  Obstructive sleep apnea (OSA) is a common sleep disorder and widening the
upper airway is often used in clinical practice. However, the success rate of
this surgery is limited; the failed surgery would even make the situation
worse, indicating the widened airway is not the unique criterion to evaluate
the breathing quality. Therefore, we carried out both experimental measurement
and numerical simulation on OSA upper airways and found that there existed an
intrinsic dominant 3-5 Hz signal and the signal-to-noise ratio (SNR) at 3-5 Hz
is inversely correlated with apnea-hypopnea index (AHI). Firstly, to validate
the suitability of simulation methods, we carried out Laser Doppler measurement
in 3D-printing OSA upper airway models, and found excellent agreement between
the measured and calculated velocity profiles in two upper airway models for
the first time. Then we carried out large eddy simulation (LES) to investigate
four pairs of OSA upper airway models with 8 different AHI values for both pre-
and post-surgery; among them, three surgeries were successful and one failed.
The decreased the pressure drop for failed case, proving that only widening
airway cannot guarantee to improve OSA. In our analysis, it is indicating that
a dominant recirculation downstream of the minimum cross-section should be a
main feature of a successful surgery, and the strength of 3-5 Hz signal induced
by flow separation in the upper airway plays an important role in appraising
breathing quality. This provides a new guideline for surgery planning.
Furthermore, we found a strong correlation between AHI and the area ratio of
minimum cross-section near the retro-palate to the maximum cross-section behind
the tongue base, and this correlation is highly significant, r = - 0.833, p =
0.01 < 0.05.
