Use of Ultrasound for Prediction of Difficult Laryngoscopy in Adult Patients

The prediction of difficult laryngoscopy remains a critical challenge in the field of anesthesiology, as it directly impacts patient safety and outcomes during general anesthesia. Difficult airway management is one of the primary causes of adverse events in surgical patients, and reliable clinical indicators for predicting difficult laryngoscopy are still lacking. In this context, the use of ultrasound has emerged as a promising tool for assessing anatomical structures that may contribute to difficult laryngoscopy. A recent study by Wang et al. explored the associations between ultrasound-measured indicators and difficult laryngoscopy, providing valuable insights into this area. However, while their findings have potential clinical implications, several methodological issues raise questions about the validity of their conclusions.

Study Design and Methodology

Wang et al. conducted a prospective, self-controlled, assessor-blinded, observational study to evaluate the utility of ultrasound in predicting difficult laryngoscopy. The study focused on measuring the angle between the epiglottis and glottis using ultrasound and its association with difficult laryngoscopy. Multivariate logistic regression analyses and receiver operating characteristic (ROC) curve analysis were employed to assess the diagnostic accuracy of this ultrasound-measured indicator. The authors concluded that an angle of less than 50° between the epiglottis and glottis was highly associated with difficult laryngoscopy.

The study used a modified Cormack-Lehane scoring system to classify the ease of direct laryngoscopy, with grades 3 and 4 defined as difficult laryngoscopy. Notably, the reported incidence of difficult laryngoscopy in the study was 9.4%, even though patients with known risk factors such as maxillofacial deformities, fractures, limited mouth opening, and limited neck movement were excluded. This high incidence raises concerns about the study design and the criteria used to define difficult laryngoscopy.

Critical Methodological Issues

One of the primary concerns with the study design is the lack of clarity regarding the experience level of the anesthesiologists performing the direct laryngoscopy and the number of attempts made during the procedure. According to the Practice Guidelines for Management of the Difficult Airway by the American Society of Anesthesiologists Task Force on Difficult Airway Management, difficult laryngoscopy is defined as the inability to visualize any portion of the glottis (grades 3 and 4) after multiple attempts by an experienced anesthesiologist. The absence of these details in Wang et al.’s study may have led to an incorrect definition of difficult laryngoscopy and an overestimation of its incidence.

Additionally, the study did not specify whether optimum external laryngeal manipulation (OELM) was allowed during the laryngoscopy procedure. OELM has been shown to improve the laryngeal view by at least one grade in adult patients, and its omission could have influenced the results. The failure to account for these factors in the study design may have compromised the accuracy of the findings.

Diagnostic Accuracy and ROC Curve Analysis

The study employed ROC curve analysis to assess the diagnostic accuracy of the angle between the epiglottis and glottis for predicting difficult laryngoscopy. The authors reported a maximal area under the ROC curve for this parameter, with a sensitivity of 81% and specificity of 89% at a cut-off point of 50°. However, the study did not provide the Youden index, positive predictive value (PPV), or negative predictive value (NPV) at this cut-off point.

The Youden index is a critical measure of diagnostic accuracy, representing the maximum overall correct classification rate that an indicator can achieve. Even an indicator with a large area under the ROC curve may have a low overall correct classification rate at the optimal cut-off point. The absence of the Youden index and predictive values in the study makes it difficult to determine whether the angle between the epiglottis and glottis has a good discriminative power for difficult laryngoscopy.

Comparison with Other Ultrasound Parameters

The study by Wang et al. focused exclusively on the angle between the epiglottis and glottis as a predictor of difficult laryngoscopy. However, other ultrasound-measured parameters have also been shown to be independent predictors of difficult laryngoscopy. These include soft tissue thickness at the level of the hyoid bone, epiglottis, and vocal cords, as well as the visibility of the hyoid bone in sublingual ultrasound examination, hyomental distance in the head-extended position, and hyomental distance ratio.

A significant limitation of the study is the lack of comparisons between the discriminative power of the angle between the epiglottis and glottis and these other ultrasound parameters. This omission leaves an important question unanswered: whether the angle between the epiglottis and glottis is a better predictor of difficult laryngoscopy than other available ultrasound indicators. Future studies should address this gap by directly comparing the diagnostic accuracy of these parameters.

Clinical Implications and Future Directions

Despite the methodological limitations, the findings of Wang et al. have potential clinical implications. The identification of a specific ultrasound-measured angle associated with difficult laryngoscopy could provide anesthesiologists with a valuable tool for preoperative assessment. However, the study’s conclusions must be interpreted with caution due to the issues discussed above.

Future research should aim to address these limitations by incorporating standardized definitions of difficult laryngoscopy, accounting for the experience level of the anesthesiologists, and allowing for OELM during the procedure. Additionally, studies should compare the diagnostic accuracy of the angle between the epiglottis and glottis with other ultrasound parameters to determine the most reliable predictor of difficult laryngoscopy.

Conclusion

The use of ultrasound for predicting difficult laryngoscopy in adult patients is a promising area of research with significant clinical implications. The study by Wang et al. highlights the potential utility of the angle between the epiglottis and glottis as a predictor of difficult laryngoscopy. However, methodological issues related to the study design, diagnostic accuracy, and comparison with other ultrasound parameters raise questions about the validity of the findings. Addressing these limitations in future research will be essential for developing reliable tools for preoperative assessment and improving patient outcomes in the field of anesthesiology.

doi.org/10.1097/CM9.0000000000000789

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