The Difficult Airway in Adult Critical Care

Gavin G. Lavery, MD, FCARSCI, MB, BCh, BAO; Brian V. McCloskey, MB, BCh, FRCA, FFARCSI, MRCP

Disclosures

Crit Care Med. 2008;36(7):2163-2173. 

In This Article

The Difficult Airway: Prediction

The conditions associated with airway difficulty are numerous[24,25] ( Table 1 ). Past airway difficulty is a significant predictor of future problems unless a temporary factor, for example, airway swelling, pharyngeal abscess, was responsible. The finding of limited mouth opening, dysphonia, dysphagia, dyspnea, or stridor suggests pharyngeal-, neck- or mediastinal-related pathology, which is often relevant. It has been stated, however, that accurate prediction of airway difficulty is a myth but that the exercise is useful in focusing our attention on potential airway strategy.[26]

Five criteria have been identified using multivariate analysis as independent risk factors for DMV[15] (age >55 yrs, body mass index >26 kg/m2, presence of beard, lack of teeth, history of snoring). The presence of two factors indicated high likelihood of DMV (sensitivity, 0.72; specificity, 0.73). Limited mandibular protrusion has been associated with both DMV and DTI.[14]

A clinical assessment developed to attempt prediction of DTI is the Mallampati test.[27] Originally this graded the patient (grades 1-3) based on the structures visible in the oropharynx under set conditions with maximal mouth opening; a fourth grade was added subsequently.[16] Although grades 3 and 4 suggest difficult tracheal intubation, grading is subject to significant interobserver variation. In a series of 1,956 adult elective surgical patients receiving general anesthesia, Cattano et al.[28] showed that, although the Mallampati scale had a good correlation (.90) with the Cormack and Lehane classification, it lacked the sensitivity to be predictive for difficult intubation and stated the score alone was insufficient for predicting difficult tracheal intubation. Other relevant anatomic indices (interincisor gap, thyromental distance, mentohyoid distance, sternomental distance, and neck mobility) were found to be of even less predictive value. The accuracy of sternomental distance as a predictive index has been described as approaching worthlessness.[29] The reported association of DTI and male gender, increased age, decreased neck mobility, history of obstructive sleep apnea, temporomandibular joint pathology, Mallampati 3 or 4, and abnormal upper teeth[30,31,32,33,34] are of little predictive value.

Disease processes such as neoplasm of the pharynx or larynx[19] may be associated with DTI. The presence of a thyroid mass has been reported to be associated with DTI[35,36,37] and would certainly make an emergency cricothyroidotomy difficult or impossible in the event of failure to ventilate. However, three studies[38,39,40] suggest only marginally increased difficulty in intubating patients presenting with thyroid disease, the most recent finding being a difficult intubation rate of 11%. Acromegaly,[41] the presence of a large or poorly compliant tongue,[42] or decreased compliance of the submental tissues may be associated with DTI.

The poor predictive ability of individual factors, tests, or measurements prompted evaluation of combinations and the development of scores and indices. Wilson et al. developed a score based on body weight, head and neck movement, jaw movement, and the presence or absence of mandibular recession and protruding teeth.[43] However, it had a false-positive rate of 12% and combining it with the Mallampati score appeared to increase false-positives.[44] More recently, this combination has shown a sensitivity of 100%, specificity of 96%, and positive predictive value of 65% in a study of 372 obstetric patients.[45] The combination of Mallampati 3 or 4, interincisor distance of 4 cm or less, and thyromental distance of 6.5 cm or less has been shown to have 85% sensitivity and 95% specificity for DTI.[46] Other scores include the Arne[47] and El-Ganzouri risk indices.[34] The former was devised from 1,200 consecutive general/ear, nose and throat surgical patients and prospectively evaluated in a further 1,090. Although the sensitivity and specificity are above 90% for most patient groups, the predictive value is still limited.

The evidence regarding obesity as a risk factor for airway difficulty is hard to interpret. Increased body mass index (BMI) is a risk factor for DMV[12] and the Wilson score is influenced unfavorably by increased body weight.[43] Older studies of normal patients suggested obesity was a risk factor for difficult intubation.[5,48] In a more recent study,[49] an IDS >5 was found in 15.5% of patients with a BMI >35 kg/m2 but only 2.2% of patients with a BMI <30 kg/m2. However, when 200 morbidly obese patients were compared with 1,272 nonobese control subjects, increased BMI had no influence on intubation difficulty.[31] Brodsky et al.,[50] in a series of 100 patients with a median weight of 137 kg and BMI >40 kg/m2, found that degree of obesity, BMI, and a history of obstructive sleep apnea were not associated with difficult intubation, but increased neck circumference (at the level of the superior border of the cricothyroid cartilage) was a predictor of potential intubation problems. This is in conflict with Komatsu et al. who found that the thickness of pretracheal soft tissue, at the level of the glottis, as measured by ultrasound, was not a predictor of difficult intubation.[51] Because most patient populations show a low prevalence of difficult airway and tests have low predictive power, a preplanned strategy is central to managing airway problems when they occur.[15,52]

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