Intermittent hypoxia, respiratory plasticity and sleep apnea in humans: present knowledge and future investigations

Assessing the importance and safety of hypoxia conditioning for patients with occupational pulmonary diseases: A recent clinical perspective

Occupational pulmonary diseases (OPDs) pose a significant global health challenge, contributing to high mortality rates. This review delves into the pathophysiology of hypoxia and the safety of intermittent hypoxic conditioning (IHC) in OPD patients. By examining sources such as PubMed, Relemed, NLM, Scopus, and Google Scholar, the review evaluates the efficacy of IHC in clinical outcomes for OPD patients. It highlights the complexities of cardiovascular and respiratory regulation dysfunctions in OPDs, focusing on respiratory control abnormalities and the impact of intermittent hypoxic exposures. Key areas include the physiological effects of hypoxia, the role of hypoxia-inducible factor-1 alpha (HIF-1α) in occupational lung diseases, and the links between brain ischemia, stroke, and OPDs. The review also explores the interaction between intermittent hypoxic exposures, mitochondrial energetics, and lung physiology. The potential of IHE to improve clinical manifestations and underlying pathophysiology in OPD patients is thoroughly examined. This comprehensive analysis aims to benefit molecular pathologists, pulmonologists, clinicians, and physicians by enhancing understanding of IHE's clinical benefits, from research to patient care, and improving clinical outcomes for OPD patients.

Effects of oropharyngeal exercises on CPAP compliance: A prospective intervention study

PURPOSE

To investigate the effects of oropharyngeal exercise on continuous positive airway pressure (CPAP) compliance in patients with moderate to severe obstructive sleep apnea over a period of 6 months.

MATERIALS AND METHODS

This study was conducted as a prospective, observational, and interventional investigation. A total of 70 patients with moderate to severe obstructive sleep apnea were randomly assigned to either the oropharyngeal exercise group (n = 44) or the sham-therapy group (n = 26). The compliance of the enrolled patients with CPAP therapy was assessed at baseline, 3-month follow-up and 6-month follow-up. Objective sleep data, questionnaire and CPAP use time were collected over a half-year period (i.e., baseline, 6 months, and 12 months).

RESULTS

The study found that the average use time of CPAP within one month was significantly longer in the oropharyngeal exercises group compared to the sham-therapy group at the 3-month assessment (5.5 ± 1.2 vs 4.8 ± 1.3 h per night; p=0.030), and much significantly longer at 6-months assessment (6.0 ± 1.4 vs 4.9 ± 1.3 h per night; p=0.001). Furthermore, the average use time of CPAP increased over time, with the oropharyngeal exercises group exhibiting a more pronounced growth from baseline to the six-month follow-up (4.8 ± 1.0 h per night to 6.0 ± 1.3 h per night, p < 0.001) compared to the sham-therapy group (4.8 ± 1.3 h per night to 4.9 ± 1.3 h per night, p=0.952). Additionally, the oropharyngeal exercise group demonstrated an improvement in the Epworth sleepiness scale compared to the sham-therapy group at the 3-month follow-up (6.0 ± 2.0 vs 8.8 ± 3.2; p < 0.001), as well as decreased significantly at 6-month follow-up (p = 0.032).

CONCLUSIONS

CPAP adherence can be improved with oropharyngeal exercises therapy among moderate to severe OSA patients. Notably, the average duration of CPAP usage and reduction in daytime sleepiness were maintained even after six months of oropharyngeal exercise therapy.

Causal Relationships Between Circulating Inflammatory Proteins and Obstructive Sleep Apnea: A Bidirectional Mendelian Randomization Study

Purpose

Clinical studies have demonstrated the intricate association between the onset and progression of obstructive sleep apnea (OSA) and the activation of the inflammatory cascade reaction. This study delves into investigating the causal links between 91 circulating inflammatory proteins (CIPs) and OSA through the application of Mendelian randomization (MR) techniques.

Methods

Utilizing genetic data on OSA sourced from the Finnish Biobank (FinnGen) Genome-wide Association Studies (GWAS) of the European population, alongside summary-level GWAS data of CIPs from 14,824 European participants, we conducted a bidirectional MR study.

Results

This study suggests that several factors may be associated with the risk of OSA. IL-17C (odds ratio (OR) = 1.090, p = 0.0311), CCL25 (OR = 1.079, p = 0.0493), FGF-5 (OR = 1.090, p = 0.0003), CD5 (OR = 1.055, p = 0.0477), and TNFSF14 (OR = 1.092, p = 0.0008) may positively correlate with OSA risk. Conversely, IL-20RA (OR = 0.877, p = 0.0107), CCL19 (OR = 0.933, p = 0.0237), MIP-1 alpha (OR = 0.906, p = 0.0042), Flt3L (OR = 0.941, p = 0.0019), CST5 (OR = 0.957, p = 0.0320), OPG (OR = 0.850, p = 0.0001), and TRAIL (OR = 0.956, p = 0.0063) may reduce the risk of OSA. Additionally, elevated levels of IL-10RA (OR = 1.153, p = 0.0478) were observed as a consequence of OSA. Conversely, OSA may potentially lead to decreased levels of CCL28 (OR = 0.875, p = 0.0317), DNER (OR = 0.874, p = 0.0324), FGF-21 (OR = 0.846, p = 0.0344), and CSF-1 (OR = 0.842, p = 0.0396).

Conclusion

Through this bidirectional MR study, we have identified 12 upstream regulatory proteins and 5 downstream effect proteins that are linked to OSA. These findings hold promise in providing potential therapeutic targets for the inflammatory mechanisms underlying OSA.

Modeling Long-Term Facilitation of Respiration During Interval Exercise in Humans

Long-term facilitation (LTF) of respiration has been mainly initiated by intermittent hypoxia and resultant chemoreceptor stimulation in humans. Comparable levels of chemoreceptor stimulation can occur in combined exercise and carbon dioxide (CO2) inhalation and lead to LTF. This possibility was supported by data collected during combined interval exercise and 3% inhaled CO2 in seven normal subjects. These data were further analyzed based on the dynamics involved using mathematical models in this study. Previously estimated peripheral chemoreceptor sensitivity during light exercise (40 W) with air or 3% inhaled CO2 approximately doubled resting sensitivity. Ventilation after a delay increased by 17.0 ± 2.48 L/min (p < 0.001) during recovery following 45% maximal oxygen uptake ([Formula: see text] ) exercise consistent with LTF which exceeded what can be achieved with intermittent hypoxia. Model fitting of the dynamic responses was used to separate neural from chemoreceptor-mediated CO2 responses. Exercise of 45% [Formula: see text] was followed by ventilation augmentation following initial recovery. Augmentation of LTF developed slowly according to second-order dynamics in accordance with plasticity involving a balance between self-excitatory and self-inhibitory neuronal pools.

Cardiovascular physiology and pathophysiology at high altitude

Oxygen is vital for cellular metabolism; therefore, the hypoxic conditions encountered at high altitude affect all physiological functions. Acute hypoxia activates the adrenergic system and induces tachycardia, whereas hypoxic pulmonary vasoconstriction increases pulmonary artery pressure. After a few days of exposure to low oxygen concentrations, the autonomic nervous system adapts and tachycardia decreases, thereby protecting the myocardium against high energy consumption. Permanent exposure to high altitude induces erythropoiesis, which if excessive can be deleterious and lead to chronic mountain sickness, often associated with pulmonary hypertension and heart failure. Genetic factors might account for the variable prevalence of chronic mountain sickness, depending on the population and geographical region. Cardiovascular adaptations to hypoxia provide a remarkable model of the regulation of oxygen availability at the cellular and systemic levels. Rapid exposure to high altitude can have adverse effects in patients with cardiovascular diseases. However, intermittent, moderate hypoxia might be useful in the management of some cardiovascular disorders, such as coronary heart disease and heart failure. The aim of this Review is to help physicians to understand the cardiovascular responses to hypoxia and to outline some recommendations that they can give to patients with cardiovascular disease who wish to travel to high-altitude destinations.

Identification and validation of ferroptosis-related hub genes in obstructive sleep apnea syndrome

Background

By 2020, the prevalence of Obstructive Sleep Apnea Syndrome (OSAS) in the US has reached 26. 6-43.2% in men and 8.7-27.8% in women. OSAS promotes hypertension, diabetes, and tumor growth through unknown means. Chronic intermittent hypoxia (CIH), sleep fragmentation, and increased pleural pressure are central mechanisms of OSAS complications. CIH exacerbates ferroptosis, which is closely related to malignancies. The mechanism of ferroptosis in OSAS disease progression remains unknown.

