Radius of curvature of endotracheal tube: Are we losing it

Dear Editor, While working in an elective operation theater (OT), frequent use of stylets was noticed for endotracheal intubations in the American Society of Anesthesiologists Physical Status I patients with modified Cormack-Lehane Grade I during direct laryngoscopy. The cuffed endotracheal tube (C-ETT) size 7.0 mm would often slide “posterior” to the vocal cords, in the esophagus, even after using the “between-the-fingers grip”[1] technique to increase the ETT curvature. On closer observation, it was noticed that 7.0-mm C-ETT from two different manufacturers were being used in the OT. As compared to the ariette™ (Ariette Health Care and Diagnostics) [Figure 1a], the C-ETT manufactured by Polymed (Poly Medicure Ltd.) had a flatter curve [Figure 1b]. The radius of curvature (ROC) was roughly calculated by a simple mathematical formula, a2 + h2/2h (where “a” is half the length of the chord of the arc and “h” is the height of the perpendicular line from the center point of the chord to the arc). The calculated ROC of ariette™ C-ETT was approximately 125 mm, while that of Polymed C-ETT was approximately 193 mm. The standard ROC of ETT, known as the Magill curve, is 140 ± 20 mm. That is why the Polymed C-ETT with a larger ROC was difficult to maneuver and required the use of a stylet for successful ET intubations.Figure 1: 7mm ETT with different ROC. a) Ariette C-ETT; b) Polymed C-ETTWith the use of different types of video laryngoscopes, the ROC of ETT is manipulated with a stylet to facilitate successful ET intubations in a shorter duration. Use of stylets is especially important during difficult airway scenarios[2] and other critical situations.[3] The tracheal tube introducers, such as fibreoptic bronchoscopes, bougies, and catheters are useful in similar situations, and the ROC of ETT is not of much relevance. The standard technique of ET intubations using direct laryngoscopy is still routinely used in developing countries with limited access to video laryngoscopes. We tend to use a stylet for simple ET intubations during direct laryngoscopy due to its ready availability. This was probably the reason why C-ETT with a larger ROC got overlooked. A systematic review by Boulton et al. concluded that the stylets might be associated with increased risk of airway trauma compared with other devices or when no stylet was used, though the quality of evidence is modest.[4] The characteristics of ETT, such as ROC, bevel angle, cuff length, etchave been specified long ago.[5] There is a need to strictly reinforce the standard guidelines with all the manufacturing companies so that they are universally incorporated into the current design of the ETT to prevent such discrepancies in future. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.