Difficult airway (defined as more than three attempts, or taking longer than 10 minutes) is the major factor in anesthesia morbidity [Caplan Anesthesiology 98: 1269, 2003]. The incidence of difficult airways is 1.1 – 3.8% [Miller]
Nasopharynx separated from the oropharynx by the soft palate. Oropharynx separated by the hypopharynx by the epiglottis. The larynx itself is located between C3 and C6. Vocal cords are made of the thyroarytenoid ligaments, and the glottis is 23 mm AP in men, and 17 mm AP in women. Cords themselves are 6 – 9 mm in the transverse plane
Classification and Grading
The original Mallampati classification consisted of 3 classes [Mallampati SR, Gatt SP, Gugino LD, et al. A clinical sign to predict difficult intubation: a prospective study.
Can Anaesth Soc J 1985; 32: 429–34]. This was subsequently expanded into the widely known 4 class version [Samsoon GL, Young JR. Difficult tracheal intubation: a retrospective study. Anaesthesia 1987; 42: 487–90].
Modified Mallampati (Samsoon and Young) grading of the upper airway
- Class I: everything visible (tonsillar pillars)
- Class II: uvula fully visible, fauces visible
- Class III: soft palate and base of uvula visible only
- Class IV: cannot see soft palate
Cormack and Lehane grading of laryngoscopic view [Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984; 39: 1105–11]
- Grade 1: entire aperture visible
- Grade 2: posterior arytenoids visible, some of glottic aperture
- Grade 3: epiglottis visible
- Grade 4: no visible structures (only can see the soft palate)
In addition to Mallampati, the following should be evaluated –
- Interincisor gap (< 3 cm correlates with difficult direct laryngoscopy [Harmer et. al. Int J Obst Anesth 6: 25, 1997])
- Thyromental (< 6 cm) and sternomental (< 12.5 cm) can be associated with difficult direct laryngoscopy
- Neck flexibility: normally 35 degrees, if reduced by 30% can lead to difficult direct laryngoscopy
- Size and position of teeth
- Conformation of the palate
- Mandibular prominence/recession (micrognathia limits the pharyngeal space)
- Body habitus: obesity can make airway management difficult
Predictive Value of Preoperative Evaluations
- Anna Lee (Hong Kong) et. al. reviewed 42 prospective observational studies totaling 34,513 patients to assess the effectiveness of various preoperative airway evaluation systems. Retrospective and case control studies were excluded. Difficult laryngoscopy (Cormack and Lehane), intubation (individually defined for each study) and ventilation (individually defined for each study) were studied. Each test was assessed by its likelihood ratio (how much the test increases/decreases the pretest probability of the target outcome). Additionally, receiver operator curves were developed for each test (Mallampati, modified Mallampati, etc.). Both the original and modified Mallampati tests had “good” accuracy with regards to direct laryngoscopy (sensitivity 0.71 and 0.55, specificity 0.89 and 0.84, respectively). With regards to difficult intubation, the overall sensitivity of the two tests was 0.50 and 0.76, while specificity was 0.89 and 0.77 (original and modified), respectively. This study was limited by the lack of any standard definition of difficult intubation [Lee et. al. Anesth Analg 102: 1867, 2006].
- A Japanese meta-analysis of 35 studies (50,760 patients) evaluated the Mallampati oropharyngeal classification, thyromental distance, sternomental distance, mouth opening, and Wilson risk score with regards to difficult intubation. The combination of the Mallampati classification and thyromental distance had a positive likelihood ratio of 9.9, however sensitivity was only 36% (sternomental alone had a sensitivity of 62%, and Mallampati had a sensitivity of 49%) [Shiga et. al. Anesthesiology 103(2):429, 2005].
- A single study of 37,482 patients at the Mayo Clinic showed that direct laryngoscopy failed in 0.43% of cases. Morbidity associated with this included 8 instances of dental/soft tissue damage, 1 cardiac arrest, and 1 possible aspiration event. In cases in which DL was inadequate, 59% of patients were successfully intubated with a flexible fiberoptic scope, 20.6% with a bougie, and 18.1% with an LMA (2/3 of which were intubating LMA) [Burkle et. al. Can J Anaesth 52(6):634, 2005].
