Difficult Airway Assessment
Several methods exist to quickly assess the probability of success during tracheal intubation. [1] One tool for rapid assessment is the LEMON law, as described below. A patient in extremis may not be able to cooperate with all the sections of the LEMON assessment.
L: Look externally
Assessing the difficulty of an airway based on external physical features is not sensitive (not all patients who have a difficult airway appear to have a difficult airway prior to intubation) but is quite specific (most patients who appear to have a difficult airway do indeed have a difficult airway). Physical features such as a small mandible, large tongue, and short bull neck are all red flags for a difficult airway.
E: Evaluate the 3-3-2 rule
The chance for success is increased if the patient is able to insert 3 of his or her own fingers between the teeth, can accommodate 3 finger breadths between the hyoid bone and the mentum (see hyomental distance in the first image below), and is able to fit 2 finger breadths between the hyoid bone and the thyroid cartilage (see thyrohyoid distance in the second image below).
M: Mallampati classification
The Mallampati assessment is ideally performed when the patient is seated with the mouth open and the tongue protruding without phonating. In many patients intubated for emergent indications, this type of assessment is not possible. A crude assessment can be performed with the patient in the supine position to gain an appreciation of the size of the mouth opening and the likelihood that the tongue and oropharynx may be factors in successful intubation (see image below).
O: Obstruction
Obstruction of the upper airway is a marker for a difficult airway. Three signs of upper airway obstruction are difficulty swallowing secretions (secondary to pain or obstruction), stridor (an ominous sign which occurs when < 10% of normal caliber of airway circumference is clear), and a muffled (hot-potato) voice.
N: Neck mobility
The inability to move the neck affects optimal visualization of the glottis during direct laryngoscopy. Cervical spine immobilization in trauma (with a C-collar) can compromise normal mobility, as can intrinsic cervical spine immobility due to medical conditions such as ankylosing spondylitis or rheumatoid arthritis.
Preparation
Confirm that intubation equipment is functional.
Assess the patient for difficult airway (see Difficult Airway Assessment section below for recommended method). If the patient meets criteria for difficult airway, rapid sequence intubation (RSI) may be inappropriate. Nonparalysis procedures may be an alternative. The assistance of anesthesia personnel may be warranted.
Establish intravenous access.
Draw up essential drugs and determine sequence of administration (induction agent immediately followed by paralytic agent).
Review possible contraindications to medications.
Attach necessary monitoring equipment.
Check endotracheal (ET) tube cuff for leak.
Ensure functioning light bulb on laryngoscope blade.
Preoxygenation
Administer 100% oxygen via a nonrebreather mask for 3 minutes for nitrogen washout. This is done without positive pressure ventilation using a tight seal.
Though rarely possible in the emergent situation, the patient can take 8 vital capacity (as deep as possible) breaths of 100% oxygen. Studies have shown this can prevent apnea-induced desaturation for 3-5 minutes. [47]
Assist ventilation with bag-valve-mask (BVM) system only if needed to obtain oxygen saturation =90%.
Pretreatment
Consider administration of drugs to mitigate the adverse effects associated with intubation.
See Anesthesia for more information.
Paralysis with induction
Administer a rapidly acting induction agent to produce loss of consciousness.
Administer a neuromuscular blocking agent immediately after the induction agent.
These medications should be administered as an intravenous push.
Protection and positioning
Though clinical dogma dictates that the Sellick maneuver (firm pressure over the cricoid cartilage to compress the proximal esophagus) be initiated to prevent regurgitation of gastric contents, literature is lacking in support of this technique and in fact may impede laryngeal view.
Initiate this maneuver upon observing the beginning of unconsciousness.
Maintain pressure throughout intubation sequence until the position of the ET tube is verified. Note that proper laryngeal view has been shown to be best accomplished by the bimanual method and should be used if the Sellick maneuver fails to show the vocal cords.
Classical teaching dictates that cricoid pressure decreases the risk of gastric regurgitation into the lungs. However, in a study by Smith et al, the esophagus was partially lateral to the trachea in more than 50% of the subjects. [48] Also, in an ultrasound study, 29 of 33 esophagi were partially displaced to the left of the trachea. [49] In a meta-analysis, Butler and Sen showed that little evidence supports the notion that cricoid pressure decreases the risk of aspiration in RSI. [9]
Placement with proof
Visualize the ET tube passing through the vocal cords.
