DOI: 10.1378/chest.130.2.350 2006;130;350-361 Chest Schwaibold

DOI: 10.1378/chest.130.2.350 2006;130;350-361 Chest

Schwaibold, Daniel Navajas and Ramon Farré Jordi Rigau, Josep M. Montserrat, Holger Wöhrle, Diana Plattner, Matthias

Continuous Positive Airway Pressure DevicesBench Model To Simulate Upper Airway Obstruction for Analyzing Automatic

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Bench Model To Simulate Upper AirwayObstruction for Analyzing AutomaticContinuous Positive Airway PressureDevices*

Jordi Rigau, PhD; Josep M. Montserrat, MD; Holger Wohrle, MD;Diana Plattner, MS; Matthias Schwaibold, MS; Daniel Navajas, PhD; andRamon Farre, PhD

Background: Automatic positive airway pressure (APAP) devices are increasingly being used inpatients with obstructive sleep apnea. Some APAP devices present an unstable behavior whensubjected to some events or artifacts. The aims were to develop a bench model capable ofreproducing real flow, snoring, and obstructive patterns and to compare the response of APAPdevices based on flow and snoring with other devices using, in addition, the forced oscillationtechnique (FOT).Methods: The bench model subjected APAP devices to apneas with and without obstruction,obstructive hypopneas with and without snoring, periods of flow limitation, and artifacts such asleaks and mouth expiration.Results: Almost all the devices increased the pressure when subjected to apneas with obstruction,but at different rates. The time required by each device to reach 10 cm H2O ranged from 2.5 to13 min. In the presence of apneas without obstruction, all the devices based on flow and snoringincreased the pressure at the same rate as during apneas with obstruction. However, the devicesusing FOT did not modify the pressure. Four devices did not modify the pressure in the presenceof obstructive hypopneas, and all but one device increased the pressure in the presence ofsnoring. Mask leaks had little effect on the response of the devices, but four devices increased thepressure during mouth expiration artifacts.Conclusions: When, in addition to the flow and snoring signals, the measurement of the upperairway resistance is included, the accuracy of the event detection algorithms is improved.

(CHEST 2006; 130:350–361)

Key words: automatic positive airway pressure; bench testing; continuous positive airway pressure; forced oscillationtechnique; sleep apnea syndrome; upper airway obstruction

Abbreviations: APAP � automatic positive airway pressure; CPAP � continuous positive airway pressure;FOT � forced oscillation technique; OSAS � obstructive sleep apnea syndrome

C ontinuous positive airway pressure (CPAP) isconventionally used for the treatment of patients

with obstructive sleep apnea syndrome (OSAS).1 Theoptimal pressure required for treatment is deter-mined during polysomnography by progressively in-creasing the pressure until all the respiratory eventsare eliminated. This treatment modality has provedto be effective in most patients with compliance ratesat approximately 75%.2–5 Noncompliance is usuallyattributed to an absence of benefits and side effectssuch as rhinitis and leaks,5–9 associated in part withexcessive mask pressures.10,11

The automatic positive airway pressure (APAP)devices currently available on the market are aimedat improving compliance and diminishing the sideeffects during treatment. These devices reduce themean pressure during the night by automaticallyadjusting the applied pressure according to thechanging requirements of the patient.9,10,12–15 Al-though there is no clear evidence that favors treat-ment with APAP with respect to fixed CPAP inunselected OSAS patients,12 a number of stud-ies16–20 have suggested that APAP devices could besuitable for treating a subgroup of patients with

Original ResearchSLEEP MEDICINE

350 Original Research



different pressure demands during sleep dependingon body posture, on sleep stage, or on night-to-nightvariability. Moreover, these devices could also be auseful tool to automatically determine the optimalpressure level, both in attended and unattended

For editorial comment see page 312

settings19–25 and, hence, to avoid a costly CPAPtitration study and reduce hospital waiting lists.

However, the APAP devices present relevant dif-ferences. Some of them are not able to appropriatelyhandle some breathing patterns and may show anunstable behavior such as transient pressure in-creases when applied to patients. These instabilitiesare probably related to an incorrect identification ofevents and/or artifacts (such as leaks or mouthexpiration). Specifically, some devices are not able tocorrectly identify central and obstructive apneas.26,27

One reason for these shortcomings could be thatmost of the commercial APAP devices base theirdetection algorithms only on the analysis of the flowand/or snoring signals. These devices can easilyidentify apneas when no flow is detected. However,they are not able to differentiate apneas with upperairway closure (either with an obstructive or central

etiology) from central apneas without airway ob-struction, since they can only use one variable (flow)for detecting these events. Consequently, these de-vices usually increase the applied pressure regardlessof the central or obstructive nature of the apnea,which may demand different pressure levels.

To improve APAP, new devices have been devel-oped for a better detection of the phenomena thatoccur in the upper airway.28,29 To this end, someAPAP devices include, in addition to the analysis ofthe flow and snoring signal, technology such as theforced oscillation technique (FOT) for identifyingupper airway obstructions that may increase therobustness and accuracy of the detection algo-rithms.30,31 The FOT is based on superimposing anoscillatory pressure onto the spontaneous breathingof the patient. The quotient between the oscillatorypressure and the associated oscillatory flow mea-sured at the entrance of the respiratory systemprovides an overall measure of total respiratoryimpedance, which, when applied to patients withOSAS, is assumed to reflect upper airway paten-cy.31,32

We previously developed a bench model to test theperformance of several APAP devices in the presenceof different breathing patterns.33 However, given thatour first model was not able to simulate upper airwaypatency, it was not suitable for testing the new gener-ation of APAP devices that include technology toevaluate upper airway resistance. Therefore, the aim ofthis study was to develop a bench model that, inaddition to reproducing different flow patterns, is ableto simulate upper airway obstructions. To evaluate itsefficacy, the response of several APAP devices includ-ing FOT was evaluated when subjected to respiratoryevents with and without obstruction and/or snoring.Moreover, the behavior of APAP devices in the pres-ence of some artifacts such as leaks and mouth expira-tion was also analyzed. Their pressure response wascompared to that of conventional APAP devices basedonly on the flow signal.

