An Automatic Positive Airway Pressure device (APAP, Auto-PAP, Auto-CPAP) adjusts the pressure delivered based on the current needs of the patient at any given time using a proprietary algorithm. The primary goal of auto-titration is to provide the minimum pressure necessary to maintain an unobstructed airway over the course of a night, thereby increasing patient comfort.
How an Auto-CPAP Functions
An APAP uses a range of pressures predefined in the clinician settings, relying on real-time feedback from the patient such as airflow limitations, snoring, apnea, hypopnea or a combination of these variables.  Once the APAP begins to detect one of these physiological events, the algorithm will begin working to respond and treat the patient. If the machine detects a flow limitation, or obstructive event, it will begin raising pressure to maintain the patency of the airway until it determines that an effective treatment pressure has been reached, or the machine has reached its limited or functional maximum pressure.  If the machine detects a clear airway event, the machine will maintain the current pressure in order to prevent causing further clear airway events. If no events are detected over a set period of time, the APAP will begin reducing pressure to attempt to return to the minimum effective pressure. 
The effectiveness of the algorithm is an oft-debated subject, because each device manufacturer uses a unique and proprietary algorithm to detect and treat apnea events. Generally speaking, the algorithms used by the top two device manufacturers (Resmed, Philips Respironics) are excellent in detecting and treating apnea events in patients, however they tend to approach patient treatment differently. Understanding these subtle differences in manufacturer algorithm plays an integral role in accurately and effectively setting treatment pressures that result in truly effective and customized treatment for each patient.
When comparing Resmed Autoset machines with that of its counterpart Philips Respironics, it is generally accepted that Resmed has an aggressive algorithm that is capable of treating a patient effectively over a wide-range of pressures. Resmed devices will also stay at treatment pressures longer before returning to the minimum pressure necessary to maintain airway stability. One of the primary complaints of the Resmed algorithm is that it can be uncomfortable for patients when the pressure remains at treatment levels longer than the patient deems necessary, or that the rapid increase in pressure can cause patient arousal.
Philips Respironics machines on the other hand, will remain at the minimum pressure necessary to maintain airway stability, and will raise pressure at a slower pace than the Resmed to reach a preventative therapy pressure. Once the Philips Respironics machine reaches the therapy pressure it will quickly begin to decrease pressure to return to the minimum pressure necessary for airway stability. Proponents of this algorithm argue that this can result in greater patient comfort and less arousal due to gradual pressure increases and more time spent at the minimum pressures. The major criticism of the Philips Respironics algorithm argue that this behavior is less effective over a wide-range of pressures because of its slower response time and can result in increased apnea events unless the pressure range is more tightly controlled.
Advantages of APAP over fixed CPAP
- Patient Comfort: The primary goal of an APAP is to maintain the lowest minimum effective pressure to maintain airway stability, which is often lower than an effective single CPAP pressure. This results in greater patient comfort.
- Pressure Requirements: Patient needs can vary over the course of the night because of sleep stage, sleeping position, health, diet, or weight related factors that can render a single CPAP pressure less-effective or even ineffective. 
- Data-Capable: Most, if not all APAP machines are data-capable and can be accessed using either proprietary software or sleepyhead. A list of data-capable machines can be found at CPAP machine choices . Note that a machine that provides only summary compliance data is not considered data-capable.
- Increased Compliance: APAP use may increase patient compliance, particularly at higher pressures. 
- Decreased Sleepiness: APAP use may decrease day-time sleepiness more effectively than fixed pressure CPAP. 
Popular APAP Manufacturers
- Philips Respironics
- Fisher & Paykel
- Asp, Kevin. What is APAP Therapy? A comprehensive breakdown. https://www.aastweb.org/blog/what-is-apap-therapy-a-comprehensive-breakdown; February, 2017.
- Berry, Richard MD, Parish, James M MD, Hatse, Kristina M PhD, The Use of Auto-Titrating Continuous Positive Airway Pressure for Treatment of Adult Obstructive Sleep Apnea. Sleep, Vol 25, No. 2. 2002.
- Nolan GM, Doherty LS, Mc Nicholas WT. Auto-adjusting versus fixed positive pressure therapy in mild to moderate obstructive sleep apnoea. Sleep. 2007;30(2):189–194
- Vennelle M, White S, Riha RL, Mackay TW, Engleman HM, Douglas NJ. Randomized controlled trial of variable-pressure versus fixed-pressure continuous positive airway pressure (CPAP) treatment for patients with obstructive sleep apnea/hypopnea syndrome (OSAHS) Sleep. 2010;33(2):267–271