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Validity and reliability of Sleep Quality Scale in subjects with obstructive sleep apnea syndrome
Journal of Psychosomatic Research 66 (2009) 85 – 88
Validity and reliability of Sleep Quality Scale in subjects with obstructive sleep apnea syndrome Hyeryeon Yi a , Kyungrim Shin b , Jinyoung Kim b,c , Jeongsun Kim d , JungBok Lee c , Chol Shin c,e,⁎ a
b
Department of Nursing, Korea Nazarene University, Cheonan-si, Chungnam, Republic of Korea Department of Nursing Science, College of Health Science, Ewha Womans University, Seodaemun-gu, Seoul, Republic of Korea c Institute of Human Genomic Study, College of Medicine, Korea University, Seongbuk-gu, Seoul, Republic of Korea d College of Nursing, Chonnam National University, Dong-gu, Gawngju, Republic of Korea e Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Korea University Ansan Hospital, Ansan-si, Gyeonggi-do, Republic of Korea Received 17 April 2007; received in revised form 16 April 2008; accepted 15 July 2008
Abstract Objective: The present study aimed to evaluate the validity and reliability of Sleep Quality Scale (SQS) in subjects with obstructive sleep apnea syndrome (OSAS). Methods: The known group technique was used for construct validity. The study sample was composed of 40 persons with OSAS and 37 normal subjects. OSAS was defined as an apnea–hypopnea index (AHI) of ≥5 plus an Epworth Sleepiness Scale (ESS) score of N10. Control group was composed of subjects who had no history of depression and sleep problems as identified by self-report and polysomnography.
A Cronbach's alpha coefficient was used for internal consistency. Results: The total SQS score of the OSAS subjects was significantly higher than that of the normal subjects. The SQS score homogeneity was high with a Cronbach's alpha coefficient of 0.90 in all subjects, 0.87 in the OSAS subjects, and 0.70 in the normal subjects. Conclusion: SQS was confirmed to be an instrument with validity and high reliability for measuring sleep quality in OSAS subjects. © 2009 Elsevier Inc. All rights reserved.
Keywords: Reliability; Sleep Quality Scale; Validity
Introduction Sleep quality is known to be associated with physical and psychological health [1–4] and quality of life [5–6]. Complaints about sleep quality are common in the general population as well as patients. There are subjective and objective methods to measure sleep quality. Although polysomnography, one of the objective methods, provides information related to sleep
⁎ Corresponding author. Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Korea University Ansan Hospital, 516, Gojan 1-dong, Danwon-gu, Ansan-si, Gyeonggi-do, 425-707, Republic of Korea. Tel.: +82 31 412 5603; fax: +82 31 412 5604. E-mail address: [email protected] (C. Shin). 0022-3999/09/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jpsychores.2008.07.008
quality, it cannot define sleep quality accurately. Sleep is a subjective experience. Elements of sleep quality and their importance vary according to individuals [7]. A self-reported method is essential to assess sleep quality. Thus health care professionals should maintain a concern about self-reported sleep quality and its evaluation. The Sleep Quality Scale (SQS) was developed in Korean by Yi et al. [9] for the comprehensive assessment of sleep quality in the general population. It includes more items related to daytime dysfunction and restorative function of sleep than any other scales including the Pittsburgh Sleep Quality Index. The validity and reliability of SQS have been confirmed in the general population and insomniacs, whereas psychometric information is limited in clinical populations. A known group technique is used to identify construct validity. It is a method of comparing scores between groups that
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are expected to have different responses to the questionnaires [8]. Obstructive sleep apnea syndrome (OSAS) is a common sleep disorder which is characterized by periodic impairment of respiration during sleep and daytime dysfunction such as excessive daytime sleepiness. As OSAS subjects are expected to have poor sleep quality, we undertook this study to evaluate the validity of SQS by comparing the SQS scores between subjects with OSAS and normal subjects, and its reliability.