Materials and Methods

Patient Simulator

We modified the patient simulator model previously de-scribed33 by including an obstructive valve servocontrolled by amotor (HS-325BB; Hitec RCD; Powey, CA). This new model wasable to reproduce not only real flow and snoring signals obtainedfrom patients’ recordings, but also the corresponding resistanceof the upper airway (Fig 1). A driving signal generated by thecomputer was fed into the analog servocontrol of the motor,which regulated the aperture of the valve. The obstructive valvewas able to modify the airway caliber according to obstructivepatterns similar to the patterns previously recorded in patients bymeans of the FOT applied simultaneously to CPAP during

*From Science and Research (Dr. Rigau, Ms. Plattner, and Mr.Schwaibold), Measure, Check & Control GmbH & Co. KG,Karlsruhe, Germany; Pneumologia, Hospital Clınic (Dr. Mont-serrat) and Unitat de Biofısica i Bioenginyeria, Facultat deMedicina (Drs. Navajas and Farre), Universitat de Barcelona,Institut d’Investigacions Biomediques August Pi i Sunyer, Bar-celona, Spain; and Department of Internal Medicine II (Dr.Wohrle), University Hospital, Ulm, Germany.Dr. Rigau was an employee of Measure, Check & Control GmbH& Co. KG at the time that this study was carried out and atpresent is employed by Sibel S.A. Josep M. Montserrat has nodeclared conflict of interest. Dr. Wohrle received 38.500 Eurosfrom Weinmann GmbH and 27.000 Euros from ResMed/MAPfor lectures between 2002 and 2004. Dr. Wohrle also received25.000 Euros from Measure, Check & Control GmbH & Co. KGas a research grant. Ms. Plattner is an employee of Measure,Check & Control GmbH & Co. KG. Mr. Schwaibold is anemployee of Measure, Check & Control GmbH & Co. KG. Dr.Navajas has no declared conflict of interest. Dr. Farre has nodeclared conflict of interest.This study was carried out in the Unitat de Biofısica i Bioeng-inyeria (Universat de Barcelona, Spain) within the framework ofa research contract with Measure, Check & Control GmbH &Co. KG.This work was supported in part by Measure, Check & ControlGmbH & Co. KG, by Fondo de Investigacion Sanitaria (V-2003-RED C11 F-O), and by Ministerio de Ciencia y Tecnologıa(SAF2002–03616 and SAF2004–00684).Manuscript received November 8, 2005; revision accepted Feb-ruary 17, 2006.Reproduction of this article is prohibited without written permissionfrom the American College of Chest Physicians (www.chestjournal.org/misc/reprints.shtml).Correspondence to: Ramon Farre, PhD, Unitat de Biofısica iBioenginyeria, Facultat de Medicina, Universitat de Barcelona,Barcelona, Spain; e-mail: [email protected]: 10.1378/chest.130.2.350

www.chestjournal.org CHEST / 130 / 2 / AUGUST, 2006 351



titration studies.34 The simulator included a flow generator33 thatwas synchronized with the obstructive valve for reproducingincreases in airway resistance during periods of obstructiveapneas or hypopneas (ie, upper airway obstruction could bemodified within the respiratory cycle). Thus, different degrees ofairway obstruction could be reproduced during apneas, hypopneas,or periods of flow limitation. Moreover, to assess the capability of theAPAP devices in detecting typical artifacts such as mouth expiration,a second breathing circuit, in parallel to the main circuit, allowed usto simulate the mouth-breathing route. An additional exhalationvalve in this second breathing circuit was electronically controlledand synchronized with the flow generator and the obstructive valve.During mouth expiration, the exhalation valve was opened allowingthe expired air to flow through the simulated mouth route while theobstructive valve was closed. A leak valve was used for simulatingleaks through the mask usually found during conventional CPAPtreatment in patients.

Patterns of Disturbed Breathing

Two different sets of flow patterns were used for evaluating theAPAP devices. The first set consisted of six different flow eventsbased on the concatenation of identical breathing cycles previouslyrecorded in patients. Five of these events were previously de-scribed.33 For each flow pattern, the obstructive valve generated aspecific pattern of airway obstruction with the appropriate magni-tude and duration for simultaneously mimicking the obstructivephenomena and the flow shapes observed in typical OSAS respira-tory events. The flow and obstructive patterns were combined forgenerating seven different breathing patterns shown in Figure 2.During normal breathing with a breathing frequency of 16 breaths/min, the obstructive valve was completely open showing low valuesof airway resistance (Fig 2, panel A). Two different types of apneawith exactly the same flow shape were reproduced. Panel B inFigure 2 shows an apnea with obstruction in which high resistance

values (approximately 75 cm H2O�s/L) were reproduced during theperiod without breathing flow. Another apnea without obstruction,as indicated by normal low values in the resistance signal during theabsence of breathing flow, is shown in panel C (Fig 2). Panel Dshows a pattern of mild obstructive hypopnea with transient in-creases in airway resistance (approximately 20 cm H2O�s/L) onlyduring the inspiratory portion of the flow-limited breathing cycles(corresponds to hypopnea with a U-shaped inspiratory flow contourand a tidal volume equal to 70% of the normal cycle, in our previouswork33 with a flow limitation index21 of 0.12). The same flow andobstructive pattern, but with the addition of snoring vibrationssynchronized with the inspirations, is shown in panel E (Fig 2). PanelF shows a prolonged period of flow limitation (based on hypopneawith a square-like inspiratory flow contour and a tidal volume equalto 70% of the normal cycle, from our previous study33 with flowlimitation index of 0.01) with mild transient increases in resistance(approximately 15 cm H2O�s/L) during inspirations. Finally, a pat-tern of mouth expiration characterized by an absence of expiratoryflow with a simultaneous increase in airway resistance duringexpiration is shown in panel G (Fig 2).

