Effects of chronic intermittent asphyxia on haematocrit, pulmonary arterial pressure and skeletal muscle structure in rats

Diaphragm Dysfunction and Rehabilitation Strategy in Patients With Chronic Obstructive Pulmonary Disease

Chronic obstructive pulmonary disease (COPD) affects the whole body and causes many extrapulmonary adverse effects, amongst which diaphragm dysfunction is one of the prominent manifestations. Diaphragm dysfunction in patients with COPD is manifested as structural changes, such as diaphragm atrophy, single-fibre dysfunction, sarcomere injury and fibre type transformation, and functional changes such as muscle strength decline, endurance change, diaphragm fatigue, decreased diaphragm mobility, etc. Diaphragm dysfunction directly affects the respiratory efficiency of patients and is one of the important pathological mechanisms leading to progressive exacerbation of COPD and respiratory failure, which is closely related to disease mortality. At present, the possible mechanisms of diaphragm dysfunction in patients with COPD include systemic inflammation, oxidative stress, hyperinflation, chronic hypoxia and malnutrition. However, the specific mechanism of diaphragm dysfunction in COPD is still unclear, which, to some extent, increases the difficulty of treatment and rehabilitation. Therefore, on the basis of the review of changes in the structure and function of COPD diaphragm, the potential mechanism of diaphragm dysfunction in COPD was discussed, the current effective rehabilitation methods were also summarised in this paper. In order to provide direction reference and new ideas for the mechanism research and rehabilitation treatment of diaphragm dysfunction in COPD.

Effect of chronic intermittent hypoxia on the reflex recruitment of the genioglossus during airway obstruction in the anesthetized rat

We sought to test the hypothesis that chronic intermittent hypoxia (CIH)-a feature of sleep-disordered breathing in humans-impairs reflex recruitment of the genioglossus (GG, pharyngeal dilator) during obstructive airway events. Adult male Wistar rats were exposed to 20 cycles of normoxia and hypoxia (5% O2 at nadir) per hour, 8h a day for 7 days (CIH, N=7). The sham group (N=7) were exposed to normoxia in parallel. Following gas treatments, rats were anesthetized with an i.p. injection of urethane (1.5g/kg; 20%, w/v). Fine concentric needle electrodes were inserted into the GG and the costal diaphragm. Discriminated GG motor unit potentials and whole electromyograph (EMG), together with arterial blood pressure and arterial O2 saturation, were recorded during quiet basal breathing and during nasal airway occlusion. Airway occlusion significantly increased GG EMG activity in all animals; but there was no difference in the reflex response to airway occlusion between sham and CIH-treated animals (+105±22% vs. +105±17%, mean±SEM for area under the curve of integrated GG EMG, % increase from baseline, p=0.99). Occluded breaths were characterized by a significant increase in the firing frequency of phasically active units and the recruitment of large motor units that were quiescent under basal conditions. Though there are reports of impaired control of the upper airway following CIH in the rat, we conclude that reflexly evoked motor discharge to the GG is not affected by 7 days of CIH, a paradigm that we have shown increases apnea index in sleeping rats.

Diaphragm muscle remodeling in a rat model of chronic intermittent hypoxia

Respiratory muscle remodeling occurs in human sleep apnea--a common respiratory disorder characterized by chronic intermittent hypoxia (CIH) due to recurrent apnea during sleep. We sought to determine if CIH causes remodeling in rat sternohyoid (upper airway dilator) and diaphragm muscles. Adult male Wistar rats were exposed to CIH (n=8), consisting of 90 sec of hypoxia (5% at the nadir; SaO₂ ~80%)/90 sec of normoxia, 8 hr per day, for 7 consecutive days. Sham animals (n=8) were exposed to alternating air/air cycles in parallel. The effect of CIH on myosin heavy-chain (MHC) isoform (1, 2a, 2x, 2b) distribution, sarcoplasmic reticulum calcium ATPase (SERCA) isoform distribution, succinate dehydrogenase activity, glycerol phosphate dehydrogenase activity, and Na⁺/K⁺ ATPase pump content was determined. Sternohyoid muscle structure was unaffected by CIH treatment. CIH did not alter oxidative/glycolytic capacity or the Na⁺/K⁺-ATPase pump content of the diaphragm. CIH significantly increased the areal density of MHC 2b fibers in the rat diaphragm, and this was associated with a shift in SERCA proteins from SERCA2 to SERCA1. We conclude that CIH causes a slow-to-fast fiber transition in the rat diaphragm after just 7 days of treatment. Respiratory muscle functional remodeling may drive aberrant functional plasticity such as decreased muscle endurance, which is a feature of human sleep apnea.

