Respiratory-associated rhythmic firing of midbrain neurones in cats: relation to level of respiratory drive

Neurophysiological mechanisms of exertional dyspnea in post-pulmonary embolism syndrome

Following pulmonary embolism (PE), a third of patients develop persistent dyspnea, which is commonly termed the post-PE syndrome. The neurophysiological underpinnings of exertional dyspnea in patients with post-PE syndrome without pulmonary hypertension (PH) are unclear. Thus, the current study determined if abnormally high inspiratory neural drive (IND) due, in part, to residual pulmonary gas-exchange abnormalities, was linked to heightened exertional dyspnea and exercise limitation, in such patients. Fourteen participants with post-PE syndrome (without resting PH) and 14 age-, sex-, and body mass index-matched healthy controls undertook pulmonary function testing and a symptom-limited cycle cardiopulmonary exercise test with measurements of IND (diaphragmatic electromyography), ventilatory requirements for CO₂ (V̇e/V̇co₂), and perceived dyspnea intensity (modified Borg 0-10 scale). Post-PE (vs. control) had a reduced resting transfer coefficient for carbon monoxide (K_CO: 84 ± 15 vs. 104 ± 14%pred, P < 0.001) and peak oxygen uptake (V̇o_2peak) (76 ± 14 vs. 124 ± 28%pred, P < 0.001). IND and V̇e/V̇co₂ were higher in post-PE than controls at standardized submaximal work rates (P < 0.05). Dyspnea increased similarly in both groups as a function of increasing IND but was higher in post-PE at standardized submaximal work rates (P < 0.05). High IND was associated with low K_CO (r = -0.484, P < 0.001), high V̇e/V̇co₂ nadir (r = 0.453, P < 0.001), and low V̇o_2peak (r = -0.523, P < 0.001). In patients with post-PE syndrome, exercise IND was higher than controls and was associated with greater dyspnea intensity. The heightened IND and dyspnea in post-PE, in turn, were strongly associated with low resting K_CO and high exercise V̇e/V̇co₂, which suggest important pulmonary gas-exchange abnormalities in this patient population.NEW & NOTEWORTHY This study is the first to show that increased exertional dyspnea in patients with post-pulmonary embolism (PE) syndrome, without overt pulmonary hypertension, was strongly associated with elevated inspiratory neural drive (IND) to the diaphragm during exercise, compared with healthy controls. The greater IND was associated with impairments in pulmonary gas exchange and significant deconditioning. Our results help to explain why many patients with post-PE syndrome report significant dyspnea at relatively low levels of physical activity.

Dyspnea

The clinical term dyspnea (a.k.a. breathlessness or shortness of breath) encompasses at least three qualitatively distinct sensations that warn of threats to breathing: air hunger, effort to breathe, and chest tightness. Air hunger is a primal homeostatic warning signal of insufficient alveolar ventilation that can produce fear and anxiety and severely impacts the lives of patients with cardiopulmonary, neuromuscular, psychological, and end-stage disease. The sense of effort to breathe informs of increased respiratory muscle activity and warns of potential impediments to breathing. Most frequently associated with bronchoconstriction, chest tightness may warn of airway inflammation and constriction through activation of airway sensory nerves. This chapter reviews human and functional brain imaging studies with comparison to pertinent neurorespiratory studies in animals to propose the interoceptive networks underlying each sensation. The neural origins of their distinct sensory and affective dimensions are discussed, and areas for future research are proposed. Despite dyspnea's clinical prevalence and impact, management of dyspnea languishes decades behind the treatment of pain. The neurophysiological bases of current therapeutic approaches are reviewed; however, a better understanding of the neural mechanisms of dyspnea may lead to development of novel therapies and improved patient care.

The physiology and pathophysiology of exercise hyperpnea

In health, the near-eucapnic, highly efficient hyperpnea during mild-to-moderate intensity exercise is driven by three obligatory contributions, namely, feedforward central command from supra-medullary locomotor centers, feedback from limb muscle afferents, and respiratory CO₂ exchange (V̇CO₂). Inhibiting each of these stimuli during exercise elicits a reduction in hyperpnea even in the continuing presence of the other major stimuli. However, the relative contribution of each stimulus to the hyperpnea remains unknown as does the means by which V̇CO2 is sensed. Mediation of the hyperventilatory response to exercise in health is attributed to the multiple feedback and feedforward stimuli resulting from muscle fatigue. In patients with COPD, diaphragm EMG amplitude and its relation to ventilatory output are used to decipher mechanisms underlying the patients' abnormal ventilatory responses, dynamic lung hyperinflation and dyspnea during exercise. Key contributions to these exercise-limiting responses across the spectrum of COPD severity include high dead space ventilation, an excessive neural drive to breathe and highly fatigable limb muscles, together with mechanical constraints on ventilation. Major controversies concerning control of exercise hyperpnea are discussed along with the need for innovative research to uncover the link of metabolism to breathing in health and disease.

Qualitative Components of Dyspnea during Incremental Exercise across the COPD Continuum

INTRODUCTION

Evaluation of the intensity and quality of activity-related dyspnea is potentially useful in people with chronic obstructive pulmonary disease (COPD). The present study sought to examine associations between qualitative dyspnea descriptors, dyspnea intensity ratings, dynamic respiratory mechanics, and exercise capacity during cardiopulmonary exercise testing (CPET) in COPD and healthy controls.

