Types of CPAP Explained: CPAP, APAP, BiPAP and ASV

By Admin

CPAP Machines and Modes

CPAP is the original method of Positive Airway Pressure (PAP) Therapy, but all types of PAP machines pressurize air using a small motor that draws the air into the machine and distributes it through a hose to a CPAP mask. What varies between types of CPAP machines are the pressure settings, which can be steady, shifting, or varied throughout the night in response to the patient’s breathing patterns.


CPAP stands for Continued Positive Airway Pressure, which is sometimes referred to as simply PAP, or Positive Airway Pressure Therapy. As the standard respiratory ventilation therapy device, the CPAP Machine emits a constant stream of air set to a chosen pressure level. Due to the constant pressure level, CPAP therapy can be more comfortable during inhalation, but the air flow during exhalation is what helps alleviate sleep apnea events, especially during obstructive apneas that can result in partial or complete collapse of the upper airway. A Ramp feature often slows the therapy’s onset to build up to the patient’s chosen pressure setting as treatment begins. As the treatment of choice for most sleep apnea patients, the CPAP machine is often less expensive but also less advanced than some of the other machine types available.


APAP, AutoPAP, AutoSet, or Auto-CPAP is Automated Positive Airway Pressure, which has more variation than the standard CPAP machine. APAP uses an algorithm to respond to patient breaths using a range of pressure settings. Breathing patterns produce wave-like levels of intensity during sleep, and APAP follows these patterns with pressure changes throughout the night, fluctuating between minimum and maximum levels set by the patient. While regular CPAP provides a constant flow after ramping up, APAP has auto-adjusting flows that react to patient breathing changes. This is called “auto-titration,” and the algorithms used may differ between machine brands, but the general advantage of APAP is its ability to provide a range of pressures that increase comfort levels for some patients.


BiPAP stands for BiLevel Positive Airway Pressure, and is sometimes called VPAP for Variable Positive Airway Pressure. This method delivers two distinct air pressure levels into the mask: one for inhalation and one for exhalation. Normally the inhalation is set a little higher than the exhalation, giving the patient less pressure to breathe against during exhalation periods. This dual system is ideal for those with higher pressure needs, as the two settings can provide the levels needed to alleviate apneas while still making it comfortable to exhale after each breath. After the exhalation periods, BiPAP machines will “wait” for the patient to start inhaling before they make the switch to the inhalation setting, and vice versa. Like APAP, BIPAP uses algorithms to respond to breathing patterns with changes in pressure. Because of its fast response times and high level of adaptability, BIPAP is considered the most advanced form of PAP therapy currently available. Note: (BiPAP is a trademark of Respironics BiLevel device.)


These terms refer not to CPAP methods but more specifically to the transitions from breathing inward (Inspiratory Positive Airway Pressure) to breathing outward (Expiratory Positive Airway Pressure). Devices often have different settings for each, and are set according to the patient's breathing patterns and range. The term EPAP is also used to describe “Nasal EPAP” appliances that fit over the nose and filter nasal breathing. These products have shown some success in alleviating obstructive sleep apneas and snoring, though treatment results vary significantly between patients.


BiPAP S/T stands for BiLevel Positive Airway Pressure Spontaneous/Timed Therapy. The Spontaneous/Timed designation refers to mode options which allow for more complex algorithms to initiate pressure changes. BiPAP S/T will initiate inhalation by triggering IPAP either spontaneously or at fixed time intervals, depending on the settings. This type of machine actually assists impaired breathing, and can be very useful for patients with central sleep apnea or mixed sleep apnea (combinations of both obstructive and central apnea), as well as for patients with respiratory insufficiencies, restrictive lung disorders, severe COPD and hypoventilation syndromes. The most advanced BiPAP ST-A machines include advanced assistance features that allow for either patient-initiated breaths or machine-initiated breaths. These systems include a set tidal volume for inhalation and will sound alarms in the case of halted or limited breathing. This type of non-invasive ventilation (NIV) adapts in real time to changes in breathing, using backup rates as a standard baselines for each patient.


ASV stands for Adaptive Servo-Ventilation, and it differs from other forms of ventilation therapy in that it assists in inspiration to ensure normal breathing. The difference is that ASV provides support to regular breathing by detecting reductions and responding with just enough added pressure to maintain the patient's breathing at their established baseline. These advanced algorithms are based on set rates of breaths per minute that the patient should be taking for normal healthy respiration. ASV units are often used for patients with complex apneas which are combined central and obstructive conditions. While ASV is similar to a BIPAP or BiPAP S/T therapies, it responds to changes in breathing by monitoring volumes as well as patterns, adjusting immediately as the change occurs. ASV units also have backup rates like the BiPAP S/T and BiPAP ST-A machines, so they can react to halted breathing with a programmed response.

Mode Terms

S (Spontaneous) Mode: In Spontaneous Mode the device will initiate IPAP spontaneously when flow sensors detect inspiratory changes. shifting to a higher level of pressure every time a breath is taken, and cycling back to EPAP for a lower level of pressure during exhalation. T (Timed) Mode: Timed Mode refers to the timing of IPAP/EPAP intervals, meaning that each cycle is initiated by the machine, typically expressed in breaths per minute (BPM). With BiPAP in Timed Mode, the switch between inhale and exhale pressures can help to initiate breaths. S/T (Spontaneous/Timed) Mode: Like Spontaneous mode, S/T Mode switches to IPAP based on patient inspiratory effort. But in Spontaneous/Timed Mode a "backup" rate is also set to ensure that patients still receive a minimum number of breaths per minute. This backup rate is based on a patient’s baseline of normal, healthy breathing. Thus, if spontaneous breaths are not taken regularly, the ventilator will initiate breaths. AC (Assist Control Mode): This mode allows either patient-initiated breaths or machine-initiated breaths and includes a set tidal volume for inhalation. AVAP (Average Volume Assured Pressure Support): AVAPS automatically adjusts pressure based on variations in inspiratory airflow to ensure a fixed target volume. AVAPS has shown considerable long-term benefits in patients with chronic respiratory issues as well as sleep apnea. IVAPS (intelligent Volume-Assured Pressure Support): A trademark of ResMed, iVAPS uses a highly intelligent algorithm to provide pressure support that stabilizes breathing volumes.


Academy for the Advancement of Science and Technology - https://www.aastweb.org/blogb/pros-and-cons-of-adaptive-servo-ventilation-asv-for-sleep-apnea

Alaska Sleep Clinic - https://www.alaskasleep.com/blogb/adaptive-servo-ventilation-asv-therapy-for-central-sleep-apnea

American Association of Sleep Technologies - https://www.aastweb.org/blogb/pros-and-cons-of-adaptive-servo-ventilation-asv-for-sleep-apnea