Fig. 3. The relationship between the outer diameter of optimally sized endotracheal tubes (ETT) in mm and subglottic diameter, also in mm: the outer ETT diameter = 0.46 × (subglottic diameter) + 1.56, R 2= 0.90 for cuffed ETTs, and the outer ETT diameter = 0.55 × (subglottic diameter) + 1.16, R 2= 0.90 for uncuffed ETTs (n = 48 in each group).