Cardiovascular consequences of sleep apnea

Sleep apnea is a common health concern that is characterized by repetitive episodes of asphyxia. This condition has been linked to serious long-term adverse effects such as hypertension, metabolic dysregulation, and cardiovascular disease. Although the mechanism for the initiation and aggravation of cardiovascular disease has not been fully elucidated, oxidative stress and subsequent endothelial dysfunction play major roles. Animal models, which have the advantage of being free of comorbidities and/or behavioral variables (that commonly occur in humans), allow invasive measurements under well-controlled experimental conditions, and as such are useful tools in the study of the pathophysiological mechanisms of sleep apnea. This review summarizes currently available information on the cardiovascular consequences of sleep apnea and briefly describes common experimental approaches useful to sleep apnea in different animal models.

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.

Mechanisms of intermittent hypoxia induced hypertension

Exposing rodents to brief episodes of hypoxia mimics the hypoxemia and the cardiovascular and metabolic effects observed in patients with obstructive sleep apnoea (OSA), a condition that affects between 5% and 20% of the population. Apart from daytime sleepiness, OSA is associated with a high incidence of systemic and pulmonary hypertension, peripheral vascular disease, stroke and sudden cardiac death. The development of animal models to study sleep apnoea has provided convincing evidence that recurrent exposure to intermittent hypoxia (IH) has significant vascular and haemodynamic impact that explain much of the cardiovascular morbidity and mortality observed in patients with sleep apnoea. However, the molecular and cellular mechanisms of how IH causes these changes is unclear and under investigation. This review focuses on the most recent findings addressing these mechanisms. It includes a discussion of the contribution of the nervous system, circulating and vascular factors, inflammatory mediators and transcription factors to IH-induced cardiovascular disease. It also highlights the importance of reactive oxygen species as a primary mediator of the systemic and pulmonary hypertension that develops in response to exposure to IH.

Vascular effects of intermittent hypoxia

Obstructive sleep apnea is characterized by repeated upper airway obstruction during sleep and affects between 5% and 20% of the population. Epidemiological studies reveal that sleep apnea and associated intermittent hypoxemia increase the risk for hypertension and vascular disease but the mechanisms underlying these effects are incompletely understood. This review reports the results of rodent models of intermittent hypoxia (IH) and relates them to the observed hemodynamic and vascular consequences of sleep apnea. These animal studies have demonstrated that IH exposure in the absence of any other comorbidity causes hypertension, endothelial dysfunction, and augmented constrictor sensitivity, all due at least in part to increased vascular oxidative stress. Animal studies have used a variety of exposure paradigms to study intermittent hypoxia and these different exposure protocols can cause hypocapnia or hypercapnia-or maintain eucapnia-with accompanying alterations in plasma pH. It appears that these different profiles of arterial blood gases can lead to divergent results but the impact of these differences is still being investigated. Overall, the studies in rodents have clearly demonstrated that the vascular and hemodynamic impact of intermittent hypoxia provides a strong rationale for treating clinical sleep apnea to prevent the resulting cardiovascular morbidity and mortality.

