Role of hypoxia on increased blood pressure in patients with obstructive sleep apnoea

Elucidation of obstructive sleep apnoea related blood pressure surge using a novel continuous beat-to-beat blood pressure monitoring system

BACKGROUND

Obstructive sleep apnoea (OSA) episode related blood pressure (BP) surge may mediate the association of OSA with cardiovascular disease. However, BP is not measured during a clinical sleep study.

METHOD

We tested the feasibility of incorporating the Caretaker physiological monitor, which utilizes a novel continuous beat-to-beat (b-b) BP monitoring technology, into polysomnography (PSG) and aimed to characterize BP surges related to obstructive respiratory events. B-b BP was concurrently collected and merged with PSG data on a posthoc basis. We compared BP surge between mean respiratory (apnoea, hypopnea and desaturation-alone events) and nonrespiratory events (spontaneous or leg movement-related arousals). We examined the association of the degree of oxygen desaturation with BP surge in a given respiratory event combining all events. A total of 17 consecutive patients (12 men, mean 52 years old, nine diagnostic and eight split-night PSGs) undergoing clinically indicated PSG were included after excluding one patient with poor signal quality due to excessive movement.

RESULTS

Caretaker was well tolerated. Mean respiratory BP surge ranged from 5 to 19 mmHg [Median (IQR) = 13.9 (9.5--16.2)]. Mean BP surge between the respiratory and nonrespiratory events was similar [13.8 (4.5) vs. 14.9 (5.3) mmHg, P = 0.13]. Accounting for the count distribution of desaturation/BP surge data pair events, there was a linear correlation between the degree of oxygen desaturation and BP surge (R = 0.57, P < 0.001). In eight patients undergoing split-night sleep studies, the number of BP surge events (≥10 mmHg/h) decreased during continuous positive airway pressure in all but one patient.

CONCLUSION

We demonstrated highly variable OSA-related BP surge patterns using the Caretaker's b-b BP monitoring technology that has the potential to be integrated into sleep studies.

Association of Craniofacial and Upper Airway Morphology with Cardiovascular Risk in Adults with OSA

BACKGROUND AND OBJECTIVE

Clinical and population-based studies have demonstrated a strong association between obstructive sleep apnea (OSA) and cardiovascular disease (CVD). Anatomical abnormalities of the craniofacial region and upper airway are important risk factors for OSA. The objective of this study was to investigate the association of craniofacial and upper airway morphology with CVD risk biomarkers.

METHODS

One hundred and sixty-nine male patients with OSA underwent in-laboratory polysomnography (PSG) and upper airway computed tomography (CT) scanning. Ten-year Framingham CVD risk score (FRS) was calculated and categorized into low- and moderate-to-high-risk groups. N-terminal pro B-type natriuretic peptide (NT-proBNP) was measured as a biomarker of increased myocardial wall stress.

RESULTS

Compared to the low-risk group, total sleep time (TST), the proportion of N3 (N3%) and mean oxygen saturation (SpO2mean) were lower, while the arousal index of non-rapid eye movement (NREM) sleep, apnea index (AI) of NREM sleep, apnea hypopnea index (AHI) of NREM sleep, oxygen desaturation index (ODI) and percentage of total sleep time spent with oxyhemoglobin saturation below 90% (TST90) were higher in the moderate-to-high risk group. The corrected upper airway length (UAL), ANB angle and gonion-gnathion-hyoid angle were larger for subjects in the moderate-to-high risk group than those in the low-risk group. In multiple regression analysis, TST, AINREM and adjusted UAL were independently associated with moderate-to-high CVD risk. Plasma NT-proBNP levels were higher in patients in the moderate- to high-risk group, and among the PSG and CT scan parameters, only SPO2mean was marginally associated with NT-proBNP (r=0.183, P=0.054).

CONCLUSION

Craniofacial and upper airway features may contain valid cues about CVD risk, and sleep duration, obstructive event type and occurrence phase may be closely related to CVD risk for patients with OSA.

Effect of Sleep Disorders on Blood Pressure and Hypertension in Children

PURPOSE OF REVIEW

In this review, we aim to discuss the pathophysiologic basis of hypertension in sleep disorders and the current evidence in the medical literature linking sleep disorders and hypertension in children.

RECENT FINDINGS

The medical literature in adults is clear about the contribution of sleep disorders, poor sleep quality, and sleep deprivation to hypertension and increased cardiovascular risk. The literature on cardiovascular consequences of sleep disorders in children is not as robust, but there is some evidence of early cardiovascular changes in children with sleep deprivation and obstructive sleep apnea. Children with obstructive sleep apnea have increased sympathetic activation during sleep, blunted dipping, or elevated systolic or diastolic pressures. Although the literature on other sleep disorders such as narcolepsy and restless legs syndrome is scarce, there is evidence in adults and some recent supportive data in children.

