JCDR_4_2_4 - Journal of Cardiovascular Disease Research

Journal of Cardiovascular Disease Research 4 (2013) 87e91
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Journal of Cardiovascular Disease Research
journal homepage: www.elsevier.com/locate/jcdr
Original article
Endothelial dysfunction and hypertension in obstructive sleep apnea e Is it due
to intermittent hypoxia?
Behrouz Jafari a, Vahid Mohsenin b, *
Section of Pulmonary, Critical Care and Sleep Medicine, University of California, Irvine, CA 90822, USA
Section of Pulmonary, Critical Care and Sleep Medicine, Yale School of Medicine and John B. Pierce Laboratory, New Haven, CT 06519, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 March 2013
Accepted 3 April 2013
Available online 18 June 2013
Background: Obstructive sleep apnea (OSA) is a prevalent disorder causing hypertension. Endothelial
dysfunction appears to underlie development of hypertension. It is not known whether hypoxia during
sleep is necessarily the prerequisite process for endothelial dysfunction and hypertension in OSA. We
therefore examined the relationship between endothelial-dependent vasodilatory capacity, hypoxia and
circulating angiogenesis inhibitors in OSA.
Methods and results: We studies 95 subjects with and without OSA and hypertension. Endothelialdependent vasodilation was assessed using brachial artery flow-mediated vasodilation method (FMD).
Plasma angiogenesis inhibitors, endoglin (sEng) and fms-like tyrosine kinase-1 (sFlt-1), were measured
using ELISA. The apneaehypopnea indexes were 41 5 and 48 4 events/hr in normotensive OSA (NOSA) and hypertensive OSA (H-OSA), respectively, indicating severe OSA. The sleep time spent with
SaO2 < 90% (T < 90%) were 34 8 and 40 9 min, respectively. FMD was markedly impaired in H-OSA
(8.0% 0.5) compared to N-OSA (13.5% 0.5, P < 0.0001), H-non-OSA (10.5% 0.8, P < 0.01), and N-nonOSA (16.1% 1.0, P < 0.0001). There was no correlation between T < 90% and FMD. Both OSA groups had
elevated levels of sFlt-1 (62.4 5.9 and 63.9 4.7 pg/ml) compared to N-non-OSA (32.1 6.5,
P ¼ 0.0008 and P ¼ 0.0004, respectively) and H-non-OSA (41.2 7.0, P < 0.05 and P ¼ 0.03, respectively).
In contrast, sEng was only elevated in H-OSA (4.20 0.17 ng/ml) compared with N-OSA (3.64 0.14,
P ¼ 0.01) and N-non-OSA (3.48 0.20, P ¼ 0.01). There was a modest but statistically significant inverse
correlation between sEng and FMD in only H-OSA group (r ¼ 0.38, P < 0.05).
Conclusion: These data show that patients with OSA and hypertension have marked impairment of FMD,
independent of hypoxia exposure, which is associated with increased sEng.
Copyright Ó 2013, SciBioIMed.Org, Published by Reed Elsevier India Pvt. Ltd. All rights reserved.
Angiogenesis inhibitors
Sleep apnea
Endothelial dysfunction
Key Messages
Obstructive sleep apnea is a highly prevalent disorder with
associated high morbidity and mortality. Obstructive sleep apnea is
now considered as one of the causes of systemic hypertension. No
all patients with obstructive sleep apnea develop hypertension.
There appears to be divergent responses to apnea-associated hypoxia. In this article a group of patients with severe obstructive sleep
apnea and hypoxia exposure during sleep had normal blood pressure and relatively preserved endothelial-dependent vasodilatory
capacity. A comparable group of patients with similar apnea
severity and hypoxia exposure had marked impairment in endothelial-dependent vasodilatory capacity and hypertension. This
group had significantly elevated circulating levels of soluble
endoglin, an angiogenesis inhibitor, with known effect on
* Corresponding author.
E-mail address: [email protected] (V. Mohsenin).
endothelial function and development of hypertension. It is
conceivable that inflammatory state of obstructive sleep apnea
provokes release of angiogenesis inhibitors causing downstream
perturbation of endothelial function.