Efeitos sistêmicos da hipoxemia noturna em pacientes com doença pulmonar obstrutiva crônica sem síndrome da apnéia obstrutiva do sono

OBJETIVO: Estudar os efeitos da hipoxemia noturna em pacientes com doença pulmonar obstrutiva crônica sem síndrome da apnéia obstrutiva do sono. MÉTODOS: Estudamos 21 pacientes-10 dessaturadores e 11 não-dessaturadores-submetidos a gasometria arterial, polissonografia, espirometria, teste de exercício cardiopulmonar (cicloergômetro), dinamometria manual e medidas de pressão inspiratória máxima, pressão expiratória máxima e proteína C reativa (PCR). Incluíram-se os pacientes com pressão parcial arterial de oxigênio > 60 mmHg; excluíram-se os com índice de apnéia-hipopnéia > 5 eventos/hora de sono. Foram medidos consumo máximo de oxigênio, potência máxima, pressão arterial sistólica, pressão arterial diastólica (PAD) e frequência cardíaca máxima durante exercício, visando detectar alterações hemodinâmicas. A PCR foi considerada positiva quando acima de 3 mg/L. RESULTADOS: A saturação periférica de oxigênio mínima durante o sono foi significativamente maior nos não-dessaturadores (p = 0,03). Mais dessaturadores apresentaram PCR > 3 mg/L (p < 0,05). Não houve diferença quanto a capacidade de exercício e demais variáveis. No entanto, PAD (p < 0,001) e pressão inspiratória máxima (p = 0,001) correlacionaram-se com saturação periférica de oxigênio média durante o sono. CONCLUSÕES: A hipoxemia noturna não reduz a capacidade de exercício e a força de preensao manual em pacientes com DPOC leve/moderada, mas o ajuste da PAD durante o exercício máximo parece depender do grau de hipoxemia. Além disso, há uma relação positiva entre pressão inspiratória máxima e saturação periférica de oxigênio média durante o sono, bem como indícios de ativação inflamatória diferenciada em pacientes com hipoxemia noturna.

Intermittent hypoxia: cause of or therapy for systemic hypertension?

During acute episodes of hypoxia, chemoreceptor-mediated sympathetic activity increases heart rate, cardiac output, peripheral resistance and systemic arterial pressure. However, different intermittent hypoxia paradigms produce remarkably divergent effects on systemic arterial pressure in the post-hypoxic steady state. The hypertensive effects of obstructive sleep apnea (OSA) vs. the depressor effects of therapeutic hypoxia exemplify this divergence. OSA, a condition afflicting 15-25% of American men and 5-10% of women, has been implicated in the pathogenesis of systemic hypertension and is a major risk factor for heart disease and stroke. OSA imposes a series of brief, intense episodes of hypoxia and hypercapnia, leading to persistent, maladaptive chemoreflex-mediated activation of the sympathetic nervous system which culminates in hypertension. Conversely, extensive evidence in animals and humans has shown controlled intermittent hypoxia conditioning programs to be safe, efficacious modalities for prevention and treatment of hypertension. This article reviews the pertinent literature in an attempt to reconcile the divergent effects of intermittent hypoxia therapy and obstructive sleep apnea on hypertension. Special emphasis is placed on research conducted in the nations of the former Soviet Union, where intermittent hypoxia conditioning programs are being applied therapeutically to treat hypertension in patients. Also reviewed is evidence regarding mechanisms of the pro- and anti-hypertensive effects of intermittent hypoxia.

The relationship of daytime hypoxemia and nocturnal hypoxia in obstructive sleep apnea syndrome

QUESTION OF THE STUDY

Prevalence and determinants of daytime hypoxemia in patients with obstructive sleep apnea (OSA) syndrome are not well established. The aims of this study, conducted in a large series of OSA patients, were to estimate the prevalence of daytime hypoxemia, to assess the reciprocal effects between daytime PaO2 and nocturnal SpO2, and to investigate the direct and indirect role of sleep apnea severity in determining feedback gas exchange abnormalities.

MATERIALS AND METHODS

In 456 patients a daytime hypoxemia-nocturnal hypoxia feedback structural equations model was designed. PaO2 adjusted for age (% of predicted), percent sleep time spent with SpO2 <90% (TST90), oxygen desaturation index and the apnea-hypopnea index, were determined as the measures of daytime hypoxemia, nocturnal hypoxia, and sleep apnea severity, respectively, after adjusting for the severity of obesity and lung volumes.

