A new practical classification of laryngeal view

Authors


T. M. Cook Consultant Anaesthetist, Royal United Hospital, Combe Park, Bath BA1 3NG, UK

Abstract

A new practical classification of laryngeal view at laryngoscopy is presented and evaluated. The best laryngeal view obtained with or without anterior laryngeal pressure is recorded. The laryngeal view is easy (E)when the laryngeal inlet is visible. The view is restricted (R) when the posterior glottic structures (posterior commissure or arytenoids) are visible or the epiglottis is visible and can be lifted; this includes some grade 2 and some grade 3 views as classified by Cormack and Lehane. A difficult (D) view is present when the epiglottis cannot be lifted or when no laryngeal structures are visible. Five hundred patients were studied. Laryngoscopy, with the patient anaesthetised and paralysed, was performed with a Macintosh laryngoscope. If the vocal cords were not visible, a gum elastic bougie was used to aid intubation. Other aids were used only if this did not allow intubation. Each laryngeal view was graded according to the new classification and that of Cormack and Lehane. Intubation was timed and the equipment needed to facilitate intubation was recorded. The new classification stratified increasing difficulty with intubation (time for intubation longer and increasingly complex methods needed) better than the Cormack and Lehane classification. The new classification is as sensitive and more specific than the Cormack and Lehane classification in predicting difficult intubation. It is also more sensitive and more specific in predicting easy intubation.

Most anaesthetists will attempt to use information from previous anaesthetic records to determine whether tracheal intubation might prove difficult [1]. Cormack and Lehane [2] described a classification of the laryngeal view during laryngoscopy. The classification was described to allow simulated difficult intubation. It has been widely adopted and is used by most anaesthetists but is applied inaccurately by the majority [1]. There have been attempts to amend the classification [3, 4] to improve its sensitivity in delineating increasing difficulty with intubation. Yentis recently evaluated a modification in which grade 2 was divided into 2a and 2b [4]. This paper presents a new classification of laryngeal view which aims to match grade of view with degree of difficulty and with the practical technique needed to achieve intubation. The classification is evaluated and compared with Cormack and Lehane's.

Method

The new classification (Fig. 1) records the best laryngeal view obtained with or without anterior laryngeal pressure. The view is easy (E)when the laryngeal inlet is visible. These views are suitable for intubation under direct vision.The view is restricted (R) when the posterior glottic structures (posterior commissure or arytenoid cartilages) are visible or the epiglottis is visible and can be lifted. These views are likely to benefit from indirect methods (e.g. gum elastic bougie). A difficult (D) view is present when the epiglottis cannot be lifted or when no laryngeal structures are visible. These views are likely to need specialist methods for intubation which may need to be performed blindly.

Figure 1.

Cormack and Lehane classification and new classification of view at laryngoscopy.

In order to explain the new classification and compare it with that of Cormack and Lehane's, it is necessary to subdivide their grade 2 and grade 3 views. Grade 2 views in which part of the vocal cords can be seen are grade 2a. Those in which the vocal cords cannot be seen are grade 2b. Grade 3 views in which the epiglottis can be seen and lifted, for instance with a gum elastic bougie, are grade 3a. When this cannot be done they are grade 3b.

Table 1 compares the new and old classification. The easy (E)view includes all grade 1 views and easier grade 2 views (2a). Restricted (R) views include some grade 2 (2b) and some grade 3 views (3a). Difficult (D) view includes the more difficult grade 3 (3b) and all grade 4.

Table 1.  Classification of laryngeal grade: Cormack and Lehane's and new classification Thumbnail image of

The study was discussed with the chairman of the Local Research and Ethics Committee: patient consent was not required as clinical practice was not altered by inclusion in the study. Five hundred consecutive patients requiring oral tracheal intubation for elective surgery were studied. Intubation was performed by myself in all cases. Patients were excluded if an awake fibreoptic technique was used. Six months of the study period was devoted to neurosurgical anaesthesia and included patients with severe cervical spine pathology presenting for cervical surgery. A further 4 months of the study took place in a hospital with a high proportion of patients with rheumatoid arthritis, presenting similar problems. Several cases identified by colleagues as difficult to intubate were studied.

