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Eur Respir J 2003; 21:688-694
Copyright ©ERS Journals Ltd 2003

The role of mean inspiratory effort on daytime sleepiness

Z. Pelin, D. Karadeniz, L. Öztürk, E. Gözükirmizi and H. Kaynak

Sleep Disorders Unit, Dept of Neurology, Cerrahpasa Medical School, University of Istanbul, Istanbul, Turkey

CORRESPONDENCE: Z. Pelin, Bayar cad. 71/9, 81090, Kozyatagi, Istanbul, Turkey. Fax: 90 2163698925. E-mail: kzpelin@turk.net

Keywords: daytime sleepiness, respiratory effort, sleep apnoea syndrome, upper airway resistance

Received: November 28, 2001
Accepted October 1, 2002


   Abstract
TOP
 Abstract
Material and methods
 Results
 Discussion
 References
 
This study has investigated the role of average maximum inspiratory effort in excessive daytime sleepiness in patients with obstructive sleep apnoea syndrome (OSAS) and upper airway resistance syndrome (UARS).

Fifteen patients diagnosed with UARS and 32 patients with OSAS, with >5.5 h total sleep time (TST) during 8 h of nocturnal polygraphic recordings, were included in the study. Demographical data, polysomnographical data and data about daytime sleepiness, including Epworth sleepiness scale (ESS) and multiple sleep latency test (MSLT), were evaluated. In order to compute the average maximum inspiratory effort from oesophageal pressure (Poes) measurements, maximum Poes was obtained from 20 representative obstructive respiratory events (obstructive apnoeas, hypopnoeas or flow limitations) for each sleep stage in both supine and side positions. From Poes measurements during sleep, the increase in Poes ({Delta}Poes) during respiratory events was also calculated.

The average maximum Poes, {Delta}Poes, respiratory disturbance index (RDI) and arousal index were significantly correlated with ESS in OSAS patients. In patients with UARS, the only significant correlation was obtained between average maximum Poes and ESS. The MSLT score did not show any significant correlation with arousal index, number of stage variations, RDI, average Poes, {Delta}Poes, minimum oxygen saturation (Sa,O2) and percentage of TST with an Sa,O2 <90% in both UARS and OSAS patients. The results of multiple regression analysis showed that average maximum Poes correlates best with the variance in ESS for OSAS patients.

In conclusion, the data from this study indicate the possible important role of average inspiratory effort in determining subjective sleepiness in both obstructive sleep apnoea syndrome and upper airway resistance syndrome patients.

Excessive daytime sleepiness (EDS) is one of the most prominent symptoms associated with obstructive upper airway disorders during sleep 1, 2. As a consequence of EDS, significant deterioration is observed in daily performance of patients, with impairment in psychosocial and cognitive functions 3. Besides affecting the quality of life, EDS also increases the risk of motor vehicle and industrial accidents 3, 4.

In obstructive sleep apnoea syndrome (OSAS), many hypotheses have been proposed to explain EDS. Different aspects of polysomnographical evaluations, such as changes in breathing (i.e. apnoea/hypopnoea 5–8 or hypoxaemia 7–9), have been postulated as the main causes of EDS. In addition, reduction in the restorative nature of sleep by arousals 10–12, fragmentation of sleep 13, 14, a lack of slow-wave sleep 15 and a reduction in total sleep time (TST) 16, have all been suggested as possible causes of EDS.

