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Year : 2018  |  Volume : 62  |  Issue : 12  |  Page : 945-950  

An observational prospective study of performance of acromioaxillosuprasternal notch index in predicting difficult visualisation of the larynx

Department of Anaesthesiology and Critical Care, Gauhati Medical College and Hospital, Guwahati, Assam, India

Date of Web Publication10-Dec-2018

Correspondence Address:
Dr. Deepjyoti Das
C/O Akhil Chandra Das, House No. 62, Bohagi Path, Bhetapara, Guwahati, Beltola - 781 028, Kamrup (Metro), Assam
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ija.IJA_480_18

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Background and Aims: Bedside screening test for predicting difficult intubation is an accepted practice, even though its clinical value remains limited. This study aimed to study the predicting value of acromioaxillosuprasternal notch index (AASI) for difficult visualisation of the larynx (DVL). Methods: After Hospital Ethical Committee approval, 440 consecutive consenting adult non-obstetric patients were included in this study. AASI, modified Mallampati class (MMT), sternomental distance (SMD), thyromental distance (TMD) and inter incisor distance (IID) were evaluated preoperatively by trained personnel. Visualisation of larynx was graded according to Cormack–Lehane grading, with grade III and IV being considered as DVL. The cut-off values for prediction of DVL were defined a priori. Direct laryngoscopy was carried out by qualified anaesthesiologists blinded to the results of the airway predictors under evaluation. Primary outcome variable was AASI as a predictor of DVL. Comparing DVL with MMT, SMD, TMD and IID were secondary objectives. Results: DVL was observed in 3.6% [95% confidence interval (1.9–5.4%)] patients. We observed that sensitivity, specificity and Area Under Curve i.e., AUC (95% confidence interval) of ROC of AASI was 81.25 (53.69-95.03), 96.7 (94.39-98.11) and 0.890 (0.777-1.000) respectively. AUC of AASI was found to better than MMT, SMD, TMD and IID. Conclusion: AASI (≥0.5) is a good predictor of difficult visualisation of the larynx at direct laryngoscopy.

Keywords: Acromioaxillosuprasternal notch index, intratracheal/methods, intubation, laryngoscopy/methods, predictive value of tests, sensitivity and specificity

How to cite this article:
Rajkhowa T, Saikia P, Das D. An observational prospective study of performance of acromioaxillosuprasternal notch index in predicting difficult visualisation of the larynx. Indian J Anaesth 2018;62:945-50

How to cite this URL:
Rajkhowa T, Saikia P, Das D. An observational prospective study of performance of acromioaxillosuprasternal notch index in predicting difficult visualisation of the larynx. Indian J Anaesth [serial online] 2018 [cited 2021 Jan 24];62:945-50. Available from: https://www.ijaweb.org/text.asp?2018/62/12/945/247123

Tejwant Rajkhowa, Priyam Saikia: Both authors made equal contribution to this study

   Introduction Top

The search for an optimum predictor of difficult tracheal intubation is a dynamic area of research. Over the years many different predictors have been proposed, but none reaches the sensitivity and specificity that a practicing anaesthesiologist desires.[1]

It was observed that patients whose neck was situated ‘deep in the chest (i.e., with a sloping clavicle)’ had more incidence of difficult visualisation of the larynx (DVL).[2] They further reported that patients with acromioaxillosuprasternal notch index (AASI) of more than 0.5 had a higher incidence of DVL.[2] Studies that evaluated AASI have been carried out in populations from Iran only.[2],[3],[4] Cephalometric studies suggest that there are definite differences of anatomy between Asian and Western population that influences airway characteristics.[5] The hypothesis for this study was that AASI more than 0.5 will predict DVL. Therefore, the study was designed to evaluate the sensitivity and specificity of AASI in prediction of DVL.

