|Year : 2020 | Volume
| Issue : 2 | Page : 76-80
Ultrasonographic prediction of difficult laryngoscopy in obese patients
Amit Sharma, Swaran Bhalla
Department of Anaesthesiology and Intensive Care, Jaipur Golden Hospital, New Delhi, India
|Date of Submission||26-Sep-2019|
|Date of Decision||13-Dec-2019|
|Date of Acceptance||26-Jan-2020|
|Date of Web Publication||26-Mar-2020|
Dr. Amit Sharma
40/3 Ashok Nagar, P.O. Tilak Nagar, New Delhi - 110 018
Source of Support: None, Conflict of Interest: None
Introduction: Anticipation of difficulties in airway management is of prime importance in avoiding potential catastrophies. Obese patients form part of a population that is believed to have difficult airway. Ultrasound imaging has recently come forth as a simple, portable, noninvasive adjunct for bedside assessment. We hypothesized that there is an association between ultrasonographic measurements of the anterior neck soft-tissue thickness at level of vocal cords (VCs) and hyoid bone (HB) in predicting difficult laryngoscopy in the obese patients and evaluated the feasibility of ultrasound in predicting difficult laryngoscopy in Indian population. Materials and Methods: A total of 70 obese patients with body mass index >30 kg/m2 and American Society of Anesthesiologists Grade I and II undergoing elective surgeries and receiving general anesthesia were enrolled in the study. Ultrasound quantification of the anterior neck soft tissue was performed using a high-frequency (13–6 MHz) linear transducer. The distance from the skin to the anterior aspect of the trachea was measured at two levels: VCs and HB. Results: The incidence of difficult laryngoscopy (III and IV) in our study population was 24.3% (17 of 70 cases). The sonographic measurements of the skin thickness of the anterior neck were greater at the level of VCs in the difficult laryngoscopy group at 13.5 ± 1.5 mm as compared to the easy laryngoscopy group at 10.2 ± 1.6 mm, with the difference being statistically significant, P < 0.001. The soft-tissue thickness at the level of HB was also greater in the difficult laryngoscopy group in contrast to the easy laryngoscopy group (16.4 ± 1.9 mm vs. 12.4 ± 2.0 mm; P < 0.001). Conclusions: Ultrasonographically measured skin thickness at the level of HB and VCs has got a good correlation with difficult intubation.
Keywords: Difficult airway, hyoid bone, ultrasound, vocal cords
|How to cite this article:|
Sharma A, Bhalla S. Ultrasonographic prediction of difficult laryngoscopy in obese patients. Indian J Med Spec 2020;11:76-80
|How to cite this URL:|
Sharma A, Bhalla S. Ultrasonographic prediction of difficult laryngoscopy in obese patients. Indian J Med Spec [serial online] 2020 [cited 2022 Dec 9];11:76-80. Available from: http://www.ijms.in/text.asp?2020/11/2/76/281368
| Introduction|| |
Upper airway is a complex anatomical area. One of the prime objectives of an anesthesiologist is the maintenance of a patent airway. The inability to maintain gas exchange even for a few minutes can lead to disastrous consequences. Thus, anticipation of difficulties in airway management is of prime importance in avoiding potential catastrophies. Obese patients form part of a population that is believed to have difficult airway. These patients are believed to have bigger neck circumference and a higher Mallampati score as compared to nonobese individuals.
Increased age, male gender, body mass index (BMI), history of sleep apnea,,, a high modified Mallampati class,, and Wilson score have all been suggested to be predictors of difficult airway in obese individuals. However, even predictors like BMI cannot be taken as an endpoint for the prediction of difficult intubation in obese patients, and the optimum way to predict a difficult laryngoscopy in obese patients remains debatable.
Ultrasound imaging has recently come forth as a simple, portable, and noninvasive adjunct for bedside assessment. Ultrasound of the upper airway is capable of giving detailed anatomical description and may have a significant role in identifying difficult laryngoscopy cases. When combined with the knowledge of regional anatomy, it can provide tremendous amount of information which can be used to improve the quality of care delivery to patients. Studies by Ezri et al. and Komatsu et al. have shed some light on relation between skin thickness at level of vocal cords (VCs) and difficult intubation. Similarly, Adhikari et al. repeated the work at the level of hyoid bone (HB).
We hypothesized that there is an association between ultrasonographic measurements of anterior neck soft-tissue thickness at the level of VCs and HB in predicting difficult laryngoscopy in the obese patients and evaluated the feasibility of ultrasound in predicting difficult laryngoscopy in the Indian population. The main objectives of our study were to determine the incidence of difficult laryngoscopy in obese individuals in study population with evaluation of the anterior neck soft-tissue thickness at the level of the HB and VCs by ultrasonography in obese patients and its association with direct laryngoscopic view of the glottis.
| Material and Methods|| |
This was a prospective observational study. The study was conducted from January 2018 to November 2018 in the department of anesthesiology and critical care at a tertiary care hospital in Delhi. After getting approval by the institutional ethical and scientific committee and obtaining informed and written consent from the patients, 70 obese patients with BMI >30 kg/m2 and American Society of Anesthesiologists Grade I and II undergoing elective surgeries and receiving general anesthesia were enrolled in the study. The study sample size of 70 patients was based on assumption of 21.51% as the incidence of difficult laryngoscopy (mean incidence of difficult laryngoscopy in obese individuals found in two previous studies),, 10% margin of error, and 5% level of significance.