- A German study of 1425 patients, comparing upper lip bite tests to Mallampati with regards to difficult direct laryngoscopy (based on Cormack and Lehane scores) showed sensitivities of 28.2 and 70.2%, respectively, and specificities of 92.5 and 61%. Positive predictive values were 33.6% and 19.5%, respectively, and negative predictive values were 90.6 and 93.8% [Eberhart LH. Anesth Analg. 101(1): 284, 2005].
- A Korean study of 90 patients with OSA suggested that the prevalence of difficult intubation was higher in the OSA group than in controls (16.7% vs 3.3%, p = 0.003). Apnea-hypopnea index was significantly higher in the difficult intubation subgroup (67.4 +/- 22.5 vs 49.9 +/- 28.0, p = 0.026), and patients with an AHI >= 40 showed a significantly higher prevalence of difficult intubation [Kim et. al. Can J Anaesth 53(4):393, 2006].
- The incidence and predictors of difficult and impossible mask ventilation Kheterpal S e al. published a study of 22,660 intubations at the University of Michigan analyzing cases of grade 3 mask ventilation (inadequate, unstable, or requiring two providers), grade 4 mask ventilation (impossible to ventilate), and difficult intubation. Univariate and multivariate analyses were undertaken. 313 cases (1.4%) of grade 3 MV, 37 cases (0.16%) of grade 4 MV, and 84 cases (0.37%) of grade 3 or 4 MV and difficult intubation were observed [Kheterpal S. et. al Anesthesiology 105(5): 885, 2006].
Presence of a beard is the only easily modifiable independent risk factor for difficult MV. The mandibular protrusion test may be an essential element of the airway examination
|BMI >= 30||< 0.0001|
|Mallampati 3-4||< 0.0001|
|Age >= 57||0.002|
|Severely limited jaw protrusion*||0.018|
- undefined distance
|Severely limited jaw protrusion||< 0.0001|
|BMI >= 30||0.053|
Rapid Sequence Induction
Often advocated for patients at risk for aspiration, ex. obstetrical patients, those with intraabdominal processes, etc., several considerations must be taken into account: first, what is the risk of aspiration? Second, how effective are techniques designed to reduce aspiration risk? Third, do these techniques require a tradeoff in terms of airway safety? Fourth, how do different airway equipment affect the risk of aspiration?
Hawkins’ analysis of maternal deaths over a 12 year period found 33 deaths from “aspiration” during general anesthesia, as compared to 37 deaths from either “induction/intubation problems” or “inadequate ventilation” [Hawkins JL et al. Anesthesiology 86: 277, 1997]. Note that these data are pre-LMA in the United States (1990 was the last year included in Hawkins’ analysis, the LMA was not available until 1991) . The roughly equal likelihood of death secondary to a cannot-intubate/cannot-ventilate situation or pulmonary aspiration, coupled with the lack of evidence supporting rapid sequence induction [Neilipovitz DT and Crosby ET. Can J Anaesth 54: 748, 2007]. Note that these data are pre-LMA in the United States (1990 was the last year included in Hawkins’ analysis, the LMA was not available until 1991).
Propofol induction with BIS/EMG monitoring without airway manipulation. Remember the tortoise & the hare. The race goes not to the swift but to the slow and steady…
Friedberg BL: The difficult airway in office-based anesthesia. Plastic & Reconstructive Surgery 2010;125: 222e-223e.
