Bipap-when cpap eliminates obstructive respiratory events in a patient with uncomplicated osa, the patient sleepy very deeply, with increased amounts of rem sleep, and usually wakes up refreshed. however, in a patient with inadequate ventilatory capability (muscle weakness or morbid obesity) hypoventilation is worsened at night by the supine position and by changes of sleep (weakened respiratory muscles and reduced chemo responsiveness). if osa is eliminated by cpap then the arousal response to airway occlusion is also eliminated and this will lead to consolidated sleep and likely increased amounts of rem sleep (rem rebound). in rem sleep, all respiratory muscles except the diaphragm are paralyzed and tehre is further natural blunting the ventilatory response to hypoxemia/hypercapnea. if this patient is not aroused from this deep sleep he will progressively hypoventilate due to his severe muscle weakness and superimposed muscle inhibition from sleep. cpap does not provide ventilatory assistance so is not adequate therapy for such a patient. bi-level positive pressure ventilatioin provides ventilatory assistance at night. because the sleep related changes of respiration in patient with hypoventilation are potentially life-threatening, any patient with an elevated co2 at baseline needs to be closely supervised during the initiation of positive pressure cpap or bipap.