The overlap syndrome: chronic obstructive pulmonary disease and obstructive sleep apnea

The clinical and hemodynamic characteristics of pulmonary hypertension in patients with OSA-COPD overlap syndrome

BACKGROUND

Our study aimed to assess the clinical and hemodynamic characteristics of pulmonary hypertension (PH) in patients with overlapping obstructive sleep apnea (OSA) and chronic obstructive pulmonary disease (COPD), referred to OSA-COPD overlap syndrome (OS).

METHODS

We enrolled a total of 116 patients with OS, COPD, or OSA who underwent right heart catheterization (RHC) due to suspected PH. We conducted a retrospective analysis of the clinical and hemodynamic characteristics of these patients.

RESULTS

Among the three groups (OS group, n = 26; COPD group, n = 36; OSA group, n = 54), the prevalence of PH was higher in the OS group (n = 17, 65.4%)compared to OSA group (n = 26,48.1%) and COPD group (n = 20,55.6 %). Among three groups with PH, the superior vena cava pressure (CVP) and right ventricular pressure (RAP) were higher in the OS group than in the OSA group (P < 0.05). Patients in the OS and COPD groups had higher pulmonary artery wedge pressure (PAWP) than in the OSA group (14.88 ± 4.79 mmHg, 13.45 ± 3.68 mmHg vs. 11.00 ± 3.51 mmHg, respectively, P < 0.05). OS patients with PH exhibited higher respiratory event index (REI), time spent with SpO2 <90%, oxygen desaturation index (ODI), minimal SpO2 (MinSpO2) and mean SpO2 (MSpO2) compared to OS patients without PH. After adjusting for potential covariates, we found that MinSpO2 (OR 0.937, 95 % CI 0.882-0.994, P = 0.032), MSpO2 (OR 0.805, 95% CI 0.682-0.949, P = 0.010), time spent with SpO2 <90% (OR 1.422, 95% CI 1.137-1.780, P = 0.002), and FEV1 % pred (OR 0.977, 95 % CI 0.962-0.993, P = 0.005) were related to the development of PH.

CONCLUSIONS

Patients with OS showed higher prevalence of PH, along with higher PAWP, CVP and RAP. Worse nocturnal hypoxemia was found in OS patients with PH.

Phosphodiesterase-4 Inhibitors for Non-COPD Respiratory Diseases

Selective phosphodiesterase (PDE) inhibitors are a class of nonsteroid anti-inflammatory drugs for treating chronic inflammatory diseases. Modulation of systemic and airway inflammation is their pivotal mechanism of action. Furthermore, PDE inhibitors modulate cough reflex and inhibit airway mucus secretion. Roflumilast, a selective PDE4 inhibitor, has been extensively studied for the efficacy and safety in chronic obstructive pulmonary disease (COPD) patients. According to the mechanisms of action, the potential roles of PDE inhibitors in treating chronic respiratory diseases including severe asthma, asthma-COPD overlap (ACO), noncystic fibrosis bronchiectasis, and chronic cough are discussed. Since roflumilast inhibits airway eosinophilia and neutrophilia in COPD patients, it reduces COPD exacerbations in the presence of chronic bronchitis in addition to baseline therapies. The clinical studies in asthma patients have shown the comparable efficacy of roflumilast to inhaled corticosteroids for improving lung function. However, the clinical trials of roflumilast in severe asthma have been limited. Although ACO is common and is also associated with poor outcomes, there is no clinical trial regarding its efficacy in patients with ACO despite a promising role in reducing COPD exacerbation. Since mucus hypersecretion is a result of neutrophil secretagogue in patients with chronic bronchitis, experimental studies have shown that PDE4s are regulators of the cystic fibrosis transmembrane conductance regulator (CFTR) in human airway epithelial cells. Besides, goblet cell hyperplasia is associated with an increased expression of PDE. Bronchiectasis and chronic bronchitis are considered neutrophilic airway diseases presenting with mucus hypersecretion. They commonly coexist and thus lead to severe disease. The role of roflumilast in noncystic fibrosis bronchiectasis is under investigation in clinical trials. Lastly, PDE inhibitors have been shown modulating cough from bronchodilation, suppressing transient receptors potential (TRP), and anti-inflammatory properties. Hence, there is the potential role of the drug in the management of unexplained cough. However, clinical trials for examining its antitussive efficacy are pivotal. In conclusion, selective PDE4 inhibitors may be potential treatment options for chronic respiratory diseases apart from COPD due to their promising mechanisms of action.

