Daytime hypercapnia that develops in morbidly obese individuals in the absence of concurrent lung or neuromuscular disease is referred to as the obesity hypoventilation syndrome (OHS). The characteristic polysomnographic (PSG) abnormality is marked sleep hypoxemia. Although the likelihood of hypoventilation increases with increasing body mass index (BMI), it is too simplistic to think of this disorder arising merely from chest wall restriction due to excess weight.

Rather, this is a disorder which emerges when the compensatory mechanisms that normally operate to maintain ventilation appropriate for the level of obesity are impaired. OHS develops from a complex interaction between abnormal respiratory function, sleep disordered breathing and diminished respiratory drive. Irrespecti...