METHODS

In this cross-sectional study, 261 patients with mild-to-very severe COPD (forced expiratory volume in 1 s, 62 ± 25%pred) and 94 age-matched controls (forced expiratory volume in 1 s, 114 ± 14%pred) completed an incremental cycle CPET to determine peak oxygen uptake (V˙O2peak). Throughout exercise, expired gases, operating lung volumes, and dyspnea intensity were assessed. At peak exercise, dyspnea quality was assessed using a modified 15-item questionnaire.

RESULTS

Logistic regression analysis revealed that among 15 dyspnea descriptors, only those alluding to the cluster "unsatisfied inspiration" were consistently associated with an increased likelihood for both critical inspiratory mechanical constraint (end-inspiratory lung volume/total lung capacity ratio ≥0.9) during exercise and reduced exercise capacity (V˙O2peak < lower limit of normal) in COPD (odds ratio (95% confidence interval), 3.26 (1.40-7.60) and 3.04 (1.24-7.45), respectively; both, P < 0.05). Thus, patients reporting "unsatisfied inspiration" (n = 177 (68%)) had an increased relative frequency of critical inspiratory mechanical constraint and low exercise capacity compared with those who did not select this descriptor, regardless of COPD severity or peak dyspnea intensity scores.

CONCLUSIONS

In patients with COPD, regardless of disease severity, reporting descriptors in the unsatisfied inspiration cluster complemented traditional assessments of dyspnea during CPET and helped identify patients with critical mechanical abnormalities germane to exercise intolerance.

Effects of wearing facemasks on the sensation of exertional dyspnea and exercise capacity in healthy subjects

Due to the currently ongoing pandemic of coronavirus disease 2019 (COVID-19), it is strongly recommended to wear facemasks to minimize transmission risk. Wearing a facemask may have the potential to increase dyspnea and worsen cardiopulmonary parameters during exercise; however, research-based evidence is lacking. We investigated the hypothesis that wearing facemasks affects the sensation of dyspnea, pulse rate, and percutaneous arterial oxygen saturation during exercise. Healthy adults (15 men, 9 women) underwent a progressive treadmill test under 3 conditions in randomized order: wearing a surgical facemask, cloth facemask, or no facemask. Experiment was carried out once daily under each condition, for a total of 3 days. Each subject first sat on a chair for 30 minutes, then walked on a treadmill according to a Bruce protocol that was modified by us. The experiment was discontinued when the subject’s pulse rate exceeded 174 beats/min. After discontinuation, the subject immediately sat on a chair and was allowed to rest for 10 minutes. Subjects were required to rate their levels of dyspnea perception on a numerical scale. Pulse rate and percutaneous arterial oxygen saturation were continuously monitored with a pulse oximeter. These parameters were recorded in each trial every 3 minutes after the start of the exercise; the point of discontinuation; and 5 and 10 minutes after discontinuation. The following findings were obtained. Wearing a facemask does not worsen dyspnea during light to moderate exercise but worsens dyspnea during vigorous exercise. Wearing a cloth facemask increases dyspnea more than wearing a surgical facemask during exercise and increases pulse rate during vigorous exercise, but it does not increase pulse rate during less vigorous exercise. Wearing a surgical facemask does not increase pulse rate at any load level. Lastly, wearing a facemask does not affect percutaneous arterial oxygen saturation during exercise at any load level regardless of facemask type.

Multi-Level Regulation of Opioid-Induced Respiratory Depression

Opioids depress minute ventilation primarily by reducing respiratory rate. This results from direct effects on the preBötzinger Complex as well as from depression of the Parabrachial/Kölliker-Fuse Complex, which provides excitatory drive to preBötzinger Complex neurons mediating respiratory phase-switch. Opioids also depress awake drive from the forebrain and chemodrive.

Mapping dependencies of BOLD signal change to end-tidal CO2: linear and nonlinear modeling, and effect of physiological noise correction

BACKGROUND

Disentangling physiological noise and signal of interest is a major issue when evaluating BOLD-signal changes in response to breath holding. Currently-adopted approaches for retrospective noise correction are general-purpose, and have non-negligible effects in studies on hypercapnic challenges.

NEW METHOD

We provide a novel approach to the analysis of specific and non-specific BOLD-signal changes related to end-tidal CO₂ (P_ETCO₂) in breath-hold fMRI studies. Multiple-order nonlinear predictors for P_ETCO₂ model a region-dependent nonlinear input-output relationship hypothesized in literature and possibly playing a crucial role in disentangling noise. We explore Retrospective Image-based Correction (RETROICOR) effects on the estimated BOLD response, applying our analysis both with and without RETROICOR and analyzing the linear and non-linear correlation between P_ETCO₂ and RETROICOR regressors.

RESULTS

The RETROICOR model of noise related to respiratory activity correlated with P_ETCO₂ both linearly and non-linearly. The correction affected the shape of the estimated BOLD response to hypercapnia but allowed to discard spurious activity in ventricles and white matter. Activation clusters were best detected using non-linear components in the BOLD response model.

COMPARISON WITH EXISTING METHOD

We evaluated the side-effects of standard physiological noise correction procedure, tailoring our analysis on challenging understudied brainstem and subcortical regions. Our novel approach allowed to characterize delays and non-linearities in BOLD response.

CONCLUSIONS