The deleterious effects of chronic obstructive pulmonary disease and obstructive sleep apnea: pathophysiology and implications on treatment

Background

Chronic obstructive pulmonary disease (COPD) and obstructive sleep apnea (OSA) are both common in the adult population. The coexistence and association of both COPD and OSA have been described as the overlap syndrome. The scope of the present review is to address the magnitude, pathophysiology, clinical presentations, and the deleterious consequences of the coexistence of COPD and OSA in the same patient as well as the current management and treatment options of this association.

Main body of abstract

Epidemiological studies showed a prevalence of the overlap syndrome of up to 1% of the adult population. However, various studies agreed that this association is due to the fact that both COPD and OSA are common rather than an interaction of pathophysiology between the two affections. Nevertheless, sleep may cause adverse effects on breathing and lung functions. While these effects can be neglected in otherwise healthy individuals, they can lead in contrast to disastrous clinical outcomes particularly in vulnerable patients with the overlap syndrome leading to increased risk of cardiovascular diseases, acute COPD exacerbations, increased rates of hospitalizations, and nocturnal deaths. Current guidelines recommend polysomnography (PSG) as the gold standard investigation to diagnose sleep disorders and overlap syndrome. Treatment may include pulmonary rehabilitation, supplemental oxygen therapy, and non-invasive ventilation. Currently, continuous positive airway pressure (CPAP) therapy is the treatment of choice for the overlap syndrome. CPAP effects include improved respiratory mechanics; sleep quality, exercise tolerance, and prolonged survival.

Conclusion

Both COPD and OSA are common in the general population and present a significant risk of increased morbidity and mortality when they coexist in the same patient. Clinicians must carefully evaluate the clinical outcomes and the high risk of cardiovascular complications related to the overlap syndrome. Current data indicate that CPAP treatment leads to amelioration of the health-related quality of life and improve survival in patients with the overlap syndrome.

Pulmonary vascular collagen content, not cross-linking, contributes to right ventricular pulsatile afterload and overload in early pulmonary hypertension

Hypoxic pulmonary hypertension (HPH) is associated with pulmonary artery (PA) remodeling and right ventricular (RV) overload. We have previously uncovered collagen-mediated mechanisms of proximal PA stiffening in early HPH by manipulating collagen degradation and cross-linking using a transgenic mouse strain and a potent collagen cross-link inhibitor, β-aminopropionitrile (BAPN). However, the roles of collagen in distal PA remodeling, overall RV afterload, and RV hypertrophy in HPH remain unknown. Here, we used the same experimental strategy to investigate the effect of pulmonary vascular collagen content and cross-linking on steady and pulsatile RV afterload and on RV hypertrophy in early HPH. Collagenase-resistant mice (Col1a1^R/R) and their littermate controls (Col1a1^+/+) were exposed to normobaric hypoxia for 10 days with or without BAPN treatment. In vivo pulmonary vascular impedance, a comprehensive measure of RV afterload, was measured via simultaneous RV catheterization and echocardiography. Morphology and collagen accumulation were examined using histological techniques and ELISA in lungs and RVs. In both mouse strains, BAPN did not limit increases in pulmonary arterial pressure or pulmonary vascular resistance, indicating a negligible effect of either collagen content or cross-linking on steady RV afterload. However, BAPN prevented the increase in pulse pressure and RV hypertrophy in Col1a1^+/+ mice and these effects were absent in Col1a1^R/R mice, suggesting a role for PA collagen content, not cross-linking, in the pulsatile RV afterload. Moreover, we found a significant correlation between pulse pressure and RV hypertrophy, indicating an important role for pulsatile RV afterload in RV overload in early HPH.

