeal pathology. One large retrospective study identified four independent risk factors of LMA Unique failure (defined as an acute airway event requiring LMA Unique removal and rescue intubation): surgical table rotation, male sex, poor dentition (missing teeth), and increased BMI.
Prediction of Difficult Videolaryngoscopy
Videolaryngoscopy is a rapidly developing technique in airway management with continuous addition of new and improved devices. But as with the supraglottic airway devices, data on prediction of difficulties with videolaryngoscopy is lacking. It is possible that predictors may be somewhat different for different groups of the videolaryngoscopes (Macintosh-type blades vs. highly curved blades vs. devices with tube-guiding channels).
It has been shown, however, that for the GlideScope most of the standard predictors of difficult laryngoscopy, with the possible exception of high ULB test score, are not predictors of intubation difficulties. The strongest predictor of the GlideScope failure is altered neck anatomy with presence of a surgical scar, radiation changes, or mass.
Prediction of Difficult Surgical Airway
Emergent surgical airway usually is the last resort for the anesthesiologist, but occasionally it may be the only option for airway management. Data on the predictors of difficult emergent tracheostomy or cricothyrotomy is very limited since the occurrence of the event is rare. Most of the difficulties are related to inaccurately localizing the trachea or a cervical spine flexion deformity.
A short thick neck, obesity, neck masses (hematoma, infectious process, goiter, packets of lymphatic nodes), burns, or radiotherapy can make localization of the trachea difficult, especially in an emergency. In such cases real-time ultrasonography of the neck may be helpful. Ultrasonography may be used to identify and mark the trachea or cricothyroid membrane or place a transtracheal catheter before attempting airway management in cases of suspected difficult airway and/or difficult surgical airway.
Airway Management
Nonintubation Airway Management Techniques and Equipment
Management of the airway during anesthesia does not always call for tracheal intubation or supraglottic airway device placement. In cases of regional anesthesia, procedural sedation, and total intravenous anesthesia with spontaneous respiration, it may be sufficient for the anesthesiologist to provide supplemental oxygen and ensure an unobstructed airway. It is important to know that any type of oxygen therapy is a potential fire hazard, especially when the surgical site is close to the airway or an oxygen source and cautery is being used.
Management of nonintubated patients with spontaneous respirations includes continuous monitoring of end-tidal CO2, respiratory pattern, and oxygen saturation. While nonintubated, the anesthesia provider must be aware of the potential for partial or total upper airway obstruction and treat it accordingly. Obstruction can happen at the pharyngeal level (loss of pharyngeal muscle tone, anatomic airway abnormalities, space-occupying lesions, foreign bodies), at the hypopharyngeal level (epiglottis obstructing the airway), and at the laryngeal level (laryngospasm, foreign bodies, secretions).
Partial airway obstruction often manifests with noisy expiration or inspiration (snoring, stridor). Complete airway obstruction is a medical emergency and manifests with absence of chest expansion with inspiratory effort, inaudible breath sounds, absence of perceivable air movement, use of accessory muscles, and sternal, epigastric, and intercostal retractions with inspiration. Airway patency may be established with simple maneuvers: head tilt-chin lift and jaw thrust. Some describe “the triple maneuver”: head tilt, jaw thrust, and mouth opening or head tilt, flexion at lower cervical spine, and jaw thrust. Head tilt-chin lift is contraindicated in patients with cervical spine instability and basilar artery syndrome; jaw thrust is contraindicated in patients with a fractured or dislocated mandible and awake patients. All secretions should be suctioned.
Oxygen Delivery Systems
Oxygen delivery systems may be divided into low-flow systems (most commonly used perioperatively) and high-flow systems. Flow systems should not be confused with delivered oxygen concentration: high-flow devices (such as a Venturi mask) can deliver FiO2 (fraction of inspired oxygen) as low as 0.24, while low-flow devices (such as a nonrebreathing mask) can deliver an FiO2 of 0.9 or more. With high-flow systems the patient’s ventilatory demand is completely met by the system, but if a system fails to meet the ventilatory demands of the patient, it is classified as a low-flow system.
