http://oig.hhs.gov/oei/reports/oei-07-99-00440.pdf
A Bi-Level Positive Airway Pressure Device with a back-up rate (HCPCS K05333) also delivers adjustable, variable levels of positive air pressure to assist spontaneous respiratory efforts and supplement the volume of air a patient inspires. In addition to the automated delivery of bi-level air pressure, this device also has a time back-up feature to detect when a patient has stopped or delayed breathing. Whenever “sufficient spontaneous inspiratory efforts fail to occur” the back-up feature ensures that the appropriate amount of air is provided.