Positive airway pressure

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A typical CPAP machine houses the air pump in a case lined with sound-absorbing material for quieter operation. A hose carries the pressurized air to a face mask or nasal pillow.

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Positive airway pressure (PAP) is a method of respiratory ventilation used primarily in the treatment of sleep apnea, for which it was first developed.

PAP ventilation is also commonly used for critically ill patients in hospital with respiratory failure, and in newborn infants (neonates). In these patients, PAP ventilation can prevent the need for endotracheal intubation, or allow earlier extubation.


A Continuous Positive Airway Pressure (CPAP) machine is used mainly by patients for the treatment of sleep apnea at home. Obstructive sleep apnea occurs when the upper airway becomes narrow as the muscles relax naturally during sleep. This reduces oxygen in the blood and causes arousal from sleep. The CPAP machine stops this phenomenon by delivering a stream of compressed air via a hose to a nasal pillow, nose mask or full-face mask, splinting the airway (keeping it open under air pressure) so that unobstructed breathing becomes possible, reducing and/or preventing apneas and hypopneas. This has the additional benefit of reducing or eliminating snoring.

The CPAP machine blows air at a prescribed pressure (also called the titrated pressure). The necessary pressure is usually determined by a sleep physician after review of a study supervised by a sleep technician during an overnight study (polysomnography) in a sleep laboratory. The titrated pressure is the pressure of air at which most (if not all) apneas and hypopneas have been prevented, and it is usually measured in centimetres of water (cm H2O). The pressure required by most patients with sleep apnea ranges between 6 and 14 cm H2O. A typical CPAP machine can deliver pressures between 4 and 20 cm H2O. More specialized units can deliver pressures up to 25 or 30 cm H2O.

CPAP treatment can be highly effective in treatment of obstructive sleep apnea. [1] For some patients, the improvement in the quality of sleep and quality of life due to CPAP treatment will be noticed after a single night's use.

Prospective CPAP candidates are often reluctant to use this therapy, since the nose mask and hose to the machine look uncomfortable and clumsy, and the airflow required for some patients can be vigorous. Some patients adjust to the treatment within a few weeks, others struggle for longer periods, and some discontinue treatment entirely.

Therapy compliance on the part of the patient can be improved with support from a durable medical equipment (DME) provider, including allowing the patient his or her choice of PAP devices. PAP manufacturers frequently offer different models at different price ranges, and PAP masks have many different sizes and shapes, so that some users need to try several masks before finding a good fit.


Professor Colin Sullivan first developed the Continuous Positive Airway Pressure (CPAP) system at Royal Prince Alfred Hospital in 1981.


  • Fixed-pressure CPAP (Continuous Positive Airway Pressure) provides one constant pressure to the patient
  • APAP or AutoPAP or AutoCPAP (Automatic Positive Airway Pressure) automatically titrates, or tunes, the amount of pressure delivered to the patient to the minimum required to maintain an unobstructed airway on a breath-by-breath basis by measuring the resistance in the patient's breathing, thereby giving the patient the precise pressure required at a given moment and avoiding the compromise of fixed pressure.
  • VPAP™ or BiPAP® (Variable/Bilevel Positive Airway Pressure) provides two levels of pressure: one for inhalation (IPAP) and a lower pressure during exhalation (EPAP)
  • xPAP ST (Spontaneous Time) is a machine that forces a number of set breaths per minute and is used to treat patients with central apneas.


  • Flow generator (PAP machine) provides the compressed air
  • Hose connects the flow generator (sometimes via a humidifer) to the inferface
  • Interface (mask) provides the connection to the user's airway

Optional features

  • Humidifier adds moisture to the air
    • Heated: Heated water chamber that can increase patient comfort by eliminating the dryness of the compressed air. The temperature can usually be adjusted or turned off to act as a passive humidifier if desired.
    • Passive: Air is blown through an unheated water chamber and is dependent on ambient air temperature. It is not as effective as the heated humidifier described above, but still can increase patient comfort by eliminating the dryness of the compressed air.
  • Ramp is used to temporarily lower the pressure to allow the user to fall asleep more easily. The pressure gradually rises to the prescribed level over a period of time that can be adjusted by the patient and/or the DME provider.
  • Exhalation pressure relief: Gives a short drop in pressure during exhalation to reduce the effort required. This feature is known by the trade name C-Flex® in some PAPs made by Respironics and EPR™ in ResMed machines.
  • Flexible chin straps are used to help the patient not breathe through the mouth, thereby keeping a closed pressure system. The straps are elastic enough that the patient can easily open his mouth if he feels that he needs to.
  • Data logging records basic compliance info or detailed event logging, allowing the sleep physician (or patient) to download and analyze data recorded by the machine to verify treatment effectiveness.

Such features generally increase the likelihood of PAP tolerance and compliance. [2]

Care and maintenance

As with all durable medical equipment, proper maintenance is essential for proper functioning, long unit life and patient comfort. The care and maintenance required for PAP machines varies with the type and conditions of use, and are typically spelled out in a detailed instruction manual specific to the make and model.

Most manufacturers recommend that the end user perform weekly maintenance. Units must be checked regularly for wear and tear and kept clean. Worn or frayed electrical connections may present a shock or fire hazard; worn hoses and masks may reduce the effectiveness of the unit. Most units employ some type of filtration, and the filters must be cleaned or replaced on a regular schedule. Hoses and masks accumulate exfoliated skin, particulate matter, and can even develop mold. Humidification units must be kept free of mold and algae. Because units use substantial electrical power, housings must be cleaned without immersion.