Comparison between blood pressure during obstructive respiratory events in REM and NREM sleep using pulse transit time

Rapid eye movement-predominant obstructive sleep apnea has been shown to be independently associated with hypertension. This study aimed to non-invasively measure blood pressure during the rapid eye movement (REM) and non-rapid eye movement (NREM) obstructive events and the post-obstructive event period. Thirty-two consecutive continuous positive airway pressure-naïve obstructive sleep apnea patients (men, 50%) aged 50.2 ± 12 years underwent overnight polysomnography. Blood pressure was assessed indirectly using a validated method based on the pulse transit time and pulse wave velocity during the NREM and REM obstructive events (both apneas and hypopneas) and the post-obstructive event period. Among the recruited patients, 10 (31.3%) had hypertension. Mean apnea-hypopnea index was 40.1 ± 27.6 events/hr. Apnea-hypopnea indexes were 38.3 ± 30.6 and 51.9 ± 28.3 events/hr for NREM and REM sleep, respectively. No differences were detected in obstructive respiratory event duration or degree of desaturation between REM and NREM sleep. Additionally, no difference in blood pressure (systolic and diastolic) was detected between REM and NREM sleep during obstructive events and post-obstructive event period. Simple linear regression identified history of hypertension as a predictor of increased systolic blood pressure during obstructive events and post-obstructive event period in both rapid eye movement and non-rapid eye movement sleep. Oxygen desaturation index was also a predictor of increased systolic blood pressure during obstructive events and post-obstructive event period in REM sleep. When obstructive event duration and the degree of desaturation were comparable, no difference in blood pressure was found between REM and NREM sleep during obstructive events and post-obstructive event period.

Inducers of post-apneic blood pressure fluctuation monitored by pulse transfer time measurement in obstructive sleep apnea varied with syndrome severity

PURPOSE

This study investigated the properties of blood pressure (BP) fluctuation and sympathovagal imbalance with the severity of OSAS.

METHODS

Nocturnal BP was continuously monitored by polysomnography for mild (n = 33), moderate (n = 34), and severe (n = 37) OSAS patients. Apnea-related systolic BP elevation (△SBP) indicated the amplitude of BP fluctuation. The SBP index, number of △SBP > 10 mmHg/h of sleep, indicated the frequency of significant BP fluctuations. The low frequency/high frequency (LF/HF) ratios indicated heart rate variability and sympathovagal imbalance.

RESULTS

△SBP and the SBP index were the highest in severe OSAS (12.9 ± 2.3 mmHg and 33.7 ± 14.7/h), followed by moderate OSAS (9.5 ± 2.6 mmHg and 7.1 ± 4.4/h), and mild OSAS (8.3 ± 1.6 mmHg and 3.4 ± 2.1/h). The LF/HF ratios in severe OSAS were significantly higher than that in moderate and mild OSAS. In mild OSAS, arousal played a more important role in BP fluctuation. In moderate OSAS, the oxygen desaturation index (ODI) and the SBP index were correlated. The difference in △SBP induced by hypoxia or by arousal was not significant. In severe OSAS, the apnea-hypopnea index (AHI) and LF/HF ratio were correlated with the SBP index, and △SBP was larger with hypoxia than arousal.

CONCLUSIONS

BP fluctuation and sympathovagal imbalance were both related to obstructive sleep apnea severity. The influence of arousal and hypoxia on BP fluctuation varied with OSAS severity.

TRIAL REGISTRATION

NCT02876471.

Pathophysiology of essential hypertension: an update

INTRODUCTION

Hypertension is caused by increased cardiac output and/or increased peripheral resistance. Areas covered: The various mechanisms affecting cardiac output/peripheral resistance involved in the development of essential hypertension are covered. These include genetics; sympathetic nervous system overactivity; renal mechanisms: excess sodium intake and pressure natriuresis; vascular mechanisms: endothelial cell dysfunction and the nitric oxide pathway; hormonal mechanisms: the renin-angiotensin-aldosterone system (RAAS); obesity, obstructive sleep apnea (OSA); insulin resistance and metabolic syndrome; uric acid; vitamin D; gender differences; racial, ethnic, and environmental factors; increased left ventricular ejection force and hypertension and its association with increased basal sympathetic activity - cortical connections. Expert commentary: Maximum association of hypertension is found with sympathetic overactivity which is directly or indirectly involved in different mechanisms of hypertension including RAAS, OSA, obesity, etc.. It is not overt sympathetic activity but disturbed basal sympathetic tone. Basal sympathetic tone arises from hypothalamus; possibly affected by cortical influences. Therefore, hypertension is not merely a disease of circulatory system alone. Its pathogenesis involves alteration in ANS (autonomic nervous system) and likely in cortical-hypothalamic connections. Assessment of ANS and cortical-hypothalamic connections may be required for better understanding of hypertension.

Augmented cardiovascular responses to episodes of repetitive compared with isolated respiratory events in preschool children with sleep-disordered breathing

BACKGROUND

Childhood sleep disordered breathing (SDB) presents as isolated respiratory events or episodes of consecutive repetitive events. We hypothesized that the surge in blood pressure (BP) and heart rate (HR) would be greater at the termination of events during episodes of repetitive events than following isolated events.

METHODS

% change in HR and pulse transit time (PTT; inverse surrogate of BP) were calculated from the last half of an event to: (i) between successive repetitive events; (ii) termination of the last repetitive event; (iii) event termination for isolated events.

RESULTS

69% of the children exhibiting both isolated and repetitive events had more repetitive than isolated events. %HR change between repetitive events (27 ± 1%) was greater than at event termination for isolated events (17 ± 1%; P < 0.001). %PTT change at the termination of the last repetitive event (-8 ± 2%) was greater than at the termination of isolated events (-2 ± 2%; P < 0.05).

