Introduction

Obstructive sleep apnea syndrome (OSAS) is associated with an increased risk of the cardiovascular complications [1, 2]. Repetitive episodes of respiratory cessation during the night resulting in nocturnal hypoxemia are considered to be responsible for the association between OSAS and cardiovascular risk. Atherosclerosis, an inflammatory process that affects the elasticity of arterial vessel wall [3], may be a mechanism which mediates the cardiovascular risk factors in OSAS [4, 5]. Arterial stiffness [6] and endothelial dysfunction [7] have been known as early markers of atherosclerosis. These parameters were altered in OSAS patients [8–11]. We have previously found that nocturnal hypoxia was a significant determinant for arterial stiffness and endothelial dysfunction in OSAS [12, 13]. Insulin resistance, dyslipidemia, and inflammatory reactions are also risk factors of cardiovascular complications [3]. It has been reported that those serum cardiovascular risk factors were associated with OSAS patients [14–16].

It is widely accepted that nasal continuous positive airway pressure (nCPAP) is most effective therapy for OSAS. The nCPAP treatment might be effective in improving arterial stiffness and endothelial dysfunction; and dyslipidemia, insulin resistance, and inflammatory reaction could be also improved by nCPAP treatment. However, several study results have been just recently reported about the effect of continuous positive airway pressure (CPAP) on arterial stiffness or endothelial dysfunction in OSAS patients [17–20]. Also, the effects of nCPAP on dyslipidemia, insulin resistance, and inflammatory reaction were conflicting and inconclusive [21]. The aim of this study was to investigate whether nCPAP treatment would improve arterial stiffness and endothelial dysfunction simultaneously in OSAS patients. We also measured cholesterol, insulin resistance, and serum level of C-reactive protein (CRP) before and after nCPAP treatment to investigate factors related with decreasing cardiovascular complications after nCPAP treatment.

Material and methods

Subjects

Study subjects were recruited among the patients referred to the sleep laboratory in Seoul National University Bundang Hospital for nocturnal polysomnography (NPSG). OSAS patients with at least moderate to severe degree (Apnea Hypopnea Index, AHI ≥15) were enrolled in this study. Based on the clinical interview and medical records, subjects suffering from inflammatory diseases, chronic obstructive pulmonary disease, or cardiovascular diseases such as coronary artery disease, myocardial infarction, or congestive heart failure were excluded. Patients taking antihypertensives, antihyperlipidemics, or hypoglycemics were included, if the medication regimens were not changed during the study period. The study protocol was approved by the Institutional Review Board in Seoul National University Bundang Hospital, and all the subjects provided written informed consents.

Polysomnography and nCPAP titration

Overnight polysomnography was performed by using an Embla™ N 7000 recording system (Embla; Reykjavik, Iceland) and standard electrodes and sensors. Electroencephalography electrodes were applied at C3/A2, O1/A2, and O2/A1, and two electrooculography electrodes applied at the sides of both eyes to record horizontal and vertical eye movements. Submental electromyography (EMG) electrodes were applied at submentalis muscle and the EMGs of both anterior tibialis muscles recorded limb movements during sleep. Strain gages were used for recording chest and abdominal respiratory movements, and nasal pressure cannulas were used to record airflow. Arterial oxygen saturation was measured using pulse oximeters applied on index fingers. Based on the criteria of Rechtschaffen and Kales [22], we scored every epoch of 30 s NPSG. Apnea was defined as complete cessation of airflow for at least 10 s. Hypopnea was defined as a substantial reduction in airflow (>50%) for at least 10 s or a moderate reduction in airflow for at least 10 s associated with electroencephalography arousals or oxygen desaturation (≥4%) [23]. AHI was defined as the total number of apnea and hypopnea per hour of sleep, and oxygen desaturation index was calculated as the number of oxygen desaturations (≥4%) per hour of sleep.

Based on the severity of OSA and physical examinations of upper airway, the study subjects were recommended to use nCPAP. The ideal pressure for each patient was determined by an additional overnight nCPAP titration study. After using the nCPAP device for at least 3 months, data for nCPAP usage and mean AHI during the nCPAP usage were obtained from data cards inside the nCPAP machine. Compliance with nCPAP was defined as percent of days with nCPAP usage for at least 4 h among total days of the study. Subjects who have used nCPAP device with good compliance (≥60%) were included in the analysis. The AHIs during the nCPAP usage obtained from CPAP machines were compared with baseline AHIs of NPSG.