Patients with upper airway resistance syndrome (UARS) also complain of daytime sleepiness 17, 18, like patients with OSAS. Nocturnal polygraphical recordings, however, indicate differences between these two groups. During apnoeic or hypopnoeic events, OSAS patients show a substantial decrease in arterial oxygen saturation (Sa,O2). Conversely, in UARS, Sa,O2 may fluctuate somewhat, but it stays at ~90%; the abnormal events are short, involving one or two breaths and consist of flow limitation but no apnoea 17, 19. Arousal reaction and consequent sleep fragmentation have been commonly thought to cause EDS in both syndromes 17, 20. However, some studies performed in patients with OSAS failed to show a strong association between sleep fragmentation and daytime somnolence 21, 22. Also in UARS, hypoxaemia, which was suggested to be an independent determinant of hypersomnolance in OSAS 7, 8, is an excluding factor contributing to daytime somnolence. Therefore, there are some questions about daytime sleepiness in patients with OSAS and UARS that remain answered. One of these questions is whether the degree of sleepiness is similar or not. If not, which variables, such as sleep structure, sleep fragmentation and respiratory variables, contribute the difference in sleepiness between UARS and OSAS. Another question is whether the amount of respiratory effort in UARS versus OSAS subjects could lead to differences in their sleepiness.

In one study, Zamagni et al. 23 demonstrated that the degree of respiratory effort during obstructive apnoeas contributes to self-rated sleep propensity in patients with OSAS. In that study obstructive apnoeas selected only from nonrapid eye movement (NREM) sleep were evaluated. However, it was reported that among 34 patients with mild OSAS, the apnoea/hypopnoea index (AHI) during rapid eye movement (REM) sleep, but not the AHI for the entire night, was well correlated with the mean sleep latency on multiple sleep latency tests (MSLT) 24. However, Chervin and Aldrich 25 suggested that apnoeic events during REM and NREM sleep probably contribute equally to sleepiness as measured by the MSLT 25. Therefore, evaluating the effect of respiratory effort on sleepiness in different sleep stages might supply further information about daytime sleepiness in obstructive respiratory disorders.

In the present study, the effects of inspiratory effort on daytime sleepiness, with the inclusion of the influences of sleeping position and sleep stages, were investigated in both UARS and OSAS. Other demographical and polysomnographical variables were also evaluated. In a previous study, the relationship between AHI supine sleep and daytime sleepiness was documented by Chervin and Aldrich 8. However, the influence of sleeping position on the amount of respiratory effort in the context of daytime sleepiness has not been investigated before. Only sleep stage dependent changes in respiratory effort were reported by Krieger et al. 26. They found that the respiratory effort was lower in REM than NREM sleep, but they did not establish a connection between this variability and EDS.

In this study, testing for associations between Epworth sleepiness scale (ESS)- and MSLT-defined sleepiness, and inspiratory effort and other variables, was reported in patients with UARS as well as OSAS.


   Material and methods
TOP
 Abstract
 Material and methods
Results
 Discussion
 References
 
Subject selection
From 150 consecutively selected patients, the clinical and polysomnographical data of 47 patients (11 females and 36 males) were evaluated. Inclusion criteria for the study included: 1) diagnosis of OSAS or UARS; and 2) >5.5 h of TST during 8 h polysomnographical recordings. Patients with restless leg syndrome and periodic leg movements during sleep were not included in the study. Fifteen patients were diagnosed as UARS with an AHI<10 and minimum Sa,O2>90%. For the diagnosis of UARS, the patient also had to have complained about excessive daytime sleepiness and have an increased number of arousals associated with increased respiratory effort. The other 32 patients were evaluated as OSAS with AHI>10 and minimum Sa,O2<90%. All patients underwent clinical interview before nocturnal polysomnography and these investigations were performed without any treatment.

Polysomnographical evaluations: sleep and respiratory variables
Nocturnal polygraphic recordings included the following variables: electroencephalogram (EEG) (C3/A2-C4/A1, according to the 10–20 international electrode placement system), electrooculogram, chin electromyogram and electrocardiogram. Respiration was analysed with a pneumotachograph attached to a face mask. Snoring was evaluated with a microphone that was placed above the larynx. Oesophageal pressure (Poes) was measured with a latex balloon placed in the lower third of the oesophagus, as described by Baydur et al. 27, connected to a pressure transducer and inflated with 1 mL of air. The saturation during sleep was measured continuously using finger oxymetry.

