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Abstract
Introduction
Materials and Me...
Discussion
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ORIGINAL ARTICLE
Year : 2017  |  Volume : 18  |  Issue : 2  |  Page : 63-68
 

Comparison of validity of airway assessment tests for predicting difficult intubation


Department of Anaesthesia, Christian Medical College and Hospital, Vellore, Tamil Nadu, India

Date of Submission17-Sep-2017
Date of Acceptance13-Nov-2017
Date of Web Publication12-Dec-2017

Correspondence Address:
Dr. Balaji Kuppuswamy
Department of Anaesthesia, Christian Medical College and Hospital, Vellore - 632 002, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/TheIAForum.TheIAForum_31_17

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  Abstract 

Aim and Objective of the Study: This prospective study was undertaken at Christian Medical College and Hospital Vellore, India. In this study we evaluated the sensitivity and specificity of airway examination tests including: modified Mallampati test (MMT) Thyromental Distance (TMD), ULBT (Upper Lip Bite Test). BMI (Body Mass Index) was also included as one of the predictors. The tests were evaluated individually and in combination.
Methods: Total of 354 patients was recruited. Preoperative airway assessments of patients by Mallampati test, Thyromental Distance measurement, ULBT were done. The BMI was also calculated. The tests were done by the investigator and Laryngoscopic grading done by another experienced Anaesthetist according to modified Cormack and Lehane classification. The sensitivity, specificity, positive predictive and negative predictive values were analyzed for the individual tests and in combination. Statistical analysis was performed using SPSS version 11.0.
Results: The modified Mallampati had the highest sensitivity of 70% which was statistically significant with a P = 0.001.
Conclusion: From this study we conclude that the modified Mallampati test was a better predictor of difficult intubation than the upper lip bite test and Thyromental Distance.


Keywords: Difficulty, modified mallampati test, prediction, thyromental distance, tracheal intubation, upper lip bite test


How to cite this article:
Srinivasan C, Kuppuswamy B. Comparison of validity of airway assessment tests for predicting difficult intubation. Indian Anaesth Forum 2017;18:63-8

How to cite this URL:
Srinivasan C, Kuppuswamy B. Comparison of validity of airway assessment tests for predicting difficult intubation. Indian Anaesth Forum [serial online] 2017 [cited 2019 Aug 26];18:63-8. Available from: http://www.theiaforum.org/text.asp?2017/18/2/63/220557





  Introduction Top


Unanticipated difficult intubation (DI) without proper evaluation can lead to adverse outcomes such as hypoxia, hypercarbia and aspiration of vomitus, and possible awareness in a paralyzed patient. A proper airway assessment plays a vital role in the prevention of airway-related adverse effects. A number of bedside tests such as measurements of head and neck and other anatomical factors were suggested by several authors. Careful preoperative evaluation of the patients allows the anesthesiologist to plan alternative methods of securing the airway and experienced help.

The most popular tests include modified Mallampati classification (MMT), thyromental distance (TMD), Wilson's criteria, sternomental distance, atlanto-occipital distance, and upper lip bite test (ULBT). Most of the available bedside tests exhibit a low sensitivity and a high specificity. Ideally, any test to predict difficult airway should have high sensitivity, specificity, and minimal false positive- and false-negative values.[1] The airway assessment tests have all been found to be of limited validity in predicting difficult airway when used alone.[2] Therefore, combinations of individual tests or risk factors are used to increase the predictive value.

The present study aims to compare the predictive value of MMT, ULBT, and TMD for assessment of the airway.[3],[4]


  Materials and Methods Top


After approval from the Institutional Review Board, this prospective blinded study was conducted in 354 patients. All patients were assessed preoperatively using the MMT, ULBT, and TMD. These findings were correlated with the ease of exposure of the glottis at direct laryngoscopy using the Cormack & Lehane (C&L) classification.[5]

The American Society of Anesthesiologists Grade 1 and 2 patients of the age groups between 16 and 70 years undergoing elective surgical procedures under general anesthesia were included in this study. The edentulous patients, patients who are unable to open mouth, patients with laryngeal masses and those patients with limited cervical movement were excluded from the study. The preoperative assessment was done by one anesthesiologist and the laryngoscopic grading was done by another anaesthesiologist with at least 2 years experience in anesthesia who was blinded to the preoperative assessment.

