Using ultrasonographic skin-to-vocal-cord distance measurements to predict difficult intubation in obese Thai patients: a prospective observational study

BACKGROUND: This study aimed to evaluate the prediction of difficult intubation using ultrasonographic skin-to-vocal-cord measurements in obese Thai patients undergoing elective general anesthesia. METHODS: ) aged 18-60 years with an American Society of Anesthesiologists physical status classification of 2-3 requiring elective surgery under general anesthesia with oral endotracheal intubation in operating theatres were included. Anesthesiologists assessed the airway and measured the skin-to-vocal-cord distance using ultrasonography in the median coronal plane in the closed-mouth supine position. The primary outcome was difficult laryngoscopy, defined as a laryngoscopic view grade 3 or 4 on the Cormack-Lehane scale. RESULTS: (36.7-47.1), respectively (p = 0.929). The mean skin-to-vocal-cord distances (DSVC) in the difficult and easy intubation groups were 20.1 ± 7.2 mm and 16.3 ± 4.9 mm (p = 0.022), respectively. The median time for tracheal intubation in the difficult and easy intubation groups was 61.5 [50.2-120] s and 40 [30-56.8] s, respectively (p < 0.001). Multivariable logistic regression revealed that male sex and skin-to-vocal-cord distance ≥ 19.2 mm (area under the curve, 0.67) were significantly associated with difficult intubation. In sensitivity analysis excluding cases performed by anesthesiologists, DSVC ≥ 19.2 mm remained the only significant predictor of difficult intubation. CONCLUSIONS: In obese Thai patients, A skin-to-vocal-cord distance of ≥ 19.2 mm measured by ultrasonography was associated with difficult laryngoscopy and remained consistent across sensitivity analyses. These factors may help predict difficult intubation. However, the modest diagnostic performance suggests that DSVC should not be used as a standalone predictor. TRIAL REGISTRATION: Thai Clinical Trials Registry (TCTR20250616007); registered on June 16, 2025.