Table 1 Components and items of SQS Components
Items
Daytime dysfunction
Difficulty in thinking due to poor sleep Difficulty in concentrating due to poor sleep Increase of mistakes due to poor sleep Irritated feeling due to poor sleep Decrease of interest in work or others due to poor sleep Getting tired easily at work due to poor sleep Sleepiness that interferes with daily life Painful life due to poor sleep Decrease of desire due to poor sleep Increase of forgetfulness due to poor sleep Headache due to poor sleep Decrease of appetite due to poor sleep Relief of fatigue after sleep Regaining vigor after sleep Clear-headed feeling after sleep Refreshed feeling of body after sleep Difficulty in getting back to sleep after nocturnal awakening Never falling asleep after awakening during sleep Difficulty in falling asleep Tossing and turning sleeplessly Wish for more sleep after getting up Difficulty in getting up after sleep Feeling unlikely to sleep after sleep Satisfaction with sleep Deep sleep Enough sleep time Waking up easily due to noise Waking up during sleep
Methods Subjects Restoration after sleep
Study subjects were selected among outpatients who underwent polysomnography to diagnose sleep problems and among the subjects of the Ansan Sleep Cohort which is purposed to identify prevalence and associated factors of sleep disorders. Subjects who had consented to the study completed a questionnaire and then underwent polysomnography in the sleep laboratory. OSAS was defined as an apnea–hypopnea index (AHI) of ≥5 plus an Epworth Sleepiness Scale (ESS) score of N10. Depressive symptom and sleep problems including insomnia, periodic leg movement disorder, or total sleep time of b5 h were excluded in the OSAS group. Normal subjects had no history of depression either, nor did they have any other sleep problems including snoring as identified in self-report and polysomnography. Subjects were 40 persons with OSAS and 37 normal controls. The mean age was 44.9 years for the OSAS group and 46.2 years for the normal subjects. The male/female ratio was 37:3 for OSAS subjects and 33:4 for normal subjects. There were no significant differences in age and sex between the two groups. Measurements The questionnaire used in the present study included SQS, ESS, Beck Depression Inventory (BDI), questions about insomnia symptoms, and demographic variables. SQS, developed by Yi et al. [9] to assess sleep quality, is composed of 28 items, graded with a four-point Likert scale, categorized into six components: daytime dysfunction, restoration after sleep, difficulty in falling asleep, difficulty in getting up, satisfaction with sleep, and difficulty in maintaining sleep (see Table 1). The scores of items in ‘restoration after sleep’ and ‘satisfaction with sleep’ have to be reversed before all the scores are summated. The range of score is from 0 to 84, with a higher score indicating a lower sleep quality. The Cronbach's alpha coefficient was 0.92 at the inception of development [9]. ESS is an eight-item scale developed by Johns [10] to measure daytime sleepiness. It is a four-point rating scale and its score ranges from 0 to 24, with a higher score indicating
Difficulty in falling asleep
Difficulty in getting up
Satisfaction with sleep
Difficulty in maintaining sleep
greater sleepiness. Excessive daytime sleepiness was defined as a total score of N10. The Cronbach's alpha coefficient of ESS was reported at its inception as 0.88 [11]. The Cronbach's alpha coefficient of ESS in the present study was 0.88. Depression was assessed by using the widely used, 21item BDI instrument. A higher score indicates more depression and a cut-off score of 16 for depressive propensity was used on the basis of the validity study by Lee and Song [12]. The Cronbach's alpha coefficient of BDI was 0.78 in the present study. In addition, the presence and frequency of four insomnia symptoms such as difficulty in initiating sleep, difficulty in maintaining sleep, early morning awakening, and nonrestorative sleep were assessed. Insomnia was defined as when any insomnia symptom had occurred three times or more per week over the previous month. Polysomnography was performed by using the computerized PSG device (Alice 4; Respironics, Atlanta, GA, USA) in a sleep laboratory to identify sleep problems. Sleep was recorded by 16 channels. All the polysomnographic results were scored manually in accordance with Rechtschaffen and Kales' [13] criteria. Apnea was defined as absence of airflow for at least 10 s. Hypopnea was
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defined as a decrease of airflow associated with reduction of oxygen saturation by 4% from the baseline. The AHI was defined as a mean of number of apnea or hypopnea per hour during sleep.
