We get this question all the time: what's the difference between a CPAP machine and a BiPAP machine?

 In fact, we field this question so often, the short and sweet answer is located on our "Frequently Asked Questions" page. You might find some other helpful information on that page as well. When in doubt, though, don't hesitate to give us a call. 

 Here's the difference between a CPAP machine and BiPAP machine.

A CPAP machine is set to a single prescribed pressure setting. Usually this pressure setting is a number between 4 and 20. When the CPAP machine is set to this pressure, the pressure is fixed at that pressure throughout your breathing cycle. For example, if your CPAP machine is set to a pressure of 10, then the machine will blow at a pressure of 10 all the time.

There are some exceptions to the fixed nature of the pressure on a CPAP machine. Almost all CPAP machines have a ramp feature, and some CPAP machines have an exhalation pressure relief feature.

The ramp feature allows the CPAP user to start therapy at a lower-than-prescribed pressure and gradually build up to the prescribed pressure over a pre-determined period of time. For example, with a prescribed pressure of 10, a CPAP user might have a ramp start pressure of 4 with a ramp time of 20 minutes. In this scenario, the CPAP machine pressure will start out at 4 and gradually increase to the prescribed pressure of 10 over the course of 20 minutes. This type of ramp feature is designed to help CPAP users fall asleep more easily.

The exhalation pressure relief feature found on some CPAP machines reduces pressure during the exhale portion of the breathing cycle. The amount of relief during exhale is limited to 3 units of pressure. This, of course, tends to make breathing against the incoming pressure a bit easier. Not all machines have an exhalation pressure relief feature. Those that do often include the word "flex" in their name, like C-Flex, A-Flex, and SmartFlex. ResMed simply calls their exhalation pressure relief feature EPR.

A BiPAP machine is similar to a CPAP machine with exhalation pressure relief. On a BiPAP machine, two pressure settings are required. These are called the EPAP and IPAP pressures, with the "E" meaning exhalation and the "I" meaning inhalation. For example, a prescription for a BiPAP machine might read something like, "EPAP-6, IPAP-12". This would indicate that the inhalation pressure should be 12, and the exhalation pressure should be 6.

You'll notice that in the example BiPAP settings the pressure difference is 6 units of pressure. This demonstrates the difference between a CPAP machine with exhalation pressure relief and a BiPAP machine. The CPAP machine with exhalation pressure relief can have a difference of only 3 units of pressure, whereas the difference can be much greater on a BiPAP machine.

Often, a BiPAP machine is prescribed when a patient shows an intolerance to high pressures during exhalation. The patient may need a relatively high pressure setting during inhalation to open the airway, but cannot breathe or sleep comfortably with a high pressure during exahalation.

Some doctors may be unfamiliar with CPAP machines with exhalation pressure relief. These doctors may prescribe a BiPAP machine to a patient with a pressure differential of 3 or less. In this case, a BiPAP machine is usually unnecessary because a CPAP machine with an exhalation pressure relief feature can handle the prescribed settings. If the pressure differential - remember, this is the difference between inhalation pressure and exhalation pressure - is 4 or more, then a BiPAP machine would be required.

A BiPAP machine might be prescribed to a patient with a relatively low pressure setting but who has some ailment which makes breathing against the incoming air more difficult. Patients with chronic obstructive pulmonary disease (COPD) or emphysema or some other similar breathing disorder might not be able to experience comfortable, effective therapy with a normal CPAP machine. This type of person might need the significant pressure reduction of a BiPAP machine.

Also, the maximum inhalation pressure on most CPAP machines is 20. In the event a pressure higher than 20 is required, a BiPAP machine would be the only option. Many BiPAP machines have a maximum pressure of 25.

Throughout this article I've used the term BiPAP machine as a generic description, when in fact the term BiPAP is a trade name owned and marketed by Philips Respironics, as in the "PR System One BiPAP Auto". The general term is actually bilevel machine, but usage of the word BiPAP has been widely adopted as the de facto standard.

So, Philips Respironics calls their bilevel machines BiPAP machines. ResMed calls their bilevel machines VPAP machines. DeVilbiss calls their bilevel machines, well... bilevel machines! Imagine that.


1) CPAP machine pressure is fixed, although some CPAP machines offer up to 3 units of pressure relief during exhalation

2) BiPAP machines have two different pressure settings - IPAP and EPAP - for inhalation and exhalation

3) Pressure differential between inhale pressure and exhale pressure can be much greater than 3 units of pressure on a BiPAP machine

4) Maximum pressure setting on all CPAP machines is 20 or lower

5) Maximum pressure setting on most BiPAP machines is 25 or higher

I hope this helps. For more info, give us a call. Our phone number is 1-888-955-2727. We're ready and waiting to help you!

Andrew Senske

CPAP-Supply.com is a leading online retailer of CPAP equipment. Located in Spokane, WA CPAP-Supply.com has been serving thousands of customers around the world since 2001. Founded on a belief that patients are their own best primary care providers, CPAP-Supply.com understands the importance of educating patients and customers on both the effects of and treatment for obstructive sleep apnea. For more information visit CPAP-Supply.com or call toll free 1-888-955-2727.

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