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* From the Departments of Anesthesia, and
Surgery, University of Lagos, Lagos, Nigeria; and
the Department of Anesthesiology, University of Toronto, Toronto, Ontario, Canada.
Address correspondence to: Dr. Nkihu A. Merah, Department of Anesthesia, College of Medical Sciences, University of Lagos, PMB 12003, Lagos, Nigeria. Phone: 234-8023130344; Fax: 234-15851096; E-mail: mnkihu{at}yahoo.com
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Abstract |
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Methods: Three hundred and eighty consecutive patients undergoing general anesthesia were evaluated using the MMT, TMD, SMD, HLM and IIG and the cut-off points for the airway predictors were Mallampati III and IV; 
6.5 cm, 13.5 cm, 9.0 cm and 4.0 cm respectively. During direct laryngoscopy, the laryngeal view was graded using the Cormack and Lehane (CL) classification. CL grades III and IV were considered difficult visualization. Sensitivity, specificity and positive predictive value for each airway predictor in isolation and in combination were determined. Logistic regression analysis was used to determine independent predictors of DVL.
Results: Difficulty to visualize the larynx was found in 13 (3.4%) patients. The sensitivity, specificity and the positive predictive value for the five airway predictors were: MMT (61.5%; 98.4%; 57.1%), TMD (15.4%; 98.1%; 22.2%), SMD (0%; 100%; 0%), HLM (30.8%, 76.0%; 4.3%) and IIG (30.8%; 97.3%; 28.6%). The best combination of predictors was MMT/TMD/IIG with a sensitivity, specificity and positive predictive value of 84.6%; 94.6%; 35.5% respectively. Logistic regression analysis showed that weight, MMT, IIG and TMD were independent predictors of DVL.
Conclusion: MMT, TMD and IIG appear to provide the optimal combination in prediction of DVL in a West African population.
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Introduction |
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Several preoperative airway assessment tests have been proposed.4–11 These tests have been used singly or in various combinations. However, they are characterized by low sensitivity, reasonable specificity and low positive predictive value. They all have significant false positives.12–14 The majority of these studies were performed in North American or European populations. At the Lagos University Teaching Hospital in West Africa, the Mallampati score is used in combination with various anatomical indices that include receding mandible, full set of teeth, short fat neck and facial deformities. Anatomically, West Africans appear to have a longer neck and sternomental distance (SMD) compared to Caucasians. The purpose of our study was to determine the ability to predict DVL in a West African population from the following preoperative airway predictors, in isolation and in combination: modified Mallampati test (MMT), thyromental distance (TMD), SMD, horizontal length of the mandible (HLM) and inter-incisor gap (IIG).
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Methods |
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Preoperative assessment
Demographic data collected included age, sex, weight, height, and body mass index (BMI). A single anesthesiologist investigator with five years’ experience in anesthesia carried out the evaluation as described by the original authors. The following five predictive test measurements were performed on all patients:
The cut-off points for the predictors were determined a priori as suggested by the originators of the tests except for the SMD cut-off that was increased from 12.5 cm to 13.5 cm after preliminary analysis of pilot data. Values below and inclusive of each cut-off point were predicted as DVL for the anthropometric variables. Values above the cut-off point were predicted as easy-visualization of the larynx (EVL). DVL was predicted with MMT III or IV, TMD 6.5 cm; SMD 13.5 cm; HLM 9.0 cm; IIG 4.0 cm.
Anesthesia induction and tracheal tube insertion
Induction of anesthesia was performed in the supine position with 5 mg•kg–1 of sodium thiopental or propofol 2 mg•kg–1 intravenously. Suxamethonium chloride 1 mg•kg–1 was administered intravenously to facilitate endotracheal intubation. After disappearance of fasciculations, the patient’s head was placed in the ‘sniffing position.’ Laryngoscopy was performed using a Macintosh #4 blade to visualize the larynx and the view was classified using the Cormack and Lehane (CL) classification:15 (I = vocal cords visible; II = only posterior commisure or arytenoids visible; III = only epiglottis visible; IV = none of the foregoing visible). DVL was defined as CL III or IV view on direct laryngoscopy. EVL was defined as CL I or II view on direct laryngoscopy. Confirmation of successful intubation was by bilateral auscultation over the lung fields and capnography.
Statistical analysis
Demographic data, value of the airway predictors were compared using t tests for continuous variables and Mann Whitney U test for MMT.
