Obstructive sleep apnoea Obstructive sleep apnoea (OSA) is a complex condition with many different phenotypes. Historically, OSA has been defined using the apnoea-hypopnoea index (AHI). However, because there is no clear relationship between the AHI and the severity of symptoms and comorbidities the degree of hypoxia is increasingly being used to define OSA severity. To reach a diagnosis of obstructive sleep apnoea syndrome (OSAS), it has to be shown that symptoms improve with therapy. The treatment of first choice for patients with severe OSA is continuous positive airway pressure (CPAP) therapy. The indication for other therapies depends upon the patient's characteristics and preferences. Treatment with a position trainer and implantation of a hypoglossal nerve stimulator are relatively new therapies. OSA is a cardiovascular risk factor, but the effect of OSA treatment on cardiovascular outcome measures and mortality has not been shown in clinical trials.