Table 3 Comparison of SQS scores between subjects with OSAS and normal subjects
Statistics
Total score 27.3±10.95 Daytime dysfunction 7.1±5.61 Restoration after sleep 8.0±3.45 Difficulty in falling asleep 0.9±1.21 Difficulty in getting up 4.5±2.55 Satisfaction with sleep 5.8±2.07 Difficulty in maintaining sleep 1.2±1.12
Data were analyzed by the SAS Enterprise Guide 3.0 software (SAS Institute, Cary, NC, USA). t Tests were performed to compare the SQS scores of the OSAS subjects with those of normal subjects for construct validity. Pearson correlations were also used to examine the relationships between SQS and polysomnographic results. Cronbach's alpha coefficients were used to identify the homogeneity of SQS in each group, as well as in all subjects.
OSAS subjects Normal subjects (n=40) (n=37) P value 9.7±6.16 1.4±1.92 2.9±3.13 0.6±0.96 1.2±1.09 2.5±2.25 1.1±1.05
.000 .000 .000 .262 .000 .000 .866
Values are shown as mean±S.D.
As a result of homogeneity testing, the Cronbach's alpha coefficient was 0.90 in all subjects, 0.87 in the OSAS subjects, and 0.70 in the normal subjects.
Results Table 2 shows the polysomnographic results of the two groups. There were also significant differences in the AHI (P=.000), arousal index (P=.000), number of awakening (P=.001), percentage of Stage 1 (P=.000), percentage of Stage 2 (P=.000), and percentage of rapid eye movement (REM) sleep (P=.004) between the two groups. The mean SQS total score was 27.3 (S.D.=10.95) for OSAS subjects and 9.7 (S.D.=6.16) for normal subjects. As shown in Table 3, the total score of OSAS subjects was significantly higher than that of normal subjects (P=.000). Among the six components of SQS, the scores of ‘daytime dysfunction’ (P=.000), ‘restoration after sleep’ (P=.000), ‘difficulty in getting up’ (P=.000), and ‘satisfaction with sleep’ (P=.000) showed significant differences between the two groups. SQS was significantly correlated with AHI (P=.000), arousal index (P=.000), percentage of Stage 1 (P=.001), percentage of Stage 2 (P=.002), and percentage of REM (P=.044) from the polysomnographic data (see Table 4). Poor sleep quality was associated with high AHI, high arousal index, high percentage of Stage 1, low percentage of Stage 2, and low percentage of REM sleep.
Table 2 Polysomnographic results of subjects with OSAS and normal subjects
Discussion As validity varies according to samples and situations, the developed scale has to be validated for various specific uses [8]. Since SQS was initially developed to assess sleep quality in the general population, its application to clinical samples requires psychometric evaluation. The validity of a scale can be tested by comparing scores between groups which are expected to be different [8]. The present study confirmed the construct validity using the known group technique that has been identified in several studies to confirm the construct validity of scales or questionnaires [7,14,15]. In the results, the total score of OSAS subjects was significantly higher than that of normal subjects, which confirmed the validity of the instrument in discriminating between the two. The result of the present study is similar to that of a previous study which confirmed the construct validity of SQS in insomniacs [9]. In addition, four of the six components also showed the significant differences between the two groups. However, two components, ‘difficulty in falling asleep’ and ‘difficulty in maintaining sleep’, did not show a significant difference. It is likely that these two components are associated with symptoms of insomnia rather than OSAS. Table 4 Correlational coefficients between SQS and polysomnographic results
Variables
OSAS subjects (n=40)
Normal subjects (n=37)
P value
Polysomnographic results
Correlational coefficients (P value)
Total sleep time (min) Stage 1 (%) Stage 2 (%) SWS (%) REM (%) Sleep onset latency (min) Sleep efficiency (%) AHI Arousal index Number of awakening
401.0±52.71 29.5±12.50 52.3±11.56 2.3±4.63 15.9±4.56 19.6±10.18 86.4±9.32 36.3±25.69 41.2±20.61 37.8±20.57
382.8±56.31 15.9±5.24 63.1±6.51 1.4±2.50 19.3±5.67 20.0±16.81 85.1±9.23 1.4±1.41 12.7±5.20 24.3±9.96
.146 .000 .000 .311 .004 .904 .546 .000 .000 .001
Total sleep time Sleep efficiency Percentage of stage 1 Percentage of stage 2 Slow wave sleep Stage REM AHI Arousal index Sleep onset latency REM latency Number of awakening
0.08 (0.492) −0.03 (0.777) 0.37 (0.001) −0.35 (0.002) 0.18 (0.119) −0.23 (0.044) 0.56 (0.000) 0.54 (0.000) 0.08 (0.471) 0.13 (0.271) 0.19 (0.103)
Values are shown as mean±S.D.
