EPO and obstructive sleep apnea syndrome: The aim of this study was to investigate the effects of mandibular advancement device (MAD) therapy for obstructive sleep apnea–hypopnea syndrome (OSAHS) on hypoxia-inducible factor-1α (HIF-1α), erythropoietin (EPO) and vascular endothelial growth factor (VEGF) in myocardial tissue.