After induction of anaesthesia, muscular paralysis was provided with suxamethonium or a nondepolarising muscle relaxant in the dose recommended in the data sheet. Laryngoscopy was performed with the Macintosh no. 3 blade either 1 min after administration of suxamethonium or 3 min after nondepolarising relaxant. A gum elastic bougie was always available and was used immediately where the vocal cords could not be seen, as recommended by Nolan [5]. Other aids were used if intubation could not be achieved with the standard blade and gum elastic bougie after three attempts. Aids included a long Macintosh blade, McCoy blade, laryngeal mask airway and stylet. A fibreoptic laryngoscope was available but not necessarily in the operating theatre for all cases. The intubating laryngeal mask became available midway through the study.

The best view of the larynx was obtained with careful head and neck positioning, correct laryngoscopy technique and anterior laryngeal pressure if the view was not grade 1. Laryngeal view was recorded according to Cormack and Lehane's classification including subdivisions and the new classification. Duration of laryngoscopy was timed from the anaesthetist being handed the laryngoscope to the time the tracheal tube passed through the vocal cords. Aids needed for intubation, including gum elastic bougie, were recorded.

Tracheal intubation was considered easy when it was performed within 30 s without a gum elastic bougie or additional intubation aids and difficult where intubation took longer than 4 min or required aids in addition to the gum elastic bougie.

The two laryngeal view classifications were compared to determine their ability to detect easy and difficult intubation (sensitivity and positive predictive value) and their ability to exclude false positives (specificity).

Differences in time to intubate between groups were determined with Kruskal–Wallis one-way anova. Intergroup differences were then contrasted with Mann–Whitney testing. Comparisons between categorical data were performed with Chi-squared or Fisher exact tests as appropriate. Statistical analysis was performed with Analyse-it statistical package (Analyse-it Software Ltd, Leeds, UK) and Microsoft Excel 97. A p value of less than 0.05 was taken to indicate statistical significance.

Results

Distribution of laryngeal grade and ease of intubation are recorded in Tables 2 and 3. There were 287 female and 213 male patients. Median (range) age was 51 (16–91) years and mean (SD) weight was 74 (16) kg.

Table 2.  Laryngeal view and intubation details. Cormack and Lehane's and new classification Thumbnail image of
Table 3.  Laryngeal view and intubation details. Subdivisions of grades 2 and 3 Thumbnail image of

All 500 patients were successfully intubated. The gum elastic bougie was used in 106 cases (21%). Other airway adjuncts were used on six occasions (1.2%), in five of which difficulty with intubation was anticipated: features included small jaw and reduced mouth opening, rheumatoid arthritis of the neck and poor dentition, cervical myelopathy, neck in ‘halo’ traction and previous known difficult intubation. Four of these patients refused awake fibreoptic intubation. The McCoy laryngoscope was used on four occasions and did not facilitate intubation. The long Macintosh blade was used on four occasions without helping. The intubating laryngeal mask was used four times and was successful on three. In two patients, fibreoptic intubation was performed asleep after other techniques failed. One patient was intubated using a gum elastic bougie via a laryngeal mask airway.

The median intubation time was 14 s with a range of 6–1500 s (median without bougie 13 s, with bougie 32 s). A total of 397 patients (79.4%) were intubated within 30 s without gum elastic bougie or other adjuncts (easy intubation). Fifty-nine patients (11.8%) took longer than 30 s to intubate. There were seven difficult intubations (1.4%) — six needed adjuncts in addition to the gum elastic bougie and all these took longer than 4 min to intubate.

Time to intubate differed significantly between groups (p < 0.0001, anova). There were statistically significant differences between grade 1 and 2, 2 and 3, easy (E) and restricted (R) and between restricted (R) and difficult (D)(p < 0.0001, Mann–Whitney test; Fig. 2). Time to intubate also differed significantly between grades 2a and 2b, 3a and 3b (p < 0.0001). There was no significant difference between grades 2b and 3a (p = 0.18). (Table 4).

Figure 2.

Time taken to intubate according to grade of laryngeal view. Range (–) and median (●).

Table 4.  Time taken for intubation. Comparisons between grades: Mann–Whitney tests p-values Thumbnail image of

The use of aids to intubation (excluding gum elastic bougie) differed significantly between grades 2 and 3, Restricted and Difficult, and grades 3a and 3b (Fisher exact test p < 0.0001, p < 0.0001 and p < 0.001, respectively).

