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PLoS One. 2019 Feb 27;14(2):e0212976. doi: 10.1371/journal.pone.0212976. eCollection 2019.

Prediction of difficult tracheal intubations in thyroid surgery. Predictive value of neck circumference to thyromental distance ratio.

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Department of Medicine-DIMED, Section of Anesthesiology and Intensive Care, University of Padova, Padova, Italy.
Department of Surgery, Oncology and Gastroenterology, Endocrine Surgery Unit, University of Padua, Padua.



Difficult tracheal intubation (DTI) contributes to perioperative morbidity and mortality. There are conflicting study results about the most predictive DTI risk criteria in patients undergoing thyroid surgery.


We conducted a prospective observational study on 500 consecutive patients aged ≥18 years to identify predictors for DTI. Body weight, body mass index (BMI), inability to prognath, head movement, mouth opening, Mallampati score, neck circumference (NC), thyromental distance (TMD), neck circumference to thyromental distance ratio (NC/TMD), tracheal deviation apparent on chest x-ray, mediastinal goiter, histology and history of DTI were measured as possible predictors of DTI. Spearman's rank correlation test and multiple logistic regression analysis were performed.


DTI was observed in 9.6% of all patients. Compared with the group of patients without DTI, the group of patients with DTI had significantly greater median values for body weight, BMI, NC, NC/TMD, Mallampati score, el-Ganzouri score, incidence of mediastinal goiter, and had reduced TMD and mouth opening. Significant correlations between BMI ≥30 kg/m2 and the Mallampati score ≥3 (R = 0.124, p = 0.00541), Cormack-Lehane ≥3 (R = 0.128, p = 0.00409), NC ≥40 cm (R = 0.376, p<0.001), and NC/TMD ≥5 (R = 0.103, p = 0.0207) were found. The logistic regression analysis revealed that an NC ≥40 cm at the goiter level, but not an NC/TMD ratio ≥5, was the strongest predictor of DTI (p<0.001). The area under the receiver operating characteristic curve for NC/TMD was better than the curve for NC. The sensitivity and specificity of NC/TMD were also greater, compared with NC. An NC of 40.00 cm and an NC/TMD of 5.85 were the estimated cut-off points.


This study found that NC was a strong predictor of DTI. The results also suggested that NC/TMD could be used as a measure to stratify the risk of DTI in patients undergoing thyroid surgery.

Conflict of interest statement

The authors have declared that no competing interests exist.

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