1. Introduction
Obstructive sleep apnea (OSA) is a highly prevalent sleep disorder that affects 15e24% of the adults and is associated with
increased morbidity and mortality.1 Individuals with OSA are
particularly at increased risk for premature atherosclerosis, coronary artery disease, stroke and hypertension.2e5 Systemic hypertension affects up to two-thirds of patients with OSA.6,7 Some
investigators have proposed that endothelial dysfunction is
mechanistically implicated in a sustained increase in blood pressure8,9 and is an early process in the development of atherosclerosis.10,11 Patients with OSA have been shown to have endothelial
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B. Jafari, V. Mohsenin / Journal of Cardiovascular Disease Research 4 (2013) 87e91
dysfunction12e15 and evidence for premature atherosclerosis.16
Current evidence suggests that inflammatory processes, oxidative
stress and endothelial dysfunction may play roles in the pathogenesis of hypertension and vascular complications in OSA.15
We have previously shown elevated concentrations of circulating soluble endoglin (sEng) and soluble fms-like tyrosine kinase1 (sFlt-1) in hypertensive OSA compared to normotensive OSA.17
These angiogenesis inhibitors are released under inflammatory
state.18,19 These circulating proteins have been shown to have
pathogenetic roles in hypertension in preeclampsia,20 in human
chronic kidney disease,21 coronary artery disease22 and in animal
models of hypertension.23,24 However, none of the studies have
examined the relationship between endothelial dysfunction and
hypertension in relationship to hypoxia exposure in OSA patients.
Further, it is not known whether endothelial dysfunction is related
to angiogenesis inhibitors.
The specific aims of the present study were to examine the
relationship between endothelial-dependent vasoregulatory capacity in OSA patients with and without hypertension, hypoxia
exposure and angiogenesis inhibitors.
depth settings were optimized to identify the lumen-vessel wall
interface. After optimal transducer positioning, the skin was
marked for reference for later measurements and the arm was kept
in the same position throughout the study. After baseline measurements of the brachial artery were recorded, the cuff was placed
on forearm and inflated to 200 mm Hg for 5 min to create forearm
ischemia. Subsequently, the cuff was deflated and the arterial
diameter was measured every 3e5 s after deflation up to 5 min.
FMD was expressed as the percentage of change in the brachial
artery diameter from baseline to following peak reactive
The artery diameters were measured independently by the two
investigators (one blinded to grouping of subjects) using a digital
caliper and were verified by an automated border recognition
software. Peak vasodilation was calculated as the percent change in
the brachial artery diameter from baseline to peak reactive hyperemia. The inter-observer and intra- observer variability in diameter
measurements were less than 5%.
2. Methods
Venous blood sample was obtained 1 h after the subjects had
been seated and rested for 60 min between 10 am and 2 pm. Plasma
and serum were separated with centrifugation at 1200 g for 10 min
at 4 C, aliquoted and stored at 80 C for further analysis.
2.1. Subjects
Patients were recruited consecutively from among those
screened for sleep-disordered breathing at Yale Center for Sleep
Medicine. Patients with newly diagnosed and untreated OSA and
those without OSA (apnea-hypopnea index, AHI < 5 events/hr)
as control group were enrolled. The subjects are a subset of a
cohort that has been published previously.25 We studied hypertensive and normotensive OSA as well as hypertensive and
normotensive non-OSA patients. Hypertension was defined by
blood pressure 140 mm Hg systolic and/or 90 mm Hg diastolic,
which had been previously documented by using appropriate sized
cuff and measurements that had been made at least in three
different occasions according to the standard criteria.26 Subjects
were excluded if they had known peripheral vascular disease, liver
disease, hemolytic anemia, inflammatory disease, active infection,
or if they were pregnant, on therapy for OSA, on chronic steroid
treatment, or younger than 18 years of age. Each subject was
informed of the experimental procedures and signed the consent
form for this study that had been approved by the Human Investigation Committee of the Yale University School of Medicine.
2.2. Sleep study
Nocturnal polysomnography was performed as previously
described.17 Respiratory events were scored according to the
American Academy of Sleep Medicine. Hypopnea was scored when
there was at least 30% decrease in airflow signal with a 4%
decrease in oxygen saturation. The percentage of total sleep time
associated with oxyhemoglobin saturation of <90% was calculated
as a measure of hypoxemia duration.