NEW & NOTEWORTHY

The present study found an important role for collagen content, but not collagen cross-linking, in the pulsatile right ventricular (RV) afterload, which is correlated with RV hypertrophy. These results uncover a new collagen-mediated mechanical mechanism of RV dysfunction in early pulmonary hypertension progression. Furthermore, our results suggest that measures and metrics of pulsatile hemodynamics such as pulse pressure and pulse wave velocity are potentially important to cardiovascular mortality in patients with pulmonary hypertension.

Predictive factors warrant screening for obstructive sleep apnea in COPD: a Taiwan National Survey

BACKGROUND AND OBJECTIVES

COPD and obstructive sleep apnea (OSA) share similar pathological processes and cardiovascular sequelae. Coexisting OSA in COPD - "overlap syndrome" - has worse prognosis than either condition alone, and appropriate treatment improves survival. Our objectives were to ascertain the frequency at which COPD coexists with the risk of OSA in Taiwan and to compare the risk factors, COPD symptoms, and life quality metrics between COPD subgroups with versus without risk of OSA.

METHODS

We conducted a random cross-sectional national telephone survey of adults >40 years old in Taiwan. Participants fulfilling an epidemiological case definition of COPD completed a questionnaire to assess COPD symptoms and OSA risk, comorbidities, and performance of daily activities. Data from COPD cohorts with and without risk of OSA were analyzed and compared.

RESULTS

Of 6,600 interviews completed, 404 subjects fit the epidemiological case definition of COPD - an overall prevalence of 6.1% in this national sample. Data on OSA risk were available for 292 of this COPD cohort, of whom 29.5% were at risk of OSA. Compared to those without risk of OSA, those with risk of OSA were significantly more likely to have hypertension or cardiovascular disease and diabetes, had significantly higher body mass index and COPD Assessment Test scores, and reported impaired work performance and leisure activities.

CONCLUSION

Among adults in Taiwan who fulfill epidemiologic criteria for COPD, 29.5% have coexisting risk of OSA. Comorbid hypertension or cardiovascular disease and diabetes are common and significantly more prevalent among the COPD population at risk of OSA than those who are not. OSA screening is warranted in patients with COPD with those risk factors that are more prevalent in COPD with risk of OSA than without, to target early interventions to reduce adverse cardiovascular sequelae from overlap syndrome.

Comorbidity in chronic obstructive pulmonary disease

Patients with chronic obstructive pulmonary diseases (COPD) often experience comorbid conditions. The most common comorbidities that have been associated with COPD include cardiovascular diseases, lung cancer, metabolic disorder, osteoporosis, anxiety and depression, skeletal muscle dysfunction, cachexia, gastrointestinal diseases, and other respiratory conditions. Not only are comorbidities common but they also considerably influence disease prognosis and patients׳ health status, and are associated with poor clinical outcomes. However, perusal of literature indicates that little has been done so far to effectively assess, manage, and treat comorbidities in patients with COPD. The aim of this review is to comprehensively narrate the comorbid conditions that often coexist with COPD, along with their reported prevalence and their significant impacts in the disease management of COPD. A perspective on integrated disease management approaches for COPD is also discussed.

Obstructive sleep apnea in the perioperative setting: complications and management strategies

Obstructive sleep apnea (OSA) is frequently encountered in an undiagnosed, untreated state in perioperative patients. It increases the risk of respiratory, cardiac, and infectious complications following surgical procedures. Patients with OSA may require additional monitoring, unplanned escalations in care, and prolonged hospitalization. It is important to identify patients at risk for OSA during the preoperative assessment so that appropriate anesthesia, postsurgical monitoring, and pain control can be planned. Herein, we discuss data regarding perioperative outcomes in patients with OSA, methods to quickly identify patients at high risk for OSA, and implementation of clinical safeguards to minimize OSA-associated complications. An algorithm is provided to guide the perioperative management of patients with OSA.

