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BiPAP Machines - Spontaneous Timed ST and Adaptive Abilities/ASV/AVAPS

BiPAP stands for bi-level positive airway pressure. Unlike manual CPAP and auto CPAP machines that use a single pressure during expiration and inspiration, BiPAP machines provide varying levels of pressure during inspiration and expiration.

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BiPAP Machines - Spontaneous Timed ST and Adaptive Abilities, ASV, AVAPS Overview

The word BiPAP is a proprietary term owned by Philips Respironics, but it has come into common usage to refer to any mode of CPAP that uses bi-level support. Other companies may use terms such as VPAP by ResMed (variable positive airway pressure) or just bi-level by DeVilbiss to refer to the same type of therapy for sleep apnea and other disorders requiring respiratory support at night. We will use the terms interchangeably in this article.

During inspiration, a BiPAP machine increases the pressure to push air in. This support on inhalation is sometimes referred to as IPAP (inspiratory positive airway pressure) or pressure support. During exhalation, the pressure drops to a lower level and stays there until the next inspiration. This lower level of exhalation pressure is sometimes called EPAP (expiratory positive airway pressure), PEEP (positive end expiratory pressure), or just CPAP.

The VPAP machines listed in this section include a timed function to deliver a breath at a fixed interval. Some bi-level machines shown here also adapt the pressure on inspiration to reach a set volume of air delivery to the lungs. This volume is ordered by the physician for ASV therapy. Machines with the ability to target the volume of a breath have terms such as ASV (automatic servo-ventilation) or AVAPS (average volume assured pressure support) in their descriptions. Spontaneous Timed or ST BiPAP therapy may be recommended for patients with central sleep apnea or certain types of lung diseases to help them take in deeper breaths. Patients with unusual breathing patterns may also benefit from timed or volume-monitored bi-level CPAP therapy. Timed or volume adaptive bi-level therapy is not usually prescribed for obstructive sleep apnea. These machines are the most technically sophisticated BiPAP machines available for home use. Bi-level CPAP machines with timed and adaptive options may provide fixed-pressure bi-level CPAP or auto bi-level CPAP.

The ASVAuto mode in the ResMed AirCurve 10 VPAP ASV machines is designed to offer specialized support for patients with sleep apnea breathing disorders such as central sleep apnea (CSA), Cheyne-Stokes respiration, and others as well.

The AirCurve 10 S/T has a total of five operational modes - spontaneous (S), spontaneous/timed (S/T), timed (T), VAuto, and conventional CPAP mode. The primary mode of this particular VPAP device - spontaneous/timed (S/T) - will initiate a breath and begin IPAP. The duration of IPAP is controlled by your machine; this VPAP device will then automatically reduce pressure to EPAP. The S/T mode provides consistent relief and minimizes discomfort during therapy.

The DreamStation by Philips Respironics also comes in AVAPS and ST mode BiPAP.

Fixed pressure bi-level CPAP: The BiPAP generates continuous pressure at the airway (CPAP). When the patient takes a breath, the machine detects that activity and increases the pressure to a fixed higher pressure. The increase in pressure creates a pressure differential and supports inspiration. In fixed pressure bi-level CPAP, the inspiratory pressure and expiratory pressures are constant values. If the inspiratory pressure is set at 15 and the expiratory pressure is set at 8, the machine always uses 15 and 8. In timed mode, the device delivers a breath if it has not detected a spontaneous breath by the patient within a set time frame.

Auto bi-level CPAP: Like auto CPAP, the machine monitors the patient's breathing activity for apneas and obstructions and moves the expiratory pressures up and down. Some bi-level machines also monitor the amount of air the patient inhales and exhales and adjust the pressure on inspiration to match a set volume ordered by the physician. Machines with these words have terms such as ASV (automatic or adaptive servo-ventilation) or AVAPS (average volume assured pressure support) in their descriptions.

Bi-level machines with timed and adaptive options can use information about numerous facets of the patient's breathing pattern to make the patient comfortable. Bi-level devices also capture data but may differ in how that data is stored and retrieved for use by you or by you and your physician.


A ramp setting creates a deliberate lag between the time when the machine is turned on and when the full prescribed pressure is delivered. The ramp gives the patient time to get in bed, read, watch TV, or do a crossword puzzle before the full flow starts. The intentional lag may make it easier to get to sleep.

A ramp feature is commonplace on bi-level machines but how it works varies by model. Some machines have a preset ramp time while other models let the patient set the timer. On some devices, the ramp phase may be gradual: the pressure initializes very low and rises incrementally over the set period. On other machines, the pressure stays dormant until the device detects an airway obstruction. The machine uses the detected obstruction as the signal to ramp up to the ordered pressure level.

Flow Shaping

Most bi-level CPAP devices have flow-shaping features to make the transition between the two different levels of pressure less abrupt. When a machine needs to move the pressure from 7 to 25, for example, it must increase the flow rate considerably. If the flow rate rises too rapidly, it can be jarring to the patient. Flow shaping allows the patient to breathe easier despite the flow and pressure changes.


Oral and nasal dryness, hoarseness, and nose bleeds are potential side effects of any mode of CPAP therapy. Humidification can prevent or remedy these concerns. Heated humidifiers may be built into a machine or available for purchase separately. Without humidification, the flow from the machine tends to be cool and dry. Humidifiers warm up the air and saturate it with water vapor.

Some machines maintain the humidity rate and air temperature at a fixed point, while others provide ambient monitoring and adjust the humidity and temperature level up or down to best meet the need at the moment. The dynamic adjustment helps reduce an effect known as “rain out,” which occurs when warm, humidified gas cools as it travels through the tubing to the patient. Some machines have heated tubing to help minimize condensation accumulation.

Humidifiers require Distilled water which you can pick up in any grocery store. Be sure to fill your Humidifier away from your machine as water damage will void the warranty.

Information tracking

All bi-level equipment tracks data regarding usage and patient breathing patterns, but there is considerable variation in what information is recorded and tracked and how it is retrieved. Increasingly, machines use a wireless connection to upload data to the internet or a smartphone application. Machines that do not have a wireless connection may have data retrieval via an SD card or USB port. The data may include things like how often the machine was in use and whether the patient had obstructive or apneic events. You or your physician can look at the data to determine if the ordered parameters are working for you. Some machines integrate with smartphone applications both for data capture and sharing and for setting adjustments.

Care and Maintenance

All BIPAP machines need a stable location near the bed with easy access to a power supply. The machine should be flat and not jostled. The user must make sure the tubing reaches their bed space without difficulty. Longer hoses are usually available for purchase if needed.

Bi-level CPAP machines pull air in from the room through a filter that removes dust and other particles. The filter needs to be replaced as recommended to prevent device obstructions and failures. The hose or tubing requires daily cleaning. The humidifier also requires cleaning and sterile water replenished.

Follow the manufacturer's recommendations for cleaning and part replacements. Adhering to the recommendations keeps the user healthy and prolongs the life of the machine.

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