Endothelial Function in Patients with Post-CPAP Residual Sleepiness

Abstract

Study Objectives:

The significance of residual excessive daytime sleepiness (EDS) on cardiovascular markers in patients with adequately treated obstructive sleep apnea (OSA) remains unclear. The objective of this study was to investigate flow-mediated dilatation (FMD) and inflammatory markers (C-reactive protein [CRP], tumor necrosis factor [TNF]-α, and interleukin [IL]-6) in continuous positive airway pressure (CPAP)-compliant patients with residual EDS compared with CPAP-compliant patients without residual EDS.

Methods:

FMD of the brachial artery was measured by ultrasound in 12 CPAP-compliant patients with OSA who had residual EDS and 12 age-, sex-, and body mass index-matched CPAP-compliant patients with OSA who did not have residual EDS on week 8 after initiation of CPAP. Twelve otherwise-healthy subjects without sleep disordered breathing were used as control subjects. Serum concentrations of CRP, TNF-α, and IL-6 were quantified by enzyme-linked immunosorbent assays.

Results:

Baseline FMD was comparable among CPAP-compliant patients with residual EDS (7.2 ± 2.3), CPAP-compliant patients without residual EDS (8.6 ± 2.1), and control subjects (7.7 ± 1.4) (p = 0.37). The concentrations of CRP, TNF-α, and IL-6 were also not significantly different between subjects with CPAP-compliant residual EDS and those without residual EDS (p = 0.44, p = 0.37, and p = 0.42; respectively).

Conclusions:

Residual EDS in patients with adequately treated OSA may not represent a risk factor for cardiovascular diseases.

Citation:

El-Solh AA; Akinnusi ME; Moitheennazima B; Ayyar L; Relia S. Endothelial function in patients with post-CPAP residual sleepiness. J Clin Sleep Med 2010;6(3):251-255.

Obstructive sleep apnea (OSA) is a common disorder characterized by recurrent upper airway collapses during sleep.¹ These recurrent episodes of upper airway collapse lead to sleep fragmentation, oxyhemoglobin desaturation, and excessive daytime sleepiness (EDS). As a result, there is a sustained activation of the sympathetic nervous system, increased expression of systemic inflammatory markers, and derangement in endothelial function.^2,3 Many of these physiologic and biochemical abnormalities are implicated in the pathogenesis of cardiovascular disease.

BRIEF SUMMARY

Current Knowledge/Study Rationale: Epidemiological studies suggest that treatment of OSA is associated with a reduction in cardiovascular events however a small group of patients continues to experience persistent excessive sleepiness despite adequate CPAP therapy. The significance of post CPAP sleepiness on cardiovascular risk profile is unknown.

Study Impact: The current study shows that residual excessive daytime sleepiness in adequately CPAP-treated sleep apnea is not associated with increased inflammatory markers or endothelial dysfunction.

The current management of OSA is largely dependent on nasal continuous positive airway pressure (CPAP), which acts to splint the upper airway open during sleep. The normalized breathing patterns translate into major improvements in a broad array of daytime measures, quality of life, and neurocognitive function,^4,5 as well as in cardiovascular risk factors.^6,7 However, 6% to 14% of compliant CPAP-treated patients with OSA continue to exhibit residual EDS.^8,9 The scores of the emotional and energy domains are 2 times worse in those patients, compared to CPAP-treated patients without residual EDS. Furthermore, residual EDS has been consistently associated with an adverse cardiovascular risk profile. ^10–12 Yet, the clinical implications of residual EDS on cardiovascular diseases in CPAP-treated patients are not fully understood. We hypothesized that residual EDS in patients with adequately treated OSA is associated with endothelial dysfunction and a heightened inflammatory state. Hence, we sought to investigate whether residual EDS after CPAP therapy elicits increased expression of circulating inflammatory markers (C-reactive protein [CRP], tumor necrosis factor [TNF]-α, and interleukin [IL]-6) and contributes to endothelial dysfunction.

MATERIALS AND METHODS

Study Population

The study protocol was approved by the Institutional Review Board, and all subjects provided written informed consent. Participants were part of a cohort group under study for the association between OSA and cardiovascular diseases. All male patients with polysomnography-proven OSA who were compliant with CPAP therapy (M series auto CPAP, Respironics, PA) at 8 weeks follow-up were considered for enrollment. All participants had received education sessions, training, and proper mask fitting. Overnight oximetry was performed for 1 night while the patients were on CPAP therapy to assess the presence of oxygen desaturation. Subjects who were actively smoking or who were receiving treatment for cardiovascular diseases or diabetes mellitus were excluded. Patients diagnosed with central sleep apnea, restless legs syndrome, narcolepsy, chronic insomnia, or depression were also excluded. Subjects with residual EDS post-CPAP therapy were matched by age (within 5 years), sex, and body mass index (BMI) (within 10%) with a group of CPAP-compliant patients without residual EDS. A control group of nonsmoking healthy subjects with no evidence of sleep apnea by polysomnography was recruited for comparison.

