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(Chest. 2001;120:170-176.)
© 2001 American College of Chest Physicians

Determinants of Continuous Positive Airway Pressure Compliance in a Group of Chinese Patients With Obstructive Sleep Apnea*

David S. C Hui, MBBS, FCCP; Dominic K. L. Choy, MBBS; Thomas S. T. Li, MBChB; Fanny W. S. Ko, MBChB; Kwok K. Wong, MPH; Joseph K. W. Chan, MBBS, FCCP and Christopher K. W. Lai, DM, FCCP

* From the Department of Medicine and Therapeutics, Centre for Clinical Trials and Epidemiological Research (Mr. Wong), Prince of Wales Hospital, and Chinese University of Hong Kong (Drs. Hui, Choy, Li, Ko, Chan, and Lai), Shatin, New Territories, Hong Kong.

Correspondence to: David S. C. Hui, MBBS, FCCP, Department of Medicine and Therapeutics, Prince of Wales Hospital, Chinese University of Hong Kong, Shatin, New Territories, Hong Kong; e-mail: dschui{at}cuhk.edu.hk


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Objective: To assess continuous positive airway pressure (CPAP) compliance and factors associated with CPAP compliance among Chinese patients with obstructive sleep apnea (OSA).

Design: A prospective study of 112 consecutive patients with newly diagnosed OSA commencing CPAP treatment.

Setting: A university teaching hospital.

Measurements and results: The following factors were evaluated for any correlation with objective CPAP compliance (effective mask pressure [hours per day]) at 1 month and 3 months: age, baseline apnea-hypopnea index (AHI), common OSA symptoms, minimum arterial oxygen saturation (SaO2), mean SaO2, arousal index (AI), Epworth sleepiness scale (ESS), education level, CPAP levels, satisfaction with CPAP, side effects, and machine cost. There were 101 male and 11 female patients, with a mean (± SD) age of 45.6 ± 1.2 years; body mass index, 29.3 ± 5.2 kg/m2; AI, 60 ± 18/h; AHI, 48 ± 24/h; minimum SaO2 of 70 ± 17%; and mean SaO2 of 86 ± 7%. ESS fell from 12.9 ± 4.0 (baseline) to 5.2 ± 4.7 at 3 months (p < 0.001). Objective CPAP compliance was 5.4 ± 1.6 h/d and 5.3 ± 1.6 h/d, while 75% and 72% of our patients were using CPAP objectively for >= 4 h/d and at least 70% of the nights per week at 1 month and 3 months, respectively. Following univariate analysis of variance, a high baseline AHI (p = 0.006 and p = 0.004) was associated with higher objective CPAP compliance at 1 month and 3 months, respectively.

Conclusion: CPAP usage and compliance were high in this patient population. A high baseline AHI was the only significant independent predictor of better CPAP compliance.

Key Words: compliance • continuous positive airway pressure • obstructive sleep apnea


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Obstructive sleep apnea (OSA) is a common disorder characterized by repetitive episodes of partial or complete upper-airway obstruction during sleep.1 Excessive daytime sleepiness is a major consequence,2 and there is also increasing epidemiologic evidence linking sleep-disordered breathing and hypertension.3 4 5 6 7

Introduced by Sullivan et al8 almost 2 decades ago as a pneumatic splint preventing collapse of the upper airway during sleep, nasal continuous positive airway pressure (CPAP) has remained the "gold standard" treatment for OSA, with robust evidence from several randomized, placebo-controlled trials9 10 11 12 13 demonstrating improvement of symptoms, cognitive function, and quality of life in OSA patients treated with nasal CPAP. Nevertheless CPAP compliance has been variable as reported by different studies,9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 and the determinants of CPAP compliance have remained largely unclear and conflicting.14 15 16 17 18 19 20 21 22 23 24 25 26 27

