European Journal of Cardiovascular Medicine

Airway treatment is a key part of safe anaesthetic care. Failure to secure the airway is still a major cause of anaesthesia-related morbidity and mortality globally. Difficult tracheal intubation is said to happen in about 1 to 15% of patients undergoing general anaesthesia and if not anticipated it has the potential for severe complications on the airway such as hypoxia, aspiration, airway trauma, or death[1] . The early recognition of patients with suspected difficult airways is therefore critical for proper preparation and minimization of the perioperative risk.

Several bedside airway evaluation tests have been formulated to predict difficult intubation, such as the Modified Mallampati classification, thyromental distance, sternomental distance, and inter-incisor gap. Most of these tests are simple and very commonly used, but their predictions are inconsistent. Numerous researches found that the sensitivity and specificity of these tests are weak when applied singly, which hampers their validity in the clinical situation [2,3]. Furthermore, these findings depend heavily on external anatomical point views and subjective interpretation, which do not reflect the actual internal airway anatomy.

Ultrasonography has recently evolved with the development of new imaging technology for airway analysis. Ultrasound has several advantages; as a non-invasive method, does not expose the patient to radiation, is portable, and facilitates the visualization of soft tissue structures in real time. For the past decade, airway ultrasonography has been used more frequently for recognizing airway structures, confirmation of endotracheal tube locations, assessment of vocal cord movement and for predicting difficult airway conditions [4].

Numerous ultrasonography parameterizations have been suggested for difficult intubation prediction. Measures including anterior neck soft tissue thickness, hyomental distance, and pre-epiglottic space [5] have provided exciting data on anatomical aspects related to difficult laryngoscopy. The hyomental distance ratio (HMDR), calculated from hyomental distances from neutral and extended neck positions, reflects both submandibular space compliance and neck mobility, two key parameters determining laryngoscopic view [6]. [Likewise, the ratio of pre-epiglottic space to epiglottis-to-vocal cord distance (PreE/E-VC) has been found to correlate with laryngoscopic grade and difficulty of intubation [7].

Despite these promising findings, the role of airway ultrasonography in routine preoperative airway assessment remains under investigation. Studies evaluating its predictive value have reported varying results due to differences in patient populations, measurement techniques, and diagnostic thresholds [8]. Therefore, further research is necessary to establish the reliability and clinical applicability of ultrasound-based airway parameters.

The purpose of this study was to evaluate the effectiveness of preoperative airway ultrasonography in predicting difficult intubation in adult patients undergoing elective surgery under general anesthesia. Specifically, the aim of this analysis was to assess hyomental distance measurement, pre-epiglottic space, and epiglottis-to-vocal cord distance as predictors of a difficult airway and to compare their diagnostic accuracy with the Cormack–Lehane grading obtained during direct laryngoscopy.