CONCLUSION

Episodes of repetitive respiratory events evoke a greater acute cardiovascular response, including surges in BP and HR between events, than do isolated events. Given that the majority of respiratory events in preschool children occur as repetitive episodes, this finding should be taken into account when assessing the impact of respiratory events for a given child.

Association of severe obstructive sleep apnea and elevated blood pressure despite antihypertensive medication use

RATIONALE

We hypothesized that untreated severe obstructive sleep apnea (OSA) is associated with elevated ambulatory blood pressure (BP) in subjects with high cardiovascular disease (CVD) risk despite medical management.

METHODS

Data from the baseline examination of the Heart Biomarker Evaluation in Apnea Treatment (HeartBEAT) study, a 4-site randomized controlled trial were analyzed. Individuals with moderate-severe OSA (apnea hypopnea index, AHI = 15-50) and cardiovascular risk were recruited from cardiology practices. Those with hypertension were included. Intensive antihypertensive regimen (IAR) was defined as ≥ 3 antihypertensives including a diuretic. Definitions were: controlled BP (BP < 130/80), uncontrolled elevated BP (BP ≥ 130/80 not on IAR) and resistant elevated BP (BP ≥ 130/80 mm Hg despite IAR). Associations of untreated severe OSA (AHI ≥ 30) and uncontrolled and resistant elevated BP were evaluated using logistic regression analyses adjusted for age, sex, race, body mass index, smoking status, diabetes, and CVD.

RESULTS

Among the 284 participants (mean age 63.1 ± 7.2 years, 23.6% with severe OSA), 61.6% had controlled BP, 28.5% had uncontrolled elevated BP, and 9.9% had resistant elevated BP. Among participants prescribed IAR, resistant elevated BP was more prevalent in those with severe compared to moderate OSA (58.3% vs. 28.6%, p = 0.01). Participants with severe OSA had a 4-fold higher adjusted odds of resistant elevated BP (OR 4.1, 95% CI: 1.7-10.2), a finding not reproduced in the absence of IAR use.

CONCLUSIONS

Among patients with increased cardiovascular risk and moderate to severe OSA, untreated severe compared to moderate OSA was associated with elevated BP despite IAR suggesting untreated severe OSA contributes to poor BP control despite aggressive medication use.

COMMENTARY

A commentary on this article appears in this issue on page 845.

Obstructive sleep apnea and diurnal nondipping hemodynamic indices in patients at increased cardiovascular risk

RATIONALE

We hypothesized increasing obstructive sleep apnea (OSA) severity would be associated with nondipping blood pressure (BP) in increased cardiovascular disease (CVD) risk.

METHODS

Baseline data from 298 cardiology patients recruited for a multicenter randomized controlled trial were examined. Dipping was defined as a sleep-related BP or heart rate (HR) reduction of at least 10%. Logistic regression models were fit, adjusting for age, sex, race, BMI, CVD risk factors, CVD, and study site.

RESULTS

There was a statistically significant 4% increase in the odds of nondipping SBP per 1-unit increase in both apnea hypopnea index (AHI) and oxygen desaturation index (ODI). There was no significant relationship between AHI and nondipping mean arterial pressure (MAP); however, a 3% increase in the odds of nondipping MAP per 1-unit increase in ODI was observed [odds ratio (OR) = 1.03; 95% confidence interval (CI) 1.00-1.05]. At severe OSA levels, a 10 and 4% increase in odds of nondipping DBP per 1-unit increase in AHI and ODI were observed, respectively. A 6% [OR = 1.06; 95% CI (1.01-1.10)] increase in nondipping HR odds was observed with each increase in ODI until the upper quartile of ODI.

CONCLUSION

In patients at cardiovascular risk and moderate-to-severe OSA, increasing AHI and/or ODI were associated with increased odds of nondipping SBP and nondipping MAP. More severe levels of AHI and ODI also were associated with nondipping DBP. These results support progressive BP burden associated with increased OSA severity even in patients managed by cardiology specialty care.

Characterization of the acute pulse transit time response to obstructive apneas and hypopneas in preschool children with sleep-disordered breathing

BACKGROUND

Surges in heart rate (HR) and blood pressure (BP) at apnea termination contribute to the hypertension seen in obstructive sleep apnea (OSA). Because childhood OSA prevalence peaks in the preschool years, we aimed to characterize the cardiovascular response to obstructive events in preschool-aged children.

METHODS

Clinically referred children aged 3-5 years were grouped by obstructive apnea-hypopnea index (OAHI) into the following: primary snoring (PS) (OAHI≤1 event/h [n=21]), mild OSA (OAHI>1-≤5 [n=32]), and moderate to severe (MS) OSA (OAHI>5 [n=28]). Beat-to-beat pulse transit time (PTT), an inverse continuous indicator of BP changes, and HR were averaged during the two halves (early and late) and during the peak after (post) each obstructive event and were expressed as percentage change from late- to post-event.

RESULTS

We analyzed 422 events consisting of 55 apneas and 367 hypopneas. A significant post-event increase in HR and fall in PTT occurred in all severity groups (P<.05 for all). A greater response was associated with OSA, nonrapid eye movement sleep (NREM), cortical arousal, hypopneas, and oxygen desaturation (P<.05 for all).

