Results of a detailed study of complex sleep apnea, the name preferred by Mayo Clinic researchers, were announced by the Mayo team in 2006. In their report, the researchers described a phenomenon in which a significant fraction of patients diagnosed with severe obstructive sleep apnea failed to breathe normally after positive airway pressure was applied and their airways were opened. Rather their sleep apnea assumed the characteristics of central sleep apnea–the sleepers made no effort to breathe during apneic episodes, as if their brains were issuing no breathe command to the lungs.

In the group of 223 sleep apnea patients the Mayo researchers studied, 15 percent responded in this way, suggesting the presence of complex sleep apnea.

Wikipedia also offers useful information on complex/mixed sleep apnea.