Determining the Mouth-to-Microphone Distance in Rigid Laryngoscopy: A Novel Solution Based on the Depth of Endoscope Insertion into the Mouth

Objective: Mouth-to-microphone (MTM) distance is important when measuring the sound of voice. However, determining the MTM distance for laryngoscope-mounted microphones during laryngoscopic examinations is cumbersome. We introduce a novel solution for such cases, using the depth of insertion of the laryngoscope into the mouth DI as a reference distance.
Methods / Design: We measured the average insertion depth DI in 60 adult women and 60 adult men for rigid laryngoscopes with 70° and 90° view. Based on the measured values, we formulated empirical rules for the MTM distance.
Results: We found the DI for the 70°/90° laryngoscope to be 9.7 ± 0.9 / 9.4 ± 0.6 cm in men, 8.9 ± 0.9 / 8.7 ± 0.7 cm in women, and 9.3 ± 0.9 / 9.0 ± 0.7 cm in all adults. Using these values, we show that, for microphones fixed at 15-40 cm from the tip of the laryngoscope, the final MTM distances are between 5 and 35 cm from the lips, and standard uncertainties of these distances are between 16 % and 2.5 %.
Conclusion: Our solution allows laryngologists and laryngoscope manufacturers to set and estimate the MTM distance for any rigid laryngeal endoscope with microphone attached with reasonable accuracy, avoiding the need for measuring this distance in vivo in routine practice.