Impact of increased hematocrit on right ventricular afterload in response to chronic hypoxia

Chronic hypoxia causes chronic mountain sickness through hypoxia-induced pulmonary hypertension (HPH) and increased hematocrit. Here, we investigated the impact of increased hematocrit and HPH on right ventricular (RV) afterload via pulmonary vascular impedance. Mice were exposed to chronic normobaric hypoxia (10% oxygen) for 10 (10H) or 21 days (21H). After baseline hemodynamic measurements, ∼500 μl of blood were extracted and replaced with an equal volume of hydroxyethylstarch to normalize hematocrit and all hemodynamic measurements were repeated. In addition, ∼500 μl of blood were extracted and replaced in control mice with an equal volume of 90% hematocrit blood. Chronic hypoxia increased input resistance (Z0 increased 82% in 10H and 138% in 21H vs. CTL; P < 0.05) and characteristic impedance (ZC increased 76% in 10H and 109% in 21H vs. CTL; P < 0.05). Hematocrit normalization did not decrease mean pulmonary artery pressure but did increase cardiac output such that both Z0 and ZC decreased toward control levels. Increased hematocrit in control mice did not increase pressure but did decrease cardiac output such that Z0 increased. The paradoxical decrease in ZC with an acute drop in hematocrit and no change in pressure are likely due to inertial effects secondary to the increase in cardiac output. A novel finding of this study is that an increase in hematocrit affects the pulsatile RV afterload in addition to the steady RV afterload (Z0). Furthermore, our results highlight that the conventional interpretation of ZC as a measure of proximal artery stiffness is not valid in all physiological and pathological states.

Effect of CPAP on long-term mortality in overlap syndrome: is hypercapnic the best appropriate determinant?

Overlap syndrome and represents a complex clinical condition in pulmonary medicine. We discuss in this letter factors related with mortality after long-term effects CPAP and key differences between hypercapnic versus hypoxemic overlap patients.

Respiratory mechanics and ventilatory control in overlap syndrome and obesity hypoventilation

The overlap syndrome of obstructive sleep apnoea (OSA) and chronic obstructive pulmonary disease (COPD), in addition to obesity hypoventilation syndrome, represents growing health concerns, owing to the worldwide COPD and obesity epidemics and related co-morbidities. These disorders constitute the end points of a spectrum with distinct yet interrelated mechanisms that lead to a considerable health burden. The coexistence OSA and COPD seems to occur by chance, but the combination can contribute to worsened symptoms and oxygen desaturation at night, leading to disrupted sleep architecture and decreased sleep quality. Alveolar hypoventilation, ventilation-perfusion mismatch and intermittent hypercapnic events resulting from apneas and hypopneas contribute to the final clinical picture, which is quite different from the “usual” COPD. Obesity hypoventilation has emerged as a relatively common cause of chronic hypercapnic respiratory failure. Its pathophysiology results from complex interactions, among which are respiratory mechanics, ventilatory control, sleep-disordered breathing and neurohormonal disturbances, such as leptin resistance, each of which contributes to varying degrees in individual patients to the development of obesity hypoventilation. This respiratory embarrassment takes place when compensatory mechanisms like increased drive cannot be maintained or become overwhelmed.

Although a unifying concept for the pathogenesis of both disorders is lacking, it seems that these patients are in a vicious cycle. This review outlines the major pathophysiological mechanisms believed to contribute to the development of these specific clinical entities. Knowledge of shared mechanisms in the overlap syndrome and obesity hypoventilation may help to identify these patients and guide therapy.

Increased prevalence of obstructive lung disease in patients with obstructive sleep apnea

STUDY PURPOSES

This study aims to determine whether there is an increased prevalence of obstructive lung diseases (OLDs) in patients with obstructive sleep apnea (OSA). We also determined whether among the OLD patients there is a difference in the prevalences of specific chronic disease co-morbidities between patients with and without OSA.

METHODS

The prevalences of COPD, asthma, and COPD combined with asthma (ICD-9 coding) were compared between 1,497 adult OSA patients and 1,489 control patients, who were matched for age, gender, geographic location, and primary care physician. The prevalences of specific co-morbidities were measured in the OLD groups between patients with OSA and the matched control group.