Methods

OSAS-related datasets (GSE135917 and GSE38792) were obtained from the GEO. Differentially expressed genes (DEGs) were screened using the R software and intersected with the ferroptosis database (FerrDb V2) to get ferroptosis-related DEGs (f-DEGs). GO, DO, KEGG, and GSEA enrichment were performed, a PPI network was constructed and hub genes were screened. The TCGA database was used to obtain the thyroid cancer (THCA) gene expression profile, and hub genes were analyzed for differential and survival analysis. The mechanism was investigated using GSEA and immune infiltration. The hub genes were validated with RT-qPCR, IHC, and other datasets. Sprague-Dawley rats were randomly separated into normoxia and CIH groups. ROS, MDA, and GSH methods were used to detect CIH-induced ferroptosis and oxidative stress.

Results

GSEA revealed a statistically significant difference in ferroptosis in OSAS (FDR < 0.05). HIF1A, ATM, HSPA5, MAPK8, MAPK14, TLR4, and CREB1 were identified as hub genes among 3,144 DEGs and 74 f-DEGs. HIF1A and ATM were the only two validated genes. F-DEGs were mainly enriched in THCA. HIF1A overexpression in THCA promotes its development. HIF1A is associated with CD8 T cells and macrophages, which may affect the immunological milieu. The result found CIH increased ROS and MDA while lowering GSH indicating that it could cause ferroptosis. In OSAS patients, non-invasive ventilation did not affect HIF1A and ATM expression. Carvedilol, hydralazine, and caffeine may be important in the treatment of OSAS since they suppress HIF1A and ATM.

Conclusions

Our findings revealed that the genes HIF1A and ATM are highly expressed in OSAS, and can serve as biomarkers and targets for OSAS.

Temporal variations in the pattern of breathing: techniques, sources, and applications to translational sciences

The breathing process possesses a complex variability caused in part by the respiratory central pattern generator in the brainstem; however, it also arises from chemical and mechanical feedback control loops, network reorganization and network sharing with nonrespiratory motor acts, as well as inputs from cortical and subcortical systems. The notion that respiratory fluctuations contain hidden information has prompted scientists to decipher respiratory signals to better understand the fundamental mechanisms of respiratory pattern generation, interactions with emotion, influences on the cortical neuronal networks associated with cognition, and changes in variability in healthy and disease-carrying individuals. Respiration can be used to express and control emotion. Furthermore, respiration appears to organize brain-wide network oscillations via cross-frequency coupling, optimizing cognitive performance. With the aid of information theory-based techniques and machine learning, the hidden information can be translated into a form usable in clinical practice for diagnosis, emotion recognition, and mental conditioning.

Breathing during sleep

The depth, rate, and regularity of breathing change following transition from wakefulness to sleep. Interactions between sleep and breathing involve direct effects of the central mechanisms that generate sleep states exerted at multiple respiratory regulatory sites, such as the central respiratory pattern generator, respiratory premotor pathways, and motoneurons that innervate the respiratory pump and upper airway muscles, as well as effects secondary to sleep-related changes in metabolism. This chapter discusses respiratory effects of sleep as they occur under physiologic conditions. Breathing and central respiratory neuronal activities during nonrapid eye movement (NREM) sleep and REM sleep are characterized in relation to activity of central wake-active and sleep-active neurons. Consideration is given to the obstructive sleep apnea syndrome because in this common disorder, state-dependent control of upper airway patency by upper airway muscles attains high significance and recurrent arousals from sleep are triggered by hypercapnic and hypoxic episodes. Selected clinical trials are discussed in which pharmacological interventions targeted transmission in noradrenergic, serotonergic, cholinergic, and other state-dependent pathways identified as mediators of ventilatory changes during sleep. Central pathways for arousals elicited by chemical stimulation of breathing are given special attention for their important role in sleep loss and fragmentation in sleep-related respiratory disorders.

Therapeutic acute intermittent hypoxia: A translational roadmap for spinal cord injury and neuromuscular disease

We review progress towards greater mechanistic understanding and clinical translation of a strategy to improve respiratory and non-respiratory motor function in people with neuromuscular disorders, therapeutic acute intermittent hypoxia (tAIH). In 2016 and 2020, workshops to create and update a "road map to clinical translation" were held to help guide future research and development of tAIH to restore movement in people living with chronic, incomplete spinal cord injuries. After briefly discussing the pioneering, non-targeted basic research inspiring this novel therapeutic approach, we then summarize workshop recommendations, emphasizing critical knowledge gaps, priorities for future research effort, and steps needed to accelerate progress as we evaluate the potential of tAIH for routine clinical use. Highlighted areas include: 1) greater mechanistic understanding, particularly in non-respiratory motor systems; 2) optimization of tAIH protocols to maximize benefits; 3) identification of combinatorial treatments that amplify plasticity or remove plasticity constraints, including task-specific training; 4) identification of biomarkers for individuals most/least likely to benefit from tAIH; 5) assessment of long-term tAIH safety; and 6) development of a simple, safe and effective device to administer tAIH in clinical and home settings. Finally, we update ongoing clinical trials and recent investigations of tAIH in SCI and other clinical disorders that compromise motor function, including ALS, multiple sclerosis, and stroke.

Activation of Astrocytes in the Persistence of Post-hypoxic Respiratory Augmentation

Acute hypoxia increases ventilation. After cessation of hypoxia loading, ventilation decreases but remains above the pre-exposure baseline level for a time. However, the mechanism of this post-hypoxic persistent respiratory augmentation (PHRA), which is a short-term potentiation of breathing, has not been elucidated. We aimed to test the hypothesis that astrocytes are involved in PHRA. To this end, we investigated hypoxic ventilatory responses by whole-body plethysmography in unanesthetized adult mice. The animals breathed room air, hypoxic gas mixture (7% O₂, 93% N₂) for 2min, and again room air for 10min before and after i.p. administration of low (100mg/kg) and high (300mg/kg) doses of arundic acid (AA), an astrocyte inhibitor. AA suppressed PHRA, with the high dose decreasing ventilation below the pre-hypoxic level. Further, we investigated the role of the astrocytic TRPA1 channel, a putative ventilatory hypoxia sensor, in PHRA using astrocyte-specific Trpa1 knockout (asTrpa1^−/−) and floxed Trpa1 (Trpa1^f/f) mice. In both Trpa1^f/f and asTrpa1^−/− mice, PHRA was noticeable, indicating that the astrocyte TRPA1 channel was not directly involved in PHRA. Taken together, these results indicate that astrocytes mediate the PHRA by mechanisms other than TRPA1 channels that are engaged in hypoxia sensing.

Pathophysiology of Obstructive Sleep Apnea in Aging Women

The following review is designed to explore the pathophysiology of sleep apnea in aging women. The review initially introduces four endotypes (i.e., a more collapsible airway, upper airway muscle responsiveness, arousal threshold, and loop gain) that may have a role in the initiation of obstructive sleep apnea. Thereafter, sex differences in the prevalence of sleep apnea are considered along with differences in the prevalence that exist between younger and older women. Following this discussion, we consider how each endotype might contribute to the increase in prevalence of sleep apnea in aging women. Lastly, we address how modifications in one form of respiratory plasticity, long-term facilitation, that might serve to mitigate apneic events in younger women may be modified in aging women with obstructive sleep apnea. Overall, the published literature indicates that the prevalence of sleep apnea is increased in aging women. This increase is linked primarily to a more collapsible airway and possibly to reduced responsiveness of upper airway muscle activity. In contrast, modifications in loop gain or the arousal threshold do not appear to have a role in the increased prevalence of sleep apnea in aging women. Moreover, we suggest that mitigation of long-term facilitation could contribute to the increased prevalence of sleep apnea in aging women.

Is Sleep Disordered Breathing Confounding Rehabilitation Outcomes in Spinal Cord Injury Research?

The purpose of this article is to highlight the importance of considering sleep-disordered breathing (SDB) as a potential confounder to rehabilitation research interventions in spinal cord injury (SCI). SDB is highly prevalent in SCI, with increased prevalence in individuals with higher and more severe lesions, and the criterion standard treatment with continuous positive airway pressure remains problematic. Despite its high prevalence, SDB is often untested and untreated in individuals with SCI. In individuals without SCI, SDB is known to negatively affect physical function and many of the physiological systems that negatively affect physical rehabilitation in SCI. Thus, owing to the high prevalence, under testing, low treatment adherence, and known negative effect on the physical function, it is contended that underdiagnosed SDB in SCI may be confounding physical rehabilitation research studies in individuals with SCI. Studies investigating the effect of treating SDB and its effect on physical rehabilitation in SCI were unable to be located. Thus, studies investigating the likely integrated relationship among physical rehabilitation, SDB, and proper treatment of SDB in SCI are needed. Owing to rapid growth in both sleep medicine and physical rehabilitation intervention research in SCI, the authors contend it is the appropriate time to begin the conversations and collaborations between these fields. We discuss a general overview of SDB and physical training modalities, as well as how SDB could be affecting these studies.

Cardiorespiratory plasticity in humans following two patterns of acute intermittent hypoxia

NEW FINDINGS

What is the central question of this study? Do cardiorespiratory experience-dependent effects (EDEs) differ between two different stimulus durations of acute isocapnic intermittent hypoxia (IHx; 5-min vs. 90-s cycles between hypoxia and normoxia)? What is the main finding and its importance? There was long-term facilitation in ventilation and blood pressure in both IHx protocols, but there was no evidence of progressive augmentation or post-hypoxia frequency decline. Not all EDEs described in animal models translate to acute isocapnic IHx responses in humans, and cardiorespiratory responses to 5-min versus 90-s on/off IHx protocols are largely similar.