For the Mallampati test, the patients were made to sit upright with the head in the neutral position and were asked to open their mouths as widely as possible and protrude their tongue to the maximum. The patient was asked not to phonate or to say “ah.” The observer sat opposite at eye level and inspected the pharyngeal structures.

The view was graded as follows:[3]

  • Class 1-Soft palate, fauces, uvula, pillars are seen
  • Class 2-Soft palate, fauces, uvula are seen
  • Class 3-Soft palate, base of uvula seen
  • Class 4-Soft palate not visible at all.


The patient was allowed to relax for a minute and test was repeated to confirm the Grading.

The ULBT was performed in all patients according to the following criteria:[4]

  • Class 1-lower incisors can bite the upper lip above the vermilion line
  • Class 2-lower incisors can bite the upper lip below the vermilion line
  • Class 3-lower incisors cannot bite the upper lip.


The TMD was measured with the head fully extended and the mouth closed. The straight distance between the thyroid notch and the bony point of the mentum was measured using a ruler.

We considered modified Mallampati (MMT) Class III and IV, ULBT class III, TMD <7 cm and BMI >30 as predictors for DI.

After induction of anesthesia, direct laryngoscopy was performed. All intubations were performed with head resting on a standard size pillow and patient in Magill position, i.e., the neck is flexed anteriorly with head extended at atlantooccipital joint using Macintosh blades.

The laryngoscopy view was graded according to modified Cormack and Lehane Classification as follows:[5]

  • Grade 1-Full view of Glottis
  • Grade II a-Partial view of Glottis seen
  • Grade II b-Only arytenoids seen
  • Grade III-Only epiglottis seen
  • Grade IV-Neither epiglottis nor glottis seen.


The initial Laryngoscopy was noted without any external laryngeal maneuver; if visualization was difficult then backward, upward, rightward pressure (BURP) external pressure was used A special note was made if airway adjuncts such as McCoy blade or bougie were used. A laryngoscopic view of IIb, III, and IV was considered as difficult.

The preoperative assessment data and the laryngoscopic findings were used to determine the accuracy of the tests in predicting DI both individually and in combination. Sensitivity, specificity, positive predictive values (PPV), and negative predictive values (NPV) were calculated for each test.

True positive = a DI that had been predicted to be difficult.

False positive = an easy intubation that had been predicted to be difficult.

True negative = an easy intubation that had been predicted to be easy.

False negative = a DI that had been predicted to be easy.

Sensitivity = the percentage of correctly predicted DIs that was truly difficult (true positives/true positives + false negatives).

Specificity = the percentage of correctly predicted easy intubations as a proportion of all intubations that were truly easy (true negative/true negatives + false positives).

PPV = the percentage of correctly predicted DIs as a proportion of all predicted DIs (true positives/true positives + false positives).

NPV = the percentage of correctly predicted easy intubations as a proportion of all predicted easy intubations (true negatives/true negatives + false negatives).

Accuracy = the percentage of correctly predicted easy or DIs as a proportion of all intubations (true positive + true negatives/true positives + true negatives + false positives + false negatives).

The data were analyzed using SPSS version 11.0 (IBM Corporation) for Microsoft Windows. Sensitivity, specificity values were calculated using STATA. P value was calculated using Pearson Chi-square and Fisher Exact test wherever applicable. P < 0.05 was considered to be statistically significant.

Observations

In the present study, a total number of 354 cases were recruited [Flow Chart 1]. Of the 354 patients, 112 were male and 242 female patients with age ranging from 16 to 66 years. The mean height, mean weight, and the mean BMI were 157.0 cm ± 7.26, 58.38 ± 12.65, and 23.64 ± 4.73 respectively. Fifty-one patients had difficult laryngoscopy with CL Grade II b and III with incidence of 14.4% of DI. There were no failed attempts at intubation.



