Complex sleep apnea and obesity hypoventilation syndrome. Opposite ends of the spectrum of obstructive sleep apnea?

In most cases, the application of continuous positive airway pressure (CPAP) during sleep in patients affected by obstructive sleep apnea (OSA) eliminates upper airway obstruction and makes breathing stable and regular. However, some OSA patients develop periodic breathing and central apneas during CPAP administration, a finding that has been labelled as ''complex sleep apnea" (complex SA). Such breathing disorder may occur only acutely after CPAP treatment initiation or sometimes persist with chronic CPAP treatment. We hypothesize that complex SA may be the consequence of mechanisms analogous to those leading to obesity hypoventilation syndrome (OHS), but operating in an opposite direction. Periodic breathing is one of the factors predisposing to OSA and is an essential factor for the recurrence of central apneas in normo or hypocapnic patients. A high ventilatory responsiveness to chemical stimuli enhances breathing periodicity. In subjects with periodic central apneas chemoresponsiveness is high, while in subjects with OSA it spans throughout a wide range, and is correlated to diurnal blood gas levels. In fact, sleep respiratory disorders may be responsible for either an augmentation in ventilatory responses to chemical stimuli consequent to chronic exposure to intermittent hypoxia, or for a decrease in ventilatory responses when prolonged exposure to hypercapnia is experienced. Among OSA subjects, those with OHS show very depressed hypercapnic responses. After chronic OSA treatment, ventilatory responses to chemical stimuli may either decrease, in previously hyperresponsive subjects, or increase, in previously hyporesponsive subjects. Most patients with OHS decrease daytime PCO2 levels and increase their ventilatory responses after chronic CPAP treatment. Complex SA could appear in those OSA subjects in whom chronic exposure to nocturnal respiratory disorders leads to the highest responsiveness to chemical stimuli, and could disappear after blunting of ventilatory responses following chronic CPAP treatment. Complex SA may be one extreme of evolutionary spectrum of OSA, the opposite end being represented by OHS