ABSTRACT

Peripheral respiratory chemoreceptors monitor breath-by-breath changes in arterial CO₂ and O₂ , and mediate ventilatory changes to maintain homeostasis. Intermittent hypoxia (IHx) elicits hypoxic ventilatory responses, with well-described experience-dependent effects (EDEs), derived mostly from animal work involving intermittent 5-min cycles of hypoxia and normoxia. These EDEs include post-hypoxia frequency decline (PHxFD), progressive augmentation (PA) and long-term facilitation (LTF). Comparisons of these EDEs between animal models and humans using similar IHx protocols are lacking. In addition, it is unknown whether shorter bouts of hypoxia, which may be more relevant to clinical conditions, elicit EDEs of similar magnitudes in humans. Respiratory (frequency, tidal volume and minute ventilation ( V ̇ I ) and cardiovascular (heart rate and mean arterial pressure (MAP)) variables were measured during and following two patterns of acute isocapnic IHx in 14 healthy human participants (four female): (1) 5 × 5 min and (2) 5 × 90 s on/off hypoxia. Participants' end-tidal P O 2 was clamped at 45 Torr during hypoxia and 100 Torr during normoxia. We found that (1) PHxFD and PA were not present in either IHx pattern (P > 0.14), (2) LTF was present in V ̇ I following both 5-min (P < 0.001) and 90-s isocapnic IHx trials (P < 0.001), and (3) LTF was present in MAP following 5-min isocapnic IHx (P < 0.001), and trended towards significance following 90-s IHx (P = 0.058). We demonstrate that acute isocapnic IHx alone may not elicit all of the EDEs that have been described in animal models. Additionally, ventilatory LTF occurred regardless of the length of hypoxia-normoxia cycles.

The combination of intermittent electrical stimulation with acute intermittent hypoxia strengthens genioglossus muscle discharge in chronic intermittent hypoxia-pretreated rats

OBJECTIVE

Exploring whether the genioglossus discharge in chronic intermittent hypoxia(CIH) - pretreated rats could be enhanced by intermittent electrical stimulation combined with acute intermittent hypoxia(AIH).

METHODS

Rats were pretreated with CIH for 4 weeks and then were randomly divided into 6 groups: time control, intermittent electric stimulation, AIH, intermittent electric stimulation + AIH, continuous electric stimulation and continuous hypoxia exposure. The genioglossus discharges were recorded and compared before and after stimulation. Normoxic-treated rats were grouped and treated with the same stimulation protocols.

RESULTS

Intermittent electrical stimulation or AIH temporarily increased the activity of the genioglossus discharge, in which the degree of the increase was significantly higher in CIH-pretreated rats than in normoxic rats.After intermittent electrical stimulation, AIH evoked a sustained elevation of genioglossus discharge activities in CIH-pretreated rats, in which the degree of the increase was significantly higher than in rats induced by a single intermittent electric stimulation.

CONCLUSION

Intermittent electrical stimulation combined with AIH strengthens the genioglossus plasticity in CIH-pretreated rats.

A comprehensive review of respiratory, autonomic and cardiovascular responses to intermittent hypoxia in humans

This review explores forms of respiratory and autonomic plasticity, and associated outcome measures, that are initiated by exposure to intermittent hypoxia. The review focuses primarily on studies that have been completed in humans and primarily explores the impact of mild intermittent hypoxia on outcome measures. Studies that have explored two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of ventilation and upper airway muscle activity, are initially reviewed. The role these forms of plasticity might have in sleep disordered breathing are also explored. Thereafter, the role of intermittent hypoxia in the initiation of autonomic plasticity is reviewed and the role this form of plasticity has in cardiovascular and hemodynamic responses during and following intermittent hypoxia is addressed. The role of these responses in individuals with sleep disordered breathing and spinal cord injury are subsequently addressed. Ultimately an integrated picture of the respiratory, autonomic and cardiovascular responses to intermittent hypoxia is presented. The goal of the integrated picture is to address the types of responses that one might expect in humans exposed to one-time and repeated daily exposure to mild intermittent hypoxia. This form of intermittent hypoxia is highlighted because of its potential therapeutic impact in promoting functional improvement and recovery in several physiological systems.

Hypoxia-inducible factors and obstructive sleep apnea

Intermittent hypoxia (IH) is a hallmark manifestation of obstructive sleep apnea (OSA), a widespread disorder of breathing. This Review focuses on the role of hypoxia-inducible factors (HIFs) in hypertension, type 2 diabetes (T2D), and cognitive decline in experimental models of IH patterned after O2 profiles seen in OSA. IH increases HIF-1α and decreases HIF-2α protein levels. Dysregulated HIFs increase reactive oxygen species (ROS) through HIF-1-dependent activation of pro-oxidant enzyme genes in addition to reduced transcription of antioxidant genes by HIF-2. ROS in turn activate chemoreflex and suppress baroreflex, thereby stimulating the sympathetic nervous system and causing hypertension. We also discuss how increased ROS generation by HIF-1 contributes to IH-induced insulin resistance and T2D as well as disrupted NMDA receptor signaling in the hippocampus, resulting in cognitive decline.

Systemic inflammation suppresses spinal respiratory motor plasticity via mechanisms that require serine/threonine protein phosphatase activity

Background

Inflammation undermines multiple forms of neuroplasticity. Although inflammation and its influence on plasticity in multiple neural systems has been extensively studied, its effects on plasticity of neural networks controlling vital life functions, such as breathing, are less understood. In this study, we investigated the signaling mechanisms whereby lipopolysaccharide (LPS)-induced systemic inflammation impairs plasticity within the phrenic motor system—a major spinal respiratory motor pool that drives contractions of the diaphragm muscle. Here, we tested the hypotheses that lipopolysaccharide-induced systemic inflammation (1) blocks phrenic motor plasticity by a mechanism that requires cervical spinal okadaic acid-sensitive serine/threonine protein phosphatase (PP) 1/2A activity and (2) prevents phosphorylation/activation of extracellular signal-regulated kinase 1/2 mitogen activated protein kinase (ERK1/2 MAPK)—a key enzyme necessary for the expression of phrenic motor plasticity.

Methods

To study phrenic motor plasticity, we utilized a well-characterized model for spinal respiratory plasticity called phrenic long-term facilitation (pLTF). pLTF is characterized by a long-lasting, progressive enhancement of inspiratory phrenic nerve motor drive following exposures to moderate acute intermittent hypoxia (mAIH). In anesthetized, vagotomized and mechanically ventilated adult Sprague Dawley rats, we examined the effect of inhibiting cervical spinal serine/threonine PP 1/2A activity on pLTF expression in sham-vehicle and LPS-treated rats. Using immunofluorescence optical density analysis, we compared mAIH-induced phosphorylation/activation of ERK 1/2 MAPK with and without LPS-induced inflammation in identified phrenic motor neurons.

Results

We confirmed that mAIH-induced pLTF is abolished 24 h following low-dose systemic LPS (100 μg/kg, i.p.). Cervical spinal delivery of the PP 1/2A inhibitor, okadaic acid, restored pLTF in LPS-treated rats. LPS also prevented mAIH-induced enhancement in phrenic motor neuron ERK1/2 MAPK phosphorylation. Thus, a likely target for the relevant okadaic acid-sensitive protein phosphatases is ERK1/2 MAPK or its upstream activators.

Conclusions

This study increases our understanding of fundamental mechanisms whereby inflammation disrupts neuroplasticity in a critical population of motor neurons necessary for breathing, and highlights key roles for serine/threonine protein phosphatases and ERK1/2 MAPK kinase in the plasticity of mammalian spinal respiratory motor circuits.

Obstructive sleep apnea and respiratory center regulation abnormality

PURPOSE

Obstructive sleep apnea (OSA) is a complex disease in which phenotypic analysis and understanding pathological mechanisms facilitate personalized treatment and outcomes. However, the pathophysiology responsible for this robust observation is incompletely understood. The objective of the present work was to review how respiratory center regulation varies during sleep and wakeness in patients with OSA.

DATA SOURCES

We searched for relevant articles up to December 31, 2019 in PubMed database.

METHODS

This review examines the current literature on the characteristics of respiratory center regulation during wakefulness and sleep in OSA, detection method, and phenotypic treatment for respiratory center regulation.

RESULTS

Mechanisms for ventilatory control system instability leading to OSA include different sleep stages in chemoresponsiveness to hypoxia and hypercapnia and different chemosensitivity at different time. One can potentially stabilize the breathing center in sleep-related breathing disorders by identifying one or more of these pathophysiological mechanisms.

CONCLUSIONS

Advancing mechanism research in OSA will guide symptom research and provide alternate and novel opportunities for effective treatment for patients with OSA.