Nocturnal recordings were scored according to the standard criteria of Rechtschaffen and Kales 28, as epochs of 30 s, and the following sleep variables were calculated: TST, sleep efficiency index (TST/time in bed), sleep continuity index (TST/total sleep period), waketime after sleep onset, percentages of sleep stages, and awakening and arousal index. Awakening was defined as transition to awake state from any NREM or REM sleep stage for ≥15 s and the awakening index was calculated as the number of awakenings per hour of the total sleep period.

Abnormal respiratory events were evaluated according to standard criteria of American Sleep Disorders Association (ASDA) 29. An apnoea was defined as a cessation of airflow with a duration of ≥10 s. A hypopnoea was defined as ≥50% reduction in airflow relative to baseline, coupled with a desaturation of ≥4%. Obstructive type apnoeas were differentiated from central and mixed apnoeas with Poes analysis. Flow limitations were differentiated from the other abnormal respiratory events as they were defined as episodes during which there was an increase of ≥20% in peak-to-peak Poes amplitude, compared with baseline, lasting ≥15 s, with a reduction in flow not >50% of baseline. These episodes were not accompanied by significant oxygen desaturation and resulted in awakening or arousal reactions and the breath following the awakening or arousal reaction showed an abrupt reduction in Poes (fig. 1Go). Baseline peak-to-peak Poes fluctuations were obtained during awake, unobstructed breathing. The AHI was calculated as the number of apnoeas and hypopnoeas per hour of sleep. However, respiratory disturbance index (RDI) was established as the number of all obstructive type respiratory events per hour of sleep including flow limitations. As an index of nocturnal Sa,O2, the minimum Sa,O2 level throughout the night was measured. For OSAS patients, percentage TST with an Sa,O2 <90% was also calculated.



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  		Fig. 1.— Oesophageal pressure (Poes) measurement for flow limitation (box). a) Flow, b) Poes and c) electroencephalogram. The minimum of the first three Poes where the crescendo pattern began was taken as minimum Poes (closed arrow; –6). The maximum of the last three Poes values just before the effort associated arousal (dashed arrow) was taken as maximum Poes (open arrow; 8).

 
Arousals were determined according to ASDA criteria 30 and the arousal index was computed as the number of arousals per hour of sleep.

Assessment of daytime vigilance
Daytime sleepiness was assessed subjectively by ESS 31 and objectively by MSLT 32.

The Turkish version of ESS was administered by two technicians. In the current authors' laboratory there are rules for the administration of ESS that limit subjective evaluations of technicians. For the ESS, 2–10 was accepted as normal and >10 was an indicator of pathological degree of sleepiness 33.

The MSLT was performed in the sleep laboratory the day following the polysomnographical examinations 32. The MSLT consisted of five naps scheduled at 10:00, 12:00, 14:00, 16:00 and 18:00 h. The mean sleep latency was calculated by averaging the individual latency scores of the five sleep latency tests. A mean latency to sleep onset of ≤5 min indicated severe sleepiness, while >10 min was taken as normal. The patient population with a mean sleep latency, in the range of 5–10 min, was classified as mild in sleepiness 32.

The cut-off points of excessive sleepiness for ESS (10 point) and MSLT (5 min) were not used for statistical analysis. They were only used for the evaluation of patients on an individual basis.

Assessment of respiratory efforts
Twenty representative obstructive respiratory events (obstructive apnoeas, hypopnoeas or flow limitations) were selected from the sleep stages (stages (st.) 1+2, 3+4) of NREM and REM sleep in both supine and side positions, and each respiratory event was evaluated to analyse inspiratory effort. Mixed and central apnoeas were not included in the analysis. As shown in figure 2Go, Poes variations (from end-expiration to peak inspiration in the wave of Poes) during the first three and the last three occluded inspiratory efforts within obstructive apnoeas were measured. The minimum value of the first three efforts was taken as the minimum Poes and the maximum of the last three occluded efforts as the maximum Poes. In order to analyse the inspiratory effort within the flow limitation episode, the Poes value at end-expiration for each breath was measured. The maximum of the three occluded inspiratory efforts before effort-associated awakening or arousal reactions was assessed as the maximum Poes (fig. 1Go). Although the crescendo pattern of inspiratory effort was not as brief as obstructive apnoeas, three inspiratory efforts were taken at the beginning of the episode that was ≥15 s before the awakening or arousal reaction, and the minimum value of these measurements was accepted as the minimum Poes. The duration of each sleep stage in certain positions was also measured manually from recordings.