A second set of five breathing patterns was used to simulate thebehavior of a real patient when subjected to treatment with an APAPdevice (Fig 3). These breathing patterns (based on previouslydescribed flow patterns33) were complete events recorded in pa-tients. Therefore, they reflect the within cycle variability found inreal events (the flow amplitude and shape progressively changedalong the event). Panel A in Figure 3 shows the same normalbreathing pattern as panel A in Figure 2. A prolonged period of flowlimitation and a mild hypopnea with transient mild increases inresistance during inspirations are shown in panels B and C, respec-tively. Panel D shows a severe hypopnea with a progressive reduc-tion in tidal volume accompanied by progressive transient increasesin resistance during inspirations. Finally, panel E shows an apneawith obstruction (approximately 70 cm H2O�s/L). Neither of thesebreathing patterns included snoring.

Figure 1. Diagram of the patient simulator. PNT � pneumotachograph; EP � exhalation port;Obstr � obstructive; Exhal � exhalation; V’ � flow; M � motor; P � pressure (modified from Rigau et al34).




APAP Devices Evaluated

Single units of 10 different APAP devices currently available onthe market were evaluated with this bench test. The only criterionused when selecting the devices was their availability. Seven of thesedevices based their detection algorithms on the analysis of the flowshape and the high frequency vibrations produced in the airwaysduring snoring: PV10i (Breas AB; Molnlycke Sweden); AutosetSpirit (Resmed; North Ryde, NSW, Australia); REMstar auto(Respironics; Murrysville, PA); Delphinus (Jaeger Tonnies/ViasysHealthcare; Hochberg, Germany); Magellan (MAP; Munich, Ger-many); Goodknight 420E (Puritan Bennett/Tyco Healthcare; Pleas-anton, CA); and Horizon LT plus (DeVilbiss/Sunrise Medical;Carlsbad, CA). These seven devices were designated as F1, F2, F3,F4, F5, F6, and F7. The three other devices, in addition to theanalysis of the flow shape and snoring, included a simplified methodof FOT for detecting increases in airway resistance. The Autoset IIPlus (Resmed) generated a small pressure oscillation (5 Hz) when nobreathing flow was detected in the course of 8 s. This capabilityallowed the device to detect obstructions only during apneas. TheSOMNOsmart 2 (Weinmann GmbH; Hamburg, Germany) and

Figure 2. Breathing patterns recorded at the nasal side of thebench model during tests 1 to 7 (panel A through panel G). Flowis given as liters per second, and resistance is given as centimetersof water per liter per second.

Figure 3. Breathing patterns reproduced by the simulatedOSAS patient during test 8 depending on the CPAP level appliedby APAP devices (panel A through panel E). Flow is given asliters per second, and resistance is given as centimeters of waterper liter per second.




the AutoTREND (Hoffrichter GmbH; Schwerin, Germany)detected airway obstructions by continuously generating asmall pressure oscillation at 20 Hz during the whole treatment.These three devices were identified as O1, O2, and O3,respectively. All devices were programmed with a minimumCPAP of 4 cm H2O, a maximum CPAP of 16 cm H2O, aninitial CPAP level of 4 cm H2O, and the initial waiting time orthe ramp period were set to 0 min (when possible). All theother parameters were set at their default value.

Measurement ProtocolEach APAP device was connected to the patient simulator with

its own tubing. A Whisper Swivel valve (Respironics) was used for

all the devices, except for the SOMNOsmart 2, in which case itsown exhalation port was used. At the beginning of each test run,the patient simulator reproduced normal breathing for at least 5min (or the minimum initial time required by each device), andthe maximum duration of the tests was 20 min. Eight indepen-dent test runs were performed based on the reproduction of theflow and obstructive patterns described above. Tests 1 and 2 (Fig4 and 5, respectively) were based on the continuous repetition ofapneas with and without obstruction, respectively (patterns frompanels B and C in Fig 2). During tests 3 and 4, the simulatorcontinuously reproduced hypopneic obstructive events withoutand with snoring (patterns from panels D and E in Fig 2),respectively. Tests 5 and 6 were based on a prolonged period of

Figure 4. Pressure response of the different APAP devices when subjected to a continuous repetitionof apneas with obstruction (test 1). R � resistance. Dashed horizontal lines indicate 10 cm H2O. SeeFigure 1 for expansion of abbreviation.




flow limitation and of mouth expiration artifact (panels F and Gin Fig 2, respectively) for � 10 min. To assess the effect of airleaks in the response of the devices, test 7 was based on apneaswith obstruction with the addition of a leak (0.5 L/s at 4 cm H2O).Finally, test 8 was based on the simulation of a real patient whensubjected to APAP treatment. To this end, a closed loop wasprogrammed in the simulator in order to change the breathingpattern of the simulated patient depending on the pressureapplied by each APAP device. As shown in Figure 3, thesimulated patient experienced apneas with obstruction when theCPAP level was � 5 cm H2O (panel E), severe hypopneas when

the pressure was between 5 and 7 cm H2O (panel D), mildhypopneas between 7 and 10 cm H2O (panel C), and prolongedperiods of flow limitation between 10 and 12 cm H2O (panel B).Finally, when the pressure applied by the devices was � 12 cmH2O, the breathing pattern of the simulated patient was normal-ized and the simulator reproduced normal breathing (panel A).