In a study28 examining the level of compliance with regard to the administration of medications in a general medical clinic in our hospital, 50% of patients receiving long-term medical therapy (at least 12 months) were noncompliant or only partially compliant. In a previous study29 comparing the effect of basic vs augmented CPAP education on compliance and outcome, we reported that augmentation of CPAP education and support did not improve CPAP compliance but led to a greater improvement of quality of life during the reinforced period. In the present study, we aimed to assess prospectively the objective compliance of nasal CPAP and factors that might predict the use of nasal CPAP among our Chinese patients with OSA.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Between October 1997 and June 1998, we recruited from our respiratory clinic 112 consecutive patients with newly diagnosed OSA for this prospective study. Significant OSA was defined as an apnea-hypopnea index (AHI) of >= 10 events per hour of sleep as shown by overnight polysomnography (Sleep Lab 1000p; Aequitron Medical; Minneapolis, MN) plus self-reported sleepiness. Overnight polysomnography recorded EEG, electro-oculogram, submental electromyogram, bilateral anterior tibial electromyogram, ECG, chest and abdominal wall movement by inductance plethysmography, airflow measured by a nasal pressure transducer (PTAF; Pro-Tech; Woodinville, WA), and backed up by an oronasal thermistor plus finger pulse oximetry. Sleep stages were scored according to standard criteria by Rechtschaffen and Kales.30 Apnea was defined as cessation of airflow for > 10 s and hypopnea as a reduction of airflow of >= 50% for > 10 s plus an oxygen desaturation of > 4% or an arousal. AHI was defined as the number of apneas and hypopneas per hour of sleep. Arousals were scored according to the American Sleep Disorders Association criteria.31 Our study was approved by the ethics committee of the Chinese University of Hong Kong, and appropriate informed consent was obtained from the subjects.

Apart from demographic data, the patients were required to complete the Sleep and Health Questionnaire (SHQ)32 at baseline. The SHQ is a modified version of the Specialized Centers of Research sleep questionnaire and contains 16 questions grouped into five factors: functional impact of sleepiness, self-reported breathing disturbances, roommates-observed breathing disturbances, driving impairment, and insomnia. The SHQ has been shown32 previously to be a valid means of characterizing symptom distribution in population surveys of sleep apnea. The responses to the SHQ, using either a 5-point frequency scale or a 6-point Likert scale grading the severity of the symptoms, were categorized into mild, moderate, and severe as follows for the purpose of data analysis: 6-point Likert scale (1 to 2 points [not affected], 3 to 4 points [mild], 5 points [moderate], and 6 points [severe]); 5-point frequency scale (never [not affected], rarely or sometimes [mild], frequently [moderate], and always [severe]). Additional questions were included to document the use of alcohol and the subjects’ sleeping habits, including the patient’s usual sleep schedule and particularly the sleep quality the night before the sleep study.

Subjective sleepiness was assessed with the Epworth sleepiness scale (ESS) at baseline and repeated at 3 months after CPAP treatment. The ESS33 is a questionnaire specific to symptoms of daytime sleepiness, and the patients are asked to score the likelihood of falling asleep in eight different situations with different levels of stimulation, adding up to a total score of 0 to 24. The ESS has been reported to be more discriminating than the Maintenance of Wakefulness Test and the Multiple Sleep Latency Test (MSLT) as a test of daytime sleepiness.34

Following confirmation of significant OSA from the overnight diagnostic sleep study, each patient was interviewed by the physician on duty and offered a trial of nasal CPAP treatment. Each patient was given CPAP education by a respiratory nurse explaining the basic operation and care of the CPAP device and the mask. An educational brochure on OSA and CPAP treatment in Chinese was given to each patient during the education session. The nurse would select and fit a comfortable CPAP mask from a wide range of choices for the patient who would then be given a short trial of CPAP therapy with the AutoSet (Resmed; Sydney, Australia) CPAP device for approximately 30 min for acclimatization in the afternoon. Attended CPAP titration was performed with the AutoSet autotitrating device on the second night of the study in our hospital. Several studies35 36 37 38 have shown that automatic CPAP titration is as effective as manual titration in correcting the obstructive respiratory events, arousal frequency, and improving oxygenation. Throughout the night and the next morning, the nurses on duty would deal with any discomfort related to the CPAP treatment. The CPAP pressure for each patient was set at the minimum pressure needed to abolish snoring, obstructive respiratory events, and airflow limitation for 95% of the night as determined by the overnight AutoSet CPAP titration study.29 37 All the patients were prescribed the Aria CPAP device (Respironics; Murrysville, PA), which automatically turns on when patients breathe into the mask and shuts off when the mask is removed. The Aria device contains a microprocessor with dual time meters recording both machine run time and time spent at effective pressure (measured by mask pressure transducer recorder).

