Predicting the Difficult Airway

Additional Information on Airways

Benumoff’s further feelings on extensive evaluation of the difficult airway

Recognition of difficult airway and preoperative evaluation: divided into 11 steps; clinician’s eyes must first focus on teeth, then mouth, oropharyngeal space, mandibular space, and neck; follows line of sight needed when placing laryngoscope blade

Teeth:

1) determine whether maxillary teeth (in involuntary state) lie anterior to mandibular teeth (ie, maxillary teeth “buck” on mandibular teeth); forces laryngoscope blade to enter mouth in more cephalad direction than normal; requires seeing around greater radius of curvature;

2) note length of upper incisors (long upper incisors force laryngoscope blade to enter mouth in more cephalad direction than normal, increasing size of curve);

3) determine whether (in voluntary state) patient can bring mandibular teeth anterior to maxillary teeth (ie, able to prognath); indicates temporomandibular joint not frozen and jaw able to move forward, moving tongue into position to be moved aside during intubation;

4) determine inter-incisor distance (greater than or equal to 3 cm required to get laryngoscope blade into mouth without hitting teeth) Pharyngeal structures:

5) ability to visualize uvula and tonsillar pillars indicates tongue small relative to size of pharyngeal cavity (therefore relatively easy to move tongue out of line of sight during laryngoscopy; does not necessarily indicate easy intubation); large tongue obscures pharyngeal structures;

6) determine height and narrowness of palate (highly arched and narrow palate can cause intubation difficulty due to smaller oropharyngeal volume)

Mandibular space:

7) determine thyromental distance (ie, distance between inside of mandible and thyroid notch); index of degree vocal cords anterior or posterior relative to other upper airway structures (vocal cords lie directly under midpoint of thyroid cartilage); if thyroid notch very posterior, indicates vocal cords under midpoint lie relatively posterior and relatively straight line from incisors to cords; greater than or equal to 3 fingerbreadths (ie, 6 cm) between inside of mandible and thyroid notch indicates vocal cords posterior, and line of sight relatively straight;

8) determine compliance and distensibility of submandibular space; laryngoscopy retracts tongue into mandibular space; mandibular space made noncompliant by, eg, inflammation, fibrosis, radiation, infection, cancer, edema

Length of neck:

9) qualitative determination; short neck causes intubation difficulty (vocal cords ride relatively high in neck; sharp radius of curvature; lining up angles more difficult); many short-neck syndromes (eg, Klippel-Feil syndrome)

Thickness of neck:

10) qualitative determination; thick neck decreases ability to align upper airway planes

Range of motion:

11) of head and neck; determine whether patient can assume sniffing position (involves 35° flexion of neck on chest; head severely extended (approximately 90°) on neck at atlanto-occipital joint)

Conclusion:

20 to 30 sec required for thorough and complete preoperative evaluation; necessary to have “prepared and educated mind”

The 6-D method of airway assessment*Sign of difficulty Description Quantitative or qualitative findings reported to be associated with difficulty Acceptable findings not usually associated with difficulty 1. Disproportion —Increased size of tongue in relation to pharyngeal size —Mallampati class III or IV —Mallampati class I or II   —Airway swelling —Possibly difficult to assess     —Airway trauma (blunt or penetrating) —Blunt or penetrating airway trauma —Midline trachea   —Tissue consolidation (e.g., secondary to radiation) —Tracheal deviation —No contractures of the neck     —Neck asymmetry —No surgical airway scar 2. Distortion —Neck mass —Voice changes —Mobile laryngeal anatomy   —Neck hematoma —Subcutaneous emphysema (crepitus) —Easily palpated thyroid cartilage   —Neck abscess —Laryngeal immobility —Easily palpated cricoid cartilage   —Arthritic changes in the neck joints —Nonpalpable thyroid cartilage     —Previous surgical airway —Nonpalpable cricoid cartilage   3. Decreased thyromental distance —Anterior larynx and decreased mandibular space —Thyromental distance >7 cm (˜3 finger breadths) measured from the superior aspect of the thyroid cartilage to the tip of the chin —Thyromental distance ≥7 cm (-3 finger breadths)     —Receding chin —No receding chin 4. Decreased interincisor gap —Reduced mouth opening —Distance between upper and lower incisors (i.e., interincisor gap) <4 cm (<2 finger breadths) —Interincisor gap <4 cm (-2 finger breadths)     —Mandibular condyle fracture       —Rigid cervical spine collar   5. Decreased range of motion in any or all of the joints of the airway (i.e., atlanto-occipital joint, temporomandibular joints, cervical spine); atlantooccipital range of motion is critical for assuming the sniffing position —Limited head extension secondary to arthritis, diabetes, or other diseases —Head extension <35° —Head extension ≥35° of atlanto-occipital extension   —Previous neck radiation and/or radical surgery —Neck flexion <35° —Cervical spine flexion ≥35°   —Neck contractures secondary to burns or trauma —Short, thick neck —Long, thin neck     —Cervical spine collar or cervical spine immobilization —No cervical spine collar or cervical spine immobilization 6. Dental overbite —Large angled teeth disrupting the alignment of the airway axes and possibly decreasing the interincisor gap —Dental overbite —No dental overbite

*Based on references 16–18, 21. The 6-D method of airway assessment helps practitioners remember to assess for each of the six signs that can be associated with a difficult intubation. Each sign begins with the letter D like the word difficult. The potential for difficult intubation is generally proportional to the number of signs observed. (Baylor Proceedings)

have patient bite their upper lip (Khan ZH, Gharabaghian M, Ferozeh N, Ghiamat M, Mohammadi M. Easy endotracheal intubation of a patient suffering from both Cushing’s and Nelson’s Syndromes predicted by the upper lip bite test despite a Mallampati class 4 airway. Anesth Analg 2007;105:786–7)

·        Atropine (No longer recommended)

o       In children less than 10 years old to prevent Sux induced Bradycardia

o       In adults and kids>10 y/o, who receive repeat Sux, for the same reason

o       As an antisalagogue if using Ketamine (can also use glycopyrolate 0.2 mg)

o       0.02 mg/kg

·        Defasiculating Dose of Paralytic (No longer recommended)

• In tight brain for adults and kids >5 y/o, when using Sux.  Dog experiments show that sux actually increases ICP not by the fasiculations, but by increased afferent input to muscle spindle fibers; vec or roc can cause a partial spinal blockade attenuating this response (Anes 1986; 64:551)
• Vec .01 mg/kg or Roc .05 mg/kg (Ron Walls ACEP 2003)
• Lidocaine and Magnesium prevent the fasiculations as well (MA of RCTs Anesthesiology 2005;103(4):877)
• Succinylcholine mini-dose can also be used (Ann Emerg Med 1992;21(8):929/47.

Can J Anaesth. 1998 Jun;45(6):521-5. Rocuronium is the best non-depolarizing relaxant to prevent succinylcholine fasciculations and myalgia. double blind randomized study, 120 female patients CONCLUSION: Among the pretreatments tested, 0.06 mg.kg-1 rocuronium was the best to prevent muscular fasciculations following succinylcholine injection. In the population studied, pretreatment did not prevent postoperative myalgia. Succinylcholine 1.5 mg.kg-1 was more effective without a non-depolarizing pretreatment.

 

A randomized, double-blind comparison of rocuronium, d-tubocurarine, and “mini-dose” succinylcholine for preventing succinylcholine-induced muscle fasciculations. (Anesth Analg. 1998 Sep;87(3):719-22.)

|      |      |