This prospective cross-sectional study was conducted in the Department of Anaesthesiology at R L Jalappa Hospital and Research Centre, Karnataka, India. The study was carried out over a period of twelve months after obtaining approval from the Institutional Ethics Committee. Written informed consent was obtained from all participants prior to enrolment. Study Population A total of 185 adult patients scheduled for elective surgical procedures under general anaesthesia with endotracheal intubation were included in the study. Patients were selected using consecutive sampling. Inclusion Criteria 1. Patients aged 18–60 years 2. American Society of Anesthesiologists (ASA) physical status I or II 3. Patients undergoing surgery requiring endotracheal intubation Exclusion Criteria 1. Edentulous patients 2. Patients with restricted neck movements 3. Patients with head and neck pathology or anatomical abnormalities that could interfere with ultrasound assessment Preoperative Airway Assessment On the day of surgery, demographic data including age, gender, height, weight, and body mass index were recorded. Conventional airway assessments were performed by an anaesthesiologist. Modified Mallampati Classification The patient was seated upright with the head in neutral position and asked to open the mouth widely and protrude the tongue without phonation. The visibility of pharyngeal structures was graded from I to IV. Inter-Incisor Gap The maximum distance between the upper and lower incisors during maximal mouth opening was measured using an inch tape and recorded in centimeters. Thyromental Distance The distance from the thyroid notch to the mentum with the head fully extended was measured using an inch tape. Ultrasonographic Airway Assessment Airway ultrasonography was performed using a SonoSite SII ultrasound machine equipped with a high-frequency linear probe (6–13 MHz). The patient was positioned supine with a pillow under the head and instructed to maintain slow breathing to minimize movement artefacts. Ultrasound measurements were obtained in the transverse plane with the following settings: • Frequency: 11 MHz • Depth: 3–4 cm • Gain: 20–30 Measured Parameters The anterior neck soft tissue thickness was measured at three levels: 1. Hyoid bone – identified as an inverted U-shaped hyperechoic structure 2. Epiglottis – visualized as a hypoechoic structure behind the thyrohyoid membrane 3. Vocal cords – identified by the hyperechoic arytenoid structures Measurements recorded included: • Hyomental distance in neutral position (HMDn) • Hyomental distance in extended position (HMDe) • Hyomental distance ratio (HMDR = HMDe/HMDn) • Pre-epiglottic space (PreE) • Epiglottis-to-vocal cord distance (E-VC) • PreE/E-VC ratio Anaesthetic Procedure After standard monitoring was established, anaesthesia was induced using intravenous propofol (2 mg/kg), fentanyl (2 µg/kg), and atracurium (0.5 mg/kg). Direct laryngoscopy was performed using a Macintosh blade (size 3 for females and size 4 for males) by an anaesthesiologist with more than five years of experience. The laryngoscopic view was graded according to the Cormack–Lehane classification: • Grade I – full view of glottis • Grade II – partial view of glottis • Grade III – only epiglottis visible • Grade IV – neither glottis nor epiglottis visible Grades I and II were classified as easy intubation, whereas Grades III and IV were considered difficult intubation. The number of attempts, use of external laryngeal pressure, and airway adjuncts were recorded. Statistical Analysis Continuous variables were expressed as mean ± standard deviation, and categorical variables expressed as frequencies and percentages. Student’s t-test for continuous variables and chi-square test or Fisher’s exact test for categorical variables were used to analyze comparison across groups. Correlation between ultrasound parameters and Cormack–Lehane grades was evaluated. Receiver operating characteristic (ROC) curves were calculated to gauge diagnostic accuracy on ultrasound measurements, with AUC calculated. Statistical significance was considered at p < 0.05.

A total of 185 patients were included in the analysis. The study population had a mean age of 39.8 ± 10.7 years, 56% were males. Based on the direct results of laryngoscopy, 160 patients (86.5%) were able to do easy intubation, and another 25 (13.5%) of the patients had difficult intubation. Patients undergoing difficult intubation showed significantly elevated anterior neck soft tissue thickness at the level of the epiglottis and vocal cords compared to patients subject to easy intubation. The hyomental distance ratio was also significantly lower in patients with difficult laryngoscopy, indicating diminished submandibular space compliance.

The PreE/E-VC ratio showed a strong correlation with Cormack–Lehane grades. Patients with higher ratios were more likely to have difficult intubation. Ultrasonographic parameters demonstrated higher predictive accuracy compared with conventional bedside airway tests.

ROC curve analysis revealed that PreE/E-VC ratio and HMDR were the most reliable ultrasound predictors of difficult airway.

Table 1: Demographic Characteristics

There were slightly higher body mass index in patients who had difficult intubation compared to easy intubation. Despite similar age and gender distribution across groups, increased BMI was found to correlate with a higher incidence of difficult airway. These results are in line with prior evidence indicating that obesity and increased anterior neck soft tissue may hinder visualization of airway structures during laryngoscopy.

Table 2: Conventional Airway Assessment

traditional airway assessment procedures including Modified Mallampati classification and thyromental distance showed a moderate correlation with difficult intubation. Patients with Mallampati grades III and IV had a higher incidence of difficult laryngoscopy. Similarly, reduced thyromental distance increased the intubation difficulty. Such tests, however, have limited predictive ability compared to ultrasound-derived measurements.

Table 3: Ultrasonographic Airway Measurements

Differences between easy and difficult intubation conditions were evident between the ultrasound measurements. The lower hyomental distance ratio pointed to limited submandibular space expansion, which may limit the alignment of airway axes during laryngoscopy. Alternatively, an increased PreE/E-VC ratio indicated increased anterior soft tissue thickness, which might block glottic opening visualization. These parameters were strongly statistically associated with difficult airway.