Definitions

Apnea was defined as the absence of airflow for at least 10 seconds. Hypopnea was defined as a reduction in airflow lasting at least 10 seconds and associated with either a 4% drop in arterial oxyhemoglobin saturation or an electroencephalogram arousal. An arousal was defined according to the criteria proposed by the Atlas Task Force.¹³ Residual EDS referred to a score of greater than 11 on the Epworth Sleepiness Scale. The baseline apnea-hypopnea index (AHI) was derived from overnight polysomnography. The residual AHIs were obtained from the auto-CPAP smart card. Adherence was assessed by use of a CPAP device with compliance software. Compliance with therapy was defined as CPAP use for at least 4 hours daily.¹⁴

Vascular Reactivity Study

The method for measuring endothelium-dependent brachial artery dilation has been described previously.¹⁵ All subjects fasted from midnight before the experimental procedure and were studied in the morning in the supine position. The room was kept quiet and at a constant temperature (22°C −24°C). After the patients rested for 30 minutes in a supine position, the brachial artery diameter was measured 6 cm proximal to the antecubital fossa using a 7- to 15-MHz linear array transducer (Hewlett Packard Sonos 5500, San Antonio, TX). An occlusion blood pressure cuff was placed over the proximal forearm just below the antecubital fossa. The protocol included initial recording of brachial artery diameter, after which a blood pressure cuff was placed around the left forearm and inflated to a pressure of 200 mm Hg for 5 minutes, followed by sudden deflation. Brachial artery diameter (expressed in millimeters up to 1 decimal place) was measured at rest and during peak hyperemia for 1 minute after a 5-minute occlusion of arterial flow. Diameter measurements were made using a computerized analyzing system based on automated detection of the echo structures, with the option of making manual corrections by the operator. One expert reader who was blinded to subjects' clinical condition made brachial arterial reactivity measurements. For assessment of the reproducibility, 5 scans were selected randomly for repeated measurements. The mean intraobserver variability for measurement of flow-mediated vasodilatation was 1.5% (SD 0.9%). The percentage of change in the diameter for flow-mediated dilatation (FMD) was calculated as follows:

Blood Collection and Biochemical Analysis

Blood samples (20 mL) were obtained from the anterior cubital vein of the contralateral arm in a resting and fasting state between 08:30 am and 09:30. All blood samples were set on ice for 5 minutes before centrifugation at 3000 g for 5 minutes at 4°C. After centrifugation, plasma was pipetted into polypropylene tubes and frozen at −70°C until thawed for analysis. Determination of high-sensitivity CRP concentrations was performed using an ultrasensitive latex-enhanced immunoassay (Dade Behring, Newark, DE). Concentrations of CRP below the detection level of the CRP assay (< 0.02 mg/dL) were assigned a value of 0.01 mg/dL. The coefficient of variation in our hands was of the order of 4.8% (in-house data). Assays of serum IL-6 and TNF-α were performed using an enzyme-linked immunosorbent assay kit (R&D Systems, Minneapolis, MN). The minimum detectable levels of IL-6 and TNF-α were less than 0.10 pg/ mL and 0.18 pg/ mL, respectively. Intraassay precision and interassay coefficients of variation of IL-6 are 4.2% and 6.4% and of TNF-α are 5.2% and 7.4%.

Sample Size

The primary endpoint of the study was the absolute change of brachial artery flow-mediated dilation between patients with OSA who had residual EDS after CPAP therapy and those without residual EDS. Based on previously published studies,¹⁵ a total of 12 participants per group were needed to have 80% power to demonstrate a difference of 2 percentage points of FMD change between groups with an average SD of 1.8%.

Statistical Analysis

All analyses were conducted by observers blinded to the subjects' identity. The results are expressed as mean ± SD. Differences between subject characteristics were determined by a student t test or Mann-Whitney rank sum test when normality testing failed, and the significance of differences between groups was performed by analysis of variance first, followed by a ttest with Bonferroni correction. A 2-sided p value of 0.05 was considered to be statistically significant.

RESULTS

Characteristics of the study population are displayed in Table 1. Patients with OSA did not differ from the control subjects with respect to age, blood pressure, glucose, or cholesterol levels. Only the BMI was higher in the OSA group. There were also no significant differences with regard to demographic, hemodynamic, and biochemical characteristics between the CPAP-compliant patients with residual EDS and those without residual EDS.

