After 25 years since the first description of CPAP, titration for treatment of obstructive sleep apnea remains surprisingly not evidence based. Adding to this observation, Rodenstein comments in his 2008 editorial1 that manual titration is still an art and auto-adjusting positive airway pressure (APAP) seems to fail so frequently that it may be unreliable as a means to improve compliance or serve as a diagnostic device. These bold statements are justified by his review of the evidence and the wide variability of performance of APAP devices that impact consistent therapy. These comments should challenge clinicians to understand the issues surrounding APAP therapy and promote more “why” and “how” questions related to product capabilities and the science behind the performance and capabilities of devices used to treat obstructive sleep apnea.

One of the first APAP devices was introduced in France in 1962.2 The intent of the device was to reduce mean airway pressure to make CPAP therapy more comfortable and improve patient compliance. The device was designed and developed based on an estimate of appropriate pressure, estimates of flow changes, possible breathing disorder, and a little SWAG (scientific wild ass guess). There was no way for the early developers to have an established base for the technology as it was a new approach to a sleep-disordered breathing event. The product was tested with positive results related to improved patient compliance. Was this exhaustive or complete evidence-based research? Further research discovered that the flow-based breathing signal could identify breathing patterns that could assist in the understanding of disorganized breathing patterns and add another feature for diagnostic as well as therapeutic value to APAP. Again, there were studies that confirmed the capabilities of the device to provide a diagnostic option. Was this exhaustive or complete evidence-based research? Series et al discussed in his 2008 article3 that there are significant differences in performance between APAP devices for both 1-night and 1-week titration and warned that important discrepancies in the effective pressure can alter CPAP compliance and efficiency. In Lofaso’s bench evaluation of five APAP devices,4 he concluded that physicians need details on the algorithm used in each APAP device and that manufacturers should provide that data. There continues to be a lack of data on both the mechanical and clinical performance of individual APAP devices, and most of the device algorithms were based on a flurry of research in 19985-7 that was used by many of the manufacturers to base their auto-adjusting algorithm with little change or adjustment since that time.

Many Questions, Few Answers

Today, in the US market, there are five commercially available APAP devices that provide clinicians with the means to diagnose and treat sleep-disordered breathing.

  • Do clinicians know of the performance differences?
  • Is there current, objective research identifying the capabilities and limitations of each device?
  • Does the clinician select the appropriate device knowing its capabilities for a specific patient and monitor the patient for the desired outcomes?
  • Is anyone asking questions about these devices related to their capabilities and the evidence-based research that has documented the specific performance of the devices before they are purchased and used on patients?

The purpose of this article is to identify performance variables of the five APAP devices and challenge the reader to ponder these questions.

APAP devices are considered a commodity by many clinicians. The general consensus is that all APAPs are capable of providing positive results and the only consideration for determining which device to choose is features and price. Our research has determined that all APAPs are different, based on the algorithm that controls the device’s response to a breathing signal.8 The devices are not commodities and are significantly different in pressure response. How would the clinician know of these performance variables? Product literature does not identify how a device will respond to a breathing signal. There are no standards established for the appropriate pressure response to a specific breathing pattern as there do not seem to be research sites or standards agencies that are looking at this issue. If there are no standards and no research, how does one know if a product is providing the results expected from these devices? Monitoring positive outcomes is the standard response from many clinicians, yet if all devices perform differently and clinicians indicate that the device they have selected provides positive outcomes, is it possible that each clinician has purchased the correct device for their patient population? Are patient populations that well segmented to allow for this phenomenon, and have all the different APAP devices been put on the proper patients for these positive results? Unlikely!

The sleep apnea therapy market is moving too quickly for research to keep up with market changes, economics, and product development. New products are entering the market at a rapid pace and being positioned as the latest and greatest products for the sleep-deprived patient. New payor regulations are challenging the provider to identify new methods of providing products and services with the goal of reduced costs. Without evidence of positive results from the therapy, overall cost may be higher with the goal of effectively treating sleep apnea falling short and the prevention of complications not achieved.

Comparative Bench Testing of Five APAP Devices

Figure 1
Figure 2
Figure 3

Bench testing was conducted a few years ago by our group on the four then commercially available APAP devices.8 Recently, a new APAP was introduced to the market and our group wanted to determine this new device’s capability and retest the other devices to see if there were any algorithm changes to the existing units. The test process was similar to the 2006 bench testing method.