RESULTS

COPD, asthma, and COPD combined with asthma were found to be more prevalent among OSA patients compared to the matched controls. Prevalences among patients with and without OSA, respectively, were COPD-7.6 and 3.7 % (P<0.0001), asthma-10.4 and 5.1 % (P<0.0001), COPD plus asthma-3.3 and 0.9 % (P<0.0001). The Charlson Comorbidity Index was greater for OSA patients (2.3 ± 0.2) than for controls (1.9 ± 1.8; P<0.0001). These trends held for all severity ranges of OSA. Patients with OSA and COPD were characterized by more severe hypoxia at night compared with the OSA patients without OLD.

CONCLUSION

OSA was associated with an increased prevalence of OLDs.

Effects of collagen deposition on passive and active mechanical properties of large pulmonary arteries in hypoxic pulmonary hypertension

Proximal pulmonary artery (PA) stiffening is a strong predictor of mortality in pulmonary hypertension. Collagen accumulation is mainly responsible for PA stiffening in hypoxia-induced pulmonary hypertension (HPH) in mouse models. We hypothesized that collagen cross-linking and the type I isoform are the main determinants of large PA mechanical changes during HPH, which we tested by exposing mice that resist type I collagen degradation (Col1a1[Formula: see text] and littermate controls (Col1a1[Formula: see text] to hypoxia for 10 days with or without [Formula: see text]-aminopropionitrile (BAPN) treatment to prevent cross-link formation. Static and dynamic mechanical tests were performed on isolated PAs with smooth muscle cells (SMC) in passive and active states. Percentages of type I and III collagen were quantified by histology; total collagen content and cross-linking were measured biochemically. In the SMC passive state, for both genotypes, hypoxia tended to increase PA stiffness and damping capacity, and BAPN treatment limited these increases. These changes were correlated with collagen cross-linking ([Formula: see text]). In the SMC active state, hypoxia increased PA dynamic stiffness and BAPN had no effect in Col1a1[Formula: see text] mice ([Formula: see text]). PA stiffness did not change in Col1a1[Formula: see text] mice. Similarly, damping capacity did not change for either genotype. Type I collagen accumulated more in Col1a1[Formula: see text] mice, whereas type III collagen increased more in Col1a1[Formula: see text] mice during HPH. In summary, PA passive mechanical properties (both static and dynamic) are related to collagen cross-linking. Type I collagen turnover is critical to large PA remodeling during HPH when collagen metabolism is not mutated and type III collagen may serve as a reserve.

Is portable monitoring accurate in the diagnosis of obstructive sleep apnea syndrome in chronic pulmonary obstructive disease?

BACKGROUND

No scientific evidence supports the use of portable devices to diagnose obstructive sleep apnea syndrome (OSAS) in patients with co-morbities. Our aim was to evaluate the accuracy of a portable monitoring device (Stardust - STD) in the detection of patients with chronic obstructive pulmonary disease (COPD).

METHODS

Patients with COPD and clinical suspicion of OSAS were recruited for a prospective randomized study. The STD was used on two different nights: (1) at home (STDHome) and (2) at the sleep laboratory simultaneous with polysomnography (PSG-STDLab).

RESULTS

A total of 72 patients underwent the proposed recordings. Forty-six volunteers were excluded due to recording problems, and data from 26 subjects were analyzed. The mean age was (mean±SD) 62.8±8.5 years, 50% were male, and the mean forced expiratory volume in the first second was 55±11%. Significant intraclass correlation was observed between apnea-hypopnea index (AHI)-PSG vs. AHI-STDLab (r=0.61, p<0.0001) and AHI-STDHome (r=0.47, p<0.007). Kappa analysis also showed a significant agreement for severe group.

CONCLUSION

Despite the agreement found in a small number of patients between AHI, a large number of failures in the recording limits the use of this portable device for the diagnosis of OSAS in patients with COPD.