Low-Flow Systems/Devices
They include nasal cannulas, simple face masks, partial rebreathing masks, nonrebreathing masks, face tent, tracheostomy collar, and transtracheal catheter.
Nasal cannulas
These are simple, easy tolerated by patients, and require that the nasal passages be patent. Nasal cannulas allow an FiO2 delivery of approximately 0.24–0.44, with oxygen flow rates from 1 to 6 L/min. For each 1 L/min increase in flow, the FiO2 increases approximately by 4 %, though the FiO2 can be inaccurate and inconsistent depending on the inspiratory demand of the patient (variable amount of room air entrained with different tidal volumes). Increasing the oxygen flow rate above 6 L/min does not increase the FiO2 much further than 0.44. The use of >4 L/min O2 flow requires a humidifier to prevent the mucous membranes from drying and crusting, epistaxis, or causing laryngitis.
Simple face masks
These allow a higher FiO2 due to increase in the size of the O2 reservoir (100–200 mL as additional O2 reservoir volume). An FiO2 of 0.4–0.6 can be achieved with O2 flows of 5–8 L/min. O2 flow should be at least 5 L/min to prevent CO2 accumulation and rebreathing. Gas flows >8 L/min do not increase FiO2 significantly over 0.6.
Partial rebreathing masks
These are simple masks with a reservoir bag (600–1,000 mL). An FiO2 of 0.6–0.8+ can be achieved with an oxygen flow of 6–10 L/min. Partial rebreathing occurs because the first 33 % of the exhaled volume derived from anatomic dead space fills the reservoir bag and subsequently gets inhaled with the fresh gas during the next respiratory cycle. To minimize rebreathing, the O2 flow should be kept at 8 L/min or more, sufficient to keep the reservoir bag 1/3 to 1/2 inflated during the entire respiratory cycle.
Nonrebreathing masks
These have three unidirectional valves allowing venting of exhaled gas and preventing room air entrainment. Oxygen flows of 10–15 L/min are used to deliver an FiO2 of 0.8–0.9. If room air is not entrained from around the mask, an FiO2 of 1.0 can be potentially achieved with 15 L/min of oxygen flow.
High-Flow Devices
They include Venturi masks, high-flow nasal cannulas, air entrainment nebulizers, and air-oxygen blenders.
Venturi masks
Two types of Venturi masks are available: a fixed FiO2 model with color-coded specific attachments and a variable FiO2 model with a graded adjustment. Venturi masks use the Bernoulli principle and constant-pressure jet mixing to entrain air and provide the needed FiO2. Alterations in the gas orifice or entrainment port size change the FiO2. The oxygen flow determines the total gas flow by the device, not the FiO2. The minimum recommended O2 flows for a certain FiO2 should be used with the standard air-O2 ratios. Venturi masks provide reliable FiO2 of 0.24–0.5 and are very useful in patients in respiratory distress, as delivered FiO2 is not dependent on the patient’s inspiratory demand. As FiO2 increases, the total gas flow decreases due to reduction in air entrainment.
High-flow nasal cannulas
Oxygen gas flow through regular low-flow nasal cannulas is limited to 16 L/min. High gas flows through regular nasal cannulas can cause patient discomfort, frontal sinus pain, irritation, and drying of the nasal mucosa because of lack of humidification. High-flow nasal cannulas (HFNC) have the advantages of providing warmed and humidified gas flows up to 50 L/min with FiO2 0.72–1.0. HFNCs offer independent adjustments of FiO2 and gas flow, a design feature which allows greater flexibility to match the needs of acutely ill patients. In addition, they generate moderate level of continuous positive airway pressure (CPAP), thereby improving pulmonary dynamics. HFNCs can be useful in patients with marginal oxygenation, for whom removing a face mask for eating, drinking, or the need to frequently expectorate to clear pulmonary secretions could precipitate hypoxemia.