Patients were excluded from study if they had gross anatomic abnormalities of the airway or any previous history of difficult laryngoscopy and intubation.
A single coinvestigator after requisite training in ultrasonography of the head and neck performed a scan of the neck in the department of anesthesia the day before the surgery. Sonosite Micromaxx ultrasonography system was used for taking sonography measurements.
The ultrasonographic examination was performed with the patient in the supine position and the head and neck in a neutral position. Ultrasound quantification of the anterior neck soft tissue was performed using a high-frequency (13–6 MHz) linear transducer. The distance from the skin to the anterior aspect of the trachea was measured at two levels: VCs and HB. The thyroid cartilage provides the best window for imaging the VCs. The true VCs appear as two hypoechoic structures (muscles) outlined medially by the hyperechoic vocal ligaments in transverse view by linear probe [Figure 1]. The thickness of soft tissue was calculated by averaging the amounts of soft tissue in mm obtained in the central axis of the neck at the level of VCs. The distance was calculated from skin to anterior commissure and 10 mm to the left and right of the central axis. The final reading was taken as the average of 3.
|Figure 1: Ultrasonographic measurement of distances at the level of vocal cords|
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The HB appears as a hyperechoic, inverted U-shaped linear structure with posterior acoustic shadow on a transverse view with a linear probe [Figure 2]. Next, the thickness of soft tissue at HB was calculated by taking the average of the thickness of the soft tissue in mm obtained in the central axis at the HB and 10 mm to the left and right of the central axis. The final reading was the average of 3.
|Figure 2: Ultrasonographic measurement of distances at the level of hyoid bone|
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Modified Mallampatti classification (MMC) was specified according to the visibility of pharyngeal structures with the patient in an upright sitting position, head in neutral position, mouth wide open, and tongue protruding to its maximum without phonation. Class I is visualization of soft palate, uvula, fauces, and pillars. Class II is visualization of soft palate, uvula, and fauces. Class III is visualization of soft palate and base of uvula. Class IV is only hard palate being visible [Figure 3].
Neck circumference was measured by a measuring tape at the level of thyroid cartilage. BMI was calculated as kg/m2.
The patient was positioned for the direct laryngoscopy using ramp position so as to align the external auditory meatus with the sternal notch. A standard anesthesia technique was followed for all patients. The patient was preoxygenated with 100% oxygen for 3 min. Injection fentanyl 2 μg/kg was administered intravenously. For intravenous (i.v.) induction, injection propofol 1.5–2.5 mg/kg was used. After ensuring adequacy of ventilation, injection succinylcholine 1.5 mg/kg i.v. was given to achieve muscle relaxation. The patient was then mask ventilated with 100% oxygen in 2% sevoflurane.
Cormack–Lehane (CL) classification involves four grades of glottic visualization: Grade 1 is visualization of the entire laryngeal aperture, Grade 2 is visualization of only posterior commissure of laryngeal aperture, Grade 3 is visualization of only epiglottis, and Grade 4 is visualization of just the soft palate [Figure 4].
Easy laryngoscopies were defined as CL Grade 1 and 2. Difficult laryngoscopy was defined as CL grade 3 and 4. All laryngoscopies were done by an experienced anesthesiologist (>10-year experience) who was unaware of ultrasonographic as well as conventional airway assessment results. Then, patient's trachea was intubated with an appropriate size endotracheal tube, and anesthesia was maintained as per surgical requirements. Difficult airway cart was made available at our disposal. A video laryngoscope was kept at standby. In case of failure to intubate in the first attempt with Macintosh blade, we would directly switch to video laryngoscope.
Statistical testing was conducted with the Statistical Package for Social Sciences 17.0 software (SPSS IBM Inc., Chicago, IL, USA). Continuous variables are presented as mean ± standard deviation or median (interquartile range) for nonnormally distributed data. Categorical variables are expressed as frequencies and percentages. The comparison of normally distributed continuous variables between the groups was performed using Student's t-test. Nominal categorical data between the groups was compared using Chi-square test or Fisher's exact test as appropriate. For all statistical tests, P < 0.05 was taken to indicate a significant difference.
| Results|| |
A total of 70 patients were enrolled in the study. The demographic characteristics and clinical airway assessment parameters are shown in [Table 1]. The incidence of difficult laryngoscopy (III and IV) in our study population was 24.3% (17 of 70 cases). No differences were noted in BMI values of difficult and easy laryngoscopy group. Patients with difficult laryngoscopy had higher Mallampati class and larger neck circumference than patients with easy laryngoscopy (P < 0.001).
|Table 1: Demographic characteristics and clinical airway assessment parameters of patients|
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The sonographic measurements of the skin thickness of the anterior neck were greater at the level of VCs in the difficult laryngoscopy group at 13.5 ± 1.5 mm as compared to the easy laryngoscopy group at 10.2 ± 1.6 mm (P < 0.001). The soft-tissue thickness at the level of HB was also greater in the difficult laryngoscopy group in contrast to the easy laryngoscopy group (16.4 ± 1.9 mm vs. 12.4 ± 2.0 mm; P < 0.001).