Grades 3 and 4 laryngeal view predicted difficult intubation with sensitivity of 100%, specificity of 96.7% and positive predictive value of 30.4%. Difficult laryngeal view predicted difficult intubation with sensitivity of 100%, specificity of 99.6% and positive predictive value of 77%. Difficult view was significantly more specific and had a higher predictive value at predicting difficult intubation than grades 3 and 4 (Fisher exact test p = 0.001 and p = 0.04, respectively).

Grade 1 laryngeal view predicted easy intubation with sensitivity 80.3%, specificity of 87.3% and positive predictive value of 95.5%. When grades 1 and 2 were combined, easy intubation was predicted with sensitivity of 100%, specificity of 22.3% and positive predictive value of 83.2%. Easy laryngeal view predicted easy intubation with sensitivity 96.2%, specificity of 70.1% and positive predictive value of 92.7%. Thus Easy view is significantly more sensitive than grade 1 view at predicting easy intubation (Fisher exact test p < 0.0001), at the cost of a small reduction in specificity and positive predictive value. Compared with grades 1 and 2, easy view is less sensitive (p < 0.001), but more specific (p < 0.0001) and with better positive predictive value (p < 0.0001).

Discussion

It is in the nature of studies requiring visual assessment that observer bias may be introduced. In devising a new classification and evaluating it myself, it is possible that my observations are biased or that the classification is only sensitive and specific to my own performance of laryngoscopy. To avoid this I tried to ensure that the new classification can be objectively applied and used semi-objective measures of difficulty with laryngoscopy (time taken and adjuncts used) and used adjuncts according to a defined regimen. Lastly, easy and difficult intubation were predefined in order to provide further objective measures. However, the possibility of bias and lack of generalisability remains and further evaluation with ‘naive’ anaesthetists would be appropriate.

Concerning the validity of the results in this study, 4.6% of patients had grade 3 views and none had grade 4. The incidence of grades 3 or 4 laryngeal views varies between 0.3% and 13.3% in unselected populations [3, 6[7][8][9]–10]. In selected populations this may rise to 20% [11]. The study population included patients presenting for cervical surgery, with rheumatoid arthritis and several known to be difficult to intubate; these factors increase the likelihood of difficult laryngoscopy [11]. Cases where no laryngeal structures can be seen are very uncommon and this is a problem with all such studies. In spite of studying a group of patients likely to include a high proportion of difficult cases, no laryngeal view was grade 4. Many such cases probably have such obvious airway abnormality that elective fibreoptic intubation is mandatory. Anterior laryngeal pressure was used in all cases unless the view was grade 1: this improves the view in up to 30% of cases [3, 12]. The time taken to intubate with and without the gum elastic bougie and the proportion of cases in which it was necessary are similar in this study to Nolan's results when evaluating the gum elastic bougie [5]. Time for intubation does not equate with increasing difficulty but is one objective measure of such difficulty.

The frequency of airway difficulty varies according to the population studied and the definition of difficult intubation used [6]. There is no universally accepted definition of difficult intubation. Definitions vary widely and include Grades 3 and 4 laryngoscopy, the need to use specialist (and variable) equipment, more than one, two or three attempts at laryngoscopy, intubation time more than 10 min and failure to intubate the trachea. Only the first definition includes the laryngoscopic grade. Intubation is often considered as ‘difficult’ or, ‘not difficult’. This dichotomy may be misinterpreted as implying two things that may not be true. Firstly that, in the absence of difficulty, intubation is easy and secondly that all difficult intubations are of equal difficulty. In reality, most would recognise a spectrum of increasing difficulty. Most truly difficult intubations occur when the glotttis cannot be seen but inability to view the whole glottis can cause some inconvenience. Rose studied 3325 cases and reported that 84% of difficult and all failed intubations, were associated with poor visualisation of the glottis [6].

It is useful to predict difficult intubation before it occurs but no pre-operative test has adequate sensitivity and specificity to identify most cases without substantial false positives [13]. Many patients about to undergo anaesthesia will have had previous operations, and information regarding previous laryngoscopy may therefore be available in anaesthetic notes. Most anaesthetists will look for this [1].

