Airway pressure machines are available with two different modes of operation, namely standard CPAP and bilevel modes. BiPAP is a proprietary name for one brand of bilevel unit. CPAP units function at a fixed pressure during both inhaling and exhaling. Bilevel units function with one pressure for inhaling, and a lower pressure for exhaling. Many patients feel that bilevel therapy is a much more comfortable breathing mode, however bilevel units are more expensive. Therefore most patients start with standard CPAP units. Ask your physician if you are a candidate for bilevel therapy. If you hare having trouble breathing with CPAP therapy, you may be eligible to upgrade to a bilevel unit.