Prior to this test, four sleep-disordered breathing patterns were taken from nasal pressure data collected on patients who had undergone a polysomnography study and scored by a professional RPSGT. Normal, apnea, flow-limitation, and hypopnea patterns were digitized for use by a mechanical test lung (Series 1101 Breathing Simulator, Hans Rudolph Inc). The amount of data converted for each pattern varied, with the flow-limitation pattern totaling approximately 8 minutes in length and the hypopnea pattern totaling approximately 12.5 minutes in length. Both the normal and apnea breathing patterns were approximately 10 minutes in length. Each file and script were set to loop back to start at the conclusion of the recorded sample to allow for a 30-minute test run. While the disordered breathing patterns are scored as apnea, flow limitation, and hypopnea, there may be incidences of one type present in another signal, such as flow-limited breaths in the hypopnea signal. Shown at left are 50-second samples of each type of sleep-disordered breathing pattern recorded and applied to each APAP device.


All APAP units were able to complete all tests as outlined in the protocol with no complications.

Common in the Respironics APAP models is a “hunt and peck” search pattern, as part of the device’s algorithm is to find optimum therapy pressures in the face of airway resistance. The unit will increase pressure by 1.5 cm H2O over the course of 3 minutes and monitor the flow to see if there is an improvement. If there is none, the unit returns to the original pressure within 1 minute. If an improvement is recorded during the 1.5 cm H2O pressure increase phase, the device will lower the pressure by only 0.5 cm H2O. As there is no physiological component in this bench test, the unit consistently returned to the original pressure setting during periods of normal breathing. These are seen in the resulting device pressure profiles as triangular pressure increases/decreases.

Apnea. All devices exhibited pressure response to the change from normal to apneic breathing (Figure 1). The SleepStyle 220 (Fisher & Paykel), Sandman (Covidien), and S8 units (ResMed) rapidly responded to the apnea pattern, achieving the 10 cm H2O default apnea cap programmed for these units within 5 minutes of the apnea pattern starting; the REMstar unit achieved 10 cm H2O within 10 minutes. The SleepStyle 200, Sandman, and S8 units all increased pressure above 10 cm H2O, suggesting a pressure response to other nonapneic events present in the signal. The intelliPAP (DeVilbiss) unit responded less quickly, achieving a maximum therapy pressure of 5.5 cm H2O by the end of the 30-minute apnea session. The REMstar (Respironics) unit showed an instantaneous drop of 2 cm H2O to 8.5 cm H2O about 12 minutes into the apnea pattern, where it remained for the next 15 minutes. This is a programmed response by the device as its algorithm has recognized that there is no response from the “patient” to the sudden increases in pressure. The drop in pressure is to ensure the patient is not receiving too high a pressure in the event that the apnea is central in nature, possibly resulting in a Hering-Breuer reflex. The return to the initial pressure once normal breathing resumed varied for each unit with quicker reductions in pressures occurring on the Sandman and REMstar units compared to the more gradual pressure decreases on the intelliPAP, S8, and SleepStyle 200 units.

Flow Limitation. Both the SleepStyle 200 and S8 units showed a rapid response to the flow-limitation breathing pattern (Figure 2). The initial peak therapy pressures were achieved within 5 minutes of the change from normal breathing to flow-limited breathing. The intelliPAP, Sandman, and REMstar unit responded much differently, showing little change in pressures over the 30 minutes, with the IPAP AA unit not changing delivered pressure at all. Based on the REMstar algorithm’s search feature, the unit appeared to note an improved flow signal, as there were two instances of the device increasing pressure by 1.5 cm H2O and then dropping pressure by 0.5 cm H2O shortly thereafter during the flow-limited segment. Again, the return to the initial therapy pressure once normal breathing resumed varied for the two units that showed response to the flow-limitation pattern, though both were more gradual in nature.

Hypopnea. All devices exhibited a response to the change from normal to hypopneic breathing (Figure 3). Again, the SleepStyle 200 and S8 units showed a rapid response, quickly reaching initial peak pressures within 5 to 7 minutes. The intelliPAP, Sandman, and REMstar unit exhibited similar, steady rises in therapy pressure throughout the 30-minute session with maximum pressures between 10 and 12 cm H2O. As with the other two disordered patterns tested, the return to the initial therapy pressure once normal breathing resumed varied for each unit, with the intelliPAP, SleepStyle 200, and S8 units employing a more gradual decrease in therapy pressure compared to the Sandman and REMstar unit.

