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All patients undergoing general anesthesia should have a complete history and anesthesia-directed physical examination. One goal of this evaluation is to predict the degree of difficulty with mask ventilation and endotracheal intubation using standard devices.
Airway history — For patients who report problems with anesthesia in the past, every effort should be made to obtain and review prior anesthesia records for details of airway management.
A number of disease states, both congenital and acquired, have been associated with difficult airway management). In addition, pulmonary problems such as asthma, recent upper respiratory infection, pneumonia, bronchitis, or presence of chronic obstructive pulmonary disease (COPD) may impact oxygenation and ventilation during induction .
Most patients who present for emergency procedures are at increased risk of aspiration during anesthesia, either because of recent oral intake or because of predisposing conditions In addition, the incidence of difficult intubation is significantly higher in the emergency department and other areas outside of the operating room.
Airway examination —
●First glance assessment – The preoperative first glance assessment provides significant useful information. Obesity, facial hair, a thick, short neck, and neck collars are immediately apparent and suggest potential difficulty with airway management.
●Mouth opening – Mouth opening is usually assessed in finger breadths. A mouth opening of less than three finger breadths is considered limited. Patients with temporomandibular joint (TMJ) disease or prior surgery may have very limited mouth opening or trismus. Radiation of the head and face can also result in trismus or scarring that distorts the anatomy or limits mobility.
Dentition should be assessed, with particular attention to the presence of caps, crowns, implants, veneers, dentures, braces, or loose teeth. These should be documented and the risk of damage discussed with the patient. Dentures should ordinarily be removed in the preoperative area before the patient is brought to the operating suite. However, if mask ventilation is planned, dentures may be left in place to improve mask fit, though they should be removed immediately prior to intubation to prevent dislodgement or damage. If the patient has braces, the risk of soft tissue injury to the lips during airway management should be discussed.
●Mallampati class – Mallampati class was first described in 1985 as a test to predict difficult laryngoscopy . The Mallampati evaluation originally included three classes based on the ability to view the tonsillar pillars, uvula, and palate with the mouth open and the tongue protruded. The more widely used modified Mallampati class includes a fourth class]:
•Class I: The entire tonsillar pillars, uvula, hard and soft palates are visualized
•Class II: Partial uvula and soft palate are visualized
•Class III: Only the soft palate is visualized
•Class IV: No visualization of any structures beyond the tongue
A Mallampati class 0 has also been described, in which part of the epiglottis can also be seen upon mouth opening in addition to all the class I structures listed above .
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Thyromental distance – (TMD) is the distance between the thyroid cartilage and the mandible, measured in full extension of the neck. Short TMD has been defined as less than 6 cm.. Historically, TMD has been used as a rough estimate of the submental space, which is the space that must accommodate the tongue during laryngoscopy .
●Sternomental distance – SMD is measured between the sternal notch and the mandible, measured in full neck extension. Short sternomental distance is defined as less than 12 cm. This parameter and TMD may be objective surrogates for adequacy of neck extension
●Neck range of motion – Both neck flexion and extension should be assessed for limitations. Patients with arthritis of the neck, cervical spine disease, or previous spine surgery may have limited neck extension. Studies have shown that neck range of motion decreases with age, and decreased neck extension has been associated with difficulty with airway management Patients with restricted neck extension may be more difficult to optimally position for induction of anesthesia and intubation.
●Mandibular protrusion – Patients are asked to protrude the lower jaw such that the mandibular teeth are in front of the maxillary teeth, as a predictor of the ability to sublux the mandible during laryngoscopy. A more objective, similar measurement is the upper lip bite test (ULBT), which assesses the patient's ability to reach and cover the upper lip with their lower incisors. ULBT grading includes :
•Grade 1: The patient can fully cover the upper lip with lower incisors
•Grade 2: The patient can partially cover the upper lip with lower incisors
•Grade 3: The patient cannot reach the upper lip with lower teeth
