|ORIGINAL CONTRIBUTION - CLINICS IN ANESTHESIOLOGY
|Year : 2016 | Volume
| Issue : 2 | Page : 90-95
Prediction of difficult laryngoscopy in patients undergoing endotracheal intubation: A comparative study of various airway assessment tests
Shoba Philip, Farah Fatima Nizar
Department of Anaesthesiology and Critical Care, Lourdes Hospital, Kochi, Kerala, India
|Date of Web Publication||30-Dec-2016|
Department of Anaesthesiology and Critical Care, Lourdes Hospital, Kochi - 682 012, Kerala
Source of Support: None, Conflict of Interest: None
Introduction: Predicting a difficult airway is one of the necessities in anesthetic practice. Multiple tests used singly and in combination have been used to predict it, with varied results. We conducted a study to compare the exactitude of various airway assessment tests individually and in combination with parameters suited to the Indian population for the prediction of difficult laryngoscopy. Aims: The aim of this study was to compare various airway assessment tests individually and to statistically analyze the three best tests from our analysis in combination with the Modified Mallampati Test (MMT) for their efficiency in predicting a difficult laryngoscopy in apparently normal individuals undergoing endotracheal intubation. Materials and Methods: This was a prospective, single-blinded, observational study, where 250 patients of American Society of Anesthesiologists Physical Status Grade I and II, without predictors for difficult airway undergoing elective surgeries, were assessed and graded for thyromental distance (TMD), ratio of height-to-TMD (RHTMD), inter-incisor gap, head and neck movement (HNM), MMT and upper lip bite test (ULBT), and correlated intraoperatively with Cormack and Lehane score, and combinations of the best three tests with MMT were then statistically analyzed using standard formulae and the IBM SPSS version 20 statistics software. Results: Analysis of tests for difficult laryngoscopy showed that RHTMD and TMD had the highest sensitivity (90.0% and 70%). ULBT and HNM had the highest specificity of 99.0% and 91.4%. ULBT and TMD were highest in positive predictive value (PPV) (90% and 58.3%) and RHTMD and TMD in negative predictive value (NPV) (97.8% and 94.1%). Of the combinations, MMT + RHTMD had the highest sensitivity (95.0%), NPV (98.4%), odds ratio (23.5), and relative risk, but MMT + ULBT had higher specificity (65.7%), PPV (32.1%), and likelihood ratio (2.5). The combination of MMT + ULBT + RHTMD had 100% sensitivity and 100% NPV and 57% specificity and 30.8% PPV. Conclusion: Ratio of height to TMD is the single best test for difficult laryngoscopy. A combination of MMT + RHTMD + ULBT would be the best option for the prediction of difficult laryngoscopy in apparently normal patients.
Keywords: Combination of tests, difficult laryngoscopy, ratio of height-to-thyromental distance, upper lip bite test
|How to cite this article:|
Philip S, Nizar FF. Prediction of difficult laryngoscopy in patients undergoing endotracheal intubation: A comparative study of various airway assessment tests. Astrocyte 2016;3:90-5
|How to cite this URL:|
Philip S, Nizar FF. Prediction of difficult laryngoscopy in patients undergoing endotracheal intubation: A comparative study of various airway assessment tests. Astrocyte [serial online] 2016 [cited 2019 Jun 15];3:90-5. Available from: http://www.astrocyte.in/text.asp?2016/3/2/90/197213
| Introduction|| |
The term "airway management" refers to the practice of establishing and securing a patient's airway and forms one of the cornerstones of anesthetic practice.
Recognition of an obviously difficult airway leads to a series of communications and preparations to assist establishment and maintenance of airway. It is essential for every anesthesiologist to have in his/her armamentarium, simple and efficient tests to quickly and accurately predict the incidence of a difficult airway.
Various studies assess a mixed population of those with known risk factors for a difficult airway along with those patients without risk factors. As it is an unanticipated difficulty which may be associated with catastrophes, our study has included only those with apparently normal airways.