RESULTS

The TST90-PaO2 feed-back structural equations modeling showed that daytime PaO2 was inversely related (P<0.001) to nocturnal hypoxia (-4.0% of PaO2 per 1 SD of TST90). The severity of OSA (-1.0%) was an indirect determinant of daytime PaO2 via the TST90 pathway. In contrast, daytime PaO2 did not influence (P>0.05) the extent of nocturnal hypoxia.

CONCLUSIONS

In OSA patients, the extent of nocturnal hypoxia seems to be both a direct determinant and a mediator of the indirect effect of sleep apnea on the development of daytime hypoxemia.

Obstructive sleep apnea and insulin resistance: a role for microcirculation?

Obstructive sleep apnea is an increasingly recognized medical problem. The recent attention to its frequency in the general population and its important role in metabolic, vascular, and behavioral aspects have sharply increased the number and nature of investigations, thereby revealing new aspects that open new approaches in research. Whereas obstructive sleep apnea is a well-known phenomenon accompanying obesity and diabetes, new findings strongly suggest that this close relationship may also operate in the opposite direction. Indeed obstructive sleep apnea may be a primary feature inducing or aggravating a series of vascular and metabolic disturbances closely resembling the metabolic syndrome. This review will discuss established and potential mechanisms responsible for these changes. Obstructive sleep apnea indeed appears to gather all the elements necessary to induce insulin resistance, hypertension, and possibly heart failure. After careful analysis of these modifications and considering how they are intertwined, we propose that microcirculation could represent the common denominator mediating the progression of this pathology, as it is eventually the case in the metabolic syndrome and diabetes domain. This plausible hypothesis is discussed in detail and should be verified by appropriate preclinical and clinical protocols, which are now achievable by using noninvasive techniques in humans.

Chronic intermittent hypoxia increases infarction in the isolated rat heart

Coronary heart disease is frequently associated with obstructive sleep apnea syndrome and treating obstructive sleep apnea appears to significantly improve the outcome in coronary heart disease. Thus we have developed a rat model of chronic intermittent hypoxia (IH) to study the influence of this condition on myocardial ischemia-reperfusion tolerance and on functional vascular reactivity. Wistar male rats were divided in three experimental groups ( n = 12 each) subjected to chronic IH (IH group), normoxia (N group), or control conditions (control group). IH consisted of repetitive cycles of 1 min (40 s with inspired O2fraction 5% followed by 20 s normoxia) and was applied for 8 h during daytime, for 35 days. Normoxic cycles were applied in the same conditions, inspired O2fraction remaining constant at 21%. On day 36, mean arterial blood pressure (MABP) was measured before isolated hearts were submitted to an ischemia-reperfusion protocol. The thoracic aorta and left carotid artery were also excised for functional reactivity studies. MABP was not significantly different between the three experimental groups. Infarct sizes (in percent of ventricles) were significantly higher in IH group (46.9 ± 3.6%) compared with N (26.1 ± 2.8%) and control (21.7 ± 2.1%) groups. Vascular smooth muscle function was similar in aorta and carotid arteries from all groups. The endothelium-dependent relaxation in response to acetylcholine was also similar in aorta and carotid arteries from all groups. Chronic IH increased heart sensitivity to infarction, independently of a significant increase in MABP, and did not affect vascular reactivity of aorta and carotid arteries.

Augmented Endothelin Vasoconstriction in Intermittent Hypoxia-Induced Hypertension