Asthma and Obstructive Sleep Apnea Overlap: What Has the Evidence Taught Us?

Obstructive sleep apnea (OSA) and asthma are highly prevalent chronic respiratory disorders. Beyond their frequent coexistence arising from their high prevalence and shared risk factors, these disorders feature a reciprocal interaction whereby each disease impacts the severity of the other. Emerging evidence implicates airway and systemic inflammation, neuroimmune interactions, and effects of asthma-controlling medications (corticosteroids) as factors that predispose patients with asthma to OSA. Conversely, undiagnosed or inadequately treated OSA adversely affects asthma control, partly via effects of intermittent hypoxia on airway inflammation and tissue remodeling. In this article, we review multiple lines of recently published evidence supporting this interaction. We provide a set of recommendations for clinicians involved in the care of adults with asthma, and identify critical gaps in our knowledge about this overlap.

Variations in loop gain and arousal threshold during NREM sleep are affected by time of day over a 24-hour period in participants with obstructive sleep apnea

We investigated whether time of day affects loop gain (LG) and the arousal threshold (AT) during non-rapid eye movement (NREM) sleep. Eleven men with obstructive sleep apnea (apnea-hypopnea index > 5 events/h) completed a constant-routine protocol that comprised 3-h sleep sessions in the evening [10 PM (1) to 1 AM], morning (6 AM to 9 AM), afternoon (2 PM to 5 PM), and subsequent evening [10 PM (2) to 1 AM]. During each sleep session LG and the AT were measured during NREM sleep with a model-based approach. Our results showed the presence of a rhythmicity in both LG (P < 0.0001) and the AT (P < 0.001) over a 24-h period. In addition, LG and the AT were greater in the morning compared with both evening sessions [6 AM vs. 10 PM (1) vs. 10 PM (2): LG (1 cycle/min): 0.71 ± 0.23 vs. 0.60 ± 0.22 (P = 0.01) vs. 0.56 ± 0.10 (P < 0.001), AT (fraction of eupneic breathing): 1.45 ± 0.47 vs. 1.28 ± 0.36 (P = 0.02) vs. 1.20 ± 0.18 (P = 0.001)]. No difference in LG and the AT existed between the evening sessions (LG: P = 0.27; AT: P = 0.24). LG was correlated to measures of the hypocapnic ventilatory response (i.e., a measure of chemoreflex sensitivity) (r = 0.72 and P = 0.045) and the critical closing pressure (i.e., a measure of airway collapsibility) (r = 0.77 and P = 0.02) that we previously published. We conclude that time of day, independent of hallmarks of sleep apnea, affects LG and the AT during NREM sleep. These modifications may contribute to increases in breathing instability in the morning compared with other periods throughout the day/night cycle in individuals with obstructive sleep apnea. In addition, efficaciousness of treatments for obstructive sleep apnea that target LG and the AT may be modified by a rhythmicity in these variables.NEW & NOTEWORTHY Loop gain and the arousal threshold during non-rapid eye movement (NREM) sleep are greater in the morning compared with the afternoon and evening. Loop gain measures are correlated to chemoreflex sensitivity and the critical closing pressure measured during NREM sleep in the evening, morning, and afternoon. Breathing (in)stability and efficaciousness of treatments for obstructive sleep apnea may be modulated by a circadian rhythmicity in loop gain and the arousal threshold.

Intermittent hypoxia modulates redox homeostasis, lipid metabolism associated inflammatory processes and redox post-translational modifications: Benefits at high altitude

Intermittent hypoxia, initially associated with adverse effects of sleep apnea, has now metamorphosed into a module for improved sports performance. The regimen followed for improved sports performance is milder intermittent hypoxic training (IHT) as compared to chronic and severe intermittent hypoxia observed in sleep apnea. Although several studies have indicated the mechanism and enough data on physiological parameters altered by IH is available, proteome perturbations remain largely unknown. Altitude induced hypobaric hypoxia is known to require acclimatization as it causes systemic redox stress and inflammation in humans. In the present study, a short IHT regimen consisting of previously reported physiologically beneficial FIO2 levels of 13.5% and 12% was administered to human subjects. These subjects were then airlifted to altitude of 3500 m and their plasma proteome along with associated redox parameters were analyzed on days 4 and 7 of high altitude stay. We observed that redox stress and associated post-translational modifications, perturbed lipid metabolism and inflammatory signaling were induced by IHT exposure at Baseline. However, this caused activation of antioxidants, energy homeostasis mechanisms and anti-inflammatory responses during subsequent high-altitude exposure. Thus, we propose IHT as a beneficial non-pharmacological intervention that benefits individuals venturing to high altitude areas.

Acute intermittent hypercapnic hypoxia and sympathetic neurovascular transduction in men

KEY POINTS

Intermittent hypoxia leads to long-lasting increases in muscle sympathetic nerve activity and blood pressure, contributing to increased risk for hypertension in obstructive sleep apnoea patients. We determined whether augmented vascular responses to increasing sympathetic vasomotor outflow, termed sympathetic neurovascular transduction (sNVT), accompanied changes in blood pressure following acute intermittent hypercapnic hypoxia in men. Lower body negative pressure was utilized to induce a range of sympathetic vasoconstrictor firing while measuring beat-by-beat blood pressure and forearm vascular conductance. IH reduced vascular shear stress and steepened the relationship between diastolic blood pressure and sympathetic discharge frequency, suggesting greater systemic sNVT. Our results indicate that recurring cycles of acute intermittent hypercapnic hypoxia characteristic of obstructive sleep apnoea could promote hypertension by increasing sNVT.

ABSTRACT

Acute intermittent hypercapnic hypoxia (IH) induces long-lasting elevations in sympathetic vasomotor outflow and blood pressure in healthy humans. It is unknown whether IH alters sympathetic neurovascular transduction (sNVT), measured as the relationship between sympathetic vasomotor outflow and either forearm vascular conductance (FVC; regional sNVT) or diastolic blood pressure (systemic sNVT). We tested the hypothesis that IH augments sNVT by exposing healthy males to 40 consecutive 1 min breathing cycles, each comprising 40 s of hypercapnic hypoxia ( P ETC O 2 : +4 ± 3 mmHg above baseline; P ET O 2 : 48 ± 3 mmHg) and 20 s of normoxia (n = 9), or a 40 min air-breathing control (n = 7). Before and after the intervention, lower body negative pressure (LBNP; 3 min at -15, -30 and -45 mmHg) was applied to elicit reflex increases in muscle sympathetic nerve activity (MSNA, fibular microneurography) when clamping end-tidal gases at baseline levels. Ventilation, arterial pressure [systolic blood pressure, diastolic blood pressure, mean arterial pressure (MAP)], brachial artery blood flow ( Q ̇ _BA ), FVC ( Q ̇ _BA /MAP) and MSNA burst frequency were measured continuously. Following IH, but not control, ventilation [5 L min^-1 ; 95% confidence interval (CI) = 1-9] and MAP (5 mmHg; 95% CI = 1-9) were increased, whereas FVC (-0.2 mL min^-1  mmHg^-1 ; 95% CI = -0.0 to -0.4) and mean shear rate (-21.9 s^-1 ; 95% CI = -5.8 to -38.0; all P < 0.05) were reduced. Systemic sNVT was increased following IH (0.25 mmHg burst^-1  min^-1 ; 95% CI = 0.01-0.49; P < 0.05), whereas changes in regional forearm sNVT were similar between IH and sham. Reductions in vessel wall shear stress and, consequently, nitric oxide production may contribute to heightened systemic sNVT and provide a potential neurovascular mechanism for elevated blood pressure in obstructive sleep apnoea.

Breathing retraining in sleep apnoea: a review of approaches and potential mechanisms

PURPOSE

Anatomically based treatments for obstructive sleep apnoea (OSA) may not completely resolve OSA. This has led to interest in exploring ways of addressing physiological risk factors. This review examines the literature for research reporting on the effects of various types of breathing training and breathing activities on sleep apnoea. It also reviews and discusses proposed therapeutic mechanisms.

METHODS

A search of electronic databases was performed using the search terms related to various breathing therapies or to activities requiring high levels of breath control such as singing and the playing of musical instruments and sleep apnoea.

RESULTS

A total of 14 suitable studies were reviewed. A diverse variety of breathing retraining approaches are reported to improve sleep apnoea, e.g., Buteyko method, inspiratory resistance training, and diaphragmatic breathing. There is also a reduced incidence of sleep apnoea with intensive and regular participation in activities that require high levels of breath control, e.g., singing and playing wind instruments. Improvements in sleep-disordered breathing are thought to be related to improvements in (1) muscle tone of the upper airway; (2) respiratory muscle strength; (3) neuroplasticity of breathing control; (4) oxygen levels; (5) hyperventilation/dysfunctional breathing; and (6) autonomic nervous system, metabolic, and inflammatory status.