Table 1.

Baseline characteristics of study population

	Residual EDS (n = 12)	No residual EDS (n = 12)	Control subjects (n = 12)
Age, y	41.5 ± 10.4	43.7 ± 12.8	41.8 ± 6.7
BMI, kg/m2	30.5 ± 3.6	31.4 ± 3.1	27.5 ± 3.4a
Neck circumference, cm	38.6 ± 2.6	38.8 ± 2.6	36.1 ± 2.1
Blood pressure, mm Hg
Systolic	118 ± 12	121 ± 7	119 ± 8
Diastolic	76 ± 5	73 ± 2	71 ± 6
Blood glucose, mg/dL	98.6 ± 11.4	101.3 ± 12.5	93.9 ± 10.8
Total cholesterol, mg/dL	192 ± 21	201 ± 29	189 ± 17

EDS refers to excessive daytime sleepiness; BMI, body mass index;

^aIndicates p < 0.05 by analysis of variance.

Table 2 depicts a comparison of the sleep characteristics between the CPAP-compliant patients with and without residual EDS. Although subjects with residual EDS had longer estimated sleep time, as compared with those patients without residual EDS (7.8 ± 2.5 vs 6.3 ± 1.9; p = 0.03), the residual AHI and the mean CPAP pressure were comparable between the 2 groups. Moreover, both groups had equivalent time spent with an SaO₂ of less than 90%.

Table 2.

Sleep characteristics of CPAP-compliant patients with OSA with and without residual EDS 8 weeks after treatment

	Residual EDS (n = 12)	No residual EDS (n = 12)	p value
ESS score	12.7 ± 1.5	7.1 ± 1.7	< 0.001
AHI, no./h
Baseline	15.3 ± 6.1	19.8 ± 7.6	0.12
Residual	2.9 ± 0.8	3.1 ± 1.1	0.60
Time spent with SaO2 < 90%, min	0.55 ± 0.4	0.59 ± 0.4	0.85
Estimated sleep time, h	7.8 ± 2.5	6.3 ± 1.9	0.03
CPAP use per night, h	5.5 ± 2.5	5.1 ± 2.8	0.38
Mean CPAP pressure, cm H2O	9.4 ± 2.7	8.8 ± 2.8	0.62

CPAP refers to continuous positive airway pressure; OSA, obstructive sleep apnea; EDS, excessive daytime sleepiness; ESS, Epworth Sleepiness Scale; AHI, apnea-hypopnea index.

The baseline brachial artery diameter between CPAP-compliant patients with residual EDS and those without residual EDS was 3.6 ± 0.3 mm and 3.7 ± 0.3 mm, respectively (p = 0.81). Brachial artery reactivity measurements are displayed in Figure 1. There was no difference in the FMD among the control subjects, CPAP-compliant patients with residual EDS, and CPAP-compliant patients without residual EDS (p = 0.37).

Figure 1.

Box-whisker plots (mean and 95% confidence interval) representing flow-mediated vasodilation in control subjects, continuous positive airway pressure (CPAP)-complaint patients with obstructive sleep apnea with residual excessive daytime sleepiness (REDS) and CPAP-compliant patients without (REDS) (p = 0.37).

Analysis of variance did not reveal any significant difference in the levels of inflammatory markers among healthy control subjects and the OSA groups (Table 3). The levels of CRP, TNF-α, and IL-6 were also not significantly different between subjects with CPAP-compliant residual EDS and those without residual EDS (p = 0.44, p = 0.37, and p = 0.42; respectively).

Table 3.

Concentrations of inflammatory markers of CPAP-compliant patients with OSA with and without residual EDS 8 weeks after treatment and of control subjects

	Residual EDS (n = 12)	No residual EDS (n = 12)	Control subjects (n = 12)
CRP, mg/mL	0.17 ± 0.08	0.16 ± 0.05	0.13 ± 0.04
TNF-α, pg/mL	3.51 ± 1.13	3.12 ± 0.95	3.42 ± 0.96
IL-6, pg/dL	2.66 ± 1.06	2.51 ± 0.81	2.18 ± 0.6

CPAP refers to continuous positive airway pressure; OSA, obstructive sleep apnea; EDS, excessive daytime sleepiness; CRP, C-reactive protein; tumor necrosis factor; IL; interleukin.

DISCUSSION

The main finding of this study is that residual EDS in patients with OSA that is adequately treated with CPAP was not associated with increased concentrations of inflammatory markers or endothelial dysfunction.