CPAP compliance
Following commencement of CPAP treatment, the patients were reviewed by physicians and nurses at the CPAP clinic at 1 month and 3 months to deal with any problem with the CPAP device or mask fit. Our patients were asked to report subjectively their average total sleep time, the amount of time they used the CPAP device per day, and to complete a follow-up questionnaire18 regarding any side effects and their satisfaction with the use of CPAP. CPAP levels would be adjusted only if necessary. The objective CPAP compliance was measured at 1 month and 3 months with the Aria CPAP data downloaded into a personal computer using Encore software (Respironics). The time spent at effective pressure (hours per day) was recorded as the objective compliance. Our patients were not aware of the capacity of their CPAP device in recording compliance.

Other Parameters of Interest
The following variables were assessed for any correlation with objective CPAP compliance at 1 month and 3 months: age, body mass index (BMI), baseline AHI, minimum arterial oxygen saturation (SaO2), mean SaO2, arousal index (AI), ESS, education level, CPAP levels, CPAP-related side effects, satisfaction with CPAP, and common OSA symptoms, such as daytime sleepiness, snoring, frequent awakenings, impaired performance ability, and witnessed apnea. In addition, we were interested to see whether patients who had to purchase or rent the CPAP units themselves would have better compliance than those whose CPAP units were funded by the government.

Statistical Methods
Variables measured at different time points were compared using paired t test. The relationship between OSA symptoms and objective CPAP compliance was analyzed by linear trend. For other continuous variables vs objective CPAP compliance, the correlation was assessed by the Pearson correlation coefficients. Univariate analysis of variance (ANOVA) was performed to look for independent factors with significant association with objective compliance.

CPAP-Related Terminology
We have adopted the following terminology as suggested by Grunstein and Sullivan39 describing patient interaction with CPAP. Acceptance refers to the proportion of patients who meet selection criteria for CPAP treatment and actually proceed to have their pressure level determined. Prescription refers to the proportion of patients who accept CPAP and commence home treatment. Adherence refers to the proportion of patients prescribed CPAP who report that they are continuing to use CPAP. Usage refers to the proportion of patients with machines switched on more than an arbitrary period of time. For the purpose of this study, we measured objective compliance by the mean rate of CPAP use (hours per day) as reflected by the effective mask pressure time and subjective compliance by the mean rate of use (hours per day) reported by the patient.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
There were totally 112 patients (101 male and 11 female patients) with the following demographic and baseline polysomnography characteristics: age (mean ± SD), 45.6 ± 11.2 years; BMI, 29.3 ± 5.2 kg/m2; neck circumference, 41.2 ± 3.2 cm; AI, 60 ± 18/h; AHI, 48 ± 24/h; sleep efficiency, 87 ± 9%; total sleep time, 7.2 ± 1.1 hours per night; mean SaO2, 86 ± 7%; and minimum SaO2, 70 ± 17%. The frequency of common symptoms related to OSA is shown in Table 1 .


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  		Table 1.. Selected Questions From the SHQ (n = 112)*

 
Self-Reported Sleep Time and Subjective and Objective CPAP Compliance
The subjective compliance reported by our patients was consistently and significantly higher than the corresponding objective compliance recorded at 1 month and 3 months (p < 0.001). Overall, 75% and 72% of our patients were using CPAP regularly for at least 4 h per night and at least 70% of the nights per week at 1 month and 3 months, respectively. There was a slight drop in CPAP levels at 1 month and 3 months from baseline (Table 2 ).


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  		Table 2.. Self-Reported Sleep Time, CPAP Levels, and Compliance (n = 112)*

 
Symptoms of OSA and Objective Compliance
Among the symptoms related to OSA, snoring was associated with better objective CPAP compliance at 1 month (p = 0.03), with a trend at 3 months (p = 0.06). Frequent awakenings and witnessed apnea were associated with better compliance at 1 month (p = 0.012 and p = 0.039, respectively) but not at 3 months (p = 0.106 and p = 0.22, respectively). Other symptoms were not associated with CPAP compliance.

