Sniffing position

The presence or absence of airway pathology does not influence the definition of difficult tracheal intubation. It occurs when multiple attempts at intubation are required. Traditional laryngoscopy is performed to visualize the laryngeal opening. The laryngoscopist is positioned outside the airway, above the patient’s head. To see the airway, light must travel from the glottic opening to the laryngoscopist’s eye. This technique requires an uninterrupted linear path between the larynx and laryngoscopist because light generally travels in straight lines. Most manipulations performed attempt to satisfy this criterion.

The airway contains three visual axes—the long axes of the mouth, oropharynx, and larynx. In the neutral position, these axes form acute and obtuse angles with one another. Light cannot bend around these angles under normal circumstances. To bring all three axes into better alignment, Magill suggested the “sniffing the morning air” position. True sniffing position requires both cervical flexion and atlanto-occipital extension. Cervical flexion approximates the pharyngeal and laryngeal axes. Atlanto-occipital extension brings the oral axis into better alignment with the other two axes. Normal atlanto-occipital extension measures 35 degrees. With optimal alignment of the airway’s visual axes, it becomes possible to look through the airway into the laryngeal opening. Reduced atlanto-occipital gap or prominent C1 spinous processes impair laryngoscopy if vigorous attempts at extension are performed because the larynx is forced anteriorly causing the trachea to bow.

Inability to assume the sniffing position is a predictor of difficult intubation. Examples of problems that prevent the sniffing position include cervical vertebral arthritis, cervical ankylosing spondylitis, unstable cervical fractures, protruding cervical disks, atlantoaxial subluxation, cervical fusions, cervical collars, and halo frames. Morbidly obese patients sometimes have posterior neck fat pads that prevent atlanto-occipital extension.

The ability to achieve the sniffing position is easily tested. The clinician has the patient flex the lower cervical vertebrae and extend at the atlanto-occipital joint. Pain, tingling, numbness, or inability to achieve these maneuvers predicts difficult intubation.

The benefits of the sniffing position have been dogma for >70 years. More recently, Adnet et al. (2001) and Chou and Wu (2001) have independently questioned its utility.

Mouth opening

Mouth opening is important because it determines the available space for placing and manipulating the laryngoscope and tracheal tube. A small mouth opening may not accommodate either one. Mouth opening also facilitates visualization of the uppermost part of the airway. Mouth opening relies on the temporomandibular joint (TMJ), which works in two ways. It has both a hingelike movement and a gliding motion, known as translation. Its hingelike movement allows the mandible to pivot on the maxilla. The more the mandible swings away from the maxilla, the bigger the mouth opening. The adequacy of mouth opening is assessed by measuring the interincisor distance. An interincisor distance of 3 cm provides sufficient space for intubation. This corresponds approximately to the width of two fingerbreadths. The two-fingerbreadth test is performed by placing the examiner’s second and third digits between the patient’s central incisors. If they fit, there should be adequate room to perform laryngoscopy. If they do not fit, laryngoscopy may be difficult.

Factors that interfere with mouth opening include masseter muscle spasm; TMJ dysfunction; and various integumentary ailments, such as burn scar contractures and progressive systemic sclerosis. Masseter muscle spasm may be relieved by induction of anesthesia and administration of muscle relaxants. TMJ mechanical problems remain unaltered by medications. Some patients demonstrate adequate mouth opening when awake but not after anesthetic induction. The problem often can be relieved by pulling the mandible forward. A mouth opening that was sufficient for a previous anesthetic may not be after temporal neurosurgical procedures.

Dentition

Instrumentation of the airway places teeth at risk for damage. Multiple problems result from dental injury. Teeth may be dislodged or broken. Such teeth cannot be used for chewing, may be painful, and are costly to repair. Beyond these issues, broken teeth can fall into the trachea, migrate to the lung, and predispose to abscesses. Poor dentition is at risk for damage as the mouth is opened and as the laryngoscope blade is introduced. Teeth that can be extracted easily with digital pressure should probably be removed. During laryngoscopy in the presence of poor dentition, extra efforts are made to avoid placing pressure on the maxillary incisors. In doing so, the laryngoscope is manipulated into a less than ideal position resulting in poor visualization of the glottis.