CONCLUSION

Breathing retraining and regular practice of breath control activities such as singing and playing wind instruments are potentially helpful for sleep apnoea, particularly for individuals with minimal anatomical deficit and daytime breathing dysfunction. Research is needed to elucidate mechanisms, to inform patient selection, and to refine clinical protocols.

In patients with heart failure the burden of central sleep apnea increases in the late sleep hours

Study Objectives

Periodic breathing with central sleep apnea (CSA) is common in patients with left ventricular systolic dysfunction. Based on the pathophysiological mechanisms underlying CSA, we hypothesized that the frequency of CSA episodes would increase in the late hours of non-rapid eye movement (NREM) of sleep.

Methods

Forty-one patients with left ventricular ejection fraction <40% underwent full-night-attended polysomnography scored by a central core lab. Because central apneas occur primarily in NREM sleep, total NREM sleep time for each patient was divided into 8 equal duration segments. Segment event counts were normalized to an events/hour index based on sleep segment duration.

Results

Central apnea index (CAI) varied among sleep segments (p = 0.001). As expected CAI was higher in segment 1 compared to segments 2 and 3, increasing during later segments. The minimum CAI occurred in segment 2 with mean ± SD of 21 ± 3 events/hour and maximum CAI was in segment 8 with 37 ± 4 events/hour. We also determined central apnea duration which varied among segments (p = 0.005), with longer durations later in the night (segment 1: 22 ± 1 seconds; segment 8: 26 ± 1 seconds, p < 0.001). Data were also analyzed including rapid eye movement (REM) sleep, with similar results. Further, comparison of CAI between the first and second half of the night showed a significant increase in the index. Circulation time did not change across the segments (p = 0.073).

Conclusions

In patients with left ventricular dysfunction and CSA, central apnea burden (number and duration) increases during later hours of sleep. These findings have pathophysiological and therapeutic implications.

Clinical Trial Registration

NCT01124370.

The physiological phenotype of obstructive sleep apnea differs between Caucasian and Chinese patients

Study Objectives

The mechanisms responsible for the development of obstructive sleep apnea (phenotypic “traits”) are known to differ between individuals and may differ across ethnicities. We aimed to examine whether loop gain, arousal threshold, pharyngeal collapsibility and muscle compensation differ between Chinese and Caucasian individuals with OSA.

Methods

We noninvasively determined the relative contribution of loop gain, arousal threshold, pharyngeal collapsibility, and muscle compensation from the ventilatory flow pattern recorded during a standard clinical polysomnography in a cohort of age and AHI matched Caucasian and Chinese patients with moderate-severe OSA (n = 90).

Results

Chinese participants had significantly more collapsible pharyngeal airways as indicated by a lower Vpassive (68.9 [51.5–75.2] vs. 74.0 [65.1–80.4] %Veupnea, U = 703, p = 0.012), but less ventilatory control instability as indicated by a lower loop gain (0.60 [0.50–0.67] vs. 0.63 [0.57–0.81], U = 762, p = 0.043) compared with Caucasian participants. Further, multiple logistic regression analyses demonstrated that the combined pharyngeal collapsibility (Vpassive) and loop gain traits help to better explain the differences between the groups beyond upper-airway collapsibility alone. No statistically significant group differences were found in muscle compensation or arousal threshold traits between groups.

Conclusion

Individuals of Chinese descent appear to have OSA that is driven much more by the relative contribution of their anatomical predisposition and to a lesser extent nonanatomical causes compared with Caucasians. Future research should focus on determining if Chinese versus Caucasian ethnicity is an important contributing factor to clinical outcomes and therapeutic responses in OSA.

Competing mechanisms of plasticity impair compensatory responses to repetitive apnoea

KEY POINTS

Intermittent reductions in respiratory neural activity, a characteristic of many ventilatory disorders, leads to inadequate ventilation and arterial hypoxia. Both intermittent reductions in respiratory neural activity and intermittent hypoxia trigger compensatory enhancements in inspiratory output when experienced separately, forms of plasticity called inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. Reductions in respiratory neural activity that lead to moderate, but not mild, arterial hypoxia occludes plasticity expression, indicating that concurrent induction of iMF and LTF impairs plasticity through cross-talk inhibition of their respective signalling pathways. Moderate hypoxia undermines iMF by enhancing NR2B-containing NMDA receptor signalling, which can be rescued by exogenous retinoic acid, a molecule necessary for iMF. These data suggest that in ventilatory disorders characterized by reduced inspiratory motor output, such as sleep apnoea, endogenous mechanisms of compensatory plasticity may be impaired, and that exogenously activating respiratory plasticity may be a novel strategy to improve breathing.

ABSTRACT

Many forms of sleep apnoea are characterized by recurrent reductions in respiratory neural activity, which leads to inadequate ventilation and arterial hypoxia. Both recurrent reductions in respiratory neural activity and hypoxia activate mechanisms of compensatory plasticity that augment inspiratory output and lower the threshold for apnoea, inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. However, despite frequent concurrence of reduced respiratory neural activity and hypoxia, mechanisms that induce and regulate iMF and LTF have only been studied separately. Here, we demonstrate that recurrent reductions in respiratory neural activity ('neural apnoea') accompanied by cessations in ventilation that result in moderate (but not mild) hypoxaemia do not elicit increased inspiratory output, suggesting that concurrent induction of iMF and LTF occludes plasticity. A key role for NMDA receptor activation in impairing plasticity following concurrent neural apnoea and hypoxia is indicated since recurrent hypoxic neural apnoeas triggered increased phrenic inspiratory output in rats in which spinal NR2B-containing NMDA receptors were inhibited. Spinal application of retinoic acid, a key molecule necessary for iMF, bypasses NMDA receptor-mediated constraints, thereby rescuing plasticity following hypoxic neural apnoeas. These studies raise the intriguing possibility that endogenous mechanisms of compensatory plasticity may be impaired in some individuals with sleep apnoea, and that exogenously activating pathways giving rise to respiratory plasticity may be a novel pharmacological strategy to improve breathing.

Diagnosis and management of central sleep apnea syndrome

Introduction: Central sleep apnea (CSA) syndrome has gained a considerable interest in the sleep field within the last 10 years. It is overrepresented in particular subpopulations such as patients with stroke or heart failure. Early detection and diagnosis, as well as appropriate treatment of central breathing disturbances during sleep remain challenging. Areas covered: Based on a systematic review of CSA in adults the clinical evidence and polysomnographic patterns useful for discerning central from obstructive events are discussed. Current therapeutic indications of CSA and perspectives are presented, according to the type of respiratory disturbances during sleep, alterations in blood gases and ventilatory control. Expert opinion: The precise identification of central events during polysomnographic recording is mandatory. Therapeutic choices for CSA depend on the typology of respiratory disturbances observed by polysomnography, changes in blood gases and ventilatory control. In CSA with normocapnia and ventilatory instability, adaptive servo-ventilation is recommended. In CSA with hypercapnia and/or rapid-eye movement sleep hypoventilation, non-invasive ventilation is required. Further studies are required as strong evidence is lacking regarding the long-term consequences of CSA and the long-term impact of current treatment strategies.

Ventilatory control sensitivity in patients with obstructive sleep apnea is sleep stage dependent

Study Objectives

The severity of obstructive sleep apnea (OSA) is known to vary according to sleep stage; however, the pathophysiology responsible for this robust observation is incompletely understood. The objective of the present work was to examine how ventilatory control system sensitivity (i.e. loop gain) varies during sleep in patients with OSA.

Methods

Loop gain was estimated using signals collected from standard diagnostic polysomnographic recordings performed in 44 patients with OSA. Loop gain measurements associated with nonrapid eye movement (NREM) stage 2 (N2), stage 3 (N3), and REM sleep were calculated and compared. The sleep period was also split into three equal duration tertiles to investigate how loop gain changes over the course of sleep.

Results

Loop gain was significantly lower (i.e. ventilatory control more stable) in REM (Mean ± SEM: 0.51 ± 0.04) compared with N2 sleep (0.63 ± 0.04; p = 0.001). Differences in loop gain between REM and N3 (p = 0.095), and N2 and N3 (p = 0.247) sleep were not significant. Furthermore, N2 loop gain was significantly lower in the first third (0.57 ± 0.03) of the sleep period compared with later second (0.64 ± 0.03, p = 0.012) and third (0.64 ± 0.03, p = 0.015) tertiles. REM loop gain also tended to increase across the night; however, this trend was not statistically significant [F(2, 12) = 3.49, p = 0.09].

Conclusions

These data suggest that loop gain varies between REM and NREM sleep and modestly increases over the course of sleep. Lower loop gain in REM is unlikely to contribute to the worsened OSA severity typically observed in REM sleep, but may explain the reduced propensity for central sleep apnea in this sleep stage.

Increased Oxidative Stress, Loop Gain And The Arousal Threshold Are Clinical Predictors Of Increased Apnea Severity Following Exposure To Intermittent Hypoxia

PURPOSE

We determined if oxidative stress prior to sleep onset is correlated to loop gain (LG) and the arousal threshold (AT) during non-rapid eye movement (NREM) sleep. We also explored if LG and AT are correlated with apnea severity and indices of upper airway collapsibility during NREM sleep.