Side Effects and Problems Related to CPAP and Compliance
Common side effects and problems related to CPAP are shown in Table 3 . Inconvenience and CPAP machine cost were the main concerns expressed by our patients. Of the 112 patients, 1 patient and 6 patients gave up the use of CPAP at 1 month and 3 months, respectively, mainly because of inconvenience. They were referred to our dental clinic for consideration of the mandibular advancement device. Other less frequent side effects reported included claustrophobia, 11% and 7%; chest discomfort, 6% and 5%; and epistaxis, 6% and 6%, at 1 month and 3 months, respectively. No consistent and significant correlation, however, was found between the side effects and average hours of CPAP use per day at both 1 month and 3 months.


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  		Table 3.. Side Effects and Problems Related to CPAP*

 
Except for 19 socially disadvantaged patients who were eligible for government support, all the other patients had to purchase or rent their CPAP units themselves. When comparing the objective CPAP compliance between patients who paid for or rented the CPAP units themselves vs those who received government support, the former appeared to have better compliance than the latter but the differences did not reach statistical significance (5.6 ± 1.5 h/d vs 5.0 ± 1.9 h/d and 5.5 ± 1.5 h/d vs 4.8 ± 1.8 h/d at 1 month and at 3 months, respectively; p = 0.14 for both).

Other Determinants of CPAP Compliance
Following CPAP treatment, the ESS fell from 12.9 ± 4.9 at baseline to 5.2 ± 4.7 at 3 months (p < 0.001). Correlations between ESS and other variables vs objective compliance are shown in Table 4 . The only factor with significant correlation with higher average CPAP use per day at both 1 month and 3 months was a high baseline AHI.


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  		Table 4.. Correlation Between Different Variables and Objective CPAP Compliance (n = 112)

 
Independent Factors Associated With Objective CPAP Compliance
After univariate ANOVA with objective CPAP compliance as a dependent variable, a high baseline AHI (p = 0.006 and p = 0.004) was the only significant factor predictive of better compliance at both 1 month and 3 months. History of snoring was no longer a significant predictor (p = 0.11 and p = 0.237 at 1 month and 3 months, respectively).


   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
OSA is a common disorder causing daytime sleepiness, cognitive deficits, and impairment of quality of life. Nasal CPAP has remained the most effective treatment since its introduction by Sullivan et al8 in 1981. Nevertheless, CPAP is a rather obtrusive and cumbersome therapy, and CPAP compliance has been highly variable, ranging from 2.8 to 6.0 h per night in different studies.9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

This prospective study has shown full CPAP acceptance among our 112 Chinese patients with newly diagnosed OSA, and all proceeded to have their CPAP pressure levels determined and commenced home treatment initially. Our study has shown high levels of CPAP adherence of 99% and 94%, objective compliances of 5.4 ± 1.6 h/d and 5.3 ± 1.6 h/d, with high levels of regular CPAP usage of 75% and 72% at 1 month and 3 months, respectively. Of all the parameters tested, the only independent factor predicting better objective CPAP compliance at both 1 month and 3 months was a high initial AHI. The following factors did not show any significant correlation with objective CPAP compliance: age, BMI, minimum SaO2, mean SaO2, AI, ESS, education level, CPAP levels, CPAP-related side effects/satisfaction, CPAP machine cost, and OSA symptoms.

Despite enormous efforts by many investigators14 15 16 17 18 19 20 21 22 23 24 25 26 27 as outlined in Table 5 , the determinants of CPAP compliance have remained conflicting and largely unclear. Snoring, AHI, and ESS have been reported by McArdle et al25 in a prospective long-term study as determinants of CPAP compliance. Snoring and baseline AHI showed strong correlation with higher average hours of CPAP use per day at both 1 month and 3 months in our initial analysis. However, after univariate ANOVA, only a high baseline AHI remained the significant predictor of better compliance. While many studies14 16 20 25 suggest that compliance improves with increased severity of daytime sleepiness, our study and others21 26 do not produce such results. Several studies15 16 21 25 have shown that CPAP use is correlated with baseline AHI, while others14 17 18 19 20 26 are against such findings.