Prominent maxillary incisors complicate laryngoscopy in another way. They protrude into the mouth and block the line of sight to the larynx. To overcome this problem, laryngoscopists must adjust their line of sight. To accomplish this, the laryngoscopist’s eye is brought to a new position that is higher than the original one. The laryngoscopist looks tangentially over the protruding maxillary incisor; this creates two new points in the adjusted line of sight and a new straight line of sight. The new line of sight brings the laryngoscopist’s view to a more posterior laryngopharyngeal position, resulting in a view that is posterior to the larynx. Consequently, the larynx is not visualized, and a difficult laryngoscopy is produced. In much the same way, edentulous patients tend to be easy intubations because the laryngoscopist can adjust the line of sight to a more advantageous angle.

Tongue

The tongue occupies space in the mouth and oropharynx. The base of the tongue resides close to the glottic aperture. During traditional direct laryngoscopy, the base of the tongue falls posteriorly obstructing the line of sight into the glottis. Visualizing the larynx requires displacing the base of the tongue anteriorly so that the line of sight to the glottis is restored. The tongue is frequently displaced with a handheld rigid laryngoscope, to which Macintosh and Miller blades are most commonly attached. Laryngoscopes push the tongue anteriorly and in so doing, move it from a posterior obstructing position to a new anterior nonobstructing position within the mandibular space. The mandibular space is the area between the two rami of the mandible. Even with the tongue maximally displaced into the mandibular space, visualization of the larynx is sometimes inadequate.

Usually, a normal-size tongue fits easily into a normal-size mandibular space, whereas a large tongue would fit poorly. After filling the space, a large tongue still occupies some of the oropharyngeal airway causing obstruction. For this reason, a large tongue (macroglossia) is a predictor of a difficult intubation. Similarly, a normal-size tongue fits poorly into a small mandibular space. Consequently, it occupies some of the oropharyngeal airway, obstructing the line of sight. For this reason, a small mandible (micrognathia) is a predictor of difficult intubation. In essence, a tongue that is large compared with the size of the mouth, oropharynx, and mandible takes up excessive space in the oropharynx and interferes with visualization.

The base of the tongue resides so close to the larynx that inability to displace it adequately anteriorly creates another problem. The base of the tongue hangs down over the larynx, and the glottis is hidden from view. The glottic aperture is anatomically anterior to the base of the tongue—hence the term “anterior larynx.” Under such circumstances, the larynx is anterior to the base of the tongue and cannot be seen because the tongue hides it. Glottic and supraglottic masses that force the base of tongue posteriorly can create difficult intubations as well. Some of the masses that may be encountered include lingual tonsils, epiglottic cysts, and thyroglossal duct cysts.

After filling the mandibular space with the tongue, additional pressure on the laryngoscope blade lifts the mandible anteriorly. In this setting, mandibular displacement is dependent on the TMJ. In addition to its hingelike motion, the TMJ works in a gliding (translational) movement. The gliding motion allows the mandible to slide anteriorly across the maxilla. If the joint does not translate, the mandible cannot be displaced anteriorly, and the tongue cannot be moved out of the line of sight.

Recognizing the implications of tongue size to successful laryngoscopy, Mallampati et al. in 1985 and Samsoon and Young in 1987 devised classification systems to predict difficult laryngoscopy using this concept. A difficult laryngoscopy occurs when it is impossible to visualize any portion of the vocal cords. Mallampati and Samsoon reasoned that a large tongue could be identified on visual inspection of the open mouth. Both classification systems relate the size of the tongue to the oropharyngeal structures identified. A normal-size tongue allows for visualization of certain oropharyngeal structures. As the tongue size increases, some structures become hidden from view. Consequently, both investigators proposed systems that reason backward from this premise.