METHODS

Thirteen male participants with obstructive sleep apnea (apnea-hypopnea index > 5 events/hr) were administered an antioxidant or placebo cocktail while exposed to mild intermittent hypoxia in the awake state. Thereafter, loop gain and measures of arousal, apnea severity and upper airway collapsibility were ascertained during NREM sleep.

RESULTS

Modification in oxidative stress (i.e., 8-hydroxy-2-deoxyguanosine) prior to sleep onset was correlated to LG (r = 0.8, P = 0.003), the number (r = 0.71, P = 0.01) and duration (r = 0.63, P = 0.04) of apneic events and the percentage of time breathing was stable (r = -0.66, P = 0.03) during sleep. Using a forward stepwise regression analysis, our results showed that LG, AT, the ventilatory response to arousal and nadir end-tidal carbon dioxide were determinants of the apnea-hypopnea index (P value range = 0.04-0.001). In addition, the AT was a predictor of measures of upper airway collapsibility, including the hypopnea/apnea + hypopnea ratio and the degree of flow reduction that accompanied hypopneic events (P < 0.001).

CONCLUSION

Modifications in oxidative stress following exposure to intermittent hypoxia during wakefulness are positively associated with loop gain and apnea severity during NREM sleep. Moreover, an increase in the arousal threshold is a predictor of increased upper airway collapsibility.

Increased tongue use enhances 5-HT2C receptor immunostaining in hypoglossal motor nucleus

Hypoglossal (XII) motoneurons are activated by type 2 receptors for serotonin (5-HT). This activation is especially strong during wakefulness which facilitates diverse motor functions of the tongue, including the maintenance of upper airway patency in obstructive sleep apnea (OSA) patients. We tested whether 5-HT₂ receptor levels in the XII nucleus vary with intensity of tongue use. Three groups of rats were housed overnight under conditions of increasing oromotor activity: W-water available ad lib; S-sweetened water to stimulate drinking; S + O-sweetened water + oil applied on fur to increase grooming. After the exposures, immunostaining for 5-HT_2C, but not 5-HT_2A, receptors was higher in the XII nucleus in S + O than in W rats (65 ± 1.8 (SE) vs. 60 ± 2.0 arbitrary units; p = 0.008). In the medullary raphé obscurus region, the percentage of c-Fos-positive 5-HT cells was 13% higher (p = 0.03) in S + O than in W rats. The positive feedback between tongue use and 5-HT_2C receptor immunostaining reveals a novel mechanism potentially relevant for OSA and neuromuscular disorders.

Neural memory of the genioglossus muscle during sleep is stage-dependent in healthy subjects and obstructive sleep apnoea patients

KEY POINTS

In most patients with obstructive sleep apnoea (OSA), there is a spontaneous resolution of the breathing disorders during slow wave sleep (SWS) for yet unknown reasons related to non-anatomical factors. Some recently identified forms of neural memory specific of upper airway muscles may play a role in this phenomenon. In the present study, we show for the first time that a form of memory of the genioglossus (tongue) muscle is greatly enhanced during SWS compared to non-rapid eye movement stage 2 sleep. The present study represents a step forward in understanding the mechanisms responsible for the spontaneous development of stable breathing during SWS in OSA patients and may help the discovery of novel therapeutic strategies for this disease.

ABSTRACT

Several studies have shown that obstructive sleep apnoea (OSA) improves during slow wave sleep (SWS) for reasons that remain unclear. Recent studies have identified forms of neural memory such as short-term potentiation or after-discharge that can occur in response to upper airway obstruction. Neural memory may play a role in the development of stable breathing during SWS by increasing upper airway muscles activity in this sleep stage. We hypothesize that the after-discharge of the genioglossus muscle following upper airway obstruction is enhanced during SWS compared to non-rapid eye movement stage 2 (N2). During sleep, we performed five-breath drops in continuous positive airway pressure (CPAP-drop) to simulate obstructive events and reflexively activate the genioglossus. Immediately afterwards, CPAP was returned to an optimal level. Once the post-drop ventilation returned to eupnoea, the genioglossus after-discharge was measured as the time it took for genioglossus activity to return to baseline levels. In total, 171 CPAP-drops were analysed from a group of 16 healthy subjects and 19 OSA patients. A mixed-model analysis showed that after-discharge duration during SWS was 208% (95% confidence interval = 112% to 387%, P = 0.022) greater than during N2 after adjusting for covariates (ventilatory drive, CPAP levels). There was also a non-significant trend for a -35% reduction in after-discharge duration following an arousal vs. no-arousal from sleep (95% confidence interval = -59.5% to 5%, P = 0.08). Genioglossus after-discharge is two-fold greater in SWS vs. N2, which could partly explain the breathing stabilization described in OSA patients during this sleep stage.

Phrenic long-term depression evoked by intermittent hypercapnia is modulated by serotonergic and adrenergic receptors in raphe nuclei

Intermittent hypercapnia evokes prolonged depression of phrenic nerve activity (phrenic long-term depression, pLTD). This study was undertaken to investigate the role of 5-HT and α2-adrenergic receptors in the initiation of pLTD. Adult male urethane-anesthetized, vagotomized, paralyzed, and mechanically ventilated Sprague-Dawley rats were exposed to a protocol of acute intermittent hypercapnia (AIHc; 5 episodes of 15% CO2in air, each episode lasting 3 min). The experimental group received microinjection of the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT), the broad-spectrum 5-HT antagonist methysergide, or the α2-adrenergic antagonist yohimbine, whereas the control group received microinjection of 0.9% saline into the caudal raphe region. Peak phrenic nerve activity (pPNA) and burst frequency ( f) were analyzed during baseline (T0), during 5 hypercapnic episodes (THc1–THc5), and at 15, 30, and 60 min after the end of the last hypercapnic episode. In the control group, pPNA decreased 60 min after the end of the last hypercapnic episode compared with baseline values, i.e., pLTD developed ( P = 0.023). In the 8-OH-DPAT group, pPNA significantly decreased at T15, T30, and T60 compared with baseline values, i.e., pLTD developed ( P = 0.01). In the methysergide and yohimbine groups, AIHc did not evoke significant changes of the pPNA at T15, T30, and T60 compared with baseline values. In conclusion, activation of 5-HT1Areceptors accentuated induction of pLTD, whereas blockade of α2-adrenergic receptors prevented development of pLTD following AIHc in anesthetized rats. These results suggest that chemical modulation of 5-HT and α2-adrenergic receptors in raphe nuclei affects hypercapnia-induced pLTD, offering important insights in understanding the mechanisms involved in development of respiratory plasticity.

NEW & NOTEWORTHY Hypercapnia is a concomitant feature of many breathing disorders, including obstructive sleep apnea. In this study, acute intermittent hypercapnia evoked development of phrenic long-term depression (pLTD) 60 min after the last hypercapnic episode that was preserved if the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide was microinjected in the caudal raphe region before the hypercapnic stimulus. This study highlights that both 5-HT and adrenergic receptor activation is needed for induction of pLTD in urethane-anesthetized rats following intermittent hypercapnia exposure.

Correlation of lung function and respiratory muscle strength with functional exercise capacity in obese individuals with obstructive sleep apnea syndrome

OBJECTIVE

To evaluate lung function and inspiratory muscle strength, correlating them with exercise tolerance, in obese individuals with obstructive sleep apnea syndrome (OSAS).

METHODS

The sample comprised 31 adult subjects with moderate-to-severe OSAS diagnosed by polysomnography. We used spirometry to measure FVC, FEV1, and FVC/FEV1 ratio, using pressure manometry to measure MIP and MEP. The incremental shuttle walk test (ISWT) and the six-minute walk test (6MWT) were used in order to determine functional exercise capacity.

RESULTS

In this sample, the mean values for FVC (% of predicted), FEV1 (% of predicted): MIP, and MEP were 76.4 ± 12.3%, 80.1 ± 6.3%, 60.0 ± 21.9 cmH2O, and 81.3 ± 22.2 cmH2O, respectively. The mean distances covered on the ISWT and 6MWT were 221 ± 97 m and 480.8 ± 67.3 m, respectively. The ISWT distance showed moderate positive correlations with FVC (r = 0.658; p = 0.001) and FEV1 (r = 0.522; p = 0.003).

CONCLUSIONS

In this sample of obese subjects with untreated OSAS, lung function, inspiratory muscle strength, and exercise tolerance were all below normal. In addition, we found that a decline in lung function, but not in respiratory muscle strength, was associated with exercise tolerance in these patients.

Correlation of lung function and respiratory muscle strength with functional exercise capacity in obese individuals with obstructive sleep apnea syndrome

OBJECTIVE

To evaluate lung function and inspiratory muscle strength, correlating them with exercise tolerance, in obese individuals with obstructive sleep apnea syndrome (OSAS).