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  		Fig. 2.— Oesophageal pressure (Poes) measurement for obstructive apnoea. a) Flow and b) Poes. The minimum of the first (closed arrow; 16) and the maximum of last three (open arrow; –42) Poes were taken.

 
In the different sleep stages and positions, maximum Poes was calculated by the average of 20 measured maximum Poes values of abnormal obstructive respiratory events. The Poes value for nocturnal polygraphic recording was calculated by using the following formula:


(001)

The same formula was also used for computing the average minimum Poes values. The difference between the average maximum and the minimum inspiratory effort was taken as a measurement of the overall increase in the inspiratory effort during obstructive respiratory event ({Delta}Poes).

Poes values were reported as absolute values to facilitate interpretation of the results. Therefore, higher Poes reflected an increase in respiratory effort.

Statistical analysis
Results are presented as mean±sd. Sleep and respiratory parameters of patients diagnosed with OSAS and UARS were compared using the unpaired t-test.

For both UARS and OSAS patients, bivariate correlation analysis using Pearson's correlation coefficient was performed, using the ESS and MSLT scores as dependent variables and variables associated with sleep structure, respiratory events, respiratory effort and severity of hypoxaemia as independent variables. The parameters that were significantly correlated and very close to significance were included in multiple regression analysis.

The backward method was used for multiple regression analysis, taking the ESS scores and mean sleep latency of MSLT as dependent variables to identify significant models with the probability of F to remove p≤0.20.

Statistical significance was accepted for a p-value of <0.05.


   Results
TOP
 Abstract
 Material and methods
 Results
Discussion
 References
 
The patient population consisted of UARS and OSAS patients. Ten of 15 patients with UARS and 26 of 32 patients with OSAS were male. There were no significant overall differences in age (45±9 versus 48±10) and body mass index (30.3±4.9 versus 31.3±4.2) between UARS and OSAS patients, respectively.

The descriptive measurements of nocturnal sleep and respiratory parameters of OSAS and UARS patients are presented in table 1Go. OSAS patients showed wide variability in severity of nocturnal hypoxaemia, with a minimum Sa,O2 ranging 49–87%, while a minimum Sa,O2 of 90% was used as a diagnostic criterion for UARS patients. Although there was a significant difference in percentage of NREM 1–2 between OSAS and UARS patients, no significant difference in REM and slow-wave sleep was found. Arousal index was significantly higher in OSAS compared to UARS patients, 32±12 and 64±14, respectively.


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  		Table 1— Nocturnal sleep and respiratory parameters

 
The ESS score showed variability, ranging 1–20, in patients with both OSAS and UARS. The mean sleepiness score of UARS patients was 8.1±3.6 (range 1–16), while the mean score of OSAS patients was 10.6±5.2 (range 3–20). There were 16 OSAS and three UARS patients with an ESS score >10, which was defined as the upper point of normal range of sleepiness 33. The MSLT scores also showed a wide distribution, ranging 2.5–14 min (mean 7.0±3.3) in UARS patients and 1–10 min (mean 4.7±2.0) in OSAS patients. The pathological degree of sleepiness for the MSLT, which was defined as <5 min, was found in 22 OSAS and four UARS patients. ESS and MSLT scores showed that OSAS patients were significantly sleepier than UARS patients.