Our preoperative airway assessments using the different predictors are documented as follows in [Table 1], [Table 2], [Table 3], [Table 4], [Table 5].
Table 1: Distribution of patients among the different airway predictors (n=354)

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Table 2: Distribution of patients according to the laryngoscopic grading

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Table 3: Combinations of the predictors

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Table 4: Sensitivity, specificity, positive predictive value, and negative predictive value of the individual predictors

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Table 5: Sensitivity, specificity, positive predictive value, and negative predictive value of the tests in combination

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Study results showed that other than Mallampati, the rest of the variables did not predict DI. The modified Mallampati had the highest sensitivity of 70% which was statistically significant (P = 0.001). The ULBT, TMD, and BMI were found to have very low sensitivity though their specificity (>90%) and PPV (>20%) and accuracy (80%) were much higher than Mallampati [Table 4].

Combination of ULBT, TMD, and BMI and MMT, TMD, and BMI resulted in very low sensitivity and high specificity [Table 5]. Both combinations had low PPV (20%) and high NPV (85%). As the above two combinations did not improve the sensitivity, we combined the most specific test the ULBT (97%) and the most sensitive test Mallampati (70%) in our study, thereby hoping to increase their predictivity. The sensitivity of combining MMT and ULBT was 83.33%, which was more than the individual tests alone. With this combination, we also found that the false positive rate was 92.4% decreasing the specificity and PPV to 52.34% and 5.78%, respectively. In correlation with BMI>30 [Table 6] and [Table 7], MMT revealed class III difficulty in 22 patients out of 35, while ULBT showed difficulty in only one out of 35 patients.
Table 6: Correlation between body mass index and upper lip bite test

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Table 7: Correlation between body mass index and Mallampati

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  Discussion Top


Airway management remains an important challenge for the anesthesiologist, and proper preoperative airway assessment enables us to take appropriate measures during DI. The reported incidence of DI varies from 0.05% to 18%.[6],[7],[8],[9],[10],[11],[12] The large variation in the incidence could be attributed to the different definitions used during similar studies and the incorporation of different grades of the Cormack–Lehane for the laryngoscopic view. DI is defined as repeated attempts at intubation, the use of a bougie or other intubation aids but the most widely used is the Cormack and Lehane classification.[13],[14]

Predicting DI in apparently normal patients is highly essential. For a predictor test to be clinically useful, it should have very high sensitivity with minimal false negative results reducing the incidence of unexpected DI for an unprepared anesthesiologist. Keeping this in mind, we compared different tests with respect to their clinical value.

The incidence of DI in our study was found to be in the higher range reported about 14.4%. There were no patients in the extreme difficult group (Grade IV), and majority of patients were found to have a Grade II b laryngoscopy. Mallampati observed a 13% incidence of DI in his study.[6] In this study, we used similar criteria to define DI. The Grade III in his study was comparable to the Grade II b of our study.

The Mallampati test, when considered as a single predictor of DI had a sensitivity of 70.5% in our study, which was comparable to the previous studies.[9],[11] Studies which used Mallampati as a single predictor had a wide range of sensitivity from 40% to 82.4%.[1],[4],[7],[8],[10] which could be attributed to the interobserver variability.[14],[15] In our study, the tests were carried out by the same investigator, thereby avoiding interobserver variability. Mallampati reported a sensitivity of 53% and a PPV of 93%.[6] The specificity of MMT (54.7%) in our study was lesser than that of previous studies when used as a single predictor.[2],[11],[16] The values for the PPV (20.8%) were similar to that obtained by the previous studies.[9],[11] The low specificity and PPV in our study can be explained by the large number of false positive cases, 137 out of 173 (79.19%).