CONCLUSIONS

Obstructive events elicit acute cardiovascular changes in preschool children. Such circulatory perturbations have been implicated in the development of hypertension, and our findings complement previous studies to suggest a cumulative impact of snoring on the cardiovascular system from childhood into adulthood.

Hypertension and obstructive sleep apnea

Obstructive sleep apnea (OSA) is increasingly being recognized as a major health burden with strong focus on the associated cardiovascular risk. Studies from the last two decades have provided strong evidence for a causal role of OSA in the development of systemic hypertension. The acute physiological changes that occur during apnea promote nocturnal hypertension and may lead to the development of sustained daytime hypertension via the pathways of sympathetic activation, inflammation, oxidative stress, and endothelial dysfunction. This review will focus on the acute hemodynamic disturbances and associated intermittent hypoxia that characterize OSA and the potential pathophysiological mechanisms responsible for the development of hypertension in OSA. In addition the epidemiology of OSA and hypertension, as well as the role of treatment of OSA, in improving blood pressure control will be examined.

Intermittent hypoxia augments pulmonary vascular smooth muscle reactivity to NO: regulation by reactive oxygen species

Intermittent hypoxia (IH) resulting from sleep apnea can lead to pulmonary hypertension. IH causes oxidative stress that may limit bioavailability of the endothelium-derived vasodilator nitric oxide (NO) and thus contribute to this hypertensive response. We therefore hypothesized that increased vascular superoxide anion (O(2)(-)) generation reduces NO-dependent pulmonary vasodilation following IH. To test this hypothesis, we examined effects of the O(2)(-) scavenger tiron on vasodilatory responses to the endothelium-dependent vasodilator ionomycin and the NO donor S-nitroso-N-acetylpenicillamine in isolated lungs from hypocapnic-IH (H-IH; 3 min cycles of 5% O(2)/air flush, 7 h/day, 4 wk), eucapnic-IH (E-IH; cycles of 5% O(2), 5% CO(2)/air flush), and sham-treated (air/air cycled) rats. Next, we assessed effects of endogenous O(2)(-) on NO- and cGMP-dependent vasoreactivity and measured O(2)(-) levels using the fluorescent indicator dihydroethidium (DHE) in isolated, endothelium-disrupted small pulmonary arteries from each group. Both E-IH and H-IH augmented NO-dependent vasodilation; however, enhanced vascular smooth muscle (VSM) reactivity to NO following H-IH was masked by an effect of endogenous O(2)(-). Furthermore, H-IH and E-IH similarly increased VSM sensitivity to cGMP, but this response was independent of either O(2)(-) generation or altered arterial protein kinase G expression. Finally, both H-IH and E-IH increased arterial O(2)(-) levels, although this response was more pronounced following H-IH, and H-IH exposure resulted in greater protein tyrosine nitration indicative of increased NO scavenging by O(2)(-). We conclude that IH increases pulmonary VSM sensitivity to NO and cGMP. Furthermore, endogenous O(2)(-) limits NO-dependent vasodilation following H-IH through an apparent reduction in bioavailable NO.

Patent foramen ovale and obstructive sleep apnoea: from pathophysiology to diagnosis of a potentially dangerous association

Patent foramen ovale and obstructive sleep apnoea are frequently encountered in the general population. Owing to their prevalence, they may coexist fortuitously; however, the prevalence of patent foramen ovale seems to be higher in patients with obstructive sleep apnoea. We have reviewed the epidemiological data, pathophysiology, and the diagnostic and therapeutic options for both patent foramen ovale and obstructive sleep apnoea. We focus on the interesting pathophysiological links that could explain a potential association between both pathologies and their implications, especially on the risk of stroke.

The association among lipoprotein-associated phospholipase A2 levels, total antioxidant capacity and arousal in male patients with OSA

BACKGROUND

The mechanisms of the increased cardiac and vascular events in patients with OSA are not well understood. Arousal which is an important component of OSA was associated with increased sympathetic activation and electrocardiographic changes which prone to arrhythmias. We planned to examine the association among arousal, circulating Lp-PLA2 and total antioxidant capacity in male patients with OSA.

METHODS

Fifty male patients with newly diagnosed OSA were enrolled the study. A full-night polysomnography was performed and arousal index was obtained. Lp-PLA2 concentrations were measured in serum samples with the PLAC Test. Total antioxidant capacity in patients was determined with Antioxidant Assay Kit.

RESULTS

Arousal was positively correlated with LP-PLA2 levels (r=0.43, p=0.002) and was negatively correlated with total antioxidant capacity (r= -0.29, p=0.04). Elevated LP-PLA2 levels and decreased total antioxidant activities were found in the highest arousal quartile compared with the lowest and 2nd quartiles (p=0.02, p=0.05, respectively). LP-PLA2 was an independently predictor of arousal index in regression model (β=0.357, p=0.002)

CONCLUSIONS

This study demonstrated a moderate linear relationship between arousal and LP-PLA2 levels. Also, total antioxidant capacities were decreased in the higher arousal index. Based on the study result, the patients with higher arousal index may be prone to vascular events.