Before each test run, the devices were switched off and on forresetting. For each device, the profile of the pressure appliedduring each test was evaluated. Moreover, the speed of pressureincrease during tests 1 to 7 was assessed by measuring the timerequired for each device to reach a certain threshold. Some

Figure 5. Pressure response of the different APAP devices when subjected to a continuous repetitionof apneas without obstruction (test 2). Dashed horizontal lines indicate 10 cm H2O. See Figures 1 and4 for expansion of abbreviations.




APAP devices included a pressure limit above which they did notfurther increase the pressure during apneas. Since most of thesedevices had this pressure limit at � 10 cm H2O, this value wasselected as the CPAP threshold for measuring the speed ofpressure increase. The response of each APAP device during test8 was analyzed by evaluating its capability of normalizing thebreathing pattern of the simulated patient and by measuring themaximum pressure applied by each device and the time to reachthis pressure. Institutional review board approval was not neces-sary in this study because it did not involve human subjects.

Results

Tables 1 and 2 summarize the results of thedifferent tests. The responses of the devices whensubjected to a continuous repetition of apneas withobstruction (test 1, panel B in Fig 2) are shown inFigure 4. All the devices increased the pressure inresponse to this event but with different strategies.Devices F1, O1, and O3 increased the pressurelinearly up to the maximum pressure allowed butwith different speeds. Devices F2, F3, F4, F5, andO2 increased the pressure stepwise with differencesin magnitude and duration. Some of these devicesreached a plateau at different pressure levels de-pending on each APAP device. Device F6 increasedthe pressure linearly up to 10 cm H2O, and deviceF7 did not increase the pressure. The time requiredfor each device to reach a pressure level of 10 cmH2O is shown in Table 1. When the APAP deviceswere subjected to the test with an identical flowpattern of apneas but without obstruction (test 2,panel C in Fig 2), all the devices based on the flowshape and snoring (F1 to F7) showed exactly thesame pressure response as during the test sequenceof apneas with obstruction (Fig 5, Table 1). How-ever, the three devices that included the oscillatory

detection of increases in airway resistance (O1 to O3)did not increase the applied pressure. Therefore, asexpected, their pressure response was modified de-pending not only on the events detected with theflow signal, but also on the presence or absence ofobstructions during these apneic events.

Table 1 also shows the results for tests 3 to 7.Devices F1, F3, F4, F5, F7, and O2 increased thepressure at different rates in the presence of obstruc-tive hypopneas (test 3, panel D in Fig 2), but devicesF2, F6, O1, and O3 did not modify the appliedpressure. All the devices except O3 increased thepressure in the presence of obstructive hypopneaswith snoring (test 4, panel E in Fig 2), some of them(devices F2, F3, F6, F7, and O1) with a higher speedthan without snoring. Those devices responding tosnoring, except F5, increased the pressure up to themaximum level allowed (16 cm H2O). During aprolonged period of flow limitation (test 5, panel F inFig 2), devices F2, F4, F5, O1, and O2 increased thepressure at different rates while the other devicesdid not show any response. When the APAP deviceswere subjected to a period of mouth expiration (test6, panel G in Fig 2), six of the devices (F2, F3, F5,F6, F7, and O2) did not increase the applied pres-sure. However, four devices (F1, F4, O1, and O3)detected this artifact as an event and increased thepressure above 10 cm H2O. During the test in thepresence of leaks (test 7), devices F3, F4, F5, F7,O2, and O3 modified their response during apneaswith obstruction. Devices F3 and O2 increased thepressure more rapidly than without leaks, devices F4and O3 slightly reduced the speed of pressureincrease, F5 did not modify the pressure and F7increased the pressure up to 5 cm H2O. The other

Table 1—Response of the APAP Devices to the Different Breathing Patterns*

Devices

Test 1,Apnea WithObstruction

Test 2,Apnea Without

Obstruction

Test 3,Mild

Hypopnea

Test 4,Mild HypopneaWith Snoring

Test 5,Prolonged Flow

LimitationWith Obstruction

Test 6,Mouth

Expiration

Test 7,Apnea WithObstructionWith Leaks

F1 8 8 7.5 7.5 NR 2.5 8F2 7 7 NR 0.5 7 (9.5) NR 7F3 11 11 9 5.5 NR NR 9†F4 4.5 4.5 4.5 4.5 3.5 15 6F5 5 5 4 (9) 5 2.5 (8.5) NR NRF6 3 3 NR 5.5 NR NR 3F7 NR NR 11 (7) 6 NR NR 6 (5)O1 4 NR NR 0.5 2.5 3 4†O2 13 NR 7.5 8.5 7.5 (9) NR 9O3 2.5 NR NR NR NR 1.5 3

*Values indicate the time in minutes required to reach 10 cm H2O for the APAP devices in each test run. When the 10 cm H2O threshold wasnot reached, the time to reach the maximum applied pressure indicated. This maximum pressure is shown in parentheses. NR � no pressureresponse.

†The leak level during these tests was 0.25 L/s instead of the 0.5 L/s used in the other tests.




devices were not affected by the presence of leaksand increased the pressure with the same rate as inthe test without leaks.