2.3. Endothelial function
Conduit vessels respond to alterations in blood flow by
increasing vessel diameter via an endothelial-dependent mechanism. Endothelial function was assessed by a standard flowmediated vasodilation (FMD) method using Doppler ultrasound
of the brachial artery between 10 am and 3 pm.25 In order to best
visualize the brachial artery, the arm was comfortably immobilized
in the extended position, and the brachial artery was scanned in the
longitudinal section 3e5 cm above the antecubital fossa. Gain and
2.4. Blood sample
2.5. Measurement of plasma sEng and sFlt-1
Circulating levels of sEng and sFlt-1 in plasma were measured
using enzyme-linked immunosorbent assay using commercially
available reagents and recombinant standards (R&D Systems,
Minneapolis, MN, USA). All samples were assayed in duplicate.
Standards and control samples were run simultaneously for validation. The minimum detection limits for sEng and sFlt-1 were
0.007 ng/ml and 3.5 pg/ml, respectively. Inter- and intra-assay coefficient of variations for both assays were <10%. The assay kit
measures total plasma sFlt-1.
2.6. Data analysis
The primary outcome was endothelial-dependent vasodilation
as measured by FMD. The required sample size to detect a significant change in FMD (delta ¼ 4, SD ¼ 2.7) was 14 per group
(alpha ¼ 0.05, power ¼ 80%). However, we over sampled the OSA
groups to account for the differential susceptibility to hypertension
and responses to OSA and hypoxia in this population. Data are
expressed as means SE. Data were analyzed using ANOVA for
simultaneous comparisons of the groups (Graphpad Prism, La Jolla,
CA). Spearman correlation was used to analyze the relationship
between FMD and sEng and sFlt-1. A multivariable linear regression
analysis was used to identify independent clinical variables associated with FMD. The following variables were considered for inclusion in the comprehensive model: age, BMI, smoking, diabetes,
dyslipidemia, hypertension and OSA. Inclusion in the final model
was determined by a backward-stepwise technique evaluating all
potential univariate variables (P < 0.20) to create a multivariable
model containing variables with P < 0.05 (SAS Institute Inc, Carey,
NC). P values were 2-sided with a level of significance of P < 0.05.
3. Results
3.1. Subjects characteristics
As shown in Table 1 both OSA groups had moderately severe
OSA with significant oxygen desaturations during sleep. The control
B. Jafari, V. Mohsenin / Journal of Cardiovascular Disease Research 4 (2013) 87e91
Table 1
Subjects’ characteristics, sleep-disordered parameters, FMD, and plasma angiogenesis inhibitors.
Age, yr
Male, n
BMI, kg/m2
SBP, mm Hg
DBP, mm Hg
AHI, event/hr
ODI > 4%/hr
SaO2 < 90%, min
Nadir SaO2, %
Arousal index,/hr
FMD, %
sFlt-1, pg/ml
sEng, ng/ml
Group 1 normotensive (n ¼ 19)
Group 2 hypertensive (n ¼ 13)
Group 3 normotensive (n ¼ 27)
Group 4 hypertensive (n ¼ 36)
47.5 2.1
29.6 1.1
115 1
77 1
1 0.3
1 0.3
88 1
30 3
16.1% 1.0
32.1 6.5
3.5 0.2
45.7 2.3
33.8 2.7
129 2q
89 2q
2 0.3
2 0.6
87 1
25 3
10.5% 0.8
41.2 7.0
3.6 0.2
47.9 2.2
36.3 1.5x
120 2
81 1
41 5x
32 5x
34 8x
76 2x
53 6x
13.5% 0.5
62.4 5.9✝✝
3.6 0.1
56.1 1.4*✝
37.5 1.2
134 2q
88 1q
48 4✝
36 4✝
40 9✝
79 1✝
55 5✝
8.0% 0.5q
63.9 4.7✝✝
4.2 0.2xx
BMI, body mass index; SBP, systolic blood pressure; DBP, diastolic blood pressure; AHI, apnea-hypopnea index; ODI, oxygen desaturation index; SaO2 < 90%, time sleeping
with oxygen saturation < 90%; FMD, flow-mediated vasodilation; Data are means SE. *P-value significant between Group 3 and 4. qP-value significant between Group 1 and 2
and between Group 3 and 4; yP-value significant between Group 1-2 and 4; xP-value significant between Group 1-2 and 3. yyP-value significant between Group 4 and Group 1-2
and between Group 3 and 1. xxP-value significant between Group 1 and 4 and between Group 3 and 4. P-value significant between Group 4 and 2 for FMD.
groups had snoring but no OSA or significant oxygen desaturation.