Data on RSI and Aspiration Risk
A large, metaanalysis of studies showed that there is no data to support or refute the use of RSI to lower aspiration risk, thus the use of RSI can only be recommended on a theoretical basis [Neilipovitz DT and Crosby ET. Can J Anaesth 54: 748, 2007]
RSI and Airway Safety
RSI, which requires paralysis and a mandatory period of apnea (no masking), results in decreased time to hypoxemia and removes the option of spontaneous respiration, at least until SCh has worn off (which can be prolonged in pregnant patients)
Aspiration Risk and Airway Equipment
In Warner’s study of over 200,000 cases, 67% of cases of aspiration occurred either during laryngoscopy or at the time of extubation [Warner MA et al. Anesthesiology 78: 56, 1993], suggesting that the majority of aspiration events would not be affected by either the use of RSI or the use of an LMA. The ProSeal LMA may be a useful airway adjuvant [Cook TM at el. Br J Anaesth 88: 527, 2002], as its esophageal port allows for suctioning. An unbiased, prospective, comparison of LMA vs. ETT is unlikely, as the incidence of aspiration is low and a controlled trial designed to have 80% power and 5% type I error, to detect a 50% reduction in aspiration risk with the PLMA compared with the cLMA would require over 2.5 million elective patients [Cook T. Br J Anaesth 94: 690, 2005]
Given the roughly equal likelihood of death secondary to a cannot-intubate/cannot-ventilate situation or pulmonary aspiration, coupled with the lack of evidence supporting rapid sequence induction [Neilipovitz DT and Crosby ET. Can J Anaesth 54: 748, 2007], it seems reasonable to consider maintaining spontaneous respiration in patients at high risk for both airway failure and aspiration (ex. obstetric patients). The two major risks in pregnant patients are failed airway and aspiration – the latter has not proven to be a modifiable risk, the former can likely be modified by maintaining spontaneous respiration or considering the use of an LMA as a backup airway. Thus, the implications of rapid sequence induction should be strongly considered prior to induction in any high risk patient
Facemask Ventilation and Preoxygenation
Absolutely critical – more so than intubation, because if you can mask someone, you can get them oxygen and remove CO2. The difficulties surrounding facemask ventilation include oxygenation, ventilation, and protection from aspiration. Ventilation should be attempted at < 20 cm H20 to avoid insufflating the stomach. Nasal airways are better tolerated than oral airways (can cause gagging or laryngospasm in lightly anesthetized patients) but are relatively contraindicated in patients with coagulation or platelet abnormalities
|Predictors for difficult facemask [Langeron Anesthesiology 92: 1229, 2000]:|
|Age > 55 yrs|
|BMI > 26 kg/m2|
|Lack of teeth|
|History of snoring|
In a time of urgency, the mode of preoxygenation is controversial [Baraka AS et al. Anesthesiology 91: 612, 1999] – three studies have shown that 4 deep breaths in 30 seconds is identical to 5 mins of normal breaths, and three others have shown that 4DB/30s is inferior [Bemunof JL. Anesthesiology 91: 603, 1999] – Bemunof believes that 30s may be inadequate as the lungs are not the only depository for oxygen – while O2 stored by FRC increases by only 300 mL when increasing pre-O2 from 60 to 180 seconds, total body storage increases by 800 mL in that same period of time [Campbell IT et al. Br J Anaesth 72: 3, 1994]. Bemunof also states that preoxygenation should be part of the ASA difficult airway algorithm, which it is not, and should be mandatory in all patients as it is impossible to tell when a cannot ventilate cannot intubate situation will arise.
Desaturation During Apnea
Farmery and Roe developed a mathematical model of oxygen delivery and use in the human body, suggesting that when preapnea FAO2 is decreased from 0.87 to 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, and 0.13 (breathing room air) for a healthy 70-kg patient, apnea times to SaO2 = 60% are decrease from 9.90 to 9.32, 8.38, 7.30, 6.37, 5.40, 4.40, 3.55 and 2.80 min, respectively [Farmery AD and Roe PG. Br J Anaesth 76: 284, 1996]. Baraka showed that eight deep breaths for 60s (using a Mapleson D circuit) is superior to 4 deep breaths for 30s (and also to 5 min of non-deep breaths) [Baraka AS et al. Anesthesiology 91: 612, 1999], but this model has to be confirmed using a circle circuit and with standardized flows
|Time to SaO2 = 60% based on FAO2 and Farmery/Roe’s Model
(healthy, 70 kg patient)
|0.13(room air)||2.80 mins|
Consider the implications of a relatively standard IV induction on oxygenation and apnea. According to Miller’s Anethesia, 7th edition (page 721), a single induction dose of propofol (2 mg/kg) will peak at 7.5 ucg/kg in less than a minute (therapeutic range 1.5-5 ucg/kg, although this is PLASMA, not EFFECT-SITE concentration) and will not drop below 1.5 ucg/kg (the point at which awakening may occur) until 8 minutes after the bolus. Heier et al. administered succinylcholine (1 mg/kg) and thiopental (5 mg/kg) to twelve volunteers. The time to desaturation was highly variable, with eight volunteers never falling below SpO2 of 80%. Four volunteers, however, fell to < 80%, and two to 70% (positive pressure ventilation was initiated at an SpO2 of 80%). The time to “bottom out” ranged between 6 and 9 minutes. Once SpO2 fell below 90%, desaturation was exceedingly rapid (see Fig 1) [Heier T et al. Anesthesiology 94: 754, 2001].
