Sternomental Displacement and Neck Circumference. A New Look for the Neck as a Di cult Airway Predictor in Obese Surgical Patients. A Cohort Study


 Background

Sternomental displacement (SMDD) is a surrogate indicator for cervical spine (C-spine) mobility. SMDD revealed good potentials to predict difficult airway, but its validity in obese patients is not evident. This study assessed the performance of SMDD with neck circumference (NC) in predicting difficult airway in obese surgical patients.
Methods

The study involved 135 adult patients with BMI ≥ 35 kg/m2, scheduled for elective surgeries under general anesthesia with endotracheal tubes (ETT) inserted using Macintosh laryngoscopes. The airway was assessed using SMDD, NC, and Modified Malampati test (MMT). Difficult laryngoscopy view (DLV) was defined as Cormack–Lehane (C-L) grade ≥ 3. The accuracy of the SMDD in predicting DLV was set as the primary endpoint, while the accuracy of the SMDD compared to that of NC, MMT, and NC/SMDD ratio in predicting difficult airway was set as the secondary endpoint.
Results

The DLV cases were 28 of 135 (20.7%), with a mean body mass index (BMI) of 41.1 ± 3.3 kg/m2. SMDD < 5cm and NC > 43 cm could predict DLV with an area under the receiver operating characteristic curve (AUROC) of 0.97 and 0.83 respectively. SMDD and NC revealed a good negative correlation (r = - 0.6; 95% CI= 0.7 to 0.4; p= 0.0001). The NC/SMDD ratio had the best predition for DLV (AUROC of 0.98 at a cut-off value > 7.8).
Conclusion

In obese surgical patients, the SMDD and NC/SMDD ratio are excellent predictors for DLV when the cut-off values are < 5cm and > 7.8, respectively.
Trial registration

: ClinicalTrials.gov identifier: ID: NCT04524546 Registered in August 2020.


Introduction
The di culty securing or maintaining the airway remains a signi cant cause of anesthesia-related hypoxic brain damage and death. Identifying situations and patients at risk of airway management problems is the key to optimal care and has been the focus of numerous publications. 1 Major airway complications in intensive care units and operating rooms involving obese patients are 47% and 40%, respectively. [2][3][4] Obesity is de ned as body mass index (BMI) > 30 kg/m 2 . Obesity causes abnormal fat distribution around the cervical region, increasing neck circumference (NC). The short neck in obese patients limits cervical spine (C-spine) mobility. Increased NC and limited C-spine mobility were reported to be independent predictors of the di cult airway. 5,6 Sternomental displacement (SMDD) is a relatively new airway measure that represents the difference between the sternomental distance (SMD) that is measured while the head is extended on the neck (SMDextension) and while the head is in a neutral position (SMD-neutral). The SMDD is a surrogate indicator of Cspine mobility, and it was proved to be a good objective predictor for di cult Laryngoscopy view (DLV) in adult patients, and its predictive ability was increased when combined with NC. 7 To the best of our knowledge, the validity of the SMDD was not previously assessed in obese patients.
Therefore, this study hypothesized that SMDD could be a good objective predictor for di cult airway in obese patients undergoing general anesthesia (GA). The accuracy of the SMDD as a predictor for DLV was set as the primary endpoint, and to compare the accuracy of SMDD to that of NC, NC/SMDD ratio, BMI, and Modi ed Malampati test (MMT), in predicting DLV and di cult intubation (DI) were set as the secondary endpoints.