The maximum Poes calculated in each sleep stage and position are presented in table 2Go. Measurements in stages 3–4 for 12 OSAS patients and in REM for three OSAS patients were not performed due to the absence of these sleep stages in the recordings. Also, in two UARS patients, Poes levels during slow-wave sleep could not be determined because of inadequate duration. These unobserved sleep stages in certain positions were not taken into consideration for the calculation of averaged maximum Poes. The maximum Poes values did not differ between supine and side positions within the same sleep stages in OSAS patients. There was only significant positional difference obtained in UARS patients during slow-wave sleep; maximum Poes during supine position was higher than side position (40.2±11.7 and 30.2±6.5, respectively; p<0.01).


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  		Table 2— The distribution of maximum oesophageal pressure (Poes) according to sleep stages and positions

 
The average maximum Poes showed a wide range of variability in UARS as well as in OSAS patients. The average maximum Poes was 27.7±8.3 (range 16–53 cmH2O) and 44.9±13.7 (range 22.8–78.3 cmH2O) in UARS and OSAS patients, respectively. Other analyses of Poes between the two groups of patients are shown in table 3Go.


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  		Table 3— Analysis of oesophageal pressure (Poes) for whole polygraphic recordings

 
The ESS and MSLT were not significantly correlated with each other in either UARS (r=–0.27, p=0.32) or OSAS patients (r=–0.32, p=0.07).

The results of bivariate correlation analysis for UARS and OSAS are presented in table 4Go. In patients with UARS, the only significant correlation was obtained with average Poes and Epworth score (r=0.48, p=0.035). ESS score was found to be positively correlated with arousal index, number of stage variations, RDI, average Poes, {Delta}Poes and percentage of TST with an Sa,O2 <90% in OSAS patients (table 4Go). There was no significant correlation for the MSLT scores in either UARS or OSAS patients.


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  		Table 4— Correlation coefficients and their significance for Epworth and multiple sleep latency test (MSLT) scores in upper airway resistance syndrome (UARS) and obstructive sleep apnoea syndrome (OSAS) patients

 
In OSAS patients, the multiple regression analysis, using the backward method, was performed to more clearly define the relationship of these correlated variables with the Epworth score. The selected model included number of stage variations and average Poes. The multiple correlation coefficient (R=0.51) was statistically significant (F=5.17, df=2, p=0.012). The only significant beta weight was for average Poes (ß=0.165, se of ß=0.06, p=0.01).


   Discussion
TOP
 Abstract
 Material and methods
 Results
 Discussion
References
 
Excessive daytime sleepiness is a common complaint among patients with OSAS 1, 2. The association between daytime sleepiness and various respiratory and/or sleep-related factors has been investigated for many years. Among these factors, the most apparent association has been established with AHI 5–8, hypoxaemia severity 7–9 and sleep fragmentation 10–14. The novelty in the current data is the demonstration that the average inspiratory effort during obstructive respiratory events, calculated with respect to sleep stages and positions, contributes to self-rated sleep propensity in both UARS and OSAS patients.

The AHI, which is a commonly accepted indicator of disease severity, did not show a persistent relationship with daytime sleepiness. Although it has been suggested that there is a significant association between sleepiness and RDI 6–8, 34, in a study of Wisconsin state employees 35, 9% of females and 24% of males had an RDI of >5, while only 2% of females and 4% of males reported daytime sleepiness. A higher percentage of reported sleepiness was found even in patients with an RDI of <5 34, 35. In this study, RDI was significantly higher in patients with OSAS than with UARS. Furthermore, OSAS patients were sleepier than patients with UARS in both the subjective and objective evaluation of sleepiness. This finding might be explained by the findings of previous studies, which showed a linear association between sleepiness and RDI 33, 34. However, a very weak correlation between RDI and ESS score in OSAS patients, and no correlation with ESS in UARS and with MSLT in both UARS and OSAS patients did not support this explanation in the current study.