Table 2 and Figure 6 show the results of test 8based on the closed loop model. Only three devices(F1, F4, and O2) were able to do the following: (1)correctly identify the respiratory events, (2) increasethe pressure above 12 cm H2O, and consequently (3)normalize the breathing pattern of the simulatedpatient (Table 2). Devices F1 and F4 increased thepressure up the 14 cm H2O in 5.5 min and 7 min,respectively. Device O2 increased the pressure up to12 cm H2O in 14.5 min. The other devices did notincrease the pressure above 12 cm H2O (the optimalCPAP level for this simulated patient) and, there-fore, were not able to normalize the breathingpattern of the patient. Figure 6 shows the responseof two APAP devices (F4 in top panel and O1 in

bottom panel) when subjected to the simulatedpatient. Device F4 was able to recognize all theevents (apneas, severe and mild hypopneas, andprolonged periods of flow limitation) and increasedthe pressure until these events were abolished andthe breathing pattern of the simulated patient wasnormalized. After 3 min of normal breathing, thedevice started to reduce the pressure slowly until thepressure was � 12 cm H2O and flow limitationreappeared (indicated by an arrow in Fig 6). At thispoint, the device increased the pressure again up to14 cm H2O and the simulated patient resumednormal breathing. Device O1 recognized apneic andhypopneic events but did not identify prolonged flowlimitation as an event. Therefore, the device in-creased the pressure only up to 11 cm H2O, and thebreathing pattern of the patient was not completelynormalized.

Table 2—Response of the APAP Devices When Subjected to a Simulated Patient Based on a Closed Loop Model*

Variables F1 F2 F3 F4 F5 F6 F7 O1 O2 O3

Normalize Yes No No Yes No No No No Yes NoMaximum pressure, cm H2O 14 11.5 11 14 6.5 10 4 11 12 6Time to reach maximum pressure, min 5.5 2.5 22 7 1.5 14 NA 5.2 14.5 7

*Normalize � ability of the device to normalize the breathing pattern of the simulated patient; NA � not applicable.

Figure 6. Flow patterns and pressure response of APAP devices F4 (top panel) and O1 (bottom panel)when subjected to a simulated OSAS patient during test 8 (flow is given as liters per second; pressureis given as centimeters of water). Tracings on the right side of the flow subpanels indicate the breathingpattern at the maximum CPAP level applied by the devices. Dashed lines at 12 cmH2O indicate theoptimal CPAP level for the simulated patient. See Figure 1 for expansion of abbreviations.




Discussion

In this study, we developed a bench model tomimic not only the flow and snoring patterns ob-tained from patients, but also the correspondingupper airway obstruction. With this model, the newgeneration of APAP devices can be tested in thebench by reproducing the phenomena that occur inobstructive events. We analyzed several APAP de-vices capable of detecting the upper airway patencywith respect to other devices based only on theanalysis of the flow shape and snoring. The responsesof the devices when subjected to the same pattern ofapneas were considerably different. As expected, thedevices that were not based on detecting increases inairway resistance responded to apneas without ob-struction with the same pressure profile as to apneaswith obstruction. However, the devices based on theforced oscillation technique modified their pressureresponse depending on the presence or absence ofairway obstruction. The responses of the devices inthe presence of obstructive hypopneas, prolongedflow limitation and snoring were also different. Ar-tifacts such as mouth expiration caused a relevantpressure increase in some devices.

APAP devices have been evaluated both in clinicalstudies and on the bench.10,13,33,35–37 Although somestudies38–41 in the literature report a brief descrip-tion of the algorithms implemented in the firstprototypes of APAP devices, the most recent ver-sions of these devices probably include a number oftechnical modifications that may be relevant in theirclinical application. Clinical studies performed todate allow the evaluation of these devices in realsituations. However, these tests cannot be per-formed under systematic and well-controlled condi-tions because of the intersubject and intrasubjectvariabilities. Bench tests have been recently used toevaluate the response of APAP devices when sub-jected to different breathing patterns.33,37 Althoughbench tests and clinical studies are both useful andshould be considered complementary when evaluat-ing a specific device,42 this study was focused ondeveloping a patient simulator for the bench evalu-ation of APAP devices.

Several approaches have been used to simulate onthe bench the typical events found in patients withOSAS.32,43 Our earlier patient simulator model wasable to reproduce any flow and snoring patternrecorded in patients,33 but it did not include airwayobstructions. The inclusion of a servocontrolled ob-structive valve in our patient simulator allowed us toreproduce any pattern of airway resistance synchro-nized with the flow pattern and previously recordedin real patients by using the forced oscillation tech-nique.34 Other authors have used a Starling resistor

to generate increased upper airway resistance duringsimulated sleep respiratory events.37 Although thismodel is able to reproduce the pathophysiology ofobstructive sleep apnea, it does not allow us toprecisely reproduce flow shapes previously recordedin patients. The shape of the inspiratory flow using aStarling resistor is indirectly controlled by thebreathing effort, the pressure surrounding the col-lapsible tube and the elastic properties of the tubewall. Since most of the APAP devices base theirdetection algorithms on the analysis of the flowshape, it is crucial to precisely mimic the flowmorphology and to independently control the degreeof airway obstruction for evaluating the devices inexactly the same conditions in terms of the physio-logic signals. This independent control of the flowshape and the obstructive pattern allowed us toadjust the degree of obstruction during the eventsand to reproduce artifacts such as mouth expiration.Moreover, a closed loop model could be pro-grammed in our simulator for reproducing thechanges in the breathing pattern of a patient withOSAS during APAP application.