Application of the Mallampati or Samsoon classification system is easy and painless. The patient is seated in the neutral position and asked to open the mouth wide and protrude the tongue as much as possible without phonation. Phonation is discouraged because it raises the soft palate and allows for visualization of additional structures. The observer looks for specified anatomic landmarks: the fauces, pillars, uvula, and soft palate. The Mallampati classification system uses three groups, and the Samsoon classification system uses four groups ( Figure 43-1 ). Both systems suggest that as the tongue size increases, fewer structures are visualized, and laryngoscopy becomes more difficult. Mallampati scores tend to be higher in pregnant compared with nonpregnant patients.

Samsoon classification of pharyngeal structures.

(From Samsoon GLT, Young JRB: Difficult tracheal intubation: a retrospective study. Anaesthesia 42:487, 1987.)

Just as the size of the tongue can be estimated, so too can the size of the mandible. This estimation is accomplished by asking the patient to extend the head at the atlanto-occipital joint and identifying the mandibular mentum and thyroid cartilage. The thyroid notch (Adam’s apple) is the most superficial structure in the neck and serves as a good landmark for the thyroid cartilage. The vocal cords lie just caudad to the thyroid notch. The distance between the thyroid cartilage and mentum (thyromental distance) is measured in one of three ways. The measurement can be made with a set of spacers, a small pocket ruler, or the observer’s fingers. The normal thyromental distance is 6.5 cm. A thyromental distance >6 cm is predictive of an easy intubation. A thyromental distance ≤6 cm is suggestive of a difficult intubation. Rulers often are not present at the bedside. In the absence of a ruler, practitioners can judge the thyromental distance with their fingers. By knowing the width of one’s middle three fingers, which frequently approximates 6 cm, the thyromental distance can be compared with the fingers’ span. In this way, clinically relevant approximations can be taken into account when examining patients for the purpose of predicting difficult intubation. The usefulness of predicting difficult intubation based on thyromental distance has been challenged. Data from et al. (1992) and El-Ganzouri (1996) show that a decreased thyromental distance (receding mandible) offers a ≤7% probability of predicting difficult intubation. Chou (1993) and Brodsky et al. (2002) described patients whose thyromental distances were >6.5 cm and who were difficult intubations.

Similar measurements and predictions have been made using the hyoid bone and mandible as well as the sternum and mentum. Chou and Wu (2001) suggested that a long mandibulohyoid distance predicts a large hypopharyngeal tongue, which hides the glottis during laryngoscopy and produces a difficult intubation. They reasoned that the tongue is hinged to the hyoid bone so that a long hyomandibular length represents a caudad-lying tongue. With the base of the tongue positioned farther inferiorly, it occupies more space in the oropharyngeal airway. Consequently, it obstructs the laryngoscopist’s line of sight. The hyoid bone is more difficult to feel than the thyroid cartilage and is often impossible to locate. The sternum and mentum are generally easy to find, but the sternomental distance has not been substantiated as a good predictor of difficult intubation by other investigators.

The ability to translate the TMJ is easily assessed before induction. The patient is asked to place the mandibular incisors (bottom teeth) in front of the maxillary incisors (upper teeth). Inability to perform this simple task is usually from one of two sources. First, the TMJ may not glide, predicting a difficult intubation. Second, some patients find it difficult to coordinate the maneuver, in which case there is no implication for a difficult intubation.

The upper lip bite test was proposed as a modification of the TMJ displacement test. The upper lip bite test is performed by asking the patient to move the mandibular incisors as high on the upper lip as possible. The maneuver is similar to biting the lip. Contact of the teeth above or on the vermilion border is thought to predict adequate laryngoscopic views. Inability to contact the vermilion border is thought to predict poor laryngoscopic views. Both the TMJ translation test and the upper lip bite test assess TMJ glide, which is an important consideration during laryngoscopy. Table 43-1 summarizes a quick, easy, bedside scheme for predicting difficult intubation.

TABLE 43-1

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