Since continuous compliance is an important factor in the success of treatment, it is of importance that patients who travel have access to portable equipment. Progressively, nPAP units are becoming lighter and more compact, and often come with carrying cases. Dual-voltage power supplies permit many units to be used internationally.

Air travel presents special considerations. Most airport security inspectors have seen the portable machines, so screening rarely presents a special problem. Increasingly, machines are capable of being powered by the 400 Hz power supply used on most commercial aircraft and include manual or automatic altitude adjustment.

Some patients on PAP therapy also use supplementary oxygen. When provided in the form of bottled gas, this can present an increased risk of fire and is subject to restrictions. (Commercial airlines generally forbid passengers to bring their own oxygen.) As of November, 2006, most airlines permit the use of oxygen concentrators.


In many countries, PAP machines are only available by prescription. A sleep study at an accredited sleep lab is usually necessary before treatment can start. This is because the pressure settings on the PAP machine must be tailored to a patient's treatment needs. A doctor, who may be a Respiratory Medicine, Ear Nose and Throat (ENT) or Neurology specialist, will interpret the results from the initial sleep study and estimate the correct pressure from experience. This is later confirmed with a follow up sleep study during which the patient wears the CPAP mask and pressure is adjusted up and down from the prescribed setting to find the optimal setting.

  • In the United States, PAP machines are often available at large discounts online, but a patient purchasing a PAP personally must handle the responsibility of securing reimbursement from his or her insurance Medicaid. Many of the internet providers that deal with insurance such as Medicare will provide upgraded equipment to a patient even if he or she only qualifies for a basic PAP. In some locations a government program, separate from Medicare, can be used to claim a reimbursement for all or part of the cost of the PAP device.
  • In the United Kingdom, PAP machines are available on National Health Service prescription after a diagnosis of sleep apnea or privately from the internet provided a prescription is supplied.
  • In Australia, PAP machines can be bought from internet or physical stores, There is no requirement for a doctor's prescription, however many suppliers will require a referral. Low-income earners who hold a Commonwealth Health Care Card should enquire with their state's health department about programs that provide free or low-cost PAP machines. Those who have private health insurance may be eligible for a partial rebate on the cost of a CPAP machine and the mask. Superannuation may be released for the purchase of essential medical equipment such as PAP machines, on the provision of letters from two doctors, one of whom must be your specialist, and an application to the Australian Prudential Regulation Authority (APRA).

In a hospital setting

PAP ventilation is often used for patients who have acute type 1 or 2 respiratory failure. Usually PAP ventilation will be reserved for the subset of patients for whom oxygen delivered via a face mask is deemed to be insufficient or deleterious to health (see CO₂ retention). Usually, patients on PAP ventilation will be closely monitored in an intensive care, high dependency, coronary care unit or specialist respiratory unit.

The most common conditions for which PAP ventilation is used in hospital are congestive cardiac failure and acute exacerbation of obstructive airways disease, most notably exacerbations of COPD and asthma. It is not used in cases where the airway may be compromised, or consciousness is impaired.

The mask required to deliver CPAP must have a tight seal, and be held on very firmly. Most people find wearing the mask uncomfortable. Breathing out against the positive pressure resistance (the expiratory positive airway pressure component, or EPAP) is also unpleasant. These factors lead to inability to continue treatment due to patient intolerance in about 20% of cases where it is initiated. Obviously those who suffer an anxiety disorder or claustrophobia are more likely to be unable to tolerate PAP treatment. Sometimes medication will be given to assist with the anxiety caused by PAP ventilation.

Unlike PAP used at home to splint the tongue and pharynx, PAP is used in hospital to improve the ability of the lung to exchange oxygen and carbon dioxide, and to decrease the work of breathing (the energy expended moving air into and out of the alveoli). This is because:

  • During inspiration, the inspiratory positive airway pressure, or IPAP, forces air into the lungs - thus less work is required from the respiratory muscles.
  • The bronchioles and alveoli are prevented from collapsing at the end of expiration. If these small airways and alveoli are allowed to collapse, significant pressures are required to re-expand them. This is because of the Young-Laplace equation (which explains why the hardest part of blowing up a balloon is the first breath).
  • Entire regions of the lung that would otherwise be collapsed are forced and held open. This process is called recruitment. Usually these collapsed regions of lung will have some blood flow (although reduced). Because these areas of lung are not being ventilated the blood passing through these areas is not able to efficiently exchange oxygen and carbon dioxide. This is called ventilation/perfusion (or V/Q) mismatch. The recruitment reduces ventilation perfusion mismatch.
  • The amount of air remaining in the lungs at the end of a breath is greater (this is called the Functional residual capacity). The chest and lungs are therefore more expanded. From this more expanded resting position, less work is required to inspire. This is due to the non-linear compliance-volume curve of the lung.


  1. Montserrat, Josep (August 2001). "Effectiveness of CPAP Treatment in Daytime Function in Sleep Apnea Syndrome". American Journal of Respiratory and Critical Care Medicine. 164 (4): 608–618. Unknown parameter |coauthors= ignored (help)
  2. Richards, Dianne (May 2007). "Increased Adherence to CPAP With a Group Cognitive Behavioral Treatment Intervention: A Randomized Trial". Journal SLEEP. 30 (05): 635–640. Unknown parameter |coauthors= ignored (help)

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