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Pearson correlations showed that SQS was significantly correlated with percentage of Stage 2. This correlation was similar to the results of the previous study. Backhaus et al. reported that a global score of PSQI was correlated with percentage of Stage 2 (r=−0.33, P=.028), total sleep time without Stage 1 (r=−0.32, P=.034), and sleep efficiency (r= −0.32, P=.034) [16]. However, SQS had moderate correlations with AHI and arousal index, while it was not correlated with sleep efficiency. The reliability of SQS was high with a Cronbach's alpha coefficient of 0.90, which confirmed the internal consistency of SQS. This reliability was similar to that (Cronbach's alpha coefficient=0.92) in the initial development sample. Although Cronbach's alpha coefficients in each group were lower than those in all subjects, they were still adequate. In conclusion, the SQS instrument was confirmed to be valid with high reliability in the assessment of sleep quality in OSAS subjects. However, there were several limitations in the present study. Firstly, the sample size was rather small, which prevented a comparison of SQS among groups according to OSAS severity. Furthermore, most study subjects were male. Therefore psychometric evaluation should be performed with a large sample, and especially with women with OSAS. Further validity studies are also suggested in other clinical samples, including depressive patients and narcolepsy patients.
References [1] Edell-Gustafsson UM, Kritz EIK, Bogren IK. Self-reported sleep quality, strain, and health in relation to perceived working conditions in females. Scand J Caring Sci 2002;16:179–87.
[2] Knutson KL, Ryden AM, Mander BA, Van Cauter E. Role of sleep duration and quality in the risk and severity of type 2 diabetes mellitus. Arch Intern Med 2006;166:1768–74. [3] Motivala SJ, Levin MJ, Oxman MN, Irwin MR. Impairments in health functioning and sleep quality in older adults with a history of depression. J Am Geriatr Soc 2006;54:1184–91. [4] Prieto-Rincon D, Echeto-Inciarte S, Faneite-Hernandez P, InciarteMundo J, Rinxon-Prieto C, Bonilla E. Quality of sleep in hospitalized psychiatric patients. Invest Clin 2006;47:5–16. [5] Brostrőm A, Strőmberg A, Dahlstrőm U, Fridlund B. Sleep difficulties, daytime sleepiness, and health related quality of life in patients with chronic heart failure. J Cardiovasc Nurs 2004;19:234–42. [6] Phillips KD, Sowell RL, Boyd M, Dudgeon WD, Hand GA, The Mindbody Research Group. Sleep quality and health-related quality of life in HIV-infected African-American women of childbearing age. Qual Life Res 2005;14:959–70. [7] Buysse DJ, Reynolds CF 3rd, Monk TH, Berman SR, Kupfer DJ. The Pittsburgh Sleep Quality Index: a new instrument for psychiatric practice and research. Psychiatry Res 1989;28:193–213. [8] Burns N, Grove SK. The practice of nursing research. Conduct, critique & utilization. 