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  		Table 5.. Summary of Some Studies on Predictors of CPAP Compliance*

 
Many studies14 16 20 21 26 have reported that the prevalence of side effects is similar in compliant and noncompliant patients, but a study by Engleman et al17 has shown that patients who complained of side effects used CPAP to a lesser extent. Nevertheless, tolerance and perception of any side effect may differ across individuals with a variable degree of impact. It is therefore not advisable to compare prevalence of a particular side effect and assume it has no impact on compliance with therapy.40

It is difficult to compare CPAP compliance and factors determining compliance between studies due to different patient populations/study designs, variable definitions of CPAP-related terminology, and different duration of follow-up. For example, while many were studies on new patients,9 10 11 12 13 17 18 19 24 26 27 some were cross-sectional studies,15 20 and others were studies on long-term CPAP acceptors.14 16 22 25 Some of the early published data14 reported compliance in terms of self-reported CPAP use or proportion of patients continuing CPAP at a certain time interval, while other data19 defined compliance as machine run time divided by reported hours of sleep. Many authors have reported machine run time15 16 17 20 21 22 23 26 or effective mask pressure time,18 19 24 27 as in our study, as measurement of objective compliance. More recently,39 compliance has been defined as the proportion of patients using CPAP machines and delivering a preset level. Nevertheless, a universally accepted definition of compliance has not been devised.

As the therapeutic success of CPAP relies ultimately on patient acceptance and compliance with the device, it is important to monitor the actual rate of use in order to ensure patients achieve the desired benefits. It is also important to include total sleep time in order to allow meaningful interpretation of compliance. Self-reported compliance, which was overestimated by our patients as in other studies,18 19 20 41 should not be considered as a reliable means to establish compliance. Although a CPAP device with a time counter does not provide information on the actual time of day and duration of CPAP used in each 24-h period, it is considerably cheaper than a CPAP device with effective mask pressure time. CPAP run time has been shown by several studies17 18 19 21 to have close correlation with effective mask pressure time with a correction factor of approximately 0.9. We therefore agree with Engleman et al17 that, as a minimum, a CPAP machine with run time be prescribed and regularly recorded in order to monitor compliance.

Despite major concern about inconvenience and CPAP machine cost, 72% of the OSA patients in our study have remained regular CPAP users at 3 months, and this is much higher than that of 46% reported by Kribbs et al18 in an American population but slightly lower than that of 79% reported by Pepin et al26 in a prospective, multicenter, European study. The objective CPAP compliance, in terms of effective mask pressure time, among our patients with newly diagnosed OSA was 5.4 ± 1.6 h/d and 5.3 ± 1.6 h/d at 1 month and 3 months, respectively, and this compared favorably with any other published data on new CPAP users.9 10 11 12 13 17 18 19 24 26 27 The high level of compliance, we believe, is the result of our good basic CPAP education program consisting of educational brochures on OSA and CPAP, practical CPAP education and acclimatization sessions conducted by our nurses, plus early CPAP clinic review at 4 weeks.29 A more intensive approach, with additional nursing input (including CPAP education at home and involving their partners, a 3-night trial of CPAP titration in a sleep center followed by additional home visits), has been shown to result in greater improvement of objective CPAP compliance, OSA symptoms, mood, and reaction time.24

In summary, this study has shown a high level of CPAP acceptance, adherence, and compliance among our patient with newly diagnosed OSA commencing CPAP treatment. The independent factor predicting better CPAP compliance at both 1 month and 3 months was a high AHI at baseline.


   Acknowledgements
 
The authors thank Sisters Mabel Tong, K. Y. Chau, and M. Y. Leung for providing CPAP education and support to our patients. We would also like to thank Erica Lee, Chris Chan, and Dr. Anthony Chan for their laboratory assistance for this project.


   Footnotes
 
Abbreviations: AHI = apnea-hypopnea index; AI = arousal index; ANOVA = analysis of variance; BMI = body mass index; CPAP = continuous positive airway pressure; ESS = Epworth sleepiness scale; MSLT = Multiple Sleep Latency Test; OSA = obstructive sleep apnea; SaO2 = arterial oxygen saturation; SHQ = Sleep and Health Questionnaire

Project funded by Health Service Research grant 711007.

Received for publication September 14, 2000. Accepted for publication February 12, 2001.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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