Materials And Methods
This observational cohort study was conducted in Cairo University hospitals, from September 2020 till July 2021, after being approved by the Research Ethics Committee of the Kasr Alainy Faculty of Medicine (ID: MS-141-2020, on May 2020, email: kasralainirec@gmail.com). The study was registered on ClinicalTrials.gov (NCT04524546) in August 2020 before patients' enrollment. All methods were carried out in accordance with relevant guidelines and regulations. The reporting of this study conformed to the Accurate Reporting of Diagnostic tests (STARD) guidelines. Written informed consent was obtained from all patients.
We inrolled 135 consecutive patients aged > 18 years, ASA physical status II & III, with BMI ≥ 35 kg/m 2 , who were scheduled for elective surgeries under GA with endotracheal tubes (ETT) inserted using Macintosh laryngoscopes. Patients with craniofacial abnormalities, lesions or scars in the head and neck, previous cervical spine surgeries, and those who needed awake intubation were excluded.
The airway examination was performed preoperatively by two research team members who have not been involved in assessing the laryngeal view and endotracheal intubation. The following tests were accomplished; Sternomental displacement (SMDD): 7 while the patient's head was neutral, SMD-neutral was measured using tape as the distance between the upper borders of the manubrium sterni and the mentum. The SMD-extension was then measured in the same way with the head extended on the neck. The difference between SMD-extension and SMD-neutral was then calculated as SMDD. Neck circumference (NC): 8 was measured using tape at the level of the thyroid cartilage. Modi ed Mallampati test (MMT): The proper classi cation was recorded while the patient was sitting in a neutral position. 9 During the statistical analysis, NC/ SMDD ratio was calculated.
In the operating room, the patient was positioned in the ramped position (by placing blankets under the patient's upper body and head until a horizontal alignment was achieved between the external auditory meatus and the sternal notch). 10 Intravenous access was secured, and Ringer acetate solution was started.
The basic monitors in the form of non-invasive blood pressure, pulse oximeter, and electrocardiogram were connected. The patients were then pre-oxygenated for 3 minutes using 100% oxygen. Anesthesia was induced intravenously using fentanyl 1 µg/kg, propofol 1.5-2 mg/kg, and succinylcholine 1 mg/kg, depending on the lean body weight. Mask ventilation was maintained until complete muscle relaxation was achieved guided by a peripheral nerve stimulator. Our team's two most senior investigators assessed the laryngoscopic view and completed the endotracheal intubation using a Macintosh laryngoscope. These investigators were blinded to the preoperative airway examination results. The possible best laryngeal view that was obtained and its grade was recorded using Cormack-Lehane (C-L) grades as grade I: the vocal cords are completely visible, grade II: only the arytenoids are visible, grade III: only the epiglottis is visible, and grade IV: the epiglottis is not visible. 11 The external neck manipulation was used to get the best view. In our study, grades I &II were categorized as easy laryngoscopy, and grades III and IV were categorized as DLV. Di cult tracheal intubation (DI) was de ned as the proper insertion of the ETT using conventional laryngoscopy that required more than two attempts or more than 10 minutes. Failed conventional tracheal intubation was de ned as intubation requiring an alternative technique to secure the airway. 12 The accuracy of the SMDD in predicting DLV with a calculated cut-off value was analyzed as the primary endpoint, while the accuracy of SMDD compared to that of NC, NC/SMDD ratio, BMI, and MMT in predicting DLV and DI were set as secondary endpoints.
Statistical analysis: The sample size was calculated using MedCalc software to detect the area under the receiver operating characteristic curve (AUROC) for SMDD of 0.7 with null hypothesis AUROC of 0.5. We considered that the DLV rate in obese patients would be 23%; 13 and we calculated a minimum number of 122 patients (with at least 28 DL cases) for a study power of 90% and an alpha error of 0.05. the number was increased to 135 for possible dropouts.
Statistical analysis was performed using the program IBM SPSS version 23 (Chicago, IL, USA). The normality of data distribution was assessed using the Kolmogorov-Smirnov and Shapiro-Wilk tests. Continuous quantitative data was expressed for all statistical comparisons as means and standard deviations (SD) or median and range as appropriate, while qualitative categorical data was expressed as a percentage or ratio.
Normally distributed data were compared using Student's t-test, while non-normally distributed data were compared using the Mann-Whitney test or the Kruskal-Wallis test as appropriate. Spearman's rank correlation coe cient evaluated the possible relationship between the SMDD and NC. A P-value <0.05 was considered statistically signi cant. The MedCalc software was used to construct the receiver operating characteristics (ROC) curve to determine the accuracy of SMDD, NC, MMT, NC, and NC/SMDD for predicting di cult airway (DLV and DI). For this purpose, binary variables were created (easy versus di cult laryngology and easy versus di cult intubation). The ideal cut-off values for all the tested variables were determined using the Youden index. In addition, the sensitivity, speci city, positive predictive value (PPV), and negative predictive value (NPV) were calculated. A P-value <0.05 was considered statistically signi cant.