Except for the individual manufacturer, no one knows how these algorithms were determined, tested, and validated for effectiveness. The variability of product performance and the unknown aspect of how the algorithms determine response suggest that the manufacturers should publicly specify the characteristics of the abnormal flow pattern detection and the algorithms used by their devices to respond to the abnormal flow.4 The sophistication of the algorithm for each device may include individual responses to a combination of signals and snoring to determine how the device will respond. This study is limited in that the units tested were provided only one particular sample of each type of disordered breathing pattern for apnea, hypopnea, and flow limitation, and none of these included a snore signal. As all units feature snoring detection capabilities, the lack of a snoring component may have contributed to the minimal response on the part of the intelliPAP, REMstar, and Sandman units in the flow-limitation pattern.

Manufacturer Response

All five of the manufacturers of auto-adjusting devices included in our comparison were given the opportunity to respond to three principal questions regarding the companies’ approaches to algorithm design. All manufacturers, with the exception of Covidien, responded to the questions. The following is a summary of the responses:

Question 1: Please generally summarize the approach used in your auto-adjust CPAP machine’s algorithm to detect sleep-disordered breathing events?

  1. All four respondents indicate that the algorithm uses a proactive approach by analyzing the airway’s potential to collapse, prior to an event being detected. This is primarily based on the detection of flow limitation or snoring by utilizing “flow based analysis.”
  2. All four respondents indicate that the algorithm is able to identify flow limitation, snoring, hypopnea, and apnea.
  3. Only one respondent (Respironics) describes a goal of always delivering the lowest CPAP pressure possible.
  4. All four respondents indicate the algorithm is capable of distinguishing central versus obstructive events in order to avoid pressure increase for nonresponding events.

Question 2: How does your product respond once a sleep-disordered breathing event is detected?

  1. All four respondents indicate that if an event such as apnea or hypopnea is detected, it is assumed that the device will have already detected the potential for the events through flow-based analysis and a pressure above the minimum will already be in effect. The device will then respond with mild increases in pressure until the events cease.
  2. One of the four respondents describes a sliding 5-minute analysis window.
  3. One of the four respondents (Fisher & Paykel) indicates the algorithm’s ability to determine the severity level of a particular event, and the response of the device will vary depending on the severity of the event.
  4. Another (ResMed) describes a five-breath moving average analysis window.

Question 3: What was the basis or reference for incorporating the above-described approach to detecting and responding to sleep-disordered breathing events?

  1. One of the four respondents (Respironics) indicates that the algorithm is largely based on clinical understanding and practices used in titrating CPAP pressures during a PSG titration process.
  2. All four respondents indicate that research published throughout the 1990s is the principal reference used in developing the algorithm.

Unanswered Questions

All of the manufacturers’ responses were largely similar relating to approaches and strategies incorporated into their respective algorithms. If the algorithms are similar, why do the devices respond in such dramatically different ways to a fixed stimulus of sleep-disordered breathing events?

The manufacturer’s description of algorithms indicates that the intended use of the device is within a long-term therapeutic application. However, these devices are commonly used by sleep labs as “bed extenders” in situations where incomplete titration data was experienced during PSG testing. An average or mean pressure reported by the device after 2 or 3 weeks of home use is then used with a fixed pressure machine for long-term therapy. Should the devices be used in this manner considering the long-term therapy-based approaches used within the algorithms?

Why did the devices’ responses change over the 3-year interval of this testing. Were the changes based on new research? Should patients using the older algorithm upgrade?


Since their introduction more than 15 years ago, APAP units have become an increasingly popular option for use in sleep therapy. Used as both a means of improving patient compliance with therapy due to an overall lower mean airway pressure or an application as a bed extender for sleep labs to identify fixed therapeutic CPAP pressures, these units are seeing more utilization. Testing has revealed that the performance characteristics of the units are significantly different in response to various sleep-disordered breathing patterns. There is little comparative research on the use and benefits of the individual devices so the scientific evidence for the devices is lacking. How then does a clinician determine which brand to select for their patient populations? What questions does the clinician ask the manufacturer related to the product’s capabilities and the objective data to support marketing claims? Especially in light of the recent growth in unattended in-home sleep testing programs that utilize APAP units exclusively, these questions should certainly be considered and addressed. Clinicians working in the OSA arena need to meet patients’ expectation that they are being treated effectively with the best diagnostic process and effective therapy.

Robert McCoy, RRT, is managing director, and Ryan Diesem, BS, is the research associate, Valley Inspired Products Inc, Apple Valley, Minn. Todd Eiken, RPSGT, is technical director, Mays and Associates Inc, St Paul, Minn. For further information, contact [email protected].


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