The hypertensive non-OSA group was considered to have essential
hypertension. There were 10 diabetics and 19 with dyslipidemia in
hypertensive OSA. There was no difference in BMI, gender distribution, AHI or degree of hypoxia exposure between normotensive
and hypertensive OSA. One subject with hypertension without OSA
was excluded because of incomplete data.
(r ¼ 0.31, P ¼ 0.003, data not shown) showing that the higher the
AHI the lower the FMD. However, there was no significant correlation between FMD and T < 90. Our FMD values are higher than some
previous reports12,13 but comparable to others27 showing internal
validity of the measurements. One reason for the difference is that
these reports had chosen a fixed time point for measurement of
vasodilation as opposed to ours that peak vasodilation was chosen.
3.2. Flow-mediated vasodilation
3.3. sFlt-1
FMD was markedly impaired in hypertensive OSA (8.0% 0.5)
compared with hypertensive non-OSA (10.5% 0.8, P < 0.01),
normotensive OSA (13.5% 0.5, P < 0.0001), and normotensive nonOSA (16.1% 1.0, P < 0.0001) (Fig. 1). Normotensive OSA had a
modest but statistically significant impairment in FMD compared to
normotensive non-OSA (P < 0.008). The multivariable analysis
including age, BMI, smoking, diabetes mellitus, dyslipidemia, statins, hypertension and OSA showed variables correlating with FMD
were OSA (parameter estimate ¼ 2.69, P ¼ 0.004) and hypertension (parameter estimate ¼ 5.37, P < 0.0001). This indicated that
impaired FMD in hypertensive OSA was not likely due to older age,
BMI, smoking, diabetes, dyslipidemia, or statin use. There was a
modest but significant negative correlation between AHI and FMD
Plasma concentrations of sFlt-1 were elevated in both normotensive (62.4 5.9 pg/ml) and hypertensive (63.9 4.7 pg/ml) OSA
compared with normotensive non-OSA (32.1 6.5 pg/ml) and hypertensive non-OSA (41.2 7.0 pg/ml) (Table 1).
3.4. sEng
Plasma concentrations of sEng were elevated in hypertensive
OSA (4.20 0.17 ng/ml) compared with normotensive OSA
(3.64 0.14 ng/ml, P ¼ 0.01) and normotensive non-OSA
(3.48 0.20 ng/ml, P ¼ 0.01). Although the mean plasma concentration of sEng in hypertensive non-OSA subjects (3.64 0.26 ng/
ml) was similar to normotensive OSA (3.64 0.14 ng/ml) it was not
statistically significant from hypertensive OSA (P ¼ 0.09) likely due
to smaller sample size (Table 1). There was a statistically significant
inverse relationship between plasma concentrations of sEng and
FMD in only hypertensive OSA group (r ¼ 0.38, P < 0.05) showing
the higher the plasma sEng the lower the FMD (Fig. 2).
4. Discussion
Fig. 1. Endothelial-dependent vasodilatory capacity as measured by flow-mediated
vasodilation is markedly impaired in subjects with both obstructive sleep apnea
(OSA) and hypertension compared with normotensive OSA, normotensive non-OSA
and hypertensive non-OSA.
Our main finding is that the patients with both OSA and hypertension had markedly impaired endothelial-dependent vasodilatory capacity that inversely correlated with plasma sEng but not
to hypoxia exposure. The impairment in vasodilatory capacity in
hypertensive OSA was significantly greater than in subjects with
hypertension or OSA alone. Patients with OSA without hypertension but with similar hypoxia exposure had relatively preserved
endothelial-dependent vasodilatory capacity suggesting divergent
vascular responses to obstructive apneas and intermittent hypoxia
in OSA population. The impairment in flow-mediated vasodilation
independent of hypoxia exposure is in accord with a larger
community-based study that flow-mediated dilation did not
correlate with the hypoxemia index after adjusting for body mass
B. Jafari, V. Mohsenin / Journal of Cardiovascular Disease Research 4 (2013) 87e91
Fig. 2. FMD as a function of plasma sEng concentrations showing a significant inverse
relationship in hypertensive OSA group.
index and other covariates across all subjects.28 Based on these
studies and our own data, endothelial function is not uniformly
affected by exposure to intermittent hypoxia or apnea events in
patient with OSA.