4th ed. Philadelphia: W. B. Saunders, 2001. [9] Yi H, Shin K, Shin C. Development of Sleep Quality Scale. J Sleep Res 2006;15:309–16. [10] Johns MW. A new method for measuring daytime sleepiness: the Epworth Sleepiness Scale. Sleep 1991;14:540–5. [11] Johns MW. Reliability and factor analysis of the Epworth Sleepiness Scale. Sleep 1992;15:376–81. [12] Lee Y, Song J. A study of the reliability and the validity of the BDI, SDS, and MMPI-D scales. Kor J Clin Psychol 1991;10:98–113. [13] Rechtschaffen A, Kales KL. A manual of standardized terminology technique and scoring system for sleep stages of human subjects. Washington (DC): US Government Printing Office, 1968 [NIH Publication No 204]. [14] Domino G, Blair G, Bridges A. Subjective assessment of sleep by sleep questionnaire. Percept Mot Skills 1984;59:163–70. [15] Webb WB, Bonnet M, Blume G. A post-sleep inventory. Percept Mot Skills 1976;43:987–93. [16] Backhaus J, Junghanns K, Broocks A, Riemann D, Hohagen F. Test– retest reliability and validity of the Pittsburgh Sleep Quality Index in primary insomnia. J Psychosom Res 2002;53:737–40.
Validity and reliability of Sleep Quality Scale in subjects with obstructive sleep apnea syndrome Hyeryeon Yi a , Kyungrim Shin b , Jinyoung Kim b,c , Jeongsun Kim d , JungBok Lee c , Chol Shin c,e,⁎ a
b
Department of Nursing, Korea Nazarene University, Cheonan-si, Chungnam, Republic of Korea Department of Nursing Science, College of Health Science, Ewha Womans University, Seodaemun-gu, Seoul, Republic of Korea c Institute of Human Genomic Study, College of Medicine, Korea University, Seongbuk-gu, Seoul, Republic of Korea d College of Nursing, Chonnam National University, Dong-gu, Gawngju, Republic of Korea e Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Korea University Ansan Hospital, Ansan-si, Gyeonggi-do, Republic of Korea Received 17 April 2007; received in revised form 16 April 2008; accepted 15 July 2008
Abstract Objective: The present study aimed to evaluate the validity and reliability of Sleep Quality Scale (SQS) in subjects with obstructive sleep apnea syndrome (OSAS). Methods: The known group technique was used for construct validity. The study sample was composed of 40 persons with OSAS and 37 normal subjects. OSAS was defined as an apnea–hypopnea index (AHI) of ≥5 plus an Epworth Sleepiness Scale (ESS) score of N10. Control group was composed of subjects who had no history of depression and sleep problems as identified by self-report and polysomnography.
A Cronbach's alpha coefficient was used for internal consistency. Results: The total SQS score of the OSAS subjects was significantly higher than that of the normal subjects. The SQS score homogeneity was high with a Cronbach's alpha coefficient of 0.90 in all subjects, 0.87 in the OSAS subjects, and 0.70 in the normal subjects. Conclusion: SQS was confirmed to be an instrument with validity and high reliability for measuring sleep quality in OSAS subjects. © 2009 Elsevier Inc. All rights reserved.