For the specific purpose of this study, two sets ofbreathing patterns were used to evaluate the APAPdevices. In the first set, the flow patterns of apneawith and without obstruction were exactly the same,with the result that the only difference was thedegree of airway obstruction generated by the ob-structive valve. The breathing cycles prior to theapneic periods did not include any volume reduc-tion, flow limitation, or snoring in order to avoidinterference of these phenomena in the detection ofapneas by the APAP devices. The degree of airwayobstruction during the inspiratory phase of the flow-limited cycles in obstructive hypopneas and pro-longed flow limitation was adjusted to reproduce theincreases in resistance observed in patient stud-ies30,31 during the simultaneous application of CPAPand FOT. Hypopneas with snoring reproducedevents in typical OSAS patients, whereas hypopneaswithout snoring could simulate patients in whomuvulopalatopharyngoplasty only succeeded in elimi-nating snoring. During the mouth expiration pattern,the obstructive valve reproduced high transient in-creases in airway resistance while expiration to sim-ulate the closure of the nasal pathway during mouthexpiration,44 allowing us to evaluate the devices notonly when subjected to typical respiratory events butalso in the presence of artifacts. The second set ofbreathing patterns was used to simulate a morerealistic situation, reproducing the within cycle vari-ability in each event observed in patients. Since mostdevices showed a high sensitivity to snoring, thisphenomenon was not included in this set of patternsin order to evaluate the devices in a more challeng-




ing situation. Many other breathing patterns andartifacts (eg, mixed apneas, other flow limitedshapes, and coughing) could be programmed in thepatient simulator to extensively evaluate APAP de-vices. However, since this was not our aim, we useda fixed number of patterns covering the most usualevents found in patients with OSAS.

As previously described in other studies,33,37 ourresults confirm the existence of a great variability inthe response of APAP devices when subjected to thesame breathing patterns under well-controlled con-ditions. Regardless of the speed of the pressureincrease during apneas, the response of the devicesthat based their detection algorithms exclusively onthe analysis of the flow shape and snoring was thesame in the presence of apneas with and withoutobstruction. However, the devices that included thedetection of increases in resistance changed theirpressure reaction according to the presence or ab-sence of airway obstructions. Most APAP devicesbased on flow and snoring included a certain pres-sure threshold above which no pressure increase wasapplied in the presence of apneas. Accordingly, ifapneas without obstruction had been applied at highpressure levels, neither device would have increasedthe pressure. The response of the devices whensubjected to obstructive hypopneas or persistent flowlimitation showed the full spectrum of possiblepressure reactions, from devices increasing the pres-sure up to the maximum level allowed at a highspeed to devices that did not modify the pressure.The more rapid increase in pressure in the majorityof the devices during snoring demonstrates a highsensitivity of these devices to this event. Thesedifferent event detection algorithms and pressureregulation strategies may have an influence when thedevices are used both for treatment (some devicesmay allow an excessive number of residual events)and for titration of CPAP (with the possibility of aninappropriate optimal pressure level recommendedby the device). Indeed, when these devices weresubjected to a simulated patient in whom the breath-ing pattern changed according to the pressure ap-plied, only three devices were able to normalize thebreathing pattern of the simulated patient. It shouldbe noted, however, that the closed loop tests werecarried out in nonsnoring breathing patterns. In thelight of the open loop results (Table 1), it is expectedthat more APAP devices would normalize breathingif snoring was included in the closed loop patterns. Itis also worth noting that, although the presence ofleaks does not have a major influence on the pres-sure reaction of the devices, four devices increasedthe pressure significantly in the presence of artifactssuch as mouth expiration. These inappropriate pres-sure increases reduce the robustness of these devices

and might induce low compliance in some patientsduring treatment with APAP45 or elevated recom-mended pressures during unattended automatic ti-tration of CPAP.

The differences in the response of APAP devicesraise the question of the best strategy for modifyingthe pressure in the presence of a specific respiratoryevent. While it is currently accepted that duringobstructive apneas the pressure applied to the pa-tient should be increased, the pressure strategy is notwell established in the presence of a central apnea.One reason for this is that central apnea can occurwith the upper airway closed or open.46 Some de-vices limit their pressure increases in the presence ofapneas to avoid an excessive pressure during centralapneas (devices F2 to F6). APAP devices using FOTor other techniques such as the detection of cardiacoscillations47,48 may be useful when central apneasoccur with the open airway. However, the effective-ness of these techniques measuring upper airwayresistance will depend on the correct application ofthis technology (eg, increases in airway resistanceshould be taken into account only during inspirationto avoid a pressure increase during mouth expira-tion), and on the percentage of central events withthe upper airway open in each patient. Therefore,although the best pressure strategy should be deter-mined with clinical studies instead of with a simplebench test, our findings reveal that an accurateidentification of sleep respiratory disturbances isnecessary for determining the suitable pressuretreatment for the patient.

The conventional methods for detecting and clas-sifying respiratory events during polysomnographyare based on the analysis of at least two independentphysiologic variables (flow and thoracoabdominalbands and/or oxygen desaturation and/or arousal).49

This strategy enables us to assess the concordancebetween two or more signals. Since the main aim ofthe APAP devices during treatment is to normalizethe breathing pattern with minimal side effects, theirdetection algorithms should be focused on improv-ing their reliability and specificity, on correctly clas-sifying the respiratory disturbances, and on reinforc-ing their robustness in the presence of artifacts. Tothis end, we strongly believe that APAP devicesshould base their detection algorithms on more thanone independent variable, as in polysomnography, inorder to analyze the concordance of the differentmeasured signals, which would allow us to betterdetect and characterize respiratory events.

The differences between commercial APAP de-vices found in this work and other studies demand aconsensus on APAP technology. This agreementcould include the following: (1) a basic definition ofthe characteristics of APAP devices (number and




type of measured signals), (2) the detection algo-rithms (detailed definition of events and artifacts),(3) the strategies to modify nasal pressure, and (4)the methodology for evaluation. This consensus, as inthe case of spirometry,50 in which repetitive clinicalor bench evaluations of devices are not required,would considerably improve the technology andoutcomes of APAP.