The results of the previous studies on this subject are varied and sometimes even contradictory as they were conducted on different races and ethnicities with different parameters to judge the success of intubation. ,,,, With this in mind, we have used parameters and criteria preferably from studies on the Indian population in our study. We also decided to make difficult laryngoscopy the end point of our study. Therefore, it was decided to specifically perform tests which assess for difficulty in laryngoscopy.
An earnest attempt was made in this study to compare newer airway assessment tests such as upper lip bite test (ULBT) and the ratio of height-to-thyromental distance (RHTMD) with already established tests such as Modified Mallampati Test (MMT), inter-incisor gap (IIG), head and neck movement (HNM), and thyromental distance (TMD).
The study aimed at comparing the above tests individually with parameters suited for the Indian population, and the three best tests from our analysis were then assessed statistically in combination with the MMT (as it is the most commonly used test by anesthesiologists) for their efficiency in predicting a difficult laryngoscopy in apparently normal individuals undergoing endotracheal intubation for elective surgeries under general anesthesia.
| Materials and Methods|| |
This prospective, observational, single-blinded, unicentric evaluation of airway screening tests for difficult laryngoscopy was conducted on 250 patients of American Society of Anesthesiologists Physical Status (ASA) Grade I and II of either sex between the ages of 18 and 70, without obvious airway difficulties posted for elective surgeries under general anesthesia with endotracheal intubation, with the approval of the Institutional Ethical Committee and written informed consent of the patients. Patients unable to stand erect, pregnant patients, those with obvious facial or airway deformities (including neck swellings and edentulous patients), body mass index (BMI) >35, those with a history of snoring or obstructive sleep apnea, and patients requiring awake intubation were excluded from the study.
Following routine preanesthetic checkup, written informed consent was obtained from the patients. The airway assessment tests and demographic variable assessment were performed by the primary investigator alone to avoid interobserver variability.
IIG was assessed by asking the patient to open the mouth as wide as possible, and the distance between the upper and lower incisor in the midline was measured using a stiff tape measure and graded as follows:
The range of HNM was measured by asking the patient to extend his/her head as far as possible and a pen was held vertically on the forehead. A rigid notebook was held at the side of the face with the long axis parallel to the pen. The patient was then asked to flex his/her neck as much as possible. If the pen was lower than the bottom of the page, it was recorded as >90° (Grade I), and if it was above the bottom of the page, it was <90° (Grade II). 
- Grade I >4 cm
- Grade II <4 cm. 
The ULBT: The patient was asked to move the mandibular incisors as high as possible on the upper lip, just like biting the upper lip, and the findings were graded as Class I - lower incisors can reach up to or above the vermillion border, Class II - lower incisors reach below the vermillion border, and Class III - lower incisor unable to reach the upper lip. 
The Mallampati-Samsoon classification system or MMT: the patient was seated in a neutral position and asked to open the mouth as wide as possible and protrude the tongue as far as possible and phonation was not allowed. The oropharyngeal structures were viewed and graded as shown in [Table 1]. 
The TMD was measured with the patient's head in full extension and mouth closed. The distance between the point of the mentum and the thyroid notch was measured using a stiff tape measure, and it is rounded off to the nearest 0.5 cm and graded as shown in [Table 1]. 
The height, weight, and BMI were assessed. The height was measured using a stadiometer.
The RHTMD was calculated and graded as follows:
RHTMD = height (cm)/TMD (cm) 
According to [Table 1] and Grade 3, observations (Grade 2 for those without three grades) were considered as predictors for difficult laryngoscopy.