Keywords: Reliability; Sleep Quality Scale; Validity
Introduction Sleep quality is known to be associated with physical and psychological health [1–4] and quality of life [5–6]. Complaints about sleep quality are common in the general population as well as patients. There are subjective and objective methods to measure sleep quality. Although polysomnography, one of the objective methods, provides information related to sleep
⁎ Corresponding author. Division of Pulmonary and Critical Care Medicine, Department of Internal Medicine, Korea University Ansan Hospital, 516, Gojan 1-dong, Danwon-gu, Ansan-si, Gyeonggi-do, 425-707, Republic of Korea. Tel.: +82 31 412 5603; fax: +82 31 412 5604. E-mail address: [email protected] (C. Shin). 0022-3999/09/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jpsychores.2008.07.008
quality, it cannot define sleep quality accurately. Sleep is a subjective experience. Elements of sleep quality and their importance vary according to individuals [7]. A self-reported method is essential to assess sleep quality. Thus health care professionals should maintain a concern about self-reported sleep quality and its evaluation. The Sleep Quality Scale (SQS) was developed in Korean by Yi et al. [9] for the comprehensive assessment of sleep quality in the general population. It includes more items related to daytime dysfunction and restorative function of sleep than any other scales including the Pittsburgh Sleep Quality Index. The validity and reliability of SQS have been confirmed in the general population and insomniacs, whereas psychometric information is limited in clinical populations. A known group technique is used to identify construct validity. It is a method of comparing scores between groups that
86
H. Yi et al. / Journal of Psychosomatic Research 66 (2009) 85–88
are expected to have different responses to the questionnaires [8]. Obstructive sleep apnea syndrome (OSAS) is a common sleep disorder which is characterized by periodic impairment of respiration during sleep and daytime dysfunction such as excessive daytime sleepiness. As OSAS subjects are expected to have poor sleep quality, we undertook this study to evaluate the validity of SQS by comparing the SQS scores between subjects with OSAS and normal subjects, and its reliability.
Table 1 Components and items of SQS Components
Items
Daytime dysfunction
Difficulty in thinking due to poor sleep Difficulty in concentrating due to poor sleep Increase of mistakes due to poor sleep Irritated feeling due to poor sleep Decrease of interest in work or others due to poor sleep Getting tired easily at work due to poor sleep Sleepiness that interferes with daily life Painful life due to poor sleep Decrease of desire due to poor sleep Increase of forgetfulness due to poor sleep Headache due to poor sleep Decrease of appetite due to poor sleep Relief of fatigue after sleep Regaining vigor after sleep Clear-headed feeling after sleep Refreshed feeling of body after sleep Difficulty in getting back to sleep after nocturnal awakening Never falling asleep after awakening during sleep Difficulty in falling asleep Tossing and turning sleeplessly Wish for more sleep after getting up Difficulty in getting up after sleep Feeling unlikely to sleep after sleep Satisfaction with sleep Deep sleep Enough sleep time Waking up easily due to noise Waking up during sleep
Methods Subjects Restoration after sleep
Study subjects were selected among outpatients who underwent polysomnography to diagnose sleep problems and among the subjects of the Ansan Sleep Cohort which is purposed to identify prevalence and associated factors of sleep disorders. Subjects who had consented to the study completed a questionnaire and then underwent polysomnography in the sleep laboratory. OSAS was defined as an apnea–hypopnea index (AHI) of ≥5 plus an Epworth Sleepiness Scale (ESS) score of N10. Depressive symptom and sleep problems including insomnia, periodic leg movement disorder, or total sleep time of b5 h were excluded in the OSAS group. Normal subjects had no history of depression either, nor did they have any other sleep problems including snoring as identified in self-report and polysomnography. Subjects were 40 persons with OSAS and 37 normal controls. The mean age was 44.9 years for the OSAS group and 46.2 years for the normal subjects. The male/female ratio was 37:3 for OSAS subjects and 33:4 for normal subjects. There were no significant differences in age and sex between the two groups. Measurements The questionnaire used in the present study included SQS, ESS, Beck Depression Inventory (BDI), questions about insomnia symptoms, and demographic