Hypertensive OSA subjects had increased plasma levels of sEng
in contrast to the normotensive OSA and the control groups. sFlt-1
was elevated in both OSA groups. In the current study, sFlt-1 was
elevated in both normotensive and hypertensive OSA compared
with non-OSA groups but lower than the prior study.17 sEng was
elevated in the hypertensive OSA and comparable to the previous
study.17 We conclude that sFlt-1 response to apneic events is
similar in normotensive and hypertensive OSA but not sufficient to
significantly affect flow-mediated vasodilation and elevated sEng is
necessary for development of vascular dysfunction. This can
explain the difference between hypertensive non-OSA and hypertensive OSA in terms of degree of impairment of endothelial
function (both sFlt-1 and sEng need to be elevated). The magnitude
of the differences in sEng concentrations between hypertensive
OSA and control groups in our study is comparable to those seen
with target organ damage and hypertension and in those with high
risk for cardiovascular adverse outcome.29,30 The mechanism for
elevated sEng levels in OSA is unknown. However, there are several
possibilities. First, increased angiotensin II has been reported in
OSA31 which can induce metalloproteinase-14 causing cleavage of
sEng from trans-membrane endoglin.32 Current evidence suggests
that inflammatory processes play critical roles in the pathogenesis
of hypertension and vascular complications in OSA15 and can provoke the release of sEng and sFlt-1 via NF-KB signaling pathways.33e35 Equally plausible explanation is that these factors are
markers of vascular injury in OSA patients with impaired endothelial function and hypertension. However, this is less likely
because both of these circulating factors alone or together have
been shown to be injurious to endothelium causing hypertension.23,36 Further, the patients with hypertension and without OSA
had impaired FMD without elevations of angiogenesis inhibitors.
The clinical implication of these finding is that increased levels
of these circulating angiogenesis inhibitors may impart increased
risks of cardiovascular complication in OSA. However, a prospective
study with a larger sample size is needed to determine the significance of these angiogenesis inhibitors as predictors of vascular
complications in OSA as has been shown in other vascular
Our research’s novelty resides in studying patients with similar
apnea severity and hypoxia exposure but with divergent vascular
responses, vis-à-vis hypertension. To the best of our knowledge no
previous study has tried to investigate the mechanism of this
divergent response. Our study has some limitations. Aging and
diabetes are associated with endothelial dysfunction. Our subjects
with both OSA and hypertension were older and had higher prevalence of diabetes. However, the multivariable analysis did not
show any significant influence of age and diabetes on FMD. The
study was designed to examine endothelial-dependent vasodilatory capacity in relationship to hypertension and OSA and was
not powered to examine effect size of angiogenesis inhibitors in the
multivariable model. But the fact that these angiogenesis inhibitors
were uniquely elevated in hypertensive OSA but not hypertension
or OSA alone supports the association with endothelial dysfunction.
In conclusion, we have shown that impairment in endothelialdependent vasodilation in OSA is not necessarily related to hypoxic exposure suggesting divergent molecular responses to
obstructive respiratory events that may explain the varying individual susceptibility to development of vascular complications and
particularly hypertension in OSA. This divergent response may be
related, at least in part, to differential release of angiogenesis inhibitors contributing to the impairment of endothelial-dependent
vasodilatory capacity and hypertension.
Supported in part by an Institutional Research Training Grant
from the National Institute of Health (5T32HL07778) and Yale
University, Section of Pulmonary, Critical Care and Sleep Medicine
intramural grant. The funders had no role in study design, data
collection and analysis, decision to publish, or preparation of the
Authors contributions
Both authors have contributed equally in the design of the study,
acquisition of the data and preparation of the manuscript.
Conflicts of interest
All authors have none to declare.
The authors thank Li Qin, PhD for assistance in statistical
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