Standardized anesthetic protocol was followed in all patients. They were given aspiration prophylaxis with tablet ranitidine 150 mg on the previous night and morning of surgery and tablet ondansetron 4 mg on the morning of surgery and antianxiety treatment with tablet midazolam 7.5 mg on the morning of surgery. In the operation theater, after establishing venous access and standard monitoring, premedication and analgesia were administered as per the case requirement. Difficult airway cart was kept ready for all cases, and the anesthesia machine and suction apparatus were checked for functionality. Following preoxygenation with 100% oxygen, anesthesia was induced with propofol (2 mg/kg) intravenous (IV), and succinyl choline (2 mg/kg) IV (for neuromuscular blockade) was given to facilitate intubation. Laryngoscopy was performed using a Macintosh #3 or #4 blade by an anesthesiologist of more than 5 years of experience who was blinded to the preoperative measurements for the sake of avoiding interobserver variability. The laryngoscopic view was graded using the Cormack and Lehane scoring as Grade 1 - most of the glottis is visible; Grade 2 - at best almost half of the glottis is seen, at worst only the posterior tip of the arytenoids is seen; Grade 3 - no part of the glottis and only the epiglottis is visible; and Grade 4 - not even the epiglottis can be seen. 
Difficult laryngoscopy may be defined as the presence of Cormack and Lehane Grade 3 or Grade 4 view on laryngoscopy.  The ASA described difficult laryngoscopy as the nonvisualization of any portion of the vocal cords after multiple attempts at conventional laryngoscopy and difficult intubation as either as three attempts at intubation when an average laryngoscope is used or an intubation attempt taking 10 min or more  or tracheal intubation requiring multiple attempts in the presence or absence of tracheal pathology. 
The statistical analysis performed included the calculation of sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), odds ratio, relative risk, positive likelihood ratio, and accuracy using standard formulae and the IBM SPSS version 20 statistics software (Armonk, NY: IBM Corp).
The tests of significance performed were Chi-square test, independent t-test, and binary logistic regression. P <0.05 was considered to be statistically significant.
| Results|| |
The incidence of difficult laryngoscopy in this study was 16% (40 cases out of 250). There was no significant difference in those having difficult or easy laryngoscopy with regard to demographic variables of sex, age, weight, height, and BMI [Table 2].
The statistical analysis of individual airway assessment tests for difficult laryngoscopy gives the following results [Table 3] and Graph 1]:
RHTMD was found to be the best single screening for difficult laryngoscopy test due to its high sensitivity and a good balance of all other predictive values. Although it was followed by TMD, it has been seen that for assessing the same aspect of the airway, the RHTMD is a better test. Further, we also found that a positive ULBT is more predictive of difficult laryngoscopy.
- The sensitivity was highest for RHTMD and TMD (90.0% and 70%). The cutoff value for difficult laryngoscopy for RHTMD was 23.5, and for TMD, it was 6 cm
- The highest specificity was seen with ULBT and HNM (99.0% and 91.4%)
- The highest PPV was seen in ULBT and TMD (90% and 58.3%)
- NPV was highest in RHTMD and TMD (97.8% and 94.1%)
- The highest accuracy was shown by ULBT (90.4%) followed by TMD (87%) and RHTMD (84%)
- The odds ratio was highest for ULBT (85.1) followed by RHTMD (43.5)
- The highest relative risk was seen with RHTMD (22.3) followed by TMD (9.8)
- The positive likelihood ratio was highest for ULBT (43.7) followed by TMD (7.4).
The above three tests were combined individually with MMT, and the analysis for the best combination of tests for difficult laryngoscopy is shown in [Table 4] and Graph 2:
MMT was combined with the newer tests, ULBT and RHTMD, and the combination had 100% sensitivity and NPV, 57% specificity, PPV of 30.8%, and an accuracy of 64%.
- The highest sensitivity and NPV were seen with MMT + RHTMD, 95.0% and 98.4%, respectively. This was followed by MMT + TMD (90% and 97%, respectively)
- The PPVs were less for all the three two-test combinations due to higher false positives
- The specificities for all three combinations were comparable
- Accuracy was highest for MMT + ULBT, 68.8%
- The odds ratio and relative risk were highest for MMT+RHTMD (25.3, 18.1)
- The likely hood ratios were comparable.