   Methods Top

After obtaining approval from the Institutional Ethics Committee and written informed consent, 440 consecutive adult patients aged 20–65 years with American Society of Anesthesiologists' grade I/II, scheduled to undergo elective surgery requiring endotracheal intubation, were enrolled in this prospective observational single-centre study. The study period was from September 2016 to July 2017. Exclusion criteria included anatomical abnormality of the head, neck and thorax, use of cervical collar or having cervical spine abnormality, history of head and neck surgery, history of difficult airway, obese patients (body mass index >30 kg/m2), obstetric patient and inability to open the mouth. Each patient underwent physical examination before surgery and AASI, modified Mallampati test (MMT), sternomental distance (SMD), thyromental distance (TMD) and inter-incisor distance (IID) were assessed by the independent researcher who was not involved in subsequent study data collection.

AASI was measured with the patients lying in supine position and their upper limbs resting by the sides of the body. Using a ruler, a vertical line was drawn from the top of the acromion process to the superior border of the axilla at the pectoralis muscle [line A; [Figure 1].[2] Then a second line was drawn perpendicular to line A from the suprasternal notch [line B; [Figure 1].[2] The portion of line A that lay above the point at which line B intersected line A was termed as line C [Figure 1].[2] AASI was calculated by dividing the length of line C by that of line A (AASI = C/A).[2]
Figure 1: Measurement of acromioaxillosuprasternal notch. White arrow = Suprasternal notch, Green arrow = Anterior fold of axilla, Yellow arrow = Superior border of acromion process

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MMT was measured while patients were sitting upright in a head neutral position. They were instructed to open their mouth as wide as possible, with a fully protruded tongue without phonating.[6] The observer inspected the pharyngeal structures from the patient's eye level.[6] MMT classification used was as follows: class I-soft palate, fauces, uvula and pillars were visible; class II-soft palate, fauces and uvula were visible; class III-soft palate and base of uvula were visible; and class IV-only hard palate was visible.[6] With the help of a ruler, TMD was calculated by measuring the distance from the mentum to the thyroid notch, while the patient's neck is fully extended, and mouth closed. TMD of <6.5 cm was regarded as a predictor for DVL.[7],[8] SMD was calculated with a ruler by measuring the straight distance from the upper border of the manubrium sterni to the mentum. The head was fully extended and mouth closed.[8] SMD of <12.5 cm was considered as predictor of DVL.[8] Using a ruler, IID was measured by taking the distance between the upper and lower incisor with the mouth fully open. IID of <4 cm was considered as DVL.[9]

All patients received premedication with intravenous (IV) fentanyl 2 μg/kg. Anaesthesia was induced with IV propofol 1%, 2 mg/kg as a starting dose and then subsequently titrated to loss of verbal response. Direct laryngoscopy and tracheal intubation were facilitated with vecuronium bromide 0.1 mg/kg IV. With the head in the morning air sniffing position, direct laryngoscopy was performed with Macintosh laryngoscope by an anaesthesiologist with more than 2 years of experience after obtaining postgraduate certificate in anaesthesiology. The laryngoscopist was masked to the airway parameters measured. The laryngeal view was graded according to the Cormack and Lehane grading system; Grade I-full view of glottis; Grade II-only posterior commissure of laryngeal aperture seen; Grade III-only epiglottis seen; Grade IV-only soft palate is seen.[10] Grades I and II were considered as easy visualisation of larynx (EVL) and Grades III and IV as DVL.[2]

Sample size was calculated using the formula provided by Karimollah Hajian-Tialaki.[11] In the first published study on AASI, Kamranmanesh et al. reported the sensitivity and specificity of AASI to be 78.9 and 89.4%, respectively.[2] With α of 0.05 and β of 0.2, to detect a sensitivity and specificity of 0.8 and 0.9, respectively, with a maximum marginal error of 10%, 440 patients were required. The prevalence of DVL in our population was taken to be 13.95%.[9] We intended to enrol every consecutive patient meeting inclusion criteria until the sample size was achieved.