PWV, FMD, and laboratory tests

PWV and FMD were measured and peripheral blood was drawn from OSAS patients in the morning following NPSG. After the nCPAP treatment for at least 3 months, these tests were repeated in the morning following the night patients slept at their home. The patients fasted for at least 8 h prior to the measurements of PWV and FMD. PWV was calculated from pulse transit time and the distance traveled by the pulse between two recording sites [24] using a non-invasive vascular screening device VP-2000 (Omron-Colin, Kyoto, Japan) system. By using this device, we measured electrocardiogram, phonocardiogram, oscillometric signals from four extremities and ankles, and tonometric signals from right common carotid and right femoral arteries [25]. The estimated difference in the distance (Δd) between from heart to carotid and from heart to femoral artery based on the Frank method [26], and time delay between the start of the sharp systolic upstroke of carotid and femoral pressure waves (∆t) were obtained. Carotid-femoral PWV was calculated using the equation: cfPWV (m/s) = Δd (m)/∆t (s).

Brachial artery FMD was measured using an Accuvix XO ultrasound unit (Medison; Seoul, Korea) equipped with a 6-12 MHz linear-array transducer and lower arm occlusion technique [27]. The baseline diameter of the brachial artery was measured from the anterior intima to the posterior intima. A blood pressure (BP) cuff was placed on the lower part of the arm and inflated to 250 mm Hg for 5 min. The BP cuff was then released and brachial artery diameters were measured three times at 40, 60, and 80 s after release by investigators unaware of subject details. After recording the maximum value of three measurements, FMD was calculated as the ratio of change in diameter (maximum − baseline) over baseline value.

Serum samples were stored at −70°C until required for laboratory tests. High sensitive CRP (hsCRP) was measured by turbidimetric immunoassay using the Vitros 5.1 FS Chemistry System (Ortho-Clinical Diagnostics; Raritan, NJ, USA). Its limit of quantification for CRP was 0.01 mg/dL. Serum levels of total cholesterol, triglyceride, high density lipoprotein (HDL) cholesterol, and glucose were measured. To assess insulin resistance, homeostasis model assessment for estimating insulin resistance (HOMA-IR) was calculated using the equation: HOMA-IR = [fasting serum insulin (U/ml) × fasting plasma glucose (mmol/L)/22.5]

Statistical analysis

SPSS Ver. 12.0 for Windows (SPSS Inc, Chicago, IL, USA) was used for the statistical analysis. Results are presented as means ± SD. The Kolmogorov-Smirnov test was used to confirm normality. Paired t test for parametric variables or Wilcoxon signed-rank test for non-parametric variables was used to compare the measurements between before and after the nCPAP treatment. The significance criterion was defined to be p < 0.05 for two-tailed tests.

Results

A total of 52 patients with moderate to severe degree of OSAS participated in the study at the baseline assessments. Among the initial participants, 27 patients were excluded from the current analysis because of partial compliance to CPAP treatment (N = 15, 55.6%), stopping nCPAP treatment (N = 10, 37.0%), or withdrawal of consent (N = 2, 7.4%). There was no difference between compliers and non-compliers in terms of serum cardiovascular risk factors, OSA severity, and demographic and clinical characteristics.

Data from 25 subjects (22 men and three women) were included in the analysis. Mean age of 25 subjects was 51.4 ± 11.5 (33-76) years old, and their mean AHI was 64.9 ± 20.0 (26.1–99.5)/h. Mean duration of nCPAP treatment was 138.7 ± 42.6 (90–250) days, and mean nCPAP compliance was 81.3 ± 11.3 (60.0–98.9)%. Body mass index (BMI), neck circumference, waist-to-hip ratio, diastolic blood pressure, systolic blood pressure, and oxygen-related variables were presented in Table 1. Of those patients, 12 patients have been taking medications which included antihypertensives (N = 11), hypoglycemics (N = 3), and antihyperipidemics (N = 3).

Table 1 Demographic and clinical characteristics of the subjects (N = 25)
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During the nCPAP treatment, mean AHI of the patients was significantly decreased from 64.9 ± 20.0/h at baseline to 4.1 ± 2.0/h (p < 0.001). The value of cfPWV was significantly changed from 11.2 ± 4.5 m/s at baseline to 9.3 ± 2.1 m/s after the CPAP treatment (p = 0.031, Fig. 1). One patient showed unusually high cfPWV value of 28.8 m/s before CPAP treatment, but the improvement in cfPWV after nCPAP treatment was still significant with excluding the patient (10.4 ± 2.7 at baseline to 9.2 ± 2.0 after nCPAP treatment, p = 0.027). The value of FMD was significantly improved from 5.52 ± 2.49% at baseline to 6.58 ± 2.50% after the CPAP treatment (p = 0.006, Fig. 1). Changes in anthropometric data and laboratory findings before and after nCPAP treatment were shown in Table 2. There were no significant changes in BMI, blood pressure, total cholesterol, triglyceride, HDL cholesterol, or glucose. Also, there was no change in the level of HOMA-IR or serum level of CRP. The standard deviation of CRP was great because of two outliers in the CRP levels. After excluding the outliers, the result of CRP analysis was not changed. When we reanalyzed each outcome measure after excluding patients taking medications that may interfere with the measure (FMD and PWV in those not taking antihypertensives/lipids and CRP in those not taking antihyperlipidemics/and glucose and HOMA-IR in those not taking hypoglycemic agents), the results were unchanged. Follow-up FMD, PWV, and laboratory tests were performed only for the CPAP compliers, therefore comparison of the results of these variables between the CPAP compliers and the CPAP non-compliers could not be obtained.

