dumb question

What is the difference in function between an APAP and a BiPAP?

The main difference is that BiPAPs (Respironics' name) or VPAPs (ResMed's name) have two pressure settings, one for inspiration and one for expiration. These pressures are usually called IPAP and EPAP. BiPAP come in auto and non-auto versions. They also can provide higher pressures than a standard CPAP machines.

There are also a couple of more exotic (and expensive) types of BiPAPs. ST machines have a backup rate and will attempt to stimulate breaths if the patient's breathing falls below the set number of breaths per minute. ASV (or Servo) machines are the most expensive and can be useful for complex sleep apnea among other conditions.

A CPAP blows one pressure while it is operating. Many of today's CPAPs provide ramp, a comfort feature, that starts at a lower pressure and over a set period of time builds to the set pressure. Many of today' CPAP machines provide some sort of exhalation relief, reducing pressure upon exhalation so there is less pressure to breathe out against.

An APAP changes the pressure it provides based on your last several breaths (according to each mfg's design) within the set range. It responds (increases pressure) to certain identified types of breathing irregularities that are believed to be precursors to events (apneas, hypopneas). It is a preventative machine in that it attempts to provide the pressure needed to prevent an event from happening. When the breathing irregularity (or event) stops, the pressure is maintained for a set period of time and then slowly lowered to the minimum pressure unless more breathing irregularities occur. Many of today' APAP machines provide some sort of exhalation relief, reducing pressure upon exhalation so there is less pressure to breathe out against.

A bilevel PAP provides one pressure during inhalation and a different (typically less) pressure during exhalation. It may be set to a single pressure for inhale and another for exhale or be an auto bilevel with set ranges that operates as described above. When a person fails CPAP (or APAP), a bilevel PAP is often prescribed to see if the two different pressures will enable the person to tolerate CPAP therapy.