Acute cardiovascular changes with obstructive events in children with sleep disordered breathing

STUDY OBJECTIVES

Obstructive apneas in adults are associated with acute changes in blood pressure (BP) and heart rate (HR) that may contribute to poor cardiovascular outcome. Children with sleep disordered breathing (SDB) are similarly at risk for cardiovascular complications. We aimed to test the hypothesis that BP and HR are augmented during obstructive events in children equivalent to levels reported in adults.

DESIGN

Beat-by-beat mean arterial pressure (MAP) and HR were analyzed over the course of obstructive events (pre, early, late, and post-event) during NREM and REM sleep and compared using 2-way ANOVA with post hoc analyses.

SETTING

Pediatric sleep laboratory.

PATIENTS OR PARTICIPANTS

30 children (15M/15F) aged 7-12 y referred for investigation of SDB INTERVENTIONS: N/A.

MEASUREMENTS AND RESULTS

All children underwent overnight polysomnography with continuous BP recording. MAP and HR increased significantly from late to post event in both sleep states (mean +/- SEM, NREM: MAP, 74 +/- 3 to 93 +/- 3 mm Hg; HR, 76 +/- 2 to 97 +/- 2 bpm. REM: MAP, 76 +/- 3 to 89 +/- 3 mm Hg; HR, 76 +/- 2 to 91 +/- 2 bpm. P < 0.05 for all). NREM sleep state and arousal from sleep were significant independent predictors of the magnitude of cardiovascular change from late to post event (P < 0.05 for all).

CONCLUSIONS

Children with SDB experience significant changes in HR and BP during obstructive events with magnitudes that are similar to levels reported in adults. These changes are more pronounced during NREM sleep and with arousal. These acute cardiovascular changes may have important implications for poor cardiovascular outcome in children with OSA as repetitive cardiovascular perturbations may contribute to the development of hypertension.

Obstructive sleep apnea leads to transient uncoupling of coronary blood flow and myocardial work in humans

STUDY OBJECTIVES

Obstructive Sleep Apnea (OSA) is associated with a poor prognosis in patients with coronary artery disease. We hypothesized that abnormalities of coronary blood flow (CBF) associated with obstructive apneas may predispose patients to ischemia. We aimed to determine CBF during respiratory events in patients with OSA.

SETTING

University Hospital.

PATIENTS

Ten subjects undergoing elective percutaneous coronary intervention

DESIGN

We measured CBF and myocardial work (rate-pressure product [RPP]) in a non-culprit coronary artery in patients sleeping in the cardiac catheterization laboratory. Hemodynamic responses were matched to spontaneously occurring respiratory events.

MEASUREMENTS AND RESULTS

Events comprised a mixture of obstructive apneas, central apneas and hypopneas. RPP increased at the termination of each type of respiratory event. Following the rise in RPP, there was a delay, identified with breakpoint analysis, before CBF began to increase (P<0.001) that differed in duration with event type: 8 sec for obstructive apnea, 5 sec for central apnea, and 4 sec for hypopnea. The delay in CBF with obstructive apnea was associated with an increase in coronary vascular resistance of 16% +/- 4% (P < 0.05). Stepwise multilinear regression analysis showed the increase in CBF was predicted by the rise in RPP (R=0.52, P<0.001) and presence of arousal from sleep (R=0.30, P<0.05), but not the degree of O2 desaturation.

CONCLUSION

Following obstructive apneas there is a transient uncoupling of CBF from myocardial work and an increase in CVR. This disturbed flow-metabolic coupling may lead to nocturnal myocardial ischemia in patients with both OSA and coronary artery disease.

DNA damage and repair capacity in lymphocytes from obstructive sleep apnea patients

Obstructive sleep apnea (OSA) syndrome is a respiratory disease that is linked to heart attacks and high blood pressure. In the present study, we used the Comet assay to compare basal DNA damage and DNA damage induction by hydrogen peroxide, ethanol, and gamma-irradiation in lymphocytes from 35 OSA patients and 35 controls. We also measured the apoptosis and necrosis produced by these agents and the ability of the lymphocytes to repair the induced DNA damage. It was found that lymphocytes isolated from OSA patients had higher basal levels of DNA damage and were more sensitive to the effects of the DNA-damaging agents than lymphocytes from controls. OSA patients also had a reduced capacity to repair the DNA damage induced by the three agents, but apoptosis and necrosis were similar in OSA patients and the controls.

Differential effects of chronic hypoxia and intermittent hypocapnic and eucapnic hypoxia on pulmonary vasoreactivity

