Which Machine Do I Need? CPAP? APAP? BiPAP? BiPAP ASV?
  • 07 Jun 2022
  • 8 Minutes to read
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Which Machine Do I Need? CPAP? APAP? BiPAP? BiPAP ASV?

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By Anne - Registered Polysomnographic Technologist

Whether you are new to CPAP or have been using one for 20 years, navigating which machine is best for you can be intimidating. In this article we will review the different modes of the machines and discuss how they work and why your doctor might have prescribed one over another. 

The decision for which machine type you require ultimately is in your doctor’s hands as the prescription you have will indicate the machine you need to effectively treat your sleep apnea.

Before we proceed, you may want to watch this short video to help you read and understand your CPAP prescription:

The Difference Between CPAP and APAP (Auto CPAP)

One of the most common things people want to know about CPAP is the difference between different types of machines. Let us start with the key differences between manual and automatic CPAP (CPAP vs APAP). 

We'll give you an example of each and explain how each works. 

The Fixed Pressure CPAP

Examples: The DreamStation CPAP Pro or the AirSense 10 Elite

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Example Prescription:

CPAP: 9cmH2O


Manual CPAP is the most basic of the CPAP machines. However, there is a bit more involved in the process to use it. 

Manual (or fixed-pressure) machines blow at one pressure all night. They may have a ramp feature to start off a bit lower at the very beginning of the night that lasts for up to 45 minutes, but beyond that, the pressure does not change at all. 

Typically, having a prescription for a fixed pressure CPAP involves both a diagnostic sleep study and a titration CPAP study to determine your therapeutic pressure – the pressure at which all or most of your apneas are effectively treated. Many doctors will forego the titration study in favor of the auto CPAP which we will discuss in the next section.

As with anything, there are pros and cons to each type of machine, and those pros and cons will vary depending on the person using it. One advantage for this type of machine is that it tends to be the cheapest. 

One drawback of this machine is that it will not respond to any apneas you have, nor will it adjust the pressure in response to sleep state or position changes.

The Automatically Adjusting CPAP

Examples: The DreamStation Auto or the AirSense 10 Auto

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Example Prescription:

APAP: 5-14cmH2O


Auto CPAP is by far the most common type of machine prescribed by doctors these days. Like the fixed pressure CPAP, it blows at one constant pressure. However, that pressure will shift throughout the night to accommodate different pressure requirements for sleep stage or sleep position. 

The pressure typically starts low, often significantly below the usual therapeutic pressure. This reduced pressure allows the patient to fall asleep comfortably. 

Then, once the patient is asleep, they will have apneas. The machine detects the apnea onset and will increase the pressure to break through that apnea. It will continue this pattern of increasing the pressure until no more apneas are detected. 

At this point, it will attempt to reduce the pressure to ensure that it is not delivering more pressure than is needed. If the apneas resume at the lower pressure, it will again increase. 

This fluctuation allows for therapeutic treatment at overall lower pressures. It can also eliminate the need for the titration sleep study as the machine will adjust on a nightly basis to ensure continued effective treatment. 

Many patients also require more pressure when lying on their back vs lying on their side. It is also quite common to require more pressure during periods of REM sleep – this machine can account for this variation and will adjust accordingly.

Typically, your doctor would want you on an automatic CPAP if: 

  • You have a great pressure requirement difference depending on the sleep stage you are in or the position you are lying in
  • You are not able to tolerate your therapeutic pressure all night
  • You have not had a titration sleep study
  • They are looking to provide you with extra flexibility in your treatment than would be provided by the fixed pressure CPAP.

The next type of a machine we'll talk about is BiPAP. A common question is:

Can I use CPAP as BiPAP?

And the answer is 'No.' Please feel free to watch the video below to understand the difference between CPAP and BiPAP:

The Automatically Adjusting BiPAP

Examples: The DreamStation BiPAP or the AirCurve 10 VAuto

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Example Prescription:

IPAP Max: 15cmH2O

EPAP Min: 7cmH2O

PS Min: 4cmH2O


Now we are starting to get into the more advanced machines. The BiPAP machine allows for two pressures - one for inhale, and one for exhale. The difference between these two is called the Pressure Support. 

Using the example prescription above, the machine would start with an exhale pressure of 7cmH2O, and an inhale pressure of 11cmH2O (allowing for the pressure support minimum of 4cmH2O difference). 

The machine would automatically adjust both the inhale pressure and the exhale pressure as needed in response to apneas up to a maximum inhale pressure of 15cmH2O while maintaining at least a 4cmH2O difference between the two.

