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.