Results
In this study, a total of 135 patients (76 females and 59 males) met our inclusion criteria and were enrolled and completed the study. Their mean age was 38.6±8.8 years, BMI 41.1 ± 3.3 kg/m 2 with ASA classi cation II/III was 111/24. The SMDD was 5.5±0.7 cm, and the NC was 42.1± 2.3 cm. The cases that encountered DLV were 28/135 (20.7%), while those who encountered DI were 10/135 (7.4%). No failed endotracheal intubation cases.
The patients were divided according to their laryngoscopic view into the easy laryngoscopy group (n=107) and the di cult laryngoscopy group (n= 28). The di cult laryngoscopy group revealed a signi cantly smaller SMDD and a larger NC than that in the easy laryngoscopy group (4.4±0.5 versus 5.7±0.4 cm; p=0.001 and 41.2±2.2 versus 45.1±3.2 cm; p=0.001 respectively). The NC/SMDD ratio was signi cantly higher in the di cult laryngoscopy group than the easy laryngoscopy group (10.3±1.5 versus 7.2±0.3; p=0.001, respectively). The MMT was signi cantly higher in the di cult laryngoscopy group, while the BMI was comparable in both groups. (Table 1) The patients were then divided according to the intubation condition into easy intubation (n=125) and di cult intubation (n=10) groups. The SMDD was signi cantly smaller, and the NC was signi cantly larger in patients with di cult intubation compared to those with easy intubation (4.4±0.5 versus 5.5±0.6 cm, p=0.001 and 45±4 versus 41.8±2.7 cm, p=0.001 respectively). NC/SMDD ratio was signi cantly higher in the di cult intubation group than the easy intubation group (10.5±1.8 versus 7.7±1.4 cm; p=0.001, respectively). The BMI and MMT were both higher in patients with di cult intubation. (Table-1   The possible correlation between SMDD and NC revealed a good negative correlation (r = -0.6; 95% CI= 0.7 to 0.4; p= 0.0001). (Figure 1). The BMI, SSMD, NC, NC/SMDD, and MMT were used to construct a ROC Curve to assess their ability to predict cases with DLV (Figure-2). The AUROC curve, sensitivity, speci city, PPV, NPV, and the cut-off value for all tested variables are presented in (Table-2). The same parameters were used to construct another ROC curve to assess their ability to predict cases with DI (Figure-3). The AUROC, sensitivity, speci city, PPV, NPP, and the cut-off value for all parameters are presented in (Table-3). Both SMDD and NC/SMDD showed a very high predictive ability for both DLV and DI.

