# Effect of Subglottic Stenosis on Expiratory Sound Using Direct Noise Calculation
## 4 Summary and Discussion
### Clinical Implications  
The current simulation results indicate that strong tonal excitation is only observed under a high SGS severity. Since high severity can cause severe consequences, urgent visual inspection that can detect the SGS directly may be needed. This may limit the usefulness of sound signals as a diagnostic tool. However, misdiagnosis has been reported even in the most severe conditions. For instance, [PERSON] [9] reported a case where a patient with exertional dyspnea and wheeze was treated as acute asthma. The patient's condition deteriorated despite the treatment. It was not until a failure to insert a ventilation tube into the patient's trachea that the authors found the severe SGS in the patient. In hindsight, the patient presented a virtually silent chest, suggesting that the source of the wheeze was in the upper airway instead of the bronchi. In such cases, it may be possible to make the correct diagnosis at an earlier stage if the acoustic information is considered.  
The tonal characteristics of the SGS sound are highly variable but when combined with factors such as the respiration phase, volume, duration, rate of onset, and associated symptoms, the tonal characteristics of the sound can serve as an indicator of the location and size of the airway obstruction [2]. This current study suggests that developing different glottal tasks can provide more acoustic information for differential diagnosis, which is encouraging since the SGS sound is more affected by glottal configuration. [PERSON] et al. [13] hypothesized and then experimentally showed that tonal sound in the bronchi is generated by nozzle-jet-obstacle structures formed in obstructed bronchial airways, which are decoupled from upper airways. Thus, we expect that respiratory sounds from lower airway sources are less influenced by glottal and vocal tract configuration. Additionally, since the tonal components predominantly come from tract harmonics, changes in vocal tract shape and length that alter the resonance frequency are expected to affect the tonal characteristics of the SGS sound. Here we suggest a simple hypothetical test for differentiating subglottic obstruction from bronchial obstruction: patients who present with tonal respiratory sounds can be asked to produce the /h/ sound during either inhalation or exhalation, which essentially modifies the glottis from being neutral to constricted. If the tonal sound changes or disappears during this operation, then the sound is likely to be from a subglottic obstruction. However, further clinical verification and testing are required to develop corresponding methods for these tasks. Cross validation between the acoustic approach and other approaches such as electromyography of the respiratory muscles (see, e.g., [56]) may lead to the development of more robust and reliable non-invasive procedures to diagnose airway obstruction.  
It is worth noting that the current simplified configuration is more likely to have orderly flow structures and tonal sound, whereas realistic cases are expected to have a much more complex acoustic spectrum.