The ULBT was proposed by Khan et al in a hope that combining two important factors affecting intubation like jaw subluxation and presence of buck teeth which interfere with intubation.[4] There have been varying reports of the predictive value of this test since its introduction. Khan et al compared it with the modified Mallampati and found it to be more specific (88.7%) and accurate (88.0%) than the Mallampati with an equally good sensitivity (76.5%).[4] The accuracy, specificity, and NPV of the ULBT in our study were 83.9%, 97%, and 85.9%, respectively. This was comparable to some previous studies.[16],[17],[18],[19],[20] The PPV of the ULBT was 25.0%. The ULBT had a very low sensitivity 5.88% in our study. The sensitivity of ULBT in our study was 5.88% similar to that obtained by some authors.[16],[21],[22] Compared to the Mallampati, the ULBT had a high false negative rate of 14.0% against 8.3% thus decreasing the sensitivity of our study. However, the false positive rate was very low which explains the high specificity and PPV than Mallampati. The upper lip test in our study was not useful in picking up difficult cases. A highly specific but not sensitive test is of little value for evaluation; although when results are negative, there is a high probability that intubation is easy. The reason for low sensitivity is due to the low incidence of patients in the ULBT Class III; out of 354 patients, there were only 12 patients in the Class III group. The ULBT was claimed to be more reliable and easy to interpret than the Mallampati with its clear demarcation between the classes. Even with this feature, the problem we encountered was that, the patients could not understand the test in spite of the demonstrations made by the investigator. In our study, even though assessment by ULBT revealed a Class III difficulty in 4 out of the 24 who had prominent incisors, intubation proved difficult in only one patient. This was in contrast to what was proposed by the Khan et al.[4]

A TMD of <6 cm is generally accepted as a predictor of DI. Few research studies agree on this measurement.[9],[10],[23],[24],[25]

In some studies, a TMD of <7 cm was taken as to predict DI.[7],[11] In our study, the sensitivity was only 7.84% with a high specificity of 96.3% and a PPV of 26.6% which were similar to the previous studies.[9],[11],[24],[25] The low sensitivity was due to less number of patients with TMD <7 cm, 15 patients of which only 4 proved difficult on laryngoscopy. TMD has been criticized of not being a good predictor; now ratio of height to TMD and ratio of neck circumference to TMD (NC/TM) have been considered as a better predictor.[26],[27]

The sensitivity and PPV of a single test have been improved by combination of many other predictors as shown in few studies.[7],[11],[28] In our study, when MMT, TMD, and BMI were combined, the sensitivity was found to decrease to 19.6%. Similarly, combining ULBT, TMD, and BMI resulted in a lower sensitivity of 13%. The reason was due to the fact, that individual factors each had a very low sensitivity. Mallampati and ULBT when combined showed a sensitivity of 83.3%; however, the PPV was 5.78%.

Obesity is a known risk factor for DI and BMI is an index of obesity.[29],[30] Considering a BMI of more than 30 to be associated with DI, we obtained a sensitivity of 13.7% and a specificity of 90.7%. The PPV was only 20%. Of the 35 patients who had a BMI of >30, only 7 were truly difficult. Even the 3 patients with a BMI of 40 and more, had easy laryngoscopy grades. Unlike other studies, there was no correlation between obesity and difficult laryngoscopic grade in our study.[31],[32]

Limitations of the study The low sensitivity of the ULBT in this study may be due to the lesser number of cases in the higher grade. Although the test was easy patients could not understand it in spite of demonstrating to them.


  Conclusion Top


The predictor tests for DI have only poor to moderate discriminative power when used alone. Combination of tests adds some incremental diagnostic value in comparison to the value of each test alone. In our study, the MMT had a higher sensitivity and much lower specificity than the other predictors the ULBT, TMD, and BMI when analyzed as single predictors of DI. The combination of MMT and ULBT showed a higher sensitivity than combination with other predictors. No test has 100% sensitivity, and inevitably some difficult tracheal intubations are missed and some false positives may occur, but they should be as few as possible. Even with the varying results of the commonly used airway assessment tests we still use them, and every anesthesiologist should be familiar with the difficult airway algorithm.

Acknowledgment

All the authors express sincere gratitude to all respondents whose honest attention help and support and the participants of the study lead the research project to the worthful outcome.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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