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

Abstract 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 This prospective observational study was conducted from December 2020 to June 2023. Total 90 obese patients (body mass index ≥ 30 kg/m2) 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 The incidence of difficult laryngoscopy was 13.33% (12/90 patients). The median (interquartile range) body mass index in the difficult and easy intubation groups was 44.7 kg/m2 (34.8–47.9) and 41.7 kg/m2 (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.