variables. SQS, developed by Yi et al. [9] to assess sleep quality, is composed of 28 items, graded with a four-point Likert scale, categorized into six components: daytime dysfunction, restoration after sleep, difficulty in falling asleep, difficulty in getting up, satisfaction with sleep, and difficulty in maintaining sleep (see Table 1). The scores of items in ‘restoration after sleep’ and ‘satisfaction with sleep’ have to be reversed before all the scores are summated. The range of score is from 0 to 84, with a higher score indicating a lower sleep quality. The Cronbach's alpha coefficient was 0.92 at the inception of development [9]. ESS is an eight-item scale developed by Johns [10] to measure daytime sleepiness. It is a four-point rating scale and its score ranges from 0 to 24, with a higher score indicating
Difficulty in falling asleep
Difficulty in getting up
Satisfaction with sleep
Difficulty in maintaining sleep
greater sleepiness. Excessive daytime sleepiness was defined as a total score of N10. The Cronbach's alpha coefficient of ESS was reported at its inception as 0.88 [11]. The Cronbach's alpha coefficient of ESS in the present study was 0.88. Depression was assessed by using the widely used, 21item BDI instrument. A higher score indicates more depression and a cut-off score of 16 for depressive propensity was used on the basis of the validity study by Lee and Song [12]. The Cronbach's alpha coefficient of BDI was 0.78 in the present study. In addition, the presence and frequency of four insomnia symptoms such as difficulty in initiating sleep, difficulty in maintaining sleep, early morning awakening, and nonrestorative sleep were assessed. Insomnia was defined as when any insomnia symptom had occurred three times or more per week over the previous month. Polysomnography was performed by using the computerized PSG device (Alice 4; Respironics, Atlanta, GA, USA) in a sleep laboratory to identify sleep problems. Sleep was recorded by 16 channels. All the polysomnographic results were scored manually in accordance with Rechtschaffen and Kales' [13] criteria. Apnea was defined as absence of airflow for at least 10 s. Hypopnea was
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defined as a decrease of airflow associated with reduction of oxygen saturation by 4% from the baseline. The AHI was defined as a mean of number of apnea or hypopnea per hour during sleep.
Table 3 Comparison of SQS scores between subjects with OSAS and normal subjects
Statistics
Total score 27.3±10.95 Daytime dysfunction 7.1±5.61 Restoration after sleep 8.0±3.45 Difficulty in falling asleep 0.9±1.21 Difficulty in getting up 4.5±2.55 Satisfaction with sleep 5.8±2.07 Difficulty in maintaining sleep 1.2±1.12
Data were analyzed by the SAS Enterprise Guide 3.0 software (SAS Institute, Cary, NC, USA). t Tests were performed to compare the SQS scores of the OSAS subjects with those of normal subjects for construct validity. Pearson correlations were also used to examine the relationships between SQS and polysomnographic results. Cronbach's alpha coefficients were used to identify the homogeneity of SQS in each group, as well as in all subjects.
OSAS subjects Normal subjects (n=40) (n=37) P value 9.7±6.16 1.4±1.92 2.9±3.13 0.6±0.96 1.2±1.09 2.5±2.25 1.1±1.05
.000 .000 .000 .262 .000 .000 .866
Values are shown as mean±S.D.
As a result of homogeneity testing, the Cronbach's alpha coefficient was 0.90 in all subjects, 0.87 in the OSAS subjects, and 0.70 in the normal subjects.
Results Table 2 shows the polysomnographic results of the two groups. There were also significant differences in the AHI (P=.000), arousal index (P=.000), number of awakening (P=.001), percentage of Stage 1 (P=.000), percentage of Stage 2 (P=.000), and percentage of rapid eye movement (REM) sleep (P=.004) between the two groups. The mean SQS total score was 27.3 (S.D.=10.95) for OSAS subjects and 9.7 (S.D.=6.16) for normal subjects. As shown in Table 3, the total score of OSAS subjects was significantly higher than that of normal subjects (P=.000). Among the six components of SQS, the scores of ‘daytime dysfunction’ (P=.000), ‘restoration after sleep’ (P=.000), ‘difficulty in getting up’ (P=.000), and ‘satisfaction with sleep’ (P=.000) showed significant differences between the two groups. SQS was significantly correlated with AHI (P=.000), arousal index (P=.000), percentage of Stage 1 (P=.001), percentage of Stage 2 (P=.002), and percentage of REM (P=.044) from the polysomnographic data (see Table 4). Poor sleep quality was associated with high AHI, high arousal index, high percentage of Stage 1, low percentage of Stage 2, and low percentage of REM sleep.