Traditional DL and Intubation
Elevate the head 8-10 cm with occipital pads and extend the A-O joint. Data supporting cricoid pressure (Sellick’s maneuver) are not available [Brimacombe et. al. Can J Anaesth 44: 414, 1997]. Problems with cricoid pressure include 1) difficulty measuring the 5 kg of weight that is recommended, as well as 2) the possibly evoking upper airway reflexes and relaxing the LES [Tournadre et. al. Anesthesiology 86: 7, 1997] and 3) displacing the esophagus laterally as opposed to compressing it. Thus for some patients, if cricoid pressure is to be applied, it should be done so before induction [Miller]. Application of thyroid pressure is intended to facilitate exposure of the glottic opening. Once in place (proximal border of cuff 1-2 cm distal to glottis), tube should be taped at 23 cm in men and 21 cm in women [Owen et. al. Anesthesiology 67: 255, 1987]
Fiberoptic intubation: recommended for unstable cervical spines, as well as those with an upper airway injury. The absolute contraindication is lack of time, and relative contraindications include pharyngeal abscess and risk of bleeding (for nasal route only). Nasal is generally preferred to oral intubation for anatomic reasons, but oral is better in patients with high risk of bleeding or who will not tolerate vasoconstrictors (ex. pregnant women, some heart disease patients)
Oral: high risk of bleeding, or cannot tolerate vasoconstrictors (pregnant, cardiac patients)
Nasal: everyone else
Patients will need 0.2 mg glycopyrrolate, topical anesthesia (or local blocks) and usually vasoconstriction of the nose, often with 3% lidocaine/0.25% phenylephrine. For the tongue/oropharynx, aerosolized local anesthetics can often be used, as can bilateral glossopharyngeal block (base of each tonsillar pillar). For the larynx and trachea, topicalization can be accomplished by spray (mostly hits the pharynx) or nebulization (more of it reaches the trachea, but also the lungs which leads to rapid absorption). Lidocaine is the preferred agent for this. Alternatively, for the larynx/trachea, one can attempt a superior laryngeal nerve block, or a transtracheal block
When attempting a nasal fiberoptic intubation, use an ETT that is 1.5 mm larger than the scope diameter. Rotation of the ETT will help it pass the nasopharynx as well as into the glottis. If there is resistance during withdrawal, the ETT and scope must be removed. Intubation under general anesthesia is complicated by relaxation of the pharyngeal tissues, thus limiting the space for visualization.
If an awake oral intubation is to be attempted, an LMA provides an excellent conduit.
Retrograde Intubation: cricothyroid membrane is punctured, wire passed cephalad, retrieved in mouth or nose. Exchange catheter placed over wire, then ETT passed over exchange catheter and wire and into the trachea
May be useful in urgent/emergent situations where visualization is poor, such as upper airway or esophageal hemorrhage
Can use standard or preformed endotracheal tube (nasal RAE), usually downsized for less trauma to the nasal passages, but remember – the tube diameter also has bearing on the length, and if considering very small tube due to small nasal aperture, should realize that the tube will need to be advanced further than if placed orally. The “Chula formula” may be useful in helping determine the appropriate depth of nasotracheal tube insertion: 9 + (Ht/10) cm. Thus, for a 5’10” patient (~178 cm), the tube will be inserted 26-27 cm for optimal placement (2 cm above the carina). This would be difficult with a smaller ETT. Magill forceps will be of assistance when performing asleep nasotracheal intubation, and some view of the tracheal aperture is required in most cases.
Blind Nasotracheal Intubation: use has decreased over the years but still can be lifesaving. A consideration for obtunded patients who cannot lay flat, especially where fiberoptic equipment may not be readily available. A “whistle” is available that can be attached to the connector of the ETT, and will make an audible sound when the tube is at or near the glottic aperture, and this may be helpful.
Remember to avoid intubation through the nose in the setting of concomitant facial fractures, as inadvertent cranial placement of nasotracheal tubes has been reported.