In conclusion, this bench study shows that the useof flow and airway resistance signals as independentphysiologic variables to identify respiratory eventscan improve the detection algorithms of APAP de-vices. Consequently, when this strategy is appropri-ately used (ie, the information obtained from FOTmeasurements are correctly interpreted), it couldallow a more suitable correction of the respiratoryevents during APAP application. Bench studiesshould be followed by clinical studies to ascertain theclinical usefulness of measuring airway obstructionduring APAP application.

ACKNOWLEDGMENT: The authors thank Mr. M. A. Rodri-guez for technical assistance.

References1 Sullivan CE, Issa FG, Berthon-Jones M, et al. Reversal of

obstructive sleep apnoea by continuous positive airway pres-sure applied through the nares. Lancet 1981; 1:862–865

2 Collard P, Pieters T, Aubert G, et al. Compliance with nasalCPAP in obstructive sleep apnea patients. Sleep Med Rev1997; 1:33–44

3 Reeves-Hoche MK, Meck R, Zwillich CW. Nasal CPAP: anobjective evaluation of patient compliance. Am J Respir CritCare Med 1994; 149:149–154

4 Krieger J, Sforza E, Petiau C, et al. Simplified diagnosticprocedure for obstructive sleep apnoea syndrome: lowersubsequent compliance with CPAP. Eur Respir J 1998;12:776–779

5 McArdle N, Devereux G, Heidarnejad H, et al. Long-termuse of CPAP therapy for sleep apnea/hypopnea syndrome.Am J Respir Crit Care Med 1999; 159:1108–1114

6 Engleman HM, Martin SE, Douglas NJ. Compliance withCPAP therapy in patients with the sleep apnoea/hypopnoeasyndrome. Thorax 1994; 49:263–266

7 Meurice JC, Dore P, Paquereau J, et al. Predictive factors oflong-term compliance with nasal continuous positive airwaypressure treatment in sleep apnea syndrome. Chest 1994;105:429–433

8 Pepin JL, Leger P, Veale D, et al. Side effects of nasalcontinuous positive airway pressure in sleep apnea syndrome:study of 193 patients in two French sleep centers. Chest 1995;107:375–381

9 Randerath WJ, Galetke W, Ruhle KH. Auto-adjusting CPAPbased on impedance versus bilevel pressure in difficult-to-treat sleep apnea syndrome: a prospective randomized cross-over study. Med Sci Monit 2003; 9:CR353–CR358

10 Massie CA, McArdle N, Hart RW, et al. Comparison betweenautomatic and fixed positive airway pressure therapy in thehome. Am J Respir Crit Care Med 2003; 167:20–23

11 Hukins C. Comparative study of autotitrating and fixed-pressure CPAP in the home: a randomized, single-blindcrossover trial. Sleep 2004; 27:1512–1523

12 Ayas NT, Patel SR, Malhotra A, et al. Auto-titrating versusstandard continuous positive airway pressure for the treat-ment of obstructive sleep apnea: results of a meta-analysis.Sleep 2004; 27:249–253

13 Behbehani K, Yen FC, Lucas EA, et al. A sleep laboratoryevaluation of an automatic positive airway pressure system fortreatment of obstructive sleep apnea. Sleep 1998; 21:485–491

14 Teschler H, Wessendorf TE, Farhat AA, et al. Two monthsauto-adjusting versus conventional nCPAP for obstructivesleep apnoea syndrome. Eur Respir J 2000; 15:990–995

15 d’Ortho MP, Grillier-Lanoir V, Levy P, et al. Constant vs.automatic continuous positive airway pressure therapy: homeevaluation. Chest 2000; 118:1010–1017

16 Oksenberg A, Silverberg DS, Arons E, et al. The sleep supineposition has a major effect on optimal nasal continuouspositive airway pressure: relationship with rapid eye move-ments and non-rapid eye movements sleep, body mass index,respiratory disturbance index, and age. Chest 1999; 116:1000–1006

17 Series F, Marc I. Importance of sleep stage- and bodyposition-dependence of sleep apnoea in determining benefitsto auto-CPAP therapy. Eur Respir J 2001; 18:170–175

18 Penzel T, Moller M, Becker HF, et al. Effect of sleep positionand sleep stage on the collapsibility of the upper airways inpatients with sleep apnea. Sleep 2001; 24:90–95

19 Noseda A, Kempenaers C, Kerkhofs M, et al. Constant vsauto-continuous positive airway pressure in patients withsleep apnea hypopnea syndrome and a high variability inpressure requirement. Chest 2004; 126:31–37

20 Marrone O, Insalaco G, Bonsignore MR, et al. Sleep struc-ture correlates of continuous positive airway pressure varia-tions during application of an autotitrating continuous posi-tive airway pressure machine in patients with obstructivesleep apnea syndrome. Chest 2002; 121:759–767

21 Teschler H, Berthon-Jones M, Thompson AB, et al. Auto-mated continuous positive airway pressure titration for ob-structive sleep apnea syndrome. Am J Respir Crit Care Med1996; 154:734–740

22 Lloberes P, Ballester E, Montserrat JM, et al. Comparison ofmanual and automatic CPAP titration in patients with sleepapnea/hypopnea syndrome. Am J Respir Crit Care Med 1996;154:1755–1758

23 Stradling JR, Barbour C, Pitson DJ, et al. Automatic nasalcontinuous positive airway pressure titration in the laboratory:patient outcomes. Thorax 1997; 52:72–75

24 Series F. Accuracy of an unattended home CPAP titration inthe treatment of obstructive sleep apnea. Am J Respir CritCare Med 2000; 162:94–97