Intermittent hypoxia (IH) resulting from sleep apnea can lead to pulmonary hypertension (PH) and right heart failure, similar to chronic sustained hypoxia (CH). Supplemental CO(2), however, attenuates hypoxic PH. We therefore hypothesized that, similar to CH, IH elicits PH and associated increases in arterial endothelial nitric oxide synthase (eNOS) expression, ionomycin-dependent vasodilation, and receptor-mediated pulmonary vasoconstriction. We further hypothesized that supplemental CO(2) inhibits these responses to IH. To test these hypotheses, we measured eNOS expression by Western blot in intrapulmonary arteries from CH (2 wk, 0.5 atm), hypocapnic IH (H-IH) (3 min cycles of 5% O(2)/air flush, 7 h/day, 2 wk), and eucapnic IH (E-IH) (3 min cycles of 5% O(2), 5% CO(2)/air flush, 7 h/day, 2 wk) rats and their respective controls. Furthermore, vasodilatory responses to the calcium ionophore ionomycin and vasoconstrictor responses to the thromboxane mimetic U-46619 were measured in isolated saline-perfused lungs from each group. Hematocrit, arterial wall thickness, and right ventricle-to-total ventricle weight ratios were additionally assessed as indexes of polycythemia, arterial remodeling, and PH, respectively. Consistent with our hypotheses, E-IH resulted in attenuated polycythemia, arterial remodeling, RV hypertrophy, and eNOS upregulation compared with H-IH. However, in contrast to CH, neither H-IH nor E-IH increased ionomycin-dependent vasodilation. Furthermore, H-IH and E-IH similarly augmented U-46619-induced pulmonary vasoconstriction but to a lesser degree than CH. We conclude that maintenance of eucapnia decreases IH-induced PH and upregulation of arterial eNOS. In contrast, increases in pulmonary vasoconstrictor reactivity following H-IH are unaltered by exposure to supplemental CO(2).

Occlusion of the upper airway does not augment the cardiovascular response to arousal from sleep in humans

The cardiovascular response to an arousal from sleep at the termination of an obstructive apnea is more than double that to a spontaneous arousal. We investigated the hypothesis that stimulation of respiratory mechanoreceptors, by inspiring against an occluded airway during an arousal from sleep, augments the accompanying cardiovascular response. Arousals (>10 s) from stage 2 sleep were induced by a 1-s auditory tone (85 dB) during a concomitant 1-s inspiratory occlusion (O) and without an occlusion [i.e., control arousal (C)] in 15 healthy men (mean ± SE: age, 25 ± 1 yr). Arousals were associated with a significant increase in mean arterial blood pressure (MAP) at 4 s ( P < 0.001) and a significant decrease in R-R interval at 3 s ( P < 0.001). However, the magnitude of the cardiovascular response was not different during C compared with O (MAP: C, 86 ± 3 to 104 ± 3 mmHg; O, 86 ± 3 to 105 ± 3 mmHg; P = 0.99. R-R interval: C, 1.12 ± 0.03 to 0.89 ± 0.04 s; O, 1.11 ± 0.02 to 0.87 ± 0.02 s, P = 0.99). Ventilation significantly increased during arousals under both conditions at the second breath ( P < 0.001); this increase was not different between the two conditions (C: 4.40 ± 0.29 to 6.76 ± 0.61 l/min, O: 4.35 ± 0.34 to 7.65 ± 0.73 l/min; P = 0.31). We conclude that stimulation of the respiratory mechanoreceptors by transient upper airway occlusion is unlikely to interact with the arousal-related autonomic outflow to augment the cardiovascular response in healthy young men.

Cardiovascular response to arousal from sleep under controlled conditions of central and peripheral chemoreceptor stimulation in humans

The cardiovascular response to an arousal occurring at the termination of an obstructive apnea is almost double that to a spontaneous arousal. We investigated the hypothesis that central plus peripheral chemoreceptor stimulation, induced by hypercapnic hypoxia (HH), augments the cardiovascular response to arousal from sleep. Auditory-induced arousals during normoxia and HH (>10-s duration) were analyzed in 13 healthy men [age 24 +/- 1 (SE) yr]. Subjects breathed on a respiratory circuit that held arterial blood gases constant, despite the increased ventilation associated with arousal. Arousals were associated with a significant increase in mean arterial blood pressure at 5 s (P < 0.001) and with a significant decrease in the R-R interval at 3 s (P < 0.001); however, the magnitude of the changes was not significantly different during normoxia compared with HH (mean arterial blood pressure: normoxia, 91 +/- 4 to 106 +/- 4 mmHg; HH, 91 +/- 4 to 109 +/- 5 mmHg; P = 0.32; R-R interval: normoxia, 1.12 +/- 0.04 to 0.90 +/- 0.05 s; HH, 1.09 +/- 0.05 to 0.82 +/- 0.03 [corrected] s; P = 0.78). Mean ventilation increased significantly at the second breath postarousal for both conditions (P < 0.001), but the increase was not significantly different between the two conditions (normoxia, 5.35 +/- 0.40 to 9.57 +/- 1.69 l/min; HH, 8.57 +/- 0.63 to 11.98 +/- 0.70 l/min; P = 0.71). We conclude that combined central and peripheral chemoreceptor stimulation with the use of HH does not interact with the autonomic outflow associated with arousal from sleep to augment the cardiovascular response.

Epidemiology of obstructive sleep apnea: a population health perspective

Population-based epidemiologic studies have uncovered the high prevalence and wide severity spectrum of undiagnosed obstructive sleep apnea, and have consistently found that even mild obstructive sleep apnea is associated with significant morbidity. Evidence from methodologically strong cohort studies indicates that undiagnosed obstructive sleep apnea, with or without symptoms, is independently associated with increased likelihood of hypertension, cardiovascular disease, stroke, daytime sleepiness, motor vehicle accidents, and diminished quality of life. Strategies to decrease the high prevalence and associated morbidity of obstructive sleep apnea are critically needed. The reduction or elimination of risk factors through public health initiatives with clinical support holds promise. Potentially modifiable risk factors considered in this review include overweight and obesity, alcohol, smoking, nasal congestion, and estrogen depletion in menopause. Data suggest that obstructive sleep apnea is associated with all these factors, but at present the only intervention strategy supported with adequate evidence is weight loss. A focus on weight control is especially important given the expanding epidemic of overweight and obesity in the United States. Primary care providers will be central to clinical approaches for addressing the burden and the development of cost-effective case-finding strategies and feasible treatment for mild obstructive sleep apnea warrants high priority.

