Discuss the risk factors for difficult intubation

Discuss the risk factors for difficult intubation.

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 in order to visualize the laryngeal opening. The laryngoscopist is positioned outside the airway, above the patient’s head. To see through the airway, light must travel from the glottic opening to the laryngoscopist’s eye. This technique requires an uninter-rupted linear path between the larynx and laryngoscopist because light generally travels in a straight line. Most manipulations performed attempt to satisfy this criterion.

The airway contains three visual axes. They are 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. In order to bring all three axes into better alignment, Magill suggested “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. Normal atlanto-occipital extension measures 35°. 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 impairs 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 sniffing position include cervical vertebral arthritis, cervical ankylosing spondylitis, unstable cervical fractures, pro-truding cervical discs, atlanto-axial 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: simply have the patient flex the lower cervical verte-brae 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 over 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 laryngo-scope 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 hinge-like move-ment and a gliding motion. The gliding motion is known as translation. Its hinge-like 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 inter-incisor distance. An inter-incisor distance of 3 cm provides sufficient space for intubation, in the absence of other complicating factors. This corresponds approximately to the width of 2 finger breadths. The 2 finger breadth test is performed by placing the examiner’s 2nd and 3rd digits between the patient’s central incisors. If they fit, there should be adequate room to perform laryngoscopy. If they do not fit, then 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 can oftentimes be relieved by pulling the mandible forward. A mouth opening that was sufficient for a previous anes-thetic 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 will be 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 laryn-goscopy 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. In order 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 then looks tangentially over the protruding maxillary incisor. This creates two new points in the adjusted line of sight and, thus, a new straight line of sight. The new line of sight brings the laryngoscopist’s view to a more posterior laryngopharyngeal position. This results 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 hand-held rigid laryngoscope, to which Macintosh and Miller blades are the most commonly attached. Laryngoscopes push the tongue anteriorly and, in so doing, move it from a posterior obstructing position to a new anterior non-obstructing position within the mandibular space. The mandibular space is that 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. It too occupies some of the oropharyngeal airway, thereby obstructing the line of sight. Consequently, a small mandible (micrognathia) is predictor of a difficult intubation. In essence, a tongue that is large compared with the size of the mouth, orophar-ynx, and mandible takes up excessive space in the orophar-ynx and interferes with visualization.

The base of the tongue resides so close to the larynx that inability to adequately displace it anteriorly creates another problem. As the base of the tongue hangs down over the larynx, the glottis is hidden from view. The glottic aperture is then anatomically anterior to the base of the tongue, hence the term “anterior larynx.” Under such circum-stances 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 the tongue poste-riorly 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 displace-ment is dependent upon the TMJ. In addition to its hinge-like motion, the TMJ also works in a gliding (translational) movement. It is the gliding motion that 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, utilizing this concept. A difficult laryngoscopy occurs when it is not possible to visualize any portion of the vocal cords. Mallampati and Samsoon reasoned that a large tongue could be identified upon visual inspection of the open mouth. Both classification systems relate the size of tongue to the oropharyngeal structures identified. A normal-size tongue allows for visu-alization of certain oropharyngeal structures. As the tongue size increases, some structures become hidden from view. Consequently, both investigators proposed systems that reason backwards from this premise.

Application of the Mallampati and/or Samsoon classifi-cation system(s) is easy and painless. The patient is seated in the neutral position. The mouth is opened as wide as it can and the tongue is protruded as far as possible. Phonation is discouraged because it raises the soft palate and allows for visualization of additional structures. The observer looks for specified anatomic landmarks. They are the fauces, pillars, uvula, and soft palate. The Mallampati classification system utilizes three groups and the Samsoon classification system employs four groups (Figure 46.1). 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 preg-nant versus nonpregnant patients.

Just as the size of tongue can be estimated, so too can the size of the mandible. This is accomplished by asking the patient to extend their head at the atlanto-occipital joint and identifying the mandibular mentum and thyroid carti-lage. The Adam’s apple (thyroid notch) is the most superfi-cial 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 with the observer’s fingers. The normal thyromental distance is 6.5 cm. A thyromental distance of greater than 6 cm is predictive of an easy intubation. A thyromental distance of 6 cm or less is suggestive of a difficult intubation. Oftentimes, rulers 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, clini-cally relevant approximations can be taken into account when examining patients for the purpose of predicting difficult intubation. The usefulness of predicting diffi-cult intubation based on thyromental distance has been challenged. Data extracted from Rocke et al.’s 1992 paper and El-Ganzouri’s 1996 paper show that thyromental distance (receding mandible) offers a 7% or less probabil-ity of predicting difficult intubation. Chou in 1993 and Brodsky in 2002 describe patients whose thyromental dis-tances were well in excess of 6.5 cm and who were difficult intubations.

Similar measurements and predictions have been made utilizing the hyoid bone and mandible, as well as the sternum and mentum. Chou and Wu (2001) suggest that a long mandibulohyoid distance predicts a large hypopharyngeal tongue, which hides the glottis during laryngoscopy and thereby produces a difficult intubation. They reason 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 oftentimes 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 prior to induction. The patient is asked to place the mandibular inci-sors (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, thereby 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 laryngo-scopic 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 46.1 summarizes a quick, easy, bedside scheme for predicting difficult intubation.