25 Masa JF, Jimenez A, Duran J, et al. Alternative methods oftitrating continuous positive airway pressure: a large multi-center study. Am J Respir Crit Care Med 2004; 170:1218–1224

26 Berry RB, Parish JM, Hartse KM. The use of auto-titratingcontinuous positive airway pressure for treatment of adultobstructive sleep apnea. Sleep 2002; 25:148–173

27 Boudewyns A, Van de Heyning P, De Backer W. Appearanceof central apnoea in a patient treated by auto-CPAP forobstructive sleep apnoea. Respir Med 1998; 92:891–893

28 Ficker JH, Fuchs FS, Wiest GH, et al. An auto-continuouspositive airway pressure device controlled exclusively by theforced oscillation technique. Eur Respir J 2000; 16:914–920

29 Randerath WJ, Schraeder O, Galetke W, et al. AutoadjustingCPAP therapy based on impedance efficacy, compliance andacceptance. Am J Respir Crit Care Med 2001; 163:652–657

30 Badia JR, Farre R, Montserrat JM, et al. Forced oscillationtechnique for the evaluation of severe sleep apnoea/hypop-




noea syndrome: a pilot study. Eur Respir J 1998; 11:1128–1134

31 Navajas D, Farre R, Rotger M, et al. Assessment of airflowobstruction during CPAP by means of forced oscillation inpatients with sleep apnea. Am J Respir Crit Care Med 1998;157:1526–1530

32 Farre R, Peslin R, Rotger M, et al. Inspiratory dynamicobstruction detected by forced oscillation during CPAP: amodel study. Am J Respir Crit Care Med 1997; 155:952–956

33 Farre R, Montserrat JM, Rigau J, et al. Response of automaticcontinuous positive airway pressure devices to different sleepbreathing patterns: a bench study. Am J Respir Crit CareMed 2002; 166:469–473

34 Rigau J, Farre R, Montserrat JM, et al. Patient simulator toreproduce the airway obstruction events found in the sleepapnea-hypopnea syndrome (SAHS) [abstract]. Eur Respir J2003; 22:181s

35 Lofaso F, Lorino AM, Duizabo D, et al. Evaluation of anauto-nCPAP device based on snoring detection. Eur Respir J1996; 9:1795–1800

36 Senn O, Brack T, Matthews F, et al. Randomized short-termtrial of two autoCPAP devices versus fixed continuous posi-tive airway pressure for the treatment of sleep apnea. Am JRespir Crit Care Med 2003; 168:1506–1511

37 Abdenbi F, Chambille B, Escourrou P. Bench testing ofauto-adjusting positive airway pressure devices. Eur Respir J2004; 24:649–658

38 Gugger M, Mathis J, Bassetti C. Accuracy of an intelligentCPAP machine with in-built diagnostic abilities in detectingapnoeas: a comparison with polysomnography. Thorax 1995;50:1199–1201

39 Berthon-Jones M, Lawrence S, Sullivan CE, et al. Nasalcontinuous positive airway pressure treatment: current reali-ties and future. Sleep 1996; 19:S131–S135

40 Scharf MB, Brannen DE, McDannold MD, et al. Comput-

erized adjustable versus fixed NCPAP treatment of obstruc-tive sleep apnea. Sleep 1996; 19:491–496

41 Sharma S, Wali S, Pouliot Z, et al. Treatment of obstructivesleep apnea with a self-titrating continuous positive airwaypressure (CPAP) system. Sleep 1996; 19:497–501

42 Montserrat JM, Farre R, Navajas D. Automatic continuouspositive airway pressure devices for the treatment of sleepapnea hypopnea syndrome. Sleep Med 2001; 2:95–98

43 Reisch S, Steltner H, Timmer J, et al. Early detection ofupper airway obstructions by analysis of acoustical respiratoryinput impedance. Biol Cyber 1999; 81:25–37

44 Badia JR, Farre R, Kimoff RJ, et al. Clinical application of theforced oscillation technique for CPAP titration in the sleepapnea/hypopnea syndrome. Am J Respir Crit Care Med 1999;160:1550–1554

45 Bachour A, Maasilta P. Mouth breathing compromises adher-ence to nasal continuous positive airway pressure therapy.Chest 2004; 126:1248–1254

46 Badr MS, Toiber F, Skatrud JB, et al. Pharyngeal narrowing/occlusion during central sleep apnea. J Appl Physiol 1995;78:1806–1815

47 Morrell MJ, Badr MS, Harms CA, et al. The assessment ofupper airway patency during apnea using cardiogenic oscilla-tions in the airflow signal. Sleep 1995; 18:651–658

48 Ayappa I, Norman RG, Rapoport DM. Cardiogenic oscilla-tions on the airflow signal during continuous positive airwaypressure as a marker of central apnea. Chest 1999; 116:660–666

49 American Academy of Sleep Medicine Task Force. Sleep-related breathing disorders in adults: recommendations forsyndrome definition and measurement techniques in clinicalresearch: the Report of an American Academy of SleepMedicine Task Force. Sleep 1999; 22:667–689

50 Miller MR, Hankinson J, Brusasco V, et al. Standardisation ofspirometry. Eur Respir J 2005; 26:319–338




DOI: 10.1378/chest.130.2.350 2006;130;350-361 Chest

Schwaibold, Daniel Navajas and Ramon Farré Jordi Rigau, Josep M. Montserrat, Holger Wöhrle, Diana Plattner, Matthias

Continuous Positive Airway Pressure DevicesBench Model To Simulate Upper Airway Obstruction for Analyzing Automatic

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DOI: 10.1378/chest.130.2.350 2006;130;350-361 Chest Schwaibold