Confirm tube placement. Observe color change on a qualitative end-tidal carbon dioxide device or utilize a continuous end-tidal carbon dioxide (ET-CO2) monitor. Use the 5-point auscultation method: Listen over each lateral lung field, the left axilla, and the left supraclavicular region for good breath sounds. No air movement should occur over the stomach. Two pilot studies have shown that ultrasonography can reliably detect passage of a tracheal tube into either the trachea or esophagus without inadvertent ventilation of the stomach. [49, 50]
See the image panel below.
Left panel: Bedside ultrasound of anterior neck for proper detection of the endotracheal tube before positive-pressure ventilation is applied. Middle panel: Proper placement of the endotracheal tube in the trachea as the esophagus is normally not visualized. Right panel: Misplacement of the endotracheal tube in the left-sided esophagus. Used with permission from Springer Publishing Company.
Postintubation management
Secure the ET tube into place.
Initiate mechanical ventilation.
Obtain a chest radiograph. Assess pulmonary status. Note this modality does not confirm placement; rather, it assesses the height above the carina. Ensure that mainstem intubation has not occurred.
Administer appropriate analgesic and sedative agents for patient comfort, to decrease O2 demand, and to decrease ICP.
Video-assisted laryngoscopy (VAL)
VAL offers the advantage of abandoning the need for alignment of the optical axes in the mouth, pharynx, and larynx in order to visualize the entrance of the glottis and therefore is more effective. Unfortunately, standard ETTI via DL, performed by untrained medical personnel and those who perform it only occasionally, carries a high risk of failure. In several studies looking at the success rate of ETTI via DL performed by medical support staff, medical students, and novice anesthesia residents, the initial success rate varied between 35% and 65%. It has been shown that in order to improve the success rate of DL to over 90%, one would require about 47-56 intubations. [51] In stark contrast, VAL has been shown to be easily learned and highly successful with minimal training necessary. A prospective trial compared 37 novice residents in VAL versus DL and found that the former yielded a 14% higher success rate and 14% fewer esophageal intubations. [52] Nouruzi-Sedeh et al evaluated medical personnel with no prior experience in ETTI (paramedic students, nurses, and medical students) and after a brief didactic/manikin session compared their laryngoscopy skills in the operating room between VAL and DL. As in many other similar studies, they showed that VAL led to a significantly higher success rate (93%) compared with DL (51%) in nonphysicians with no prior laryngoscopy experience. Subjects were also noted to have a dramatic improvement after only five ETTIs; they neared a 100% success rate using VAL. [53] A meta-analysis looked at VAL compared with DL in 17 trials with 1,998 patients. The pooled relative risk for nondifficult intubations was 1.5 and for difficult intubations was 3.5; the authors concluded that VAL improves glottic visualization, particularly in patients with potentially difficult airways. [54]
VAL is now available as both a portable unit that is attached to a laryngoscope and as a stand-alone unit that is wheeled to the bedside. Utilization is often one of personal preference or institutional availability.
See the images and video below.
Set up for video-assisted laryngoscopy. Used with permission from Springer Publishing Company.
Glottic view via video-assisted laryngoscopy. Used with permission from Springer Publishing Company.
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Proper alignment of the axes for tracheal intubation.
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Hyomental distance (3 finger breadths).
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Thyrohyoid distance (2 finger breadths).
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Laryngoscope handle, No. 3 Macintosh (curved) blade, and No. 3 Miller (straight) blade.
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Mallampati classification.
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Three-axis theory. OA is oral axis, PA is pharyngeal axis, and LA is laryngeal axis. Used with permission from Springer Publishing Company.
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Left panel: Bedside ultrasound of anterior neck for proper detection of the endotracheal tube before positive-pressure ventilation is applied. Middle panel: Proper placement of the endotracheal tube in the trachea as the esophagus is normally not visualized. Right panel: Misplacement of the endotracheal tube in the left-sided esophagus. Used with permission from Springer Publishing Company.
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Set up for video-assisted laryngoscopy. Used with permission from Springer Publishing Company.
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Video demonstration of the ease of video-assisted laryngoscopy in aligning the oral, pharyngeal, and laryngeal airway axis and glottic view. Used with permission from Springer Publishing Company.
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Glottic view via video-assisted laryngoscopy. Used with permission from Springer Publishing Company.