Statistical analyses were carried out with Statistical Package for the Social Sciences (SPSS) 21.0 software (IBM Corporation, USA). Statistical tools in Excel sheet® were also used. Percentages were generated for qualitative variable and compared by Chi-square test. For quantitative variables such as age, height and weight, mean and standard deviation were computed after checking for normal distribution and compared by using t-test. Sensitivity, specificity, positive and negative predictive values were calculated for MMT, AASI, TMD, SMD and IID with 95% confidence interval (CI) using laryngoscopic view as gold standard. Receiver operating characteristics (ROC) curves with calculation of area under the curve (AUC) were also computed for the airway predictors. A P value of <0.05 was taken as statistically significant.

   Results Top

In this present study, data of the reference and index tests parameters of 440 patients are presented. The demographic parameters are presented in [Table 1]. Among the patients, 63% were females. There was no statistically significant difference among the DVL and EVL groups in terms of demographic parameters. Sixteen patients [3.6%, 95% CI (1.9–5.4%)] had DVL, 56% being female. Among those patients with EVL, 64% were female.
Table 1: Demographic characteristics of the patients

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Predictive values of the airway predictors are shown in [Table 2]. Compared to specificity and negative predictive value, the 95% CI of sensitivity and positive predictive value of all the evaluated airway predictive parameters were rather wide. It was observed that AASI had the highest predictive value among airway predictors. Comparison of the ROC and AUC of AASI and MMT is presented in [Figure 2]. Comparison of ROC curves of TMD, SMD and IID is presented in [Figure 3]. It can be seen from [Figure 2] and [Figure 3] that the ROC of MMT, SMD, TMD and IID is close to the reference line.
Table 2: Sensitivity, specificity, positive predictive and negative predictive value of the airway assessment tests

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Figure 2: Comparison of ROC curve of AASI and MMT. AASI – Acromioaxillosuprasternal notch index, MMT – Modified Mallampati test, ROC – Receiver operating characteristics

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Figure 3: Comparison of ROC curve of IID, SMD and TMD. The ROC of TMD falls on the reference line (purple line), thus the colour of the ROC curve of TMD (green line) may not be distinct. IID – Interincisor distance, SMD – Sternomental distance, TMD – Thyromental distance

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The AUC of ROC curves of the airway assessment tests are presented in [Table 3]. The AUC of AASI is highest (0.890) among the predictors. The AUC of other parameters is close to 0.5.
Table 3: AUC of ROC curves of different airway predictors

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The observed cumulative frequencies of AASI at different cut-off points are given in [Table 4]. A cut-off point of ≥0.5 has the best trade-off between false and true positive.
Table 4: Observed frequencies and cumulative rates of AASI at different cut-off points

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   Discussion Top

In our study, AASI has the best predictive performance among MMT, SMD, TMD and IID for prediction of DVL. Our finding concurs with the previous research findings on AASI.[2],[3],[4] Even though one study used a cut-off point of 0.5 for AASI, a cut-off of 0.6 was used in other studies. Published manuscripts do not provide enough information to understand the rationale of the cut-off values used.[2],[3],[4] On the basis of best possible trade-off between the cumulative rates of false and true positive cases [Table 4], we also suggest a cut-off value of ≥0.5 for AASI.

Sensitivity and specificity of AASI vary across different studies.[2],[3],[4] Among other factors, sensitivity and specificity depend on the prevalence of the condition under evaluation.[12] The prevalence of DVL in studies evaluating AASI in non-obstetric population are 2.9 and 6.3%.[2],[4] Although the authors did not provide the margin of error of the prevalence, we assume that it may play role for the small differences in predictive performance of AASI.[2],[4],[12]