METHODS

The sample comprised 31 adult subjects with moderate-to-severe OSAS diagnosed by polysomnography. We used spirometry to measure FVC, FEV1, and FVC/FEV1 ratio, using pressure manometry to measure MIP and MEP. The incremental shuttle walk test (ISWT) and the six-minute walk test (6MWT) were used in order to determine functional exercise capacity.

RESULTS

In this sample, the mean values for FVC (% of predicted), FEV1 (% of predicted): MIP, and MEP were 76.4 ± 12.3%, 80.1 ± 6.3%, 60.0 ± 21.9 cmH2O, and 81.3 ± 22.2 cmH2O, respectively. The mean distances covered on the ISWT and 6MWT were 221 ± 97 m and 480.8 ± 67.3 m, respectively. The ISWT distance showed moderate positive correlations with FVC (r = 0.658; p = 0.001) and FEV1 (r = 0.522; p = 0.003).

CONCLUSIONS

In this sample of obese subjects with untreated OSAS, lung function, inspiratory muscle strength, and exercise tolerance were all below normal. In addition, we found that a decline in lung function, but not in respiratory muscle strength, was associated with exercise tolerance in these patients.

Pharmacological modulation of hypoxia-induced respiratory neuroplasticity

Hypoxia elicits complex cell signaling mechanisms in the respiratory control system that can produce long-lasting changes in respiratory motor output. In this article, we review experimental approaches used to elucidate signaling pathways associated with hypoxia, and summarize current hypotheses regarding the intracellular signaling pathways evoked by intermittent exposure to hypoxia. We review data showing that pharmacological treatments can enhance neuroplastic responses to hypoxia. Original data are included to show that pharmacological modulation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) function can reveal a respiratory neuroplastic response to a single, brief hypoxic exposure in anesthetized mice. Coupling pharmacologic treatments with therapeutic hypoxia paradigms may have rehabilitative value following neurologic injury or during neuromuscular disease. Depending on prevailing conditions, pharmacologic treatments can enable hypoxia-induced expression of neuroplasticity and increased respiratory motor output, or potentially could synergistically interact with hypoxia to more robustly increase motor output.

The impact of intermittent or sustained carbon dioxide on intermittent hypoxia initiated respiratory plasticity. What is the effect of these combined stimuli on apnea severity?

The following review explores the effect that intermittent or sustained hypercapnia coupled to intermittent hypoxia has on respiratory plasticity. The review explores published work which suggests that intermittent hypercapnia leads to long-term depression of respiration when administered in isolation and prevents the initiation of long-term facilitation when administered in combination with intermittent hypoxia. The review also explores the impact that sustained hypercapnia alone and in combination with intermittent hypoxia has on the magnitude of long-term facilitation. After exploring the outcomes linked to intermittent hypoxia/hypercapnia and intermittent hypoxia/sustained hypercapnia the translational relevance of the outcomes as it relates to breathing stability during sleep is addressed. The likelihood that naturally induced cycles of intermittent hypoxia, coupled to oscillations in carbon dioxide that range between hypocapnia and hypercapnia, do not initiate long-term facilitation is addressed. Moreover, the conditions under which intermittent hypoxia/sustained hypercapnia could serve to improve breathing stability and mitigate co-morbidities associated with sleep apnea are considered.

Intermittent but not sustained moderate hypoxia elicits long-term facilitation of hypoglossal motor output

Phrenic long-term facilitation (pLTF) is a form of serotonin-dependent respiratory motor plasticity induced by moderate acute intermittent hypoxia (AIH), but not by moderate acute sustained hypoxia (ASH) of similar cumulative duration. Thus, moderate AIH-induced pLTF is sensitive to the pattern of hypoxia. On the other hand, pLTF induced by severe AIH protocols is neither pattern sensitive nor serotonin dependent (it converts to an adenosine-dependent mechanism). Although moderate AIH also induces hypoglossal LTF (hLTF), no data are available concerning its sensitivity/insensitivity to the pattern of hypoxia. Since hLTF following moderate hypoxia is serotonin-dependent, we hypothesized that hLTF is pattern-sensitive, similar to serotonin-dependent pLTF. Integrated hypoglossal nerve activity was recorded in urethane-anesthetized, vagotomized, paralyzed, and ventilated rats exposed to isocapnic AIH (3, 5min episodes of 11% O₂) or ASH (a single 25min episode of 11% O₂). Similar to previous studies of pLTF, hypoglossal motor output was elevated for more than 1h following AIH (50±20%, p<0.01), but not ASH (-6±9%, p>0.05). Frequency LTF was not observed following either hypoxic exposure. Thus, in agreement with our hypothesis, hypoglossal LTF following moderate AIH is pattern-sensitive, similar to phrenic LTF.

Enhancement of phrenic long-term facilitation following repetitive acute intermittent hypoxia is blocked by the glycolytic inhibitor 2-deoxyglucose

Moderate acute intermittent hypoxia (mAIH) elicits a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). Preconditioning with modest protocols of chronic intermittent hypoxia enhances pLTF, demonstrating pLTF metaplasticity. Since "low-dose" protocols of repetitive acute intermittent hypoxia (rAIH) show promise as a therapeutic modality to restore respiratory (and nonrespiratory) motor function in clinical disorders with compromised breathing, we tested 1) whether preconditioning with a mild rAIH protocol enhances pLTF and hypoglossal (XII) LTF and 2) whether the enhancement is regulated by glycolytic flux. In anesthetized, paralyzed, and ventilated adult male Lewis rats, mAIH (three 5-min episodes of 10% O₂) elicited pLTF (pLTF at 60 min post-mAIH: 49 ± 5% baseline). rAIH preconditioning (ten 5-min episodes of 11% O₂/day with 5-min normoxic intervals, 3 times per week, for 4 wk) significantly enhanced pLTF (100 ± 16% baseline). XII LTF was unaffected by rAIH. When glycolytic flux was inhibited by 2-deoxy-d-glucose (2-DG) administered via drinking water (~80 mg·kg^-1·day^-1), pLTF returned to normal levels (58 ± 8% baseline); 2-DG had no effect on pLTF in normoxia-pretreated rats (59 ± 7% baseline). In ventral cervical (C_4/5) spinal homogenates, rAIH increased inducible nitric oxide synthase mRNA vs. normoxic controls, an effect blocked by 2-DG. However, there were no detectable effects of rAIH or 2-DG on several molecules associated with phrenic motor plasticity, including serotonin 2A, serotonin 7, brain-derived neurotrophic factor, tropomyosin receptor kinase B, or VEGF mRNA. We conclude that modest, but prolonged, rAIH elicits pLTF metaplasticity and that a drug known to inhibit glycolytic flux (2-DG) blocks pLTF enhancement.

Exposure to intermittent hypoxia and sustained hypercapnia reduces therapeutic CPAP in participants with obstructive sleep apnea

Our purpose was to determine whether exposure to mild intermittent hypoxia leads to a reduction in the therapeutic continuous positive airway pressure required to eliminate breathing events. Ten male participants were treated with twelve 2-min episodes of hypoxia ([Formula: see text] ≈50 mmHg) separated by 2-min intervals of normoxia in the presence of [Formula: see text] that was sustained 3 mmHg above baseline. During recovery from the last episode, the positive airway pressure was reduced in a stepwise fashion until flow limitation was evident. The participants also completed a sham protocol under normocapnic conditions, which mimicked the time frame of the intermittent hypoxia protocol. After exposure to intermittent hypoxia, the therapeutic pressure was significantly reduced (i.e., 5 cmH2O) without evidence of flow limitation (103.4 ± 6.3% of baseline, P = 0.5) or increases in upper airway resistance (95.6 ± 15.0% of baseline, P = 0.6). In contrast, a similar decrease in pressure was accompanied by flow limitation (77.0 ± 1.8% of baseline, P = 0.001) and an increase in upper airway resistance (167.2 ± 17.5% of baseline, P = 0.01) after the sham protocol. Consistent with the initiation of long-term facilitation of upper airway muscle activity, exposure to intermittent hypoxia reduced the therapeutic pressure required to eliminate apneic events that could improve treatment compliance. This possibility, coupled with the potentially beneficial effects of intermittent hypoxia on comorbidities linked to sleep apnea, suggests that mild intermittent hypoxia may have a multipronged therapeutic effect on sleep apnea.

NEW & NOTEWORTHY Our new finding is that exposure to mild intermittent hypoxia reduced the therapeutic pressure required to treat sleep apnea. These findings are consistent with previous results, which have shown that long-term facilitation of upper muscle activity can be initiated following exposure to intermittent hypoxia in humans.

Impact of arousal threshold and respiratory effort on the duration of breathing events across sleep stage and time of night

PURPOSE

The frequency and duration of breathing events are influenced by sleep stage and time of day. In the present study we examined if these modifications are linked to adaptations in the arousal threshold and/or the magnitude of respiratory effort during and immediately after breathing events.

