Force, Torque, and Stress Relaxation with Direct Laryngoscopy

To the Editor: Hastings et al. [1] state that they measured torque, while others did not. We think that the reverse is true. This can be concluded from their Figures 1, 2, and 3 Figure 2Bshows how the axial force on the handle is measured, which allows for the calculation of the torque exerted by the tissue force on the laryngoscope. Measurement of axial force alone is sufficient for the calculation of the torques in Figure 1. This measurement is used with a fixed value of the moment arm (B) of the tissue force with respect to the axis of the handle and is assumed to present the loading mode during direct laryngoscopy. However, the authors introduced a second force transducer to measure the same torque. First, this is redundant, and second, this redundant transducer must produce the same torque on the handle, taking the fixed geometry in consideration. However, according to Figure 3, the redundant torque measurements show dramatic widespread deviations, contradicting the assumption of a fixed value of the moment arm (B). Assuming B is not a fixed value, Figure 3allows us to calculate these values for each set of data, as torque is the product of axial force and moment arm. This results in values varying between approximately 3 cm and 35 cm, the latter obviously falling outside the length of a laryngoscope blade. The dramatic variation of torques and forces in Figure 3, however, can be explained when forces are exerted on the maxillary incisors, which has been shown to be common during routine laryngoscopy [2,3]. This, however, would invalidate the majority of their data. In summary, we think that it is impossible to measure torques with the instrument of Hastings et al. in the complex loading mode that is common during direct laryngoscopy. Consequently, the majority of their conclusions are questionable. Martin J. L. Bucx, MD, PhD Department of Anaesthesia Academic Medical Center University of Amsterdam Amsterdam, The Netherlands Chris J. Snijders, MEng, PhD Department of Biomedical Physics and Technology Erasmus University Rotterdam, The Netherlands