There are many reasons that your doctor may recommend a BiPAP over a CPAP machine for you. Some of the most common include:

  • Difficulty exhaling when higher pressures are required to treat your apneas
  • Incomplete ventilation (meaning, you are not able to exhale fully and are therefore holding on to too much carbon dioxide)
  • Treating central or complex sleep apnea – when you have both obstructive and neurologically based apneas.

The major disadvantage of the more advanced machines is price. The cost for the machines increases dramatically when looking at BiPAPs vs CPAPs. The major benefit is that BiPAPs will treat more advanced conditions that would be left untreated by the CPAP machines. 

Again, it all comes down to what your doctor states you need. BiPAP or VPAP machines are for more severe obstructive sleep apnea and central sleep apnea. 

If you are well treated with a CPAP machine, there is no real advantage to getting a BiPAP, but if you need a BiPAP to treat your apneas, a CPAP will not suffice.

The BiPAP-ST

Examples: The DreamStation BiPAP-ST or the AirCurve 10 ST

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Example Prescription:

IPAP Max: 15cmH2O

EPAP Min: 7cmH2O

PS Min: 4cmH2O

Rate: 14 bpm


Like the automatically adjusting BiPAP, the BiPAP-ST (ST stands for Spontaneously Timed) allows for both an inspiratory and expiratory pressure, but it also maintains a minimum breath rate. 

If the patient is not breathing frequently enough, the machine will prompt a breath to get them back on schedule. This is a common machine for patients with central or complex sleep apnea – where they suffer from both obstructive and central apneas throughout the night.

The BiPAP ASV

Examples: The DreamStation BiPAP ASV or the AirCurve 10 ASV

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Example Prescription:

IPAP Max: 25cmH2O

EPAP Max: 11cmH2O

EPAP Min: 7cmH2O

PS Max: 15cmH2O

PS Min: 4cmH2O

Rate: 14 bpm


The ASV machine, or Adaptive Servo-Ventilator, tracks normal breathing in a 3-4 minute window and determines the average peak flow or minute volume of the patient. 

From there, it will maintain the minimum levels of inspiratory and expiratory pressures so long as the patient maintains the threshold of peak flow and minute volume. 

If the patient begins to fall below this threshold, it will increase the inspiratory and/or expiratory pressures enough to get the patient back on target. 

The machine is primarily used to treat periodic respiration and is often prescribed to patients with conditions like Cheyne-Stokes Respiration, central sleep apnea, or opioid-induced apneas. 

Patients with damage to the respiratory center of the brain from stroke or trauma, or with conditions that impact the delivery of the carbon dioxide reading to the respiratory center, such as hypertension or heart failure, may also benefit from a more advanced machine like the ASV.

The BiPAP-AVAPS

Examples: The DreamStation BiPAP-AVAPS or the AirCurve 10 ST-A

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Example Prescription:

IPAP Max: 15cmH2O

EPAP Min: 7cmH2O

PS Min: 4cmH2O

VT: 450ml

Breath rate: 14 bpm

Ti: 2.0 sec

Rise Time: 300ms


AVAPS, or Average Volume-Assured Pressure Support, ensures that the volume of air moving with each breath is maintained. 

The most notable setting for this machine is the VT or tidal volume measurement. It is typically based on the patient’s BMI and is used to ensure that enough air is moving with each breath to prevent hypoventilation or hypercapnia – an excess of carbon dioxide in the blood. 

Like the BiPAP-ST, the machine also allows for a target breath rate. It also monitors the shape of each breath, allowing for settings for the Ti (inspiratory time) and Rise time (the amount of time it takes the device to change from the expiratory pressure setting to the inspiratory pressure setting).

This machine is primarily used to treat inadequate ventilation. Patients with obesity hypoventilation, neuromuscular disorders, or chronic obstructive pulmonary disease may be best suited to an AVAPS machine.In Conclusion

There are many different types of CPAP or BiPAP machines, and the one you require is determined entirely by your specific needs. 

The type of machine you have really does matter, but it is not a decision you have to make for yourself. Rather, it is a decision that is going to be made by your doctor based on your medical history and often by your titration sleep study as well. 

If your prescription does not look exactly like one of the examples above due to extra or missing information, do not worry. Often, doctors will leave off information if they are happy with the default setting, or they will add extra settings if they want to change something that is typically left on default or is a comfort setting. 

We at RespShop are always available to help and can direct you to the machine that your doctor wants you to be on.


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