Positive correlation was observed between difficult CL grade with all of the parameters, namely Skin–VC (r = 0.721; P < 0.001), Skin–hyoid (r = 0.64; P < 0.001), MMC (r = 0.482; P < 0.001), and neck circumference (r = 0.645; P < 0.001). To further assess the roles of MMC, neck circumference, skin–hyoid distance, and skin–VCs distance in predicting difficult laryngoscopy, the receiver operating characteristic curves were drawn [Figure 5], and various parameters were deduced [Table 2]. The optimal cutoff values (with sensitivity and specificity in parentheses) for Skin–HB, Skin–VC, MMC, and neck circumference were ≥1.42 cm (88.2%, 88.7%), ≥1.23 cm (82.4%, 92.5%), ≥ III (58.8%, 94.3%), and ≥41.8 (88.2%, 94.3%), respectively.
|Figure 5: Receiver operating characteristic analysis for skin–hyoid bone distance and skin–vocal cord distance|
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|Table 2: Diagnostic validity profiles of clinical and sonographic tests for predicting difficult laryngoscopy|
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| Discussion|| |
The incidence of difficult intubation has a strong association with increasing age. In the present study, patients with difficult laryngoscopy were significantly older with mean age of 52.47 (±4.53) years in contrast to 40.42 (±7.15) years in easy laryngoscopy group. Komatsu et al. also reported difficult laryngoscopy higher in older patients. The result can be explained by multiple age-related changes, such as limited extension at the cervical spine because of the osteophytes being formed in older age group, a change in the shape of the mandible, and a higher incidence of edentulous patients.
Overall, out of 70 patients, 17 had difficult laryngoscopy. Of these 17 cases, ten cases (58.8%) were males with the gender difference being statistically significant. Conventionally, male sex is associated with a difficult airway. The incidence of difficult intubation found in our study was 24.3% in obese individuals. Komatsu et al. found the incidence of difficult intubation to be 31.25%, whereas Ezri et al. found the incidence to be 18% in obese individuals. The wide variations in the incidence of difficult laryngoscopy may be related to factors such as age and ethnic differences among patients chosen in the study.
The mean soft-tissue thickness at the level of VCs as assessed by ultrasonography in the difficult laryngoscopy group was significantly more (13.5 ± 1.5 mm) than in the easy laryngoscopy group (10.2 ± 1.6 mm). Study by Ezri et al. reported that the range of soft-tissue thickness, i.e. 24–32 mm was associated with difficult laryngoscopy, while the thickness of 15–22 mm was associated with easy laryngoscopy, and soft-tissue thickness of 23 mm at the level of the VCs completely separated the easy and difficult laryngoscopy. Another study by Komatsu et al. (with the ultrasound on-site assistance provided by Dr. Ezri) showed strikingly opposite results, as the patients in the difficult laryngoscopy group had a soft-tissue thickness at the level of VCs of 20.4 ± 3.0 mm, while the ones in the easy laryngoscopy group had 22.3 ± 3.8 mm, with no statistically significant difference. Our study suggests that more thickness at the level of VCs is a reliable indicator of difficult intubation, in accordance to the findings of Ezri et al.
Wu et al. measured the distance from the skin to the anterior commissure of VCs in midline and found the distance in the difficult laryngoscopy group (13.0 ± 0.31 mm) to be significantly more than the easy laryngoscopy group (9.2 ± 0.20 mm). The difference of objective measurements in above studies may be due to ethnic variation in the study population.
The soft-tissue thickness at the level of HB was found significantly greater in the difficult laryngoscopy group at 16.4 (±1.9) mm in contrast to the easy laryngoscopy group at 12.4 (±2.0) mm. Our results are in concurrence with the pilot study for this measurement by Adhikari et al., which reported significantly greater thickness in the difficult laryngoscopy group (mean: 16.9 mm) compared to the easy laryngoscopy group (mean: 13.7 mm).
Wu et al. in 2014 evaluated the distance of the skin to HB in nonobese Chinese population, but by a different method than our study. A total of 28 patients were found to have a difficult laryngoscopy. They also found a significantly higher soft-tissue thickness at the level of HB (1.51 ± 0.27 cm) in the difficult laryngoscopy group as compared to 0.98 ± 0.26 cm in the easy group.
Bedside-performed ultrasound of the upper airway shows promise in being an effective method for airway management. Its practicality and efficacy in evaluation of complex anatomy like in obese patients can lead to increased usage in clinical scenarios. However, further studies are needed to validate the use of ultrasonography in facilitating clinical decisions in the management of difficult airways.
| Conclusions|| |
Ultrasonographically measured skin thickness at the level of HB and VCs has got a good correlation with difficult intubation. Ultrasound is safe, portable, noninvasive, painless, and widely available. Ultrasound can be a useful adjunct in difficult airway situationsa.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2]