CPAP is considered to be the treatment of choice for OSA. The use of nasal CPAP has been associated with significant improvement in daytime sleepiness and improved quality of life, daytime functioning, and control of blood pressure in select patients with hypertension.¹⁶ However, a subgroup of patients with adequately treated OSA continues to exhibit persistent fatigue and unrefreshing sleep.^8,9 In these cases, the underlying cause of persistent sleepiness remains unclear. It has been suggested that exposure to long-term intermittent hypoxia could be responsible for neuronal changes in the brain, such as proinflammatory activation, reduced extracellular dopamine levels, increased oxidative stress, apoptosis, and gliosis.^17,18 Such abnormalities are thought to lead to neuronal dysfunction and persistent sleepiness. These observations have been reported primarily in a rodent model. Whether similar brain pathology occurs in patients genetically predisposed to having hypoxic insults requires further investigation.

Previous studies have reported that patients with OSA develop systemic inflammation, with increased levels of circulating proinflammatory cytokines, such as TNF-α, IL-6, and others, as compared with healthy subjects, and CPAP therapy reduces the levels of OSA-induced inflammatory cytokines.^19,20 In tandem, endothelial dysfunction, which is considered to be an important precursor of cardiovascular diseases, has also been observed in patients with OSA.^21,22 The rapid fluctuation of oxygen saturation during and after obstructive events has been compared with the reoxygenation/perfusion phenomenon, whereby excess reactive oxygen species are produced via the electron reduction of molecular oxygen. These molecules quench nitric oxide,²³ inducing a state of impaired relaxation.²⁴ Treatment with nasal CPAP reverses the impaired endothelial responses.²⁵ In each of these instances, the oxygen desaturation index has been the consistent parameter that correlates with the extent of vascular reactivity and the expression of inflammatory molecules.²⁵ In our patients, the effectiveness of auto-CPAP in eliminating cyclical hypoxia reoxygenation was confirmed by the data from nocturnal oximetry. The fact that there was absence of an impairment in vascular function and lack of significant differences in inflammatory markers between CPAP-compliant individuals with residual EDS and those without residual EDS lends further support to the hypothesis linking oxidative stress in OSA to the development of cardiovascular diseases.

In previous studies, the association of EDS with mortality was not consistently observed. An independent association was found in the Duke EPSE study and in the Cardiovascular Health Study,^11,26 whereas, in the Canadian study, the association was no longer significant after adjustment for classic cardiovascular risk factors.²⁷ Although none of these studies examined the coexistence of OSA, a recent prospective study involving more than 9200 elderly subjects found no significant difference in the atherosclerotic burden, as measured by the presence of carotid plaques and intima media thickness, in subjects with and without EDS.²⁸

Our study population included only 3 patients with severe OSA because choosing a study population without underlying cardiovascular diseases has virtually excluded most patients who have severe OSA. Hence, it is plausible that our results may reflect a lack of endothelial dysfunction and inflammatory burden in mild to moderate OSA. It should be noted, however, that the study was designed as to eliminate all residual sleep disturbances, including oxygen desaturations irrespective of baseline AHI. In fact, none of our participants had abnormal AHIs or persistent hypoxemia following treatment. Moreover, previous studies that included participants with AHIs of less than 30 established the benefits of CPAP in ameliorating endothelial function and in reducing the risk of cardiovascular diseases.^15,29

A limitation of our study includes the fact that residual EDS was obtained by subjective measurements. Questionnaires utilized in the assessment of subjective sleepiness (e.g., the Epworth Sleepiness Scale or Stanford Sleepiness Scale) do not always correlate with objective quantifications. A prior investigation examining the role of modafinil in treating EDS following CPAP therapy showed that only 50% of the patients had abnormal objective sleepiness (sleep onset latency < 11 min).³⁰ We acknowledge also that EDS is commonly viewed as a cardinal sign of disturbed or inadequate sleep and can be directly linked to depression.^31,32 Although there was no mention in the records of these patients of their having depression or having received prior treatment for this condition, we have not performed an accurate evaluation of this condition in the study population. It could be argued that our observations were caused by the relatively short duration of treatment.⁹ However, the lack of statistical significance in inflammatory markers and vascular reactivity between the study participants on CPAP therapy with and without residual EDS and the control group without OSA suggests otherwise.

In summary, levels of inflammatory markers in CPAP-compliant patients with residual EDS were comparable with those of CPAP-compliant patients without residual EDS. Endothelial function was also not impaired. These findings suggest that residual excessive sleepiness in patients with adequately treated OSA may not constitute a risk factor for cardiovascular diseases.

DISCLOSURE STATEMENT

This was not an industry supported study. The authors have indicated no financial conflicts of interest.