Prospective study of the association between sleep-disordered breathing and hypertension

BACKGROUND

Sleep-disordered breathing is prevalent in the general population and has been linked to chronically elevated blood pressure in cross-sectional epidemiologic studies. We performed a prospective, population-based study of the association between objectively measured sleep-disordered breathing and hypertension (defined as a laboratory-measured blood pressure of at least 140/90 mm Hg or the use of antihypertensive medications).

METHODS

We analyzed data on sleep-disordered breathing, blood pressure, habitus, and health history at base line and after four years of follow-up in 709 participants of the Wisconsin Sleep Cohort Study (and after eight years of follow-up in the case of 184 of these participants). Participants were assessed overnight by 18-channel polysomnography for sleep-disordered breathing, as defined by the apnea-hypopnea index (the number of episodes of apnea and hypopnea per hour of sleep). The odds ratios for the presence of hypertension at the four-year follow-up study according to the apnea-hypopnea index at base line were estimated after adjustment for base-line hypertension status, body-mass index, neck and waist circumference, age, sex, and weekly use of alcohol and cigarettes.

RESULTS

Relative to the reference category of an apnea-hypopnea index of 0 events per hour at base line, the odds ratios for the presence of hypertension at follow-up were 1.42 (95 percent confidence interval, 1.13 to 1.78) with an apnea-hypopnea index of 0.1 to 4.9 events per hour at base line as compared with none, 2.03 (95 percent confidence interval, 1.29 to 3.17) with an apnea-hypopnea index of 5.0 to 14.9 events per hour, and 2.89 (95 percent confidence interval, 1.46 to 5.64) with an apnea-hypopnea index of 15.0 or more events per hour.

CONCLUSIONS

We found a dose-response association between sleep-disordered breathing at base line and the presence of hypertension four years later that was independent of known confounding factors. The findings suggest that sleep-disordered breathing is likely to be a risk factor for hypertension and consequent cardiovascular morbidity in the general population.

Effects of nasal continuous positive airway pressure on awake ventilatory responses to hypoxia and hypercapnia in patients with obstructive sleep apnea

This study was aimed to examine the short- and long-term effects of nasal continuous positive airway pressure (CPAP) on the chemosensitivity to hypoxia and hypercapnia in the patients with obstructive sleep apnea (OSA). Awake ventilatory responses to hypoxia and hypercapnia were examined in 28 patients (3 female) with moderate to severe OSA. All these tests were examined before and after 2 weeks of nasal CPAP. In 10 patients these tests were repeated after 3-6 months of nasal CPAP. All were also tested for spirometry and arterial blood gas analysis. Patients were middle-aged (48.9 +/- 9.9 years) and their mean apnea-hypopnea index was 58.3 +/- 20.4/hour. After 2 week of nasal CPAP, PaO2 significantly increased (77.7 +/- 11.8 vs. 84.6 +/- 9.8 mmHg) and PaCO2 significantly decreased (44.9 +/- 3.8 vs. 42.3 +/- 3.7 mmHg). The ventilatory response to hypoxia significantly decreased (0.80 +/- 0.51 vs. 0.61 +/- 0.51 liter/min/%) whereas the ventilatory response to hypercapnia significantly increased after 2 weeks (1.47 +/- 0.73 vs. 1.80 +/- 0.76 liter/min/mmHg). Similar findings were also observed after 3-6 months of nasal CPAP in 10 OSA patients. Nasal CPAP treatment can alter the ventilatory responses in patients with OSA.

New strategies of screening and treatment for sleep apnea syndrome

The prevalence of sleep apnea syndrome (SAS) is approximately 7.5% in Japanese adults aged 18-68 years old. SAS is characterized by repeated episodes of apnea, especially obstructive apnea, during sleep. Severe SAS has life-threatening complications such as pulmonary hypertension, arrhythmias, right heart failure or brain damage, which could be caused by hypoxemia and/or hypercapnia. Upper airway relaxation is responsible for the obstruction during apnea, and an increase in the activities of the upper airway muscles dilates and stiffens the upper airway wall. Maintaining the activities of the upper airway muscles may contribute to keeping the airway patent. Submental electrical stimulation of the upper airway muscles would be a novel treatment method for obstructive apnea.