Pharyngeal Airways
Oropharyngeal and nasopharyngeal airways of different sizes (correct size—distance from lip to ear lobe) are available to assist in establishing the upper airway patency in the nonintubated patient (Fig. 4.2). Oropharyngeal airways, if used in lightly anesthetized patients with intact pharyngeal and laryngeal reflexes, may lead to airway hyperreactivity (coughing, gagging with emesis, laryngospasm, bronchospasm). Oropharyngeal airways can cause trauma to oropharyngeal structures, including dental trauma.
Fig. 4.2
(a) Nasal and oral airways of different sizes. (b) Insertion technique of a nasal airway. The nasal airway is always lubricated prior to insertion. (c) Sizing of an oral airway (distance from lip to ear lobe). (d) Insertion technique of an oral airway. Once the airway touches the hard palate, it is rotated 180° and seated in the mouth. If a tongue depressor is used to insert an oral airway, then the oral airway is inserted with the airway’s curvature following the curvature of the patient’s airway
Nasopharyngeal airways are inserted with adequate lubrication and are better tolerated than oral airways by awake or lightly anesthetized patients. They may be preferable in cases of oropharyngeal trauma. Complications of nasopharyngeal airways include epistaxis, submucosal tunneling, avulsion of the turbinates, and pressure ulcers. There are some contraindications (absolute and relative) to the use of nasopharyngeal airways: nasal fractures, known nasal airway occlusion, coagulopathy, cerebrospinal fluid rhinorrhea, known or suspected basilar skull fracture, adenoid hypertrophy, and prior transsphenoidal hypophysectomy.
Mask Ventilation
A proper bag-mask ventilation technique is one of the fundamental skills required for every anesthesiologist (Fig. 4.3).
Fig. 4.3
Bag-mask ventilation. (a) Aligning the external auditory meatus with the sternal notch. (b) One-provider technique: the “EC” hand position sealing the mask on the face. (c) Two-provider technique: one provider holds the mask with both hands, while the second provider squeezes the bag
Uses
Mask ventilation technique is minimally invasive and is used for assisted or controlled ventilation during resuscitation, for preoxygenation with spontaneous ventilation; during sedation with inadequate spontaneous ventilation, as a transitional airway technique after induction; and before intubation or after extubation, for general anesthesia by mask, and in case of failed endotracheal intubation. It is minimally stimulating and can be performed even on an awake patient and does not require neuromuscular blockers.
Characteristics of a face mask
The standard face mask has three parts: a body, an air-filled cushion rim, and a connector. The most common style of mask used nowadays is a disposable, transparent plastic mask (allows to see the condensation from exhalation, the presence of any secretions or vomiting, and the patient’s color). Masks come in different sizes, are designed to fit different contours of the patient’s face, and provide adequate seal for leak-free ventilation (spontaneous and controlled). Some masks still come with a collar around the connector and hooks to allow attachment of straps for hands-free airway maintenance. With the wide use of supraglottic airway devices, such a technique is now largely of historical interest. Most of the masks are made to cover both the nose and the mouth of the patient, but there are also nasal masks covering only the nose and potentially creating a better seal and causing less obstruction during controlled ventilation even in the neutral position.
Prerequisites for mask ventilation
Mask ventilation requires a few things for success: the airway must be patent, the seal between the mask and the patient’s face must be effective, and the mask should be attached to a bag-valve system (anesthesia circle system in the operating room or air-mask-bag unit; “Ambu” bag outside the operating room).
Techniques of mask ventilation
There are two techniques for mask ventilation: the “one person” technique and the “two person” technique. With the one-person technique, an anesthesia provider uses one hand to hold the mask while the second hand squeezes the bag to provide positive pressure ventilation. Usually, the thumb and index finger are placed on the body of the mask to apply downward pressure to achieve a good seal, at the same time using the middle and ring fingers to lift the chin and pull the mandible toward the mask, while the little finger hooks under the angle of the mandible to lift it anteriorly. These maneuvers lead to upper cervical extension as well. With the two-person technique, one person applies the mask and establishes a patent airway with a good seal using both hands, while the second person squeezes the bag.
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