Force, Torque, and Stress Relaxation with Direct Laryngoscopy

Abstract

In Response: We appreciate the interest in our report of force and torque with direct laryngoscopy. Our laryngoscope handle measured both axial force and torque as shown in Figure 2of our paper [1]. We calibrated the handle with known torques of 0 and 5.6 newton-meters (n-m) by applying a known force, 44.5 newtons (n), at a known moment arm, 0.125 m. Static load testing using varying forces and moment arms confirmed accurate torque readings over the entire calibration range. Dr. Bucx and Dr. Snijders are correct that axial force is sufficient to calculate torque when the effective moment arm of the applied force is known. However, there is no empiric or a priori argument to justify a fixed value of the moment arm during laryngoscopy in patients. Oral structures can exert force along the length of the blade in an unrestricted fashion. For this reason, resolution of force and torque requires at least two transducers. No previous investigators have reported the torque involved in direct laryngoscopy of human subjects. The laryngoscope handle using three transducers described by Chilcoat et al. [2] could measure torque, but to our knowledge, the investigators never reported torque measurements in human subjects. Bucx et al. [3] reported measurements of axial force and discussed how torque could be calculated. Bucx and Snijders correctly state that exerting force on the maxillary incisors would invalidate our data. However, in no case did the laryngoscope blade contact the maxillary incisors during the measurement period. Edward D. Hon, MD Carson City, NV 89702 Randolph H. Hastings, MD, PhD Department of Anesthesiology Department of Veterans Affairs University of California, San Diego San Diego, CA 92161-5085