First, univariate analyses were performed to assess the association of each airway predictor to DVL. Chi-square analyses were used. Sensitivity, specificity, and positive predictive value were obtained and compared amongst predictors. Secondly, combinations of predictors were formulated. Likewise, the sensitivity, specificity, and positive predictive value were obtained and compared amongst the combinations. Lastly, demographic and patient variables, and airway predictors were entered into a multivariate logistic regression analysis to determine independent predictors of DVL. Odds ratio, 95% confidence intervals and P values were obtained for independent predictors derived.
The data were analyzed using the statistical package for social studies version 11.0 (SPSS® Inc., Chicago, IL, USA).
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Results |
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. There were 90 males and 290 females. There were 306 (80.6%) ASA I, 64 (16.8%) ASA II, and ten (2.6%) ASA III patients. DVL was observed in 13 patients (3.4%). There was no failed intubation. There were significant differences in weight and BMI between the DVL and EVL patients (Table I).
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). Sensitivity, specificity and positive predictive value of the five single predictors are shown in Table III.
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Combined predictive tests of (EVL) and DVL; Table IV
The combination of the five tests increased the sensitivity to 84.6% but decreased the specificity to 73.3% and the positive predictive value to 10.1%. The combination with the best results was the Mallampati test, TMD and IIG with a sensitivity, specificity and positive predictive value of 84.6%, 94.6%, and 35.5% respectively. The various other combinations resulted in an increased sensitivity at the expense of lowering the positive predictive value.
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Discussion |
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A screening test for prediction of DVL must be rapid and provide reliable results. It should reliably identify patients with a truly DVL (good sensitivity) and not falsely label patients as DVL who actually are EVL (low false positives). No screening test is 100% sensitive and 100% specific. The ideal test should be easy to perform, highly sensitive, specific and possess a high positive predictive value with few false positive predictions.
In this study, it was found that the MMT was the most useful single predictor with a sensitivity, specificity and positive predictive value of 61.5%, 98.4%, and 57.1% respectively. These results are similar to the studies done by Tse et al. and Ramadhani and colleagues.16,17 A sensitivity of greater than 80% was reported by Frerk in a European population, Savva from the United Arab Emirates and also Ita and colleagues in Nigeria.11,12,18 Mallampati et al. reported a sensitivity of 53% and a positive predictive value of 93%, however, repeated studies have not obtained this high positive predictive value.4,16 The wide range of results has been attributed to inter-observer variability as reported by Karkouti et al.19 Furthermore, prevention of phonation was shown by Tham et al. to be a critical factor in achieving a reliable score.20
The TMD had a very low sensitivity of 15% in this our study. Tse et al. reported a similarly low sensitivity of 32% while in the majority of reports the sensitivity was reported to be above 60%. This could possibly be due to anthropometric peculiarities in the study population, a postulate that should be validated in future studies.
The SMD was unable to predict any of the difficult laryngoscopies despite a statistically significant difference between DVL and EVL patients (P < 0.022). In the EVL the average distance was 18.5 ± 2.1 cm while in the DVL it was 17.1 ± 1.9 cm. The cut-off point used in this study was 13.5 cm as described by Ramadhani who increased the original distance from 12.5 cm, following the recommendation by Savva after discriminant analysis. The SMD had not been studied in the West African people and it appears that the West African population might have different anthropometric characteristics from the results obtained. An increase in the cut-off point to 15 cm may increase the sensitivity of this test in these people, but this remains to be demonstrated.
The HLM was found to have a low sensitivity and positive predictive value and was the least useful of the test performed. This can be attributed to the large number of positives obtained in the study. The IIG was the second most sensitive of all the five tests with a sensitivity of 30.8% and a positive predictive value of 28.6%. This result is contradictory to the work done by Savva who reported that there was no correlation between IIG and view at laryngoscopy.
The difference between the means of the IIG between DVL and EVL was statistically significant. This is in agreement with results obtained by both Wilson et al. and Nath.8,21
There is little if any documentation of the five-test combination we used in the world literature though various other five-test or more combinations have been used previously.8,21–23 The combination of all five tests increased the sensitivity to 84.6% at the expense of the specificity and positive predictive value. The combination providing the best prediction in our study involved the MMT, TMD and IIG with a sensitivity, specificity and positive predictive value of 84.6%, 94.6%, and 35.5%. The MMT and TMD is the most common combination used in the prediction of difficult laryngoscopy (Table VI). In our study we obtained a sensitivity of 76.9% and a specificity of 96.7%, which is similar to the result obtained by Frerk - 81.2% and 97.8%. Tse et al. obtained a value of 21% and 92% respectively. The discrepancy in the results may be attributed to Frerk’s definition of difficult laryngoscopy. Nath and Sekar, as well as Wilson et al., who used combinations with five tests and over had similar results to our five-test combination (Table VI).
