The purpose of the project is:

1. to determine the incidence of complex sleep apnea
2. to determine what percentage of cases will resolve over time with therapy with Continuous Positive Airway Pressure
3. Determine whether there is any difference in outcome, in those with persistent complex sleep apnea on CPAP, between those treated with CPAP or adaptive servo-ventilation.

Patients with suspected obstructive sleep apnea (OSA) are studied overnight in a sleep laboratory, as part of their routine clinical management. If they are found to have OSA during the first half of the night, they are started on continuous positive airway pressure (CPAP) by nasal mask as part of their routine clinical management. A certain percentage of these patients (best estimate from the literature 15%) will develop central sleep apnea (CSA) during their treatment with CPAP.

Participants will be recruited from patients treated with Continuous Positive Airway Pressure (CPAP) for obstructive sleep apnea who develop central sleep apnea (CSA) on CPAP, and are willing to give informed consent .

Patients will be treated with the "best CPAP" pressure, as determined by the sleep physician reading their sleep study. This is defined as the minimum pressure associated with elimination of obstructive events. Patients will be treated with CPAP for 12 weeks. Following treatment, patients will be brought back for a second sleep study to be assessed on CPAP.

Those that demonstrate resolution of CSA on the sleep study will continue on CPAP for an additional 12 weeks. Those that demonstrate persistent CSA will be randomized to either "best CPAP" or Adaptive Servo-ventilation (ASV) therapy for an additional 12 weeks. ASV will be titrated during the second sleep study to determine optimal settings. Patients will be compared at the end of 12 and 24 weeks of treatment regarding response to therapy. This will be assessed by compliance with therapy (as monitored by a smart card within the positive pressure machine), improvement in sleepiness as measured by the Epworth Sleepiness Score, and change in quality of life as measured by the Sleep Apnea Quality of Life Index (SAQLI) a validated quality of life instrument specific to sleep apnea.

In addition, in those who demonstrated persistent central apneas after 12 weeks of CPAP therapy, and were randomized to CPAP or ASV, a third and final sleep study will be done to assess the residual apnea-hypopnea index (the number of respiratory events divided by the number of hours of sleep) on therapy. In those randomized to CPAP who have a persistent poor clinical response associated with an abnormal residual AHI after 24 weeks of treatment, a further 12 week trial of ASV will be undertaken to assess whether it offers any benefit.

In addition to assessing the effect of treatment, the original diagnostic polysomnogram of all patients with complex sleep apnea will be analyzed to determine if there are any polysomnographic features that could be utilized to predict complex sleep apnea before CPAP is applied, (for example: 1) evidence of mixed apneas; or 2) a component of central apneas in addition to the predominant pattern of OSA.)

Inclusion Criteria:

• Patient with obstructive sleep apnea who develop central sleep apnea once treated with CPAP

Exclusion Criteria:

• Inability to give informed consent, inability to tolerate positive pressure ventilation