Fig. 1
figure 1

Changes in cfPWV and FMD after nCPAP treatment. The cfPWV decreased from 11.2 ± 4.5 m/s to 9.3 ± 2.1 m/s, and FMD was improved from 5.52 ± 2.49% to 6.58 ± 2.50% after nCPAP treatment. cfPWV carotid-femoral pulse wave velocity, FMD flow-mediated dilatation

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Table 2 Anthropometric data (N = 25) and laboratory findings (N = 24) of the subjects before and after nCPAP treatment
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The proportions of patients whose value of cfPWV or FMD was improved after nCPAP treatment were 64.0% (16/25) and 72.0% (18/25), respectively. There was no significant difference in age, anthropometric variables, baseline AHI, baseline serum levels of lab tests and nCPAP related variables such as compliance, mean pressure, and mean duration of usage between the patients with improvement and without improvement in cfPWV or FMD.

Discussion

It is widely known that nCPAP treatment of OSAS patients has an impact on cardiovascular or cerebrovascular diseases such as hypertension [28], coronary artery disease [29], or cerebral infarct [30]. Management of moderate-to-severe degree of OSAS is mainly dependent on nCPAP, which improves nocturnal hypoxia by splinting the upper airway and counteracting the inspiratory negative pressure during sleep. We have demonstrated that nCPAP treatment could improve arterial stiffness and endothelial dysfunction in patients with OSAS. Also, these improvements were not related with changes in BMI, lipid profile, or insulin resistance after nCPAP treatment.

Arterial stiffness and endothelial dysfunction have been considered as early markers of atherosclerosis [7, 31]. The values of PWV and FMD might be worsened simultaneously by systemic progression of atherosclerosis [32], although these parameters are used to evaluate different aspects of atherosclerotic change. Recent studies have demonstrated that the bioavailability of nitric oxide (NO) and endothelial dysfunction were implicated in the regulation of arterial stiffness [33]. Consequently, it seems that FMD and PWV are simultaneously influenced by decreased NO bioavailability induced by nocturnal hypoxemia during sleep in OSAS patients [34, 35]. Previously, we have reported that nocturnal hypoxemia-related polysomnographic variables were significant determinants of impaired FMD and PWV in OSAS patients [12, 13]. These findings have the implication that correction of nocturnal hypoxemia during sleep might improve endothelial dysfunction and nocturnal stiffness in OSAS patients. In this study, AHI of the patients was significantly decreased during the nCPAP treatment and the reduced AHI may potentially be associated with the improvement of oxygen saturation during sleep, although we did not measure oxygen saturation during the nCPAP treatment. The effects of nCPAP on arterial stiffness and endothelial dysfunction seem to be mediated by resolution of the nocturnal hypoxia rather than changes in BMI, lipid profile, inflammatory reaction, or insulin resistance.

There were no significant changes in dyslipidemia, insulin resistance, or inflammatory reaction after nCPAP treatment in this study. These parameters are another important factor associated with atherosclerosis progression [3]. However, usage of nCPAP in a relatively short period does not seem to have significant effect on insulin resistance or lipid profile in the previous studies [36–38]. Results of nCPAP effect on CRP were also inconclusive [39, 40] in OSAS patients. Furthermore, dyslipidemia, insulin resistance, or CRP in OSAS was previously reported to be dependent on obesity [13, 41–43] rather than the severity of OSAS or nocturnal hypoxia, although these arguments are debatable. Considering those findings, it is predictable that CPAP could not improve dyslipidemia, insulin resistance, or inflammatory reaction without weight reduction in patients with OSAS.

We included the patients taking antihypertensives, antihyperlipidemics, or hypoglycemics, although it might be desirable to exclude patients taking those medications. However, the dosage and kind of medications were not changed during the study period; and in real clinical setting, it is very common for patients with OSAS of severe degree to take those medications. Expectedly, the results were not changed after excluding patients taking medications. The non-randomized design, small sample size and lack of sham-CPAP group were the main limitations of this study. Because of relatively small sample size, very small numbers of outliers might produce large effect size without statistical significance as in the level of CRP. That the effect of nCPAP was not so much prominent as expected might be another limitation of this study. The improvements in early markers of atherosclerosis were not observed in about one-third of patients. We could not find any differences in clinical and polysomnographical characteristics between patients with improvement and without improvement; therefore, further study is needed to clarify factors that mediate improvement in early markers of atherosclerosis.

In conclusion, nCPAP could decrease the development of cardiovascular complications in OSAS through improving impaired vascular functions, which was not accompanied by changes in BMI or improvement of dyslipidemia, inflammation or insulin resistance.