Role of hypoxemia and hypercapnia in acute cardiovascular response to periodic apneas in sedated pigs

The effects of hypoxemia and hypercapnia in acute cardiovascular response to periodic non-obstructive apneas were explored in seven preinstrumented, sedated paralyzed and ventilated pigs under three conditions: room air breathing (RA), O2 supplementation (O2), and supplementation with O2 and CO2 (CO2). EEG monitoring showed no arousal under any conditions. RA apneas increased mean arterial pressure (MAP, from baseline 95.9 +/- 4.5 to late apnea 124.4 +/- 7.8 Torr, P < 0.01), left ventricular end-diastolic pressure, end-diastolic and end-systolic myocardial fiber lengths and systemic vascular resistance, but decreased cardiac output (CO, 3.09 +/- 0.34-2.37 +/- 0.26 L/min, P < 0.01), heart rate (HR, 115.1 +/- 7.5-102.0 +/- 7.8 bpm, P < 0.01), and stroke volume (SV, 29.6 +/- 0.7 21.1 +/- 1.8 ml, P < 0.01). 02 apneas produced similar decreases in HR (114.0 +/- 11.8-105.4 +/- 8.7 bpm, P < 0.05) as with RA apneas, but smaller increases in MAP (94.5 +/- 1.8-103.4 +/- 2.8 Torr, P < 0.01) and in the variables of pre- and after-load. CO and SV remained unchanged with O2 apneas. CO2 was associated with higher MAP, CO, and HR at baseline relative to RA, but similar cardiovascular response during apneas in direction and magnitude to those of O2 apneas. We conclude that in this model hypoxemia is a major but not the sole determinant of the pressor response during apneas. Hypercapnia cannot explain the pressor response seen when hypoxemia is abolished. The HR fall during apneas is independent of hypoxemia, hypercapnia and the pressor response.

Prevalence of patent foramen ovale and its contribution to hypoxemia in patients with obstructive sleep apnea

STUDY OBJECTIVES

The objectives of this study were (1) to assess the prevalence of patent foramen ovale (PFO) by means of contrast transesophageal echocardiography (TEE) in patients with obstructive sleep apnea, and (2) to determine the potential contribution of right to left interatrial shunting to systemic oxygen desaturation following the performance of Valsalva maneuver.

DESIGN

Performance of contrast TEE during Valsalva maneuver with simultaneous measurement of systemic arterial oxygen saturation (SaO2) by means of pulse oximetry in patients with obstructive sleep apnea and a control group.

SETTING

Government teaching hospital, university hospital affiliate.

PATIENTS

Study group comprised 48 patients with documented obstructive sleep apnea and 24 control subjects.

INTERVENTIONS

Sleep studies, contrast TEE, Valsalva maneuver, pulse oximetry.

MEASUREMENTS AND RESULTS

Thirty-three of 48 patients with obstructive sleep apnea compared with 4 of 24 control patients had a detectable PFO (69% vs 17%; p < 0.0001). All sleep apnea patients had comparable baseline SaO2 regardless of the presence of a PFO (93.9+/-1.7% vs 95+/-1.2%; p=not significant). After performance of a Valsalva maneuver, however, a significantly greater fall in SaO2 was observed in patients with obstructive sleep apnea and PFO compared with patients with obstructive sleep apnea without PFO (-2.4 +/- 1.5% vs -1.3 +/- 0.6%; p=0.007). A statistically significant fall in SaO2 (defined as > 4 SD of recorded SaO2 values after Valsalva maneuver in patients without PFO) was found in one third of patients with sleep apnea and PFO.

CONCLUSION

We conclude that there is an increased prevalence of PFO in patients with obstructive sleep apnea that could contribute to significant hypoxemia after a Valsalva maneuver in approximately one third of these patients.

Role of hypoxia on increased blood pressure in patients with obstructive sleep apnoea

BACKGROUND

Cyclical changes in systemic blood pressure occur during apnoeic episodes in patients with obstructive sleep apnoea (OSA). Although several factors including arterial hypoxaemia, intrathoracic pressure changes, and disruption of sleep architecture have been reported to be responsible for these changes in blood pressure, the relative importance of each factor remains unclear. This study assessed the role of hypoxaemia on the increase in blood pressure during apnoeic episodes.

METHODS

The blood pressure in apnoeic episodes during sleep and the blood pressure response to isocapnic intermittent hypoxia whilst awake were measured in 10 men with OSA. While asleep the blood pressure was measured non-invasively using a Finapres blood pressure monitor with polysomnography. The response of the blood pressure to hypoxia whilst awake was also measured while the subjects intermittently breathed a hypoxic (5% or 7% oxygen) gas mixture. Each hypoxic gas exposure was continued until a nadir arterial oxygen saturation (nSaO2) of less than 75% was reached, or for a period of 100 seconds. The exposure was repeated five times in succession with five interposed breaths of room air in each run.

RESULTS

The mean (SD) increase in blood pressure (delta MBP) during apnoeic episodes was 42.1 (17.3) mm Hg during rapid eye movement (REM) sleep and 31.9 (12.5) mm Hg during non-REM sleep. The delta MBP during apnoeic episodes showed a correlation with the decrease of nSaO2 (delta SaO2) (r2 = 0.30). The change in blood pressure in response to intermittent hypoxia whilst awake was cyclical and qualitatively similar to that during apnoeic episodes. Averaged delta MBP at an SaO2 of 7% and 5% oxygen was 12.6 (5.7) and 13.4 (3.6) mm Hg, respectively, whereas the averaged delta MBP at the same delta SaO2 during apnoeic episodes was 38.4 (15.5) and 45.2 (20.5) mm Hg, respectively.

CONCLUSIONS

The blood pressure response to desaturation whilst awake was about one third of that during apnoeic episodes. These results suggest that factors other than hypoxia may play an important part in raising the blood pressure during obstructive sleep apnoea.