Dev Banerjee, Brendon J. Yee, Amanda J. Piper, Clifford W. Zwillich and Ronald R. Grunstein Levels of obesity are increasing in all age groups, and studies have indicated a high prevalence of obstructive sleep apnea (OSA) in the extremely obese population (body mass index [BMI] 50 kg/m2). The majority of obese subjects with OSA have normal alveolar ventilation when awake. However, in a subgroup of subjects, hypoventilation while awake will be present. The term obesity-hypoventilation syndrome (OHS) describes those subjects with obesity, daytime hypercapnia (Paco2 45 mm Hg), and hypoxia (Pao2 70 mm Hg) in the absence of significant lung or respiratory muscle disease.3 Subjects with OSA plus OHS also exhibit severe prolonged oxygen desaturations during sleep. The mechanisms that contribute toward the development of OHS are multiple, and include abnormal pulmonary mechanics, altered hypoxic and hypercapnic ventilatory responses (possibly explained by the chronic hypoxemia and poor sleep quality), upper airway obstruction, and possibly the influence of leptin.Obesity Hypoventilation Syndrome: Hypoxemia During Continuous Positive Airway Pressure