MMT + RHTMD is the best two-test combination for the prediction of difficult laryngoscopy. However, the best option would be a combination of three tests assessing different aspects of the airway due to the increased sensitivity of these combinations.
| Discussion|| |
One of the primary responsibilities of an anesthesiologist is to establish and maintain airway patency. Airway difficulties and disasters are associated with increased morbidities and even mortality.  In elective surgeries, the anesthesiologist has the advantage of being able to evaluate the patient in advance and be prepared for possible difficulties with regard to airway management and subsequently managing them successfully. Conducting a preoperative physical examination may identify the presence of upper airway pathologies or anatomical anomalies which may help predict airway difficulties.
The ASA Task Force on Difficult Airway has recommended that a patient must be assessed preoperatively for airway difficulties.  There are numerous and often contradictory studies regarding whether to use a single parameter or multiple parameters for the assessment of airway. ,,,, Most of the studies and parameters were defined for Western population whose anthropometry differs significantly from our population. Furthermore, many studies included obese individuals in their study population even though increased BMI is a known predictor of difficult airway in and off itself. ,,
Preoperative airway assessment tests are screening tests, and therefore should be highly sensitive to predict the maximum number of patients of difficult laryngoscopy correctly and highly specific to predict easy laryngoscopy correctly. Test should also have a high PPV with few false-negative predictions. NPV is the probability that patients with a negative screening test truly do not have difficult laryngoscopy. The accuracy of a predictive test (percentage of results correctly identified by the test) also indicates how useful it would be in daily practice.
The incidence of difficult laryngoscopy was 16% in our study; it correlates with the incidences reported in earlier studies. ,,, There was no significant difference in the demographic variables in those with difficult and easy laryngoscopy in the apparently normal individuals.
The TMD had a sensitivity, specificity, NPV, and accuracy in our study. The high sensitivity correlates with studies by Schmitt et al., Krishna et al., and Shah et al., but it is significantly higher than in studies by el-Ganzouri et al.  and Tse et al. This is probably due to anthropometric differences in the various populations. The high specificity of this test has been corroborated by these older studies. ,,,,, Cutoff values of TMD for predicting difficult laryngoscopy range from 5.5 to 7 cm in different studies. TMD is related to body size and proportion. The mean height of our patients (161.2 ± 6.2) in the difficult laryngoscopy group is shorter than that reported in the study by Schmitt et al.  (171 ± 9 cm), so smaller cutoff values may increase the predictive accuracy of this test in our population. 
Taking anthropometric differences between different populations into consideration, Schmitt et al.  designed a new test for difficult airway prediction, the ratio of the patient's height to TMD.  Cutoff values for RHTMD recommended for predicting difficult laryngoscopy are 25 in Caucasians,  24 in Iranian patients,  and 23.5 in Thai patients.  There are anthropometric differences between the Indian population and the American or European (Caucasian) population. The average height of an American adult male and female (1.776 and 1.632 m, respectively) is considerably greater than that of an Indian male and female (1.612 and 1.521 m, respectively). , Safavi et al. concluded that the cutoff point for RHTMD for the prediction of direct laryngoscopy is race dependent and recommend calculating cutoff point for each population separately.  In our study, the cutoff of 23.5 was obtained from a recent study by Shah et al. 
RHTMD had a high sensitivity, specificity, NPV, and accuracy of 90%, 82.9%, 97.8%, and 84%, respectively. This correlates well with previous studies. , Krishna et al. used a cutoff value suggested by Schmitt et al.  (≥25) in their study which is higher than our cutoff (≥23.5), and this may be the reason for the reduced sensitivity and higher specificity in that study when compared to ours.  Overall, the statistical analysis of RHTMD has shown that this test is better than its predecessor, the TMD.
The frequently used measurements of IIG and HNM had low sensitivities, high specificities, NPV, and good accuracy in our study. The odds ratio, relative risk, and likelihood ratio were on the lower end of the spectrum for both these tests. The high specificity and NPV make these two tests more valuable in ruling out a difficult laryngoscopy. Therefore, they would serve better as add-on tests rather than individual tests.