Laryngeal Mask Airway: because the factors that lead to difficult ETT and LMA placement are not the same, the incidence of difficulty with both is low [Bogetz Anesth Clin North Am 20: 863, 2002]
Intubating LMAs can be optimized by the Chandy maneuver (lift and posterior rotate) before attempting tracheal intubation. ProSeal LMAs have a second lumen which acts as an esophageal vent
CombiTube: double-lumen device that can be both an ETT or an esophageal obturator. This is passed blindly – the oropharyngeal cuff is inflated first (blue pilot balloon), followed by the distal cuff. Ventilation is then attempted through the longer (blue) lumen [blue first!!!], which outputs in between the two cuffs. If no breath sounds are heard, attempt ventilating through the other lumen. This device is designed to be used with minimal training but should not be used for extended periods of time (ischemia to tongue, edema formation [Agro et. al. J Clin Anesth 14: 307, 2002])
Laryngeal Tube: single-lumen dual cuff system with fenestrations between the two. Has been shown to have a high success rate when used by non-anesthetists. A supraglottic ventilatory device consisting of an airway tube at a 130 degree angle, with two low pressure cuffs and an oval aperture between them. The distal balloon is intended to be esophageal, with the proximal, larger balloon sealing the oropharyngeal cavity. Requires 23 mm of mouth opening and is placed blindly. Comes in six sizes. The Laryngeal Tube was tested in 175 ASA 1 or 2 patients for elective surgery (avg. duration 63 +/- 23 minutes) and was successfully inserted and maintained in 96.6% of patients. In 94% of patients, success was achieved on the first attempt. In six patients (3.4%) the device was unsuccessful – three because of airway pressures > 40 cm H20, and 3 because of “clinically unacceptable ventilation.” Four patients displayed evidence of Grade I upper airway trauma [Gaitini et. al. Anesth Analg 96: 1750, 2003].
Cricothyrotomy: can be done in < 30 seconds and used for up to 72 hours, after which the incidence of vocal cord dysfunction and tracheal stenosis increases [Mallampati et. al. Can Anaesth Soc J 32: 429, 1985]. Kits usually rely on air aspiration through a needle, followed by the Seldinger technique. Risks include pneumothorax, pneumomediastinum, bleeding, infection, SQ emphysema
Transtracheal Jet Ventilation
Transtracheal Jet Ventilation: risks are identical to a cricothyrotomy (pneumothorax, pneumomediastinum, bleeding, infection, SQ emphysema). Upper airway obstruction or disruption of the airway are contraindications, as TTJV relies on the patient’s airway for exhalation. Furthermore, these devices use airway pressures of ~ 50 psi, which can lead to disconnections or mechanical failure
Patients must be wide awake or deeply anesthetized, not in a light anesthetic plane (dysconjugate gaze, breath-holding, coughing, but not responsive to command) otherwise they are at increased risk for laryngospasm. Reaching for the ETT is not a reliable sign of sufficient alertness. Deep extubation is less hemodynamically traumatic but is contraindicated if mask ventilation is (or entotracheal intubation was) difficult, aspiration is a risk, or significant airway edema is expected
Patients should be ventilated on 100% O2 prior to extubation, NMBDs reversed, trachea suctioned, and the tube removed during positive pressure. Extubation over a fiberoptic scope or a bougie/exchange catheter may be prudent
Complications of Tracheal Intubation
Dental trauma that requires further treatment and/or extraction occurs in 1:4500 cases [Owen et. al. Anesthesiology 67: 255, 1987]. Use of a plastic shield on the teeth can help with this. Common complications include HTN and tachycardia, which can jeopardize myocardial oxygen supply in at-risk patients [Miller]. The two most serious complication after intubation are laryngospasm and aspiration. Treatment of laryngospasm is via PPV (with facemask if post-extubation, also jaw thrust), and possibly succinylcholine 0.1 mg/kg.
Note that flexion of the patient’s head can advance the ETT as far as 1.9 cm, leading to endobronchial intubation. Extension can remove the ETT 1.9 cm, and lateral rotation can adjust the ETT by 0.7 cm.
Prolonged tracheal intubation (> 48 hours) can damage the tracheal mucosa, leading to tracheal stenosis (which is clinically significant when the lumen is < 5 mm)
Airway Management in Children
By 10 years of age, most children have an adult-like airway. Prior to that, there are significant differences with adults. Infants have a larynx at C3-4 (not C4-5 as in adults), pushing the tongue, which is larger, superiorly. Epiglottis is larger, stiffer, and angled posteriorly (often advantageous to use the Miller blade). Large thyroid cartilage, narrow cricoid (most narrow portion of the airway), can use an uncuffed tube. Cuffed tubes can be used if pressures < 20-25 cm H20, although if N2O is used the pressure has to be monitored. Infants require shoulder or neck rolls for facemask ventilation an