Ultrasound for Diagnosing New Difficult Laryngoscopy Indicator: A Prospective, Self-Controlled, Assessor Blinded, Observational Study
Introduction
Airway management remains a critical aspect of anesthesia, with approximately 30% of anesthesia-related deaths attributed to airway management failures. Unanticipated difficult airways are a significant source of peri-operative complications and mortality. The incidence of difficult airways in the general population undergoing anesthesia is estimated to be between 1% and 4%, with tracheal intubation failure rates of approximately 1 in 2000 in elective settings and 1 in 300 during rapid sequence induction in obstetric settings. Difficult glottic exposure, contributing to difficult airway management, has an incidence rate of 6.1% to 10.1%.
Traditional methods for assessing difficult laryngoscopy (DL) include the Mallampati airway classification, thyromental distance, degree of mouth opening, and neck range of motion classification. However, these methods suffer from imperfect inter-observer reliability and limited predictive power, with sensitivities ranging from 20% to 62% and misdiagnosis rates between 38% and 80%.
The development of ultrasound technology has provided a new tool for airway assessment. Ultrasound offers high-resolution images of the upper airway’s anatomic structures, comparable to computed tomography (CT) and magnetic resonance imaging (MRI). Several ultrasonographic parameters, such as soft tissue thickness at the level of the hyoid bone, epiglottis and vocal cords, and the visibility of the hyoid bone in sub-lingual ultrasound, have been found to be independent predictors of DL.
This study aimed to verify the association between ultrasound-measured indicators and DL, focusing on the thickness and width of the base of the tongue, the angle between the epiglottis and glottis, the length of the thyrohyoid membrane, and the thickness of the lateral pharyngeal wall. The angle between the epiglottis and glottis was identified as an innovative factor after observing the ultrasonic static pharyngeal structures and analyzing the pharyngeal dynamic process during tracheal intubation.
Methods
Ethical approval for this study was obtained from the Ethics Committee at Union Hospital, Tongji Medical College, Huazhong University of Science and Technology. The study was registered in the Chinese Clinical Trial Registry (ChiCTR-DDT-13004102), and all patients provided informed consent.
Adult patients undergoing elective surgery under general anesthesia with endotracheal intubation at Union Hospital were recruited. Exclusion criteria included maxillofacial deformities and fractures, limited mouth opening, limited neck movement, non-endotracheal intubation, and agomphiasis. Patients were divided into DL and non-DL groups based on the Modified Cormack-Lehane Scoring System (MCLS).
Ultrasound measurements were performed before anesthesia induction using a LOGIQ_E portable color ultrasound diagnostic system. A 12L-RS high-frequency linear array probe and a 4C-RS low-frequency convex array probe were used to measure the thickness and width of the base of the tongue, the angle between the epiglottis and glottis, the length of the thyrohyoid membrane, and the thickness of the lateral pharyngeal wall.
Anesthesia was induced with propofol, fentanyl, and rocuronium. A No. 3 or 4 Macintosh laryngoscope was used for tracheal intubation, and the MCLS and tracheal intubation results were recorded. An MCLS score of ≥3 was used as the standard for DL.
Results
A total of 499 patients were enrolled, with 47 (9.4%) in the DL group and 452 (90.6%) in the non-DL group. Univariate analysis identified six significant factors associated with DL: age, body weight, BMI, thickness of the base of the tongue, angle between the epiglottis and glottis, and length of the thyrohyoid membrane.
Multivariate logistic regression analysis revealed that the angle between the epiglottis and glottis was the only independent risk factor for DL. The receiver operating characteristic (ROC) curve analysis showed that the area under the curve (AUC) was maximum (0.902) when the angle between the epiglottis and glottis was 50°, with a sensitivity of 81% and specificity of 89%.
Discussion
The angle between the epiglottis and glottis was found to be highly associated with DL, suggesting that this measurement is an effective indicator of DL. When the angle between the epiglottis and glottis is less than 50°, DL may occur. This finding aligns with the dynamic process of glottic exposure during tracheal intubation, where a smaller angle makes it more difficult to open the epiglottis and expose the glottis.
Ultrasound technology provides a non-invasive, simple, and effective tool for airway assessment, enhancing the ability to diagnose DL. The study’s results suggest that ultrasound-measured factors, particularly the angle between the epiglottis and glottis, can be used to predict difficult laryngoscopy and improve airway management.
Future research should focus on determining ultrasound-measured factors in predicting difficult mask ventilation and intubation, as well as validating these findings in different populations. The study’s findings highlight the potential of ultrasound in improving the safety and efficacy of airway management in clinical anesthesia practice.
Conclusion
In summary, the angle between the epiglottis and glottis, as measured by ultrasound, is highly associated with difficult laryngoscopy. This measurement serves as an effective indicator of DL, with DL likely to occur when the angle is less than 50°. Ultrasound technology offers a valuable tool for enhancing airway assessment and improving patient outcomes in anesthesia.
doi.org/10.1097/CM9.0000000000000393
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