Another possible explanation of the difference in sleepiness between UARS and OSAS patients is the contribution of hypoxaemia to the sleepiness of OSAS patients. Several investigators 36–38 have suggested that nocturnal hypoxaemia may be the primary cause of hypersomnolance in OSAS patients. In two separate reports, Mendelson 36, 37 has shown that measures of oxyhaemoglobin desaturation were the most significant predictors of daytime sleepiness as measured by MSLT. In support of these findings, there are further studies suggesting the importance of nocturnal hypoxaemia as an independent contributor to sleepiness in OSAS patients 7, 8. Interestingly, no correlation was found between measures of hypoxaemia severity and MSLT score in OSAS patients, which might be due to the difference in measure of hypoxaemia. Punjabl et al. 7 categorised the severity of oxyhaemoglobin desaturation as ≤5%, 5.1–10%, 10.1–15% and >15%, and found an inverse relationship between the degree of drop in saturation and hypersomnolance. However, Chervin and Aldrich 8 suggested that minimum Sa,O2 had an independent contribution to sleepiness in OSAS patients. However, minimum Sa,O2 as a measure of hypoxaemia severity in the present study did not show any significant association between ESS score and MSLT. The only significant finding was the correlation between percentage of TST with an Sa,O2 <90% and ESS in OSAS patients. The current authors believe that the effect of hypoxaemia on sleepiness deserves further investigation, but it does not seem to be an independent contributor to sleepiness in sleep disordered breathing if UARS is thought of as a physiological continuum between primary snoring and OSAS 39.

The average inspiratory effort, which was found as an explanatory variable to sleepiness in patients with UARS as well as OSAS, might be another confounding factor making OSAS patients more sleepy than patients with UARS. In this study, it was found that OSAS patients had more inspiratory effort throughout the night than the UARS patients. This finding could be accepted as one of the explanations of the difference in sleepiness between the two groups.

This explanation would take into account the most currently accepted explanation of daytime sleepiness in patients with UARS as well as OSAS, which relates daytime sleepiness to the consequences of abnormal respiratory events on sleep structure, causing arousal reactions and sleep fragmentation 10–12,14. However, Berg et al. 40 reported that nonapnoeic snorers complaining of excessive daytime sleepiness have similar total numbers of arousal to asymptomatic controls. It was also suggested that this difference in daytime symptom is created by the type of arousals (pure EEG versus respiratory). Roehrs et al. 13 also found a relationship between respiratory arousal index and daytime sleepiness. It has also been suggested that an increased effort of breathing is a more important stimuli for arousal than either hypercarbia or hypoxia 41. If daytime sleepiness was affected by arousals, especially respiratory arousals, there might be other determining factors identifying the arousal as a respiratory outcome. Therefore, inspiratory effort might be one of the designating factors for arousals.

The lack of correlation between arousal index and any sleepiness index in patients with UARS and MSLT of OSAS patients was surprising. This may be due to the monitoring technique used or insufficiency of ASDA criteria for designating arousals rather than the actual absence of electrocortical activation. In the current study, central electrodes were used for EEG recordings and frontal electrodes were used to improve the detection of respiratory-related arousal reactions 42. This might be one explanation for the failure to note visible arousal reactions in this study. Conversely, it has been proposed that there are some microarousals 43 and non-EEG indices of arousal-like autonomic activation (i.e. changes in blood pressure and heart rate) 14, 43, and increasing the number of these type of arousals has been shown to impair daytime functions in normal subjects 15 and OSAS patients 14, 43. As a result it can be considered that detection of arousals according to ASDA criteria, as well as other parameters, would supply better information about the effects of arousals on daytime sleepiness.

In this study, another point to be discussed is the evaluation of arousal reactions relating to lower leg movements. Mendelson 44 demonstrated that the index of periodic limb movement (PLM) with movement arousals was not different in patients with and without excessive daytime sleepiness. In addition, Coleman et al. 45 reported that there was no correlation between PLM index and MSLT in patients with primary insomnia and PLM during sleep (PLMS). Since the patients with restless leg syndrome and PLMS were excluded at the beginning of the study, it was expected that analysing the lower limb movement and associated movement arousals would not give any extra information about the sleepiness of patients with obstructive upper airway disorders during sleep.