Discussion
The main ndings of our study were that in obese patients undergoing GA with ETT inserted using Macintosh laryngoscope, SMDD was a good predictor of DLV and DI. A large NC was a fair predictor, but the NC/SMDD ratio provided an excellent prediction of DLV and DI. Despite the increased BMI in patients with DLV, BMI showed a weak predictive ability.
The C-spine mobility is a fundamental component in determining the ease of laryngeal exposure during direct laryngoscopy and endotracheal intubation. During direct laryngoscopy, the C-spine extension occurred mainly at the occipito-atlanto-axial (OAA) complex with minimal movement at the level of subaxial cervical segments. 14 A patient may be di cult to intubate because the head cannot be su ciently extended on the neck, as in patients with a cervical spine injury, rheumatoid arthritis, and ankylosing spondylitis. However, it has been demonstrated that even a simple C-spine extension could improve the visualization of the glottic opening, so the sni ng position was shown to effectively improve the laryngeal view primarily due to its ability to facilitate the angulation and extension at the level of the OAA complex. 14 In 2008, a retrospective study was conducted by Mashour GA et al., 15 including the records of the preoperative airway evaluation of 14053 surgical patients. The authors identi ed that 8.1% of patients were reported to have limited C-spine mobility, and the incidences of di cult laryngoscopy and di cult intubation were twice more than that in normal patients.
The degree of C-spine mobility is expressed either as a full range of neck movement, from maximum exion to maximum extension, or as the degree of either extension or exion. Several methods were described to measure the degree of C-spine movement. Wilson et al. 16 have described how to measure the full range of Cspine mobility by using a pencil applied vertically on the forehead that moves from maximum exion to maximum extension, and they identi ed three levels of C-spine mobility; <80°, 80-90°, and >90°. Di cult laryngoscopy could be expected when the range of C-spine mobility is less than 80°. Some studies used a goniometer or clinometer for their measurements. 17,18 In a previous study that included 190 patients in whom the clinometer measured neck extension and exion, the study revealed that di cult laryngoscopy should be expected when the neck extension from the neutral position is ≤ 37.5°. 18 In the previously mentioned techniques, 16-18 the C-spine mobility was described as angles' degrees.
The SMDD is considered a surrogate indicator of C-spine mobility. The authors measured the SMD ratio (SMDR), which represents the ratio between SMD in extension to SMD in neutral neck positions. The authors noticed a negative correlation between the SMDR and the C-L grade. A SMDR below 1.55 was associated with DLV incidence ranging between 33-53% (C-L III-IV glottic views), while a SMDR >1.9 means no likelihood expected DLV. The SMDD and SMDR are easier to measure, and the calculated difference or ratio between two straight lengths might be more accurate than angles' degrees.
Another privilege of both SMDD and SMDR is to avoid using the absolute values of the SMD, which revealed a wide range of cut-off values for predicting DLV as 12.5cm, 20 13.5cm, 21 , and 15cm 22 due to the anthropometric differences among the population. Our study is the rst study that assessed the performance of SMDD as a predictor of di cult airway in the obese population, with promising results for predicting both DLV and DI. Furthermore, the high speci city (95%) and high NPV (99%) for predicting DLV means that 99% for tested cases will be easy to intubate when the test is negative. . Other studies revealed a positive correlation between NC and the incidence of DLV and DI. The highest ability of NC to predict DLV and DI was achieved at a cut-off value > 50cm. 13,23 It was previously concluded that when a single airway test is used, the value of screening for DLV or DI is restricted, and combining individual tests can provide some incremental diagnostic bene ts. 24 Our study assumed that obese patients with both large NC and impaired neck mobility would be more di cult to intubate when compared with patients with large NC or impaired neck mobility alone. The new predictor, NC/SMDD ratio, provided an excellent predictive ability for DLV and DI. The NC/SMDD ratio revealed an AUROC of 98% with 99% sensitivity and 99% speci city at a cut-off value > 7.8 for predicting DLV and an AUROC of 92% with 90% sensitivity and 83%sepci cty at a cut-off value >8.8 for predicting DI. A study by Kim WH et al. 25 assumed that NC and Thyromental (TMD) could represent thick and short necks characters, respectively; therefore, authors provided NC/TMD ratio as a new predictor for DI with a better predictive ability compared to other established indices.
Because di cult laryngoscopy is more common than di cult intubation, it was chosen as the primary outcome in our study. Furthermore, di cult laryngoscopy is a direct cause of di cult tracheal intubation, and the degree of the DL is an important predictor of di cult intubation.
Our study has limitations; we included patients scheduled for elective surgeries, so our results cannot be extrapolated to the emergency, obstetric, or ICU setting. In addition, the in uence of age and gender was not taken into considerations. 26

Conclusion
In obese surgical patients, the SMDD as a surrogate for neck extension and the NC/SMDD ratio for both NC and extension provide strong DLV and DI predictive abilities.
AA was responsible for the conception and design of the study, analysis of the data, and writing the manuscript. AG and MA were responsible for preoperative evaluation of the airway. AA and NE were responsible for intraoperative assessment of the laryngeal view and endotracheal intubation. HN and WS shared in data collection and writing the manuscript. All authors had read, revised and approved the nal manuscript.

Figure 1
A scattered plot shows the correlation between NC and SMDD.