Even though AASI has good predictive value in obstetric populations also, its performance is inferior compared to non-obstetric population.[2],[3],[4] Airway dimensions in obstetric patient change with progress of pregnancy, and MMT is known to progress to higher grades as patients progress through labour.[13] We are not aware whether the landmarks that are used to measure AASI undergo any change during pregnancy. It is interesting that AASI had inferior predictive performance compared to non-obstetric population even with a higher (0.6 vs 0.5) cut-off point.[2],[3]

Among other evaluated airway predictors, MMT was consistently found to be a poor predictor of DVL compared to AASI in all other studies.[2],[3],[4] Although our finding concurs with these studies, they reported different predictive values of MMT.[2],[3],[4] This could be due to wide inter-observer variability of MMT.[14],[15] Moreover, one of those studies was carried out in obstetric patients.[3]

We also observed that the predictive value AASI is better than SMD, TMD and IID. No study has yet compared AASI with these parameters. Further studies are needed to understand the differences, if any, of predictive performance of these parameters.

Although not detailed by Kamranmanesh et al., we present our understanding of the probable anatomical basis of this index. Acromion process is at the lateral most end of the clavicle. The line joining acromion process and superior border of axilla (Line A) represents the ‘arm chest junction’.[2] The Line A can be regarded as a representative measurement of the portion of the ‘chest’ that continues as the arm. On the ventral side, the ‘neck’ extends caudally to the clavicles and suprasternal notch. Suprasternal notch lies caudad to acromion. Thus, the length from ‘acromion process to suprasternal notch’ may represent the ‘part of neck’ that lies ‘deep in the chest’. To quantify the degree by which the ‘neck is situated deep into chest’, it is necessary to obtain the ratio C/A. Thus, the higher the C/A, the more deep the neck is situated into the chest.

Compared with other commonly used predictors of DVL, AASI is a unique airway predictor. Most of these bedside predictors measure different parameters in the head and neck region. Measurement of AASI does not involve this anatomical region.

Allometry is a method that examines the biological scaling relationships between various traits, be it morphological, physiological or ecological.[16] Relationship between two different traits that represents ‘size’ can be explained by either a bivariate or multivariate model.[16] Thus, there exists a definable relationship between two morphological attributes, even though they may belong to different body regions. Size of a trait is predictably associated with shape.[16] Thus, we postulate that, even though measurement of AASI does not involve structures in and around ‘airway’, it is able to provide information about the size and shape of the airway. Future studies to substantiate or refute our postulation are needed.

In our study, assessment of airway predictors was performed by a single primary investigator. This reduced the risks of inter-observer variation.

Our study has some limitations. The grading of laryngeal view was performed by different anaesthesiologists. Laryngeal view is influenced by many factors, namely, technique, posture while performing and height of operating table.[17],[18] Thus, there is a chance of inter-observer bias. Prevalence of DVL used for calculation of sample size was higher than the observed prevalence. Considering the lower prevalence DVL compared to the value used during sample size calculation and the wide 95% CI of sensitivity, there is a need of a study with larger sample size to estimate the sensitivity with higher accuracy. Nonetheless, the 95% CI reported in our study will help in deciding the sample size of future studies.[19] Recently, the predictive performance of combination of different bedside predictive tests are being evaluated and certain combinations have better predictive performances.[20] We did not evaluate combination of different airway predictors. The aim of our study was to evaluate the predictive performance of a new screening index. We plan to evaluate the predictive performance of combination of tests in recent future. We must also appreciate that although combination of tests results in larger positive predictive values compared to individual airway predictors, this is accomplished at the cost of reduced sensitivity and a greater incidence of false negative predictions.[21]

   Conclusion Top

AASI (≥0.5) is a good predictor of difficult visualisation of the larynx at direct laryngoscopy. As a secondary outcome variable, AASI is a better predictor of DVL than MMT, SMD, TMD and IID.

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Conflicts of interest

There are no conflicts of interest.

   References Top

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  [Figure 1], [Figure 2], [Figure 3]

  [Table 1], [Table 2], [Table 3], [Table 4]


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