MMP9 and obstructive sleep apnea syndrome: The serum levels of MMP-9 were significantly positively correlated with the AHI and T90 in the patients with OSA (r = 0.35, p < 0.001, Fig. 2A; r = 0.36, p < 0.001, respectively, Fig. 2B) but negatively correlated with the mean SpO2 and lowest SpO2 (r = − 0.23, p = 0.017, Fig. 2C; r = − 0.11, p = 0.279, Fig. 2D, respectively).