Although the MMT is ubiquitous in anesthetic practice, our study found it to be an average predictive test. It had the least positive likelihood ratio, relative risk, and accuracy in our study. Our findings correlate well with studies by Honarmand et al. and Eberhart et al. , The high NPV indicates that it is more useful in ruling out a difficult intubation. Karkouti et al. found that the MMT had had the worst interobserver reliability, and mouth opening and chin protrusion tests have the best reliability.  This may be the reason for the varied results in multiple studies regarding the predictive value of MMT. The sensitivity and specificity of this test range between 20% and 80% in various studies. ,,,, The above reasons make the MMT an unreliable test when used alone.
The ULBT is a relatively new test which received a lot of interest due to its simplicity, reproducibility, reduced interobserver variability, and the high sensitivity and specificity seen in the initial studies.  It had a poor sensitivity in our study (45%) despite scoring high in all the other statistical tests. The poor sensitivity in our study correlates with studies by Myneni et al. (8.1%), Honarmand et al. (66%), Hester et al. (35%), and Eberhart et al.  (28%). The high NPVs correlate with many previous studies. ,,, Khan et al. and Honarmand et al. demonstrated a high sensitivity (76.5% and 90%), specificity (88.7% and 59.4%), and NPV (98.4% and 98.3). , Similar findings were also echoed in Shah et al.'s study and also Allahyary et al.'s study on obstetric patients. The sensitivity of this test in these studies was significantly higher than ours. , The variations in different studies are probably due to dental (maxillary and mandibular morphology and morphometry) and craniofacial differences in different ethnicities.  Therefore, from our study, it can be inferred that ULBT would not be a good standalone screening test to predict difficult laryngoscopy due to its poor sensitivity. The high specificity and NPV make it useful in ruling out a difficult laryngoscopy. But having said that, it can also be seen that if a patient is ULBT Grade III, the likelihood of him/her having a difficult laryngoscopy is 47.3, which is the highest in this study. This means that if ULBT is Grade III, there is a very high chance of the laryngoscopy being difficult. Hence, it can be safely concluded that this would be a useful second or third test for the prediction of difficult laryngoscopy if other tests turn out to be equivocal.
The best individual test for difficult laryngoscopy in our study was RHTMD followed by TMD. The low sensitivity of the other tests makes them poor screening tests individually.
Due to the inadequacies of individual predictive tests, many authors have recommended combining airway examination tests to better predict the possible occurrence of difficult airway such as Benumof's 11 parameter analysis, Wilson's score, and total airway score. Many studies suggest combining routinely used tests to improve the sensitivity of each. ,,,,,, This can be explained by the fact that various factors determine difficulty in airway management and a single test would be unable to assess all of them, therefore various tests will be needed to assess these multiple features of the airway.
In our study, we evaluated the following combinations: the MMT + TMD, MMT + RHTMD, MMT + ULBT, and MMT + RHTMD + ULBT. All the combinations had high sensitivity (90.0%, 95.0%, 85.0%, and 100.0%, respectively), NPV (97.0%, 98.4%, 95.8%, and 100.0%, respectively), moderate specificity (61.9%, 57.1%, 65.7%, and 57.1%, respectively), and accuracy (66.%, 63.2%, 68.8 and 64.0%, respectively), while the PPVs (31.0%, 29.7%, 32.1% and 30.8%, respectively) and positive likelihood ratios (2.4, 2.2, 2.5, and 2.3, respectively) were on the lower side. This is because the number of false positives increased in our study when the tests were combined. Apart from the minor variations in the combinations of tests used in some studies  and the lower sensitivities in Honarmand et al.'s study,  our findings largely correlated with previous studies. The variations can be explained by anthropometric differences and differing cutoff values used in these studies. , The high sensitivities decrease the chance of missing out on a difficult laryngoscopy, and the high specificity and NPV indicates that if a patient tests negative, there is very little chance of him/her having a difficult laryngoscopy. In our study, MMT + RHTMD was the best two-test combination for difficult laryngoscopy prediction and MMT + RHTMD + ULBT is the best possible option all around.
The authors thank Dr. Ashish Alexander, DA, DNB, EDAIC, for supporting and helping in the study.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4]