AHI as a determinant of EDS has been investigated previously within the aspects of body position 8 and sleep stages 24, 25. As a possible determinant of subjective sleepiness, inspiratory effort has been implicated by Zamagni et al. 23, who evaluated obstructive apnoeas during NREM sleep. However, evaluation of respiratory effort considering body position and sleep stages has not been performed previously. In the current study, inspiratory effort for all obstructive respiratory events, including flow limitations in all sleep stages and in different positions, were evaluated.

Chervin and Aldrich 8 suggested that excessive daytime sleepiness was better explained if sleeping position, type and character of abnormal respiratory event were taken into consideration during the evaluation of apnoeas and hypopnoeas. In that study, it was reported that the rate of apnoeic events in the supine position correlated better with daytime sleepiness. In this study, no position-dependent change in respiratory effort was found within the same sleep stage, except slow-wave sleep of UARS patients. The findings indicate that respiratory effort in both the supine and nonsupine positions probably contribute equally to the role of respiratory effort on daytime sleepiness. Moreover, the importance of apnoeas and hypopnoeas during different sleep stages, either REM sleep alone 24 or REM sleep as well as NREM sleep 25, on the determination of daytime sleepiness, suggests a role for sleep stages in sleepiness. The authors found that inspiratory effort was lower in REM sleep than NREM sleep in patients with both UARS and OSAS. This conclusion is similar to previous findings 26. The authors consider that evaluation of inspiratory effort in such large aspects, including body position and sleep stages, supply enlarged vision to the evaluation of sleepiness in sleep disordered breathing. The significant correlation between average inspiratory effort and ESS in patients with UARS also supported its important role in daytime sleepiness.

Some limitations of using Poes should be considered. Analysing the representative 20 obstructive respiratory events for each sleep stage and position totalled 80–120 respiratory events for each patient. This selection and positional differences between patients may have created variability in mean inspiratory effort. Therefore, using this formula in a study over consecutive nights and using a randomised selection of abnormal respiratory events may provide more information about the validity of this formula.

A possible explanation of the result of the current study is that inspiratory effort itself may increase sleepiness by way of energy expenditure throughout the night. The level of daytime sleepiness was described by Johns 46, 47 as a function of the total sleep drive to the total wake drive with which it competes, throughout the 24-h sleep/wake cycle. According to this theory, the majority of these drives are under the influence of the central nervous system. However, some behavioural and physical conditions, such as posture, feelings, mental and physical activity, have a role, especially on wake drive. Inspiratory effort, creating energy expenditure throughout the night, might be considered to affect the balance between sleep and wake drive in patients with UARS or OSAS.

In contrast with the Epworth score, any parameter showed significant correlation with MSLT scores in both UARS and OSAS patients. The lack of association between MSLT and any polysomnographic variables is also supported by other studies in which such associations are absent 48, ambiguous 20 or weak 13, 49. The authors also failed to find a significant correlation between ESS and MSLT scores. Similarly, many investigators have reported poor or no association between sleep latency of MSLT and ESS and other subjective tests, such as Stanford sleepiness scale 50, 51. A possible explanation might be that subjective and objective tests evaluate different aspects of sleepiness. In fact MSLT measures the speed with which a sleep episode occurs under standardised laboratory conditions, whereas ESS evaluates the frequency of unwanted sleep episodes under naturalistic conditions of daily life 52. Under this definition, the term "average sleep propensity" might be more preferable for the parameter that ESS measures, as suggested by Johns 47.

In conclusion, by taking into consideration the effect of sleep stages and body position, this study provides new evidence for average inspiratory effort as a contributing factor to the self-rated sleep propensity in patients with upper airway resistance syndrome as well as obstructive sleep apnoea syndrome. Finally, measuring sleepiness with a more reliable test may provide better information for understanding the relationship between respiratory effort and daytime sleepiness in obstructive breathing disorders during sleep.


   References
TOP
 Abstract
 Material and methods
 Results
 Discussion
 References
 

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