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Safe outcome of anesthesia remains the goal of every anesthesiologist. There is still no test or group of tests that can predict 100% of difficult laryngoscopies. MMT, TMD and IIG appear to provide the optimal combination to predict DVL in a West African population. Our experience and review of the literature suggest that optimal predictors are similar in Caucasians and West Africans.
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Footnotes |
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References |
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2 Crosby ET, Cooper RM, Douglas MJ, et al. The unanticipated difficult airway with recommendations for management. Can J Anaesth 1998; 45: 757–76.
3 Caplan RA, Posner KL, Ward RJ, Cheney FW. Adverse respiratory events in anesthesia: a closed claims analysis. Anesthesiology 1990; 72: 828–33.[Medline]
4 Mallampati SR, Gatt SP, Gugino LD, et al. A clinical sign to predict difficult tracheal intubation: a prospective study. Can Anaesth Soc J 1985; 32: 429–34.[Medline]
5 Benumof JL. Management of the difficult adult airway. With special emphasis on awake tracheal intubation. Anesthesiology 1991; 75: 1087–110.[Medline]
6 White A, Kander PL. Anatomical factors in difficult direct laryngoscopy. Br J Anaesth 1975; 47: 468.
7 Cobley M, Vaughan RS. Recognition and management of difficult airway problems. Br J Anaesth 1992; 68: 90–7.
8 Wilson ME, Spiegelhalter D, Robertson JA, Lesser P. Predicting difficult intubation. Br J Anaesth 1988; 61: 211–6.
9 King TA, Adams AP. Failed tracheal intubation. Br J Anaesth 1990; 65: 400–14.
10 Mathew M, Hanna LS, Aldrete JA. Pre-operative indices to anticipate difficult tracheal intubation. Anesth Analg 1989; 68: S187 (abstract).
11 Frerk CM. Predicting difficult intubation. Anaesthesia 1991; 46: 1005–8.[Medline]
12 Savva D. Prediction of difficult tracheal intubation. Br J Anaesth 1994; 73: 149–53.
13 Rose DK, Cohen MM. The airway: problems and predictions in 18,500 patients. Can J Anaesth 1994; 41: 372–83.
14 Samsoon GL, Young JR. Difficult trachea intubation: a retrospective study. Anaesthesia 1987; 42: 487–90.[Medline]
15 Cormack RS, Lehane J. Difficult tracheal intubation in obstetrics. Anaesthesia 1984; 39: 1105–11.[Medline]
16 Tse JC, Rimm EB, Hussain A. Predicting difficult endotracheal intubation in surgical patients scheduled for general anesthesia: a prospective blind study. Anesth Analg 1995; 81: 254–8.[Abstract]
17 Ramadhani SA, Mohamed LA, Rocke DA, Gouws E. Sternomental distance as the sole predictor of difficult laryngoscopy in obstetric anaesthesia. Br J Anaesth 1996; 77: 312–6.
18 Ita CE, Eshiet AI, Akpan SG. Recognition of the difficult airway in normal Nigerian adults (a prospective study). West Afr J Med 1994; 13: 102–4.[Medline]
19 Karkouti K, Rose DK, Ferris LE, Wigglesworth DF, Meisamai-Fard T, Lee H. Inter-observer reliability of ten tests used for predicting difficult tracheal intubation. Can J Anaesth 1996; 43: 554–9.
20 Tham EJ, Gildersleve CD, Sanders LD, Mapleson WW, Vaughan RS. Effects of posture, phonation and observer on Mallampati classification. Br J Anaesth 1992; 68: 32–8.
21 Nath G, Sekar M. Predicting difficult intubation - a comprehensive scoring system. Anaesth Intensive Care 1997; 25: 482–6.[Medline]
22 Bilgin H, Ozyurt G. Screening test for predicting difficult intubation. A clinical assessment in Turkish patients. Anaesth Intensive Care 1998; 26: 382–6.[Medline]
23 Arne J, Descoins P, Fusciardi J, et al. Preoperative assessment for difficult intubation in general and ENT surgery: predictive value of a clinical multivariate risk index. Br J Anaesth 1998; 80: 140–6.
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