Table 4: Diagnostic Accuracy (ROC Analysis)

Outcome analysis indicated that the PreE/E-VC ratio had the highest diagnostic accuracy for predicting difficult intubation based on receiver operating characteristic analysis. The hyomental distance ratio also had good predictive value. Both parameters performed significantly better than the traditional airway assessment tests and highlighted an increase in the accuracy of the predicted difficult laryngoscopy when imaging an airway through ultrasound.

FIGURE 1: ROC CURVE FOR HMDR PREDICTING DIFFICULT INTUBATION

The ROC curve shows the diagnostic impact of the hyomental distance ratio on difficult airway prediction. An area under the curve of 0.82 reflects good discriminative ability. The best cut-off demonstrated balanced sensitivity and specificity, indicating that HMDR is a useful ultrasonographic predictor of difficult laryngoscopy during preoperative airway assessment.

FIGURE 2: ROC CURVE FOR PREE/E-VC PREDICTING DIFFICULT INTUBATION

The ROC curve for the PreE/E-VC ratio demonstrates superior predictive performance compared with other ultrasound parameters. The AUC of 0.86 indicates excellent diagnostic capability. A higher ratio was associated with increased likelihood of difficult intubation, highlighting the clinical relevance of measuring anterior neck soft tissue thickness using ultrasonography during preoperative airway evaluation.

Precise prediction of difficult airway is an important part of anaesthetic perioperative protection. This study evaluated airway ultrasonography ability to predict difficult intubation and revealed that ultrasonographic parameters including HMDR and PreE/E-VC ratio provide significant predictive value.

The incidence of difficult intubation in the current study was 13.5%, and found in similar studies to be between 10% and 15% in elective surgical patients [9] . Reddy et al. reported a similar incidence of difficult airway in their prospective study evaluating ultrasound parameters for airway prediction [10] .

We observed that the hyomental distance ratio was significantly reduced in patients with difficult laryngoscopy. Such finding is consistent with the reports of Wojtczak et al., who showed that reduced HMDR reflects limited mandibular mobility and reduced submandibular space compliance, potentially leading to aberrant alignment of airway axes during laryngoscopy [11] . Similarly, Ezri et al. found that smaller submandibular space dimensions correlate with greater difficulty during tracheal intubation [12] .

PreE/E-VC ratio was the strongest predictor of difficult airway in our study. Pinto et al. found similar results, indicating that the increase in thickness of anterior neck soft tissue as measured by ultrasound correlates strongly with difficult laryngoscopy [13] . Moreover, Falcetta et al. validated that ultrasound assessment of pre-epiglottic space can guide Cormack–Lehane grade III and IV views during laryngoscopy [14] .

Conventional airway assessment tests like the Modified Mallampati classification and thyromental distance were not particularly good predictors in our study. This is a result observed in multiple previous studies. In a large meta-analysis, Shiga et al. found that individual bedside airway tests possess poor sensitivity and should not be used alone to predict difficult airway [15] . In similar fashion, Lundstrøm et al. stressed the need for combined or adjunctive methods for accurate airway prediction [16] .

Airway ultrasonography offers several advantages over conventional assessment methods. It provides direct visualization of internal airway structures and allows measurement of soft tissue thickness, which cannot be evaluated through external examination alone. Furthermore, ultrasound is portable, non-invasive, and easily repeatable at the bedside [17] .

However, there are certain limitations to maintain the advantage described above. Ultrasound measurements are operator-dependent and require adequate training to obtain reproducible results. Furthermore, there might be a difference in body habitus and anatomical characteristics of study populations for predictive thresholds for ultrasound parameters in different populations. The findings were limited to a single centre with a limited sample size that may hinder generalizability of the findings.

Future research involving larger multicentric studies is necessary to establish standardized cutoff va.lues and validate the routine use of airway ultrasonography in clinical practice. Integration of ultrasound with existing airway assessment protocols may enhance prediction accuracy and improve patient safety.

The current study shows the usefulness of airway ultrasonography in predicting difficult intubation. Ultrasonographic parameters like hyomental distance ratio and pre-epiglottic space to epiglottis-vocal cord ratio showed strong correlation with Cormack–Lehane grading and provided better diagnostic accuracy than conventional airway assessment methods. The incorporation of airway ultrasonography into routine preoperative examination may improve the ability of anaesthesiologists to anticipate difficult airway scenarios and apply appropriate airway management strategies. Additional large-scale studies are needed to standardize measurement techniques and to validate ultrasound-based airway assessment in diverse patient populations.

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