METHODS

Participants with sleep apnea slept for 3h in the evening and morning. For breathing events detected during these sessions the rate of change of respiratory effort, maximum respiratory effort immediately prior to termination of an event, and the maximum tidal volume and the minimum partial pressure of end-tidal carbon dioxide (P_ETCO₂) immediately following an event were measured.

RESULTS

The rate of change of respiratory effort was similar in N2 compared to N1 but the maximum respiratory effort immediately prior to event termination was greater (-10.7±1.2 vs. -9.6±1.0cmH₂O/s, P<0.05). Likewise, tidal volume was increased (1169±105 vs. 1082±100ml, P<0.05) and P_ETCO₂ was decreased (37.0±0.8 vs. 37.7±0.8mmHg P<0.05) following events in N2 compared to N1. A similar tidal volume and P_ETCO₂ response was evident following events in the morning compared to the evening independent of sleep stage.

CONCLUSIONS

We conclude that alterations in the arousal threshold, reflected by an increase in respiratory effort at event termination, coupled to increases in tidal volume and reductions in P_ETCO₂ contribute to modifications in event duration and frequency associated with variations in sleep state or time of night.

Sex differences in sleep disordered breathing in adults

The prevalence of sleep disordered breathing is greater in men compared to women. This disparity could be due to sex differences in the diagnosis and presentation of sleep apnea, and the pathophysiological mechanisms that instigate this disorder. Women tend to report more non-typical symptoms of sleep apnea compared to men, and the presentation of apneic events are more prevalent in rapid compared to non-rapid eye movement sleep. In addition, there is evidence of sex differences in upper airway structure and mechanics and in neural mechanisms that impact on the control of breathing. The purpose of this review is to summarize the literature that addresses sex differences in sleep-disordered breathing, and to discuss the influence that upper airway mechanics, chemoreflex properties, and sex hormones have in modulating breathing during sleep in men and women.

Clinical analysis of pharyngeal musculature and genioglossus exercising to treat obstructive sleep apnea and hypopnea syndrome

OBJECTIVE

To evaluate the effect of pharyngeal musculature and genioglossus exercising on obstructive sleep apnea and hypopnea syndrome (OSAHS).

METHODS

We conducted a non-randomized retrospective clinical trial of 75 patients with OSAHS. Fifty-four patients were managed by exercising of the pharyngeal musculature and genioglossus (exercising group). Twenty-one patients, who refused to undertake any treatment, were defined as the control group. We took the Epworth Sleepiness Scale (ESS), checked patients' polysomnography, and took 320-detector computed tomography (CT) before treatment. Six and twelve months later, we made records of apnea hypopnea index (AHI), lowest arterial oxygen saturation (LSaO2), body mass index (BMI), the shortest sagittal diameter, and transverse diameter, and the effective rates of exercising were calculated and compared with the 21 patients without any treatment (control group) at the same time. SPSS 10.0 was used to analyze the data.

RESULTS

Before treatment, the ESS value was 7.67; 6 and 12 months later, the values were 3.54 and 3.25, respectively in the exercising group. AHI was decreased to 15.36 after 6 months and 13.79 after 12 months from 22.84 at the beginning. LSaO2 values were up to 81.18% after 6 months and 81.93% after 12 months from 74.05% at the beginning. There were significant differences in ESS scores, AHI, and LSaO2 between pre-treatment and post-treatment in the exercising group (P<0.05). However, there was no statistical difference in all the parameters between 6 and 12 months of exercising. The effective rates were 70.37% and 74.07% after 6- and 12-month exercising, respectively. There were significant differences between the exercising and control groups (P<0.0001). There was no statistical difference in the effective rate of the exercising group between 6 and 12 months of exercising (P>0.05). At 12 months of exercising, the compliance of the anteroposterior pharyngeal wall of the retropalatal area was lower (P<0.01) than that before treatment. There was no significant change of BMI in either group.

CONCLUSIONS

Exercising pharyngeal musculature and genioglossus is a kind of non-invasive and cost-effective method to treat some OSAHS patients, especially those who are old, without surgical complications, and especially mild and moderate OSAHS patients who do not want to take surgery and continuous positive airway pressure (CPAP) treatment. In addition, exercising pharyngeal musculature and genioglossus can be considered as remedial treatment of OSAHS to surgery and other therapies.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Intermittent hypoxia promotes recovery of respiratory motor function in spinal cord-injured mice depleted of serotonin in the central nervous system

We examined the effect of repeated daily exposure to intermittent hypoxia (IH) on the recovery of respiratory and limb motor function in mice genetically depleted of central nervous system serotonin. Electroencephalography, diaphragm activity, ventilation, core body temperature, and limb mobility were measured in spontaneously breathing wild-type (Tph2+/+) and tryptophan hydroxylase 2 knockout (Tph2−/−) mice. Following a C2 hemisection, the mice were exposed daily to IH (i.e., twelve 4-min episodes of 10% oxygen interspersed with 4-min normoxic periods followed by a 90-min end-recovery period) or normoxia (i.e., sham protocol, 21% oxygen) for 10 consecutive days. Diaphragm activity recovered to prehemisection levels in the Tph2+/+ and Tph2−/− mice following exposure to IH but not normoxia [Tph2+/+ 1.3 ± 0.2 (SE) vs. 0.3 ± 0.2; Tph2−/− 1.06 ± 0.1 vs. 0.3 ± 0.1, standardized to prehemisection values, P < 0.01]. Likewise, recovery of tidal volume and breathing frequency was evident, although breathing frequency values did not return to prehemisection levels within the time frame of the protocol. Partial recovery of limb motor function was also evident 2 wk after spinal cord hemisection. However, recovery was not dependent on IH or the presence of serotonin in the central nervous system. We conclude that IH promotes recovery of respiratory function but not basic motor tasks. Moreover, we conclude that spontaneous or treatment-induced recovery of respiratory and motor limb function is not dependent on serotonin in the central nervous system in a mouse model of spinal cord injury.

Reduced respiratory neural activity elicits a long-lasting decrease in the CO2 threshold for apnea in anesthetized rats

Two critical parameters that influence breathing stability are the levels of arterial pCO₂ at which breathing ceases and subsequently resumes - termed the apneic and recruitment thresholds (AT and RT, respectively). Reduced respiratory neural activity elicits a chemoreflex-independent, long-lasting increase in phrenic burst amplitude, a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF). The physiological significance of iPMF is unknown. To determine if iPMF and neural apnea have long-lasting physiological effects on breathing, we tested the hypothesis that patterns of neural apnea that induce iPMF also elicit changes in the AT and RT. Phrenic nerve activity and end-tidal CO₂ were recorded in urethane-anesthetized, ventilated rats to quantify phrenic nerve burst amplitude and the AT and RT before and after three patterns of neural apnea that differed in their duration and ability to elicit iPMF: brief intermittent neural apneas, a single brief "massed" neural apnea, or a prolonged neural apnea. Consistent with our hypothesis, we found that patterns of neural apnea that elicited iPMF also resulted in changes in the AT and RT. Specifically, intermittent neural apneas progressively decreased the AT with each subsequent neural apnea, which persisted for at least 60min. Similarly, a prolonged neural apnea elicited a long-lasting decrease in the AT. In both cases, the magnitude of the AT decrease was proportional to iPMF. In contrast, the RT was transiently decreased following prolonged neural apnea, and was not proportional to iPMF. No changes in the AT or RT were observed following a single brief neural apnea. Our results indicate that the AT and RT are differentially altered by neural apnea and suggest that specific patterns of neural apnea that elicit plasticity may stabilize breathing via a decrease in the AT.

Intermittent hypoxia initiated plasticity in humans: A multipronged therapeutic approach to treat sleep apnea and overlapping co-morbidities

Over the past three decades exposure to intermittent hypoxia (IH) has generally been considered a stimulus associated with a number of detrimental outcomes. However, there is sufficient evidence to link IH to many beneficial outcomes but they have largely been ignored, particularly in the field of sleep medicine in the United States. Recent reviews have postulated that this apparent contradiction is related to the severity and duration of exposure to IH; mild forms of IH initiate beneficial outcomes while severe forms of IH are coupled to detrimental consequences. In the present review we explore the role that IH has in initiating respiratory plasticity and the potential this form of plasticity has to mitigate obstructive sleep apnea (OSA) in humans. In taking this approach, we address the possibility that IH could serve as an adjunct therapy coupled with continuous positive airway pressure (CPAP) to treat OSA. Our working hypothesis is that exposure to mild IH leads to respiratory plasticity that manifests in increased stability of the upper airway, which could ultimately reduce the CPAP required to treat OSA. In turn, this reduction could increase CPAP compliance and extend the length of treatment each night, which might improve the magnitude of outcome measures. Improved treatment compliance coupled with the direct effect that IH has on numerous overlapping conditions (i.e. asthma, chronic obstructive pulmonary disease, spinal cord injury) may well lead to substantial improvements that exceed outcomes following treatment with CPAP alone. Overall, this review will consider evidence from the published literature which suggests that IH could serve as an effective multipronged therapeutic approach to treat sleep apnea and its overlapping co-morbidities.