We reported previously that simulating sleep apnea in rats by exposing them 7 hours per day to intermittent hypoxia/hypercapnia (IH) elevates plasma endothelin-1 and causes hypertension, which is reversed by an endothelin-1 antagonist. We hypothesized that in this model of sleep apnea–induced hypertension, vascular sensitivity to endothelin-1 is increased in combination with the elevated plasma endothelin-1 to cause the endothelin-1–dependent hypertension. In small mesenteric arteries with endothelial function disabled by passing air through the lumen, diameter and vessel wall [Ca 2+ ] were recorded simultaneously. IH arteries demonstrated increased constrictor sensitivity to endothelin-1 (percentage max constriction 100±0% IH versus 80±10% Sham; P <0.05). This was accompanied by increased calcium sensitivity of IH arteries. In contrast, constrictor sensitivity and increases in vessel wall [Ca 2+ ] to KCl and phenylephrine were not different between IH and Sham arteries. We have shown previously that endothelin-1 constriction in mesenteric arteries is mediated by endothelin A receptors. In the current study, the selective increase in endothelin-1 constriction in IH resistance arteries was accompanied by increased expression of endothelin A receptor expression (densitometry units 271±23 IH versus 158±25 Sham; P <0.05). Thus, IH hypertension appears to cause alterations in signaling components unique to endothelin-1 at the receptor level and in postreceptor signaling that increases calcium sensitivity during endothelin A activation. Future studies will determine the specific changes in vascular smooth muscle signaling in IH hypertension causing this augmented contractile phenotype.

Geniohyoid muscle properties and myosin heavy chain composition are altered after short-term intermittent hypoxic exposure

Patients with obstructive sleep apnea (OSA) often exhibit fatigued or inefficient upper airway dilator and constrictor muscles; an upper airway dilator, the geniohyoid (GH) muscle, is a particular example. Intermittent hypoxia (IH) is a frequent concomitant of OSA, and it may trigger muscle fiber composition changes that are characteristic of a fatigable nature. We examined effects of short-term IH on diaphragmatic and GH muscle fiber composition and fatigue properties by exposing 24 rats to alternating 10.3% O2-balance N2 and room air every 480 s (240 s duty cycle) for a total duration of 5, 10, 15, 20, or 30 h. Sternohyoid fiber composition was also examined. Control animals were exposed to room air on the same schedule. Single-fiber analyses showed that GH muscle fiber types changed completely from myosin heavy chain (MHC) type 2A to MHC type 2B after 10 h of exposure, and the conversion was maintained for at least 30 h. Sternohyoid muscle fibers showed a delayed transition from MHC type 2A/2B to MHC type 2B. In contrast, major fiber types of the diaphragm were not significantly altered. The GH muscles showed similar tension-frequency relationships in all groups, but an increased fatigability developed, proportional to the duration of IH treatment. We conclude that short-term IH exposure alters GH muscle composition and physical properties toward more fatigable, fast-twitch types and that it may account for the fatigable upper airway fiber types found in sleep-disturbed breathing.

Cardioprotective and vasomotor effects of HO activity during acute and chronic hypoxia

Prolonged hypoxia leads to the development of pulmonary hypertension. Recent reports have suggested enhancement of heme oxygenase (HO), the major source of intracellular carbon monoxide (CO), prevents hypoxia-induced pulmonary hypertension and vascular remodeling in rats. Therefore, we hypothesized that inhibition of HO activity by tin protoporphyrin (SnPP) would exacerbate the development of pulmonary hypertension. Rats were injected weekly with either saline or SnPP (50 micromol/kg) and exposed to hypobaric hypoxia or room air for 5 wk. Pulmonary and carotid arteries were catheterized, and animals were allowed to recover for 48 h. Pulmonary and systemic pressures, along with cardiac output, were recorded during room air and acute 10% O2 breathing in conscious rats. No difference was detected in pulmonary artery pressure between saline- and SnPP-treated animals in either normoxic or hypoxic groups. However, blockade of HO activity altered both systemic and pulmonary vasoreactivity to acute hypoxic challenge. Despite no change in baseline pulmonary artery pressure, all rats treated with SnPP had decreased ratio of right ventricular (RV) weight to left ventricular (LV) plus septal (S) weight (RV/LV + S) compared with saline-treated animals. Echocardiograms suggested dilatation of the RV and decreased RV function in hypoxic SnPP-treated rats. Together these data suggest that inhibition of HO activity and CO production does not exacerbate pulmonary hypertension, but rather that HO and CO may be involved in mediating pulmonary and systemic vasoreactivity to acute hypoxia and hypoxia-induced RV function.