To be useful, a classification of laryngeal view should predict difficulty (or ease) of tracheal intubation. This requires that the classification stratifies views associated with increasing degrees of intubation difficulty. This is problematic as difficulty increases in a continuous rather than stepwise manner. Grade changes should correspond with a clear increase in difficulty or alternatively with a change in practical approach. The classification should also allow for unambiguous communication of information to colleagues. Fewer than 50% of a sample of British anaesthetists were able to correctly classify laryngeal views using Cormack and Lehane's classification [6]. No classification will be perfect as some cases of difficult intubation arise from factors unrelated to laryngeal view, such as awkward teeth [6].

The new classification evaluated in this paper was previously suggested [4, 14] and arose during discussions with M. E. Wilson and J. P. Nolan. It has previously been suggested that grade 2 should be divided [3, 4]. Recently, Yentis [7] demonstrated that grade 2b views were more frequently associated with difficult intubation (67%) than grade 2a (4.3%). Most grade 3 intubations are easily achieved if a gum elastic bougie or stylet is available [7, 8]. However, grade 3 views that cannot be intubated with the gum elastic bougie are very difficult as they require intubation without visualisation of the glottis. Aoyama recently confirmed this, reporting that fibreoptic laryngoscopy during attempts to pass a gum elastic bougie under direct laryngoscopy in grade 3 cases showed a closed glottis when the gum elastic bougie would not readily pass. In these cases, the risk of failure and laryngeal damage is high [15]. This suggests that a division of grade 3 view is also needed. Dividing grade 2 and grade 3 produces a six-point scale, which may be of use for research purposes. However, much of this classification is practically redundant: grade 2a views are usually easy to intubate so could be classified with grade 1; grade 2b and grade 3a views require a similar intubation strategy — indirect intubation (e.g. use of a bougie) — and therefore can also be combined; grade 3 views where the bougie does not help (3b) require the same advanced techniques to achieve intubation as grade 4 views and so may also be grouped together. This is important as failed intubation is more frequently associated with grade 3 than grade 4 views [16], because of the rarity of the latter. This provides a three-point classification: grade 1 and 2a (easy, E), grade 2b and 3a (restricted, R) and grade 3b and 4 (difficult, D).

Ninety-seven per cent of grade 1 and 2a patients were intubated without a gum elastic bougie. All grade 2b and 3a patients were intubated with no more than the gum elastic bougie and times taken did not differ between these grades. Two-thirds of grade 3b patients needed additional aids and intubation took significantly longer than grade 3a. All patients who were difficult to intubate were graded as grade 3b. Time taken and techniques used confirm little increase in difficulty between grades 1 and 2a, an increase in difficulty between grades 2a and 2b (confirming Yentis's findings [7]) and little difference between grades 2b and 3a. Finally, there are substantial differences both in time taken and in equipment needed between grades 3a and 3b. The results therefore support the practical usefulness of the reclassification as easy/restricted/difficult (E/R/D).

The new classification also allows better prediction of both difficult and easy intubation. As with difficult intubation, there is no agreed definition of easy intubation. Empirical definitions for both were adopted on the basis of time taken and need to use adjuncts. More than 75% of intubations were easy and only 1.4% difficult by these criteria. Comparing the ability of ‘grade 3 and 4’ with difficult views in their ability to predict difficult intubation, the new classification has better specificity and positive predictive value without loss of sensitivity. For predicting easy intubation, the new classification is also better than Cormack and Lehane's. Grade 1 laryngeal view has only moderate predictive ability, while combining grades 1 and 2 results in poor specificity. Using the new classification, easy view has a sensitivity of 96% without loss of specificity: nearly all the easy intubations are predicted while most false positives (unexpectedly difficult intubations) are excluded.

The classification of Cormack and Lehane is widely, but inaccurately, used. I have demonstrated that increases in grade correlate poorly with increases in intubation difficulty. The classification can be improved by dividing grades 2 and 3 into 2a, 2b, 3a and 3b. However, this leads to a six-point scale. In practical terms several divisions are redundant. The proposed easy/restricted/difficult classification is of more practical value and has greater discrimination than Cormack and Lehane's. An easy view predicts easy intubation in 95% of cases and less than 3% need any intubation adjuncts. A difficult view is associated with difficult intubation in three-quarters of cases and specialist intubation techniques are likely to be required. Between these extremes, a restricted view is likely to require the use of a gum elastic bougie but no other adjuncts.

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