Sleep deprivation in critical illness: its role in physical and psychological recovery

Critically ill patients frequently experience poor sleep, characterized by frequent disruptions, loss of circadian rhythms, and a paucity of time spent in restorative sleep stages. Factors that are associated with sleep disruption in the intensive care unit (ICU) include patient-ventilator dysynchrony, medications, patient care interactions, and environmental noise and light. As the field of critical care increasingly focuses on patients' physical and psychological outcomes following critical illness, understanding the potential contribution of ICU-related sleep disruption on patient recovery is an important area of investigation. This review article summarizes the literature regarding sleep architecture and measurement in the critically ill, causes of ICU sleep fragmentation, and potential implications of ICU-related sleep disruption on patients' recovery from critical illness. With this background information, strategies to optimize sleep in the ICU are also discussed.

Role of collagen content and cross-linking in large pulmonary arterial stiffening after chronic hypoxia

Chronic hypoxic pulmonary hypertension (HPH) is associated with large pulmonary artery (PA) stiffening, which is correlated with collagen accumulation. However, the mechanisms by which collagen contributes to PA stiffening remain largely unexplored. Moreover, HPH may alter mechanical properties other than stiffness, such as pulse damping capacity, which also affects ventricular workload but is rarely quantified. We hypothesized that collagen content and cross-linking differentially regulate the stiffness and damping capacity of large PAs during HPH progression. The hypothesis was tested with transgenic mice that synthesize collagen type I resistant to collagenase degradation (Col1a1(R/R)). These mice and littermate controls (Col1a1(+/+)) were exposed to hypoxia for 10 days; some were treated with β-aminopropionitrile (BAPN), which prevents new cross-link formation. Isolated PA dynamic mechanical tests were performed, and collagen content and cross-linking were measured. In Col1a1(+/+) mice, HPH increased both collagen content and cross-linking, and BAPN treatment prevented these increases. Similar trends were observed in Col1a1(R/R) mice except that collagen content further increased with BAPN treatment. Mechanical tests showed that in Col1a1(+/+) mice, HPH increased PA stiffness and damping capacity, and these increases were impeded by BAPN treatment. In Col1a1(R/R) mice, HPH led to a smaller but significant increase in PA stiffness and a decrease in damping capacity. These mechanical changes were not affected by BAPN treatment. Vessel-specific correlations for each strain showed that the stiffness and damping capacity were correlated with the total content rather than cross-linking of collagen. Our results suggest that collagen total content is critical to extralobar PA stiffening during HPH.

[Importance of fatigue, sleep quality and mood in patients with chronic obstructive pulmonary disease and the need for measurement instruments]

The effect of chronic obstructive pulmonary disease (COPD) on patients and their perception of wellbeing goes beyond functional respiratory alterations. Fatigue, mood and sleep alterations constitute a complex of interrelated symptoms affecting perception of health status. Fatigue is a highly important symptom due to its frequency, clinical repercussions and association with other manifestations. Functional alterations during sleep are of various degrees of severity and are most important in sleep apnea-hypopnea syndrome. Mood alterations are important because of their frequency, clinical repercussion and prognostic implications. In the last few years, several measurement instruments have been developed to quantify these manifestations, such as the Living with COPD scale (LCOPD), the COPD and Asthma Fatigue Scale (CAFS) and the COPD and Asthma Sleep Impact Scale (CASIS).The present article reviews the clinical relevance of these manifestations and the measurement instruments available for their evaluation.

The role of collagen in extralobar pulmonary artery stiffening in response to hypoxia-induced pulmonary hypertension

