Airway Management (Anesthesia Text)

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Airway Management (Anesthesia Text)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]

Anatomical PointsNasopharynx separated from the oropharynx by the soft palate. Oropharynxseparated 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 tran sverse plane

Classification and Grading

Mallampati Score

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The Glottis

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 visibleClass 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])


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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 EvaluationsAnna 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/decreasesthe 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 and0.76, 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


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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 significantlyhigher 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


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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

Grade 3 Mask Ventilation (inadequate, unstable, or requiring

two providers)

Predictor p value

BMI >= 30 < 0.0001

Beard < 0.0001

Mallampati 3-4 < 0.0001

Age >= 57 0.002

Severely limited jaw protrusion* 0.018

Snoring 0.019

undefined distance

Grade 3 or 4 Mask Ventilation and Difficult Intubation (> 3

attempts)

Predictor p value

Severely limited jaw protrusion < 0.0001

Thick/obese neck 0.019


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Sleep apnea 0.036

Snoring 0.002

BMI >= 30 0.053

Rapid Sequence InductionOften advocated for patients at risk for aspiration, ex. obstetrical patients, those

with intraabdominal processes, etc., several considerations must be taken intoaccount: 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?

Aspiration RiskHawkins’ 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 …


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References

http://www.youtube.com/watch?v=_MluHXACKJI

Friedberg BL: The difficult airway in office-based anesthesia. Plastic &

Reconstructive Surgery 2010;125: 222e-223e.

Data on RSI and Aspiration RiskA 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 SafetyRSI, 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 EquipmentIn 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 ProSealLMA 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]

Summary

http://www.youtube.com/watch?v=_MluHXACKJI


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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 andPreoxygenationMask VentilationAbsolutely 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 :

Age > 55 yrs

BMI > 26 kg/m2

Beard

Lack of teeth

[Langeron Anesthesiology 92: 1229, 2000]


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History of snoring

Preoxygenation

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 inthe 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


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Time to SaO2 = 60% based on FAO2 and Farmery/Roe’s Model (healthy, 70 kg

patient)

0.87 9.90 mins

0.8 9.32 mins

0.7 8.38 mins

0.6 7.30 mins

0.5 6.37 mins

0.4 5.40 mins

0.3 4.40 mins

0.2 3.55 mins

0.13 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

(room air)


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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].

Endotracheal IntubationTraditional DL and Intubation

Macintosh Blades of Different Sizes

Miller Blads of Different Sizes

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.


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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 intubationFiberoptic 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 nerveblock, or a transtracheal block


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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 IntubationRetrograde 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

Nasotracheal IntubationCan 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, especiallywhere 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


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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.

Supraglottic AirwaysLaryngeal 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]

LMA size

The Laryngeal Mask Airway

LMA size:

3 30-50 kg

4 50-70 kg

5 70-100 kg

Intubating LMAs can be optimized by the Chandy maneuver (lift and posterior

rotate) before attempting tracheal intubation. ProSeal LMAs have a second lumenwhich acts as an esophageal vent


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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

ExtubationPatients 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 IntubationDental 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,


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Filed Under: A, Critical Care and Perioperative Medicine , Respiratory System

leading to tracheal stenosis (which is clinically significant when the lumen is < 5

mm)

Airway Management in ChildrenBy 10 years of age, most children have an adult-like airway. Prior to that, there aresignificant 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

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