Table 2 Polysomnographic results of subjects with OSAS and normal subjects
Discussion As validity varies according to samples and situations, the developed scale has to be validated for various specific uses [8]. Since SQS was initially developed to assess sleep quality in the general population, its application to clinical samples requires psychometric evaluation. The validity of a scale can be tested by comparing scores between groups which are expected to be different [8]. The present study confirmed the construct validity using the known group technique that has been identified in several studies to confirm the construct validity of scales or questionnaires [7,14,15]. In the results, the total score of OSAS subjects was significantly higher than that of normal subjects, which confirmed the validity of the instrument in discriminating between the two. The result of the present study is similar to that of a previous study which confirmed the construct validity of SQS in insomniacs [9]. In addition, four of the six components also showed the significant differences between the two groups. However, two components, ‘difficulty in falling asleep’ and ‘difficulty in maintaining sleep’, did not show a significant difference. It is likely that these two components are associated with symptoms of insomnia rather than OSAS. Table 4 Correlational coefficients between SQS and polysomnographic results
Variables
OSAS subjects (n=40)
Normal subjects (n=37)
P value
Polysomnographic results
Correlational coefficients (P value)
Total sleep time (min) Stage 1 (%) Stage 2 (%) SWS (%) REM (%) Sleep onset latency (min) Sleep efficiency (%) AHI Arousal index Number of awakening
401.0±52.71 29.5±12.50 52.3±11.56 2.3±4.63 15.9±4.56 19.6±10.18 86.4±9.32 36.3±25.69 41.2±20.61 37.8±20.57
382.8±56.31 15.9±5.24 63.1±6.51 1.4±2.50 19.3±5.67 20.0±16.81 85.1±9.23 1.4±1.41 12.7±5.20 24.3±9.96
.146 .000 .000 .311 .004 .904 .546 .000 .000 .001
Total sleep time Sleep efficiency Percentage of stage 1 Percentage of stage 2 Slow wave sleep Stage REM AHI Arousal index Sleep onset latency REM latency Number of awakening
0.08 (0.492) −0.03 (0.777) 0.37 (0.001) −0.35 (0.002) 0.18 (0.119) −0.23 (0.044) 0.56 (0.000) 0.54 (0.000) 0.08 (0.471) 0.13 (0.271) 0.19 (0.103)
Values are shown as mean±S.D.
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Pearson correlations showed that SQS was significantly correlated with percentage of Stage 2. This correlation was similar to the results of the previous study. Backhaus et al. reported that a global score of PSQI was correlated with percentage of Stage 2 (r=−0.33, P=.028), total sleep time without Stage 1 (r=−0.32, P=.034), and sleep efficiency (r= −0.32, P=.034) [16]. However, SQS had moderate correlations with AHI and arousal index, while it was not correlated with sleep efficiency. The reliability of SQS was high with a Cronbach's alpha coefficient of 0.90, which confirmed the internal consistency of SQS. This reliability was similar to that (Cronbach's alpha coefficient=0.92) in the initial development sample. Although Cronbach's alpha coefficients in each group were lower than those in all subjects, they were still adequate. In conclusion, the SQS instrument was confirmed to be valid with high reliability in the assessment of sleep quality in OSAS subjects. However, there were several limitations in the present study. Firstly, the sample size was rather small, which prevented a comparison of SQS among groups according to OSAS severity. Furthermore, most study subjects were male. Therefore psychometric evaluation should be performed with a large sample, and especially with women with OSAS. Further validity studies are also suggested in other clinical samples, including depressive patients and narcolepsy patients.
References [1] Edell-Gustafsson UM, Kritz EIK, Bogren IK. Self-reported sleep quality, strain, and health in relation to perceived working conditions in females. Scand J Caring Sci 2002;16:179–87.
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