Hypoxic pulmonary hypertension (HPH) causes extralobar pulmonary artery (PA) stiffening, which potentially impairs right ventricular systolic function. Changes in the extracellular matrix proteins collagen and elastin have been suggested to contribute to this arterial stiffening. We hypothesized that vascular collagen accumulation is a major cause of extralobar PA stiffening in HPH and tested our hypothesis with transgenic mice that synthesize collagen type I resistant to collagenase degradation (Col1a1(R/R)). These mice and littermate controls that have normal collagen degradation (Col1a1(+/+)) were exposed to hypoxia for 10 days; some were allowed to recover for 32 days. In vivo PA pressure and isolated PA mechanical properties and collagen and elastin content were measured for all groups. Vasoactive studies were also performed with U-46619, Y-27632, or calcium- and magnesium-free medium. Pulmonary hypertension occurred in both mouse strains due to chronic hypoxia and resolved with recovery. HPH caused significant PA mechanical changes in both mouse strains: circumferential stretch decreased, and mid-to-high-strain circumferential elastic modulus increased (P < 0.05 for both). Impaired collagen type I degradation prevented a return to baseline mechanical properties with recovery and, in fact, led to an increase in the low and mid-to-high-strain moduli compared with hypoxia (P < 0.05 for both). Significant changes in collagen content were found, which tended to follow changes in mid-to-high-strain elastic modulus. No significant changes in elastin content or vasoactivity were observed. Our results demonstrate that collagen content is important to extralobar PA stiffening caused by chronic hypoxia.

The effects of vasoactivity and hypoxic pulmonary hypertension on extralobar pulmonary artery biomechanics

Loss of large artery compliance is an emerging novel predictor of cardiovascular mortality. Hypoxia-induced pulmonary hypertension (HPH) has been shown to decrease extralobar pulmonary artery (PA) compliance in the absence of smooth muscle cell (SMC) tone and to increase SMC tone in peripheral PAs. We sought to determine the impact of HPH on extralobar PA tone and the impact of SMC activation on extralobar PA biomechanics. To do so, C57BL6 mice were exposed to 0 (CTL) or 10 days (HPH) of hypoxia and isolated vessel tests were performed on extralobar PAs using either a physiological saline solution (PSS), a vasoconstrictor (U46619), two vasodilators (SNP and Y27632) or calcium free medium (relaxant solution; VBRS). The vasodilators and relaxant solution had no effect on extralobar artery diameter suggesting that basal SMC tone is essentially zero in CTL conditions and does not increase with HPH. HPH caused narrowing, decreased circumferential stretch (lambda; p<0.0001), decreased local area compliance (C(A); p<0.0005) and increased incremental elastic modulus (E(inc); p<0.05) in the normal tone state (with PSS). In both CTL and HPH conditions, SMC activation decreased E(inc) (p<0.0005) but also increased wall thickness (p<0.05) such that changes in C(A) with SMC constriction were minimal; only in HPH PAs was a significant decrease with SMC constriction observed (p<0.05). Our results demonstrate that 10 days of hypoxia does not increase extralobar PA SMC tone and that HPH-induced decreases in compliance are caused by narrowing, wall thickening and increases in modulus, not persistent vasoconstriction.

Firing probability and mean firing rates of human muscle vasoconstrictor neurones are elevated during chronic asphyxia

Elevated muscle sympathetic nerve activity (MSNA) features in many cardiovascular diseases, but how this sympathoexcitation is brought about differs across pathologies. Unitary recordings from post-ganglionic muscle vasoconstrictor neurones in human subjects have shown that the augmented MSNA in the obstructive sleep apnoea syndrome (OSAS) is associated with an increase in firing probability and mean firing rate, and an increase in multiple within-burst firing. Here we characterize the firing properties of muscle vasoconstrictor neurones in patients with chronic obstructive pulmonary disease (COPD), who are chronically asphyxic. We tested the hypothesis that this elevated chemical drive would shift the firing pattern from that seen in healthy subjects to that seen in OSAS. The mean firing probability (52%) and mean firing rate (0.92 Hz) of 17 muscle vasoconstrictor neurones recorded in COPD were comparable to those previously recorded in OSAS (51% and 0.96 Hz), but significantly higher than those recorded in a group of healthy subjects with high levels of resting MSNA (35% and 0.33 Hz). In COPD single neurones fired once in 63% of cardiac intervals, comparable to OSAS (59%), but significantly lower than in the healthy group (78%). Conversely, single neurones fired twice in 25% of cardiac intervals, similar to OSAS (27%), but significantly higher than in the healthy group (18%). We conclude that the chronic asphyxia associated with COPD results in an increase in the firing probability and mean firing frequency of muscle vasoconstrictor neurones and causes a shift towards multiple firing, reflecting an increase in central muscle vasoconstrictor drive.