What's the difference between CPAP and BiPAP?

A fellow PAP user asked me this the other day. The terms CPAP and BiPAP are used frequently in the sleep apnea world but rarely defined. I'll first define CPAP and then go into BiPAP.

CPAP stands for Continuous Positive Airway Pressure. That is, the machine pumps out the same pressure constantly. The user inhales at a predetermined air pressure and then exhales at the same pressure. This leads us to BiPAP.

BiPAP is almost the same as CPAP but with one difference. The user inhales at a certain predetermined pressure and when the user exhales, the BiPAP machine senses this and lowers the pressure to a predetermined pressure. Hence the "Bi" in BiPAP stands for bi-level or two pressures. The inhale pressure is termed IPAP ("I" is for inspiratory). The exhale pressure is termed EPAP ("E" is for expiratory). The spread or difference between IPAP and EPAP is usually 3 or 4 (cmH2O). I say "usually" because there are special circumstances when the spread is larger.

xPAP is another term used frequently on internet forums. xPAP refers to CPAP or BiPAP. It is just a simple and quick way to refer to CPAP or BiPAP when the distinction is not important for the idea at hand.

Some of the newer xPAP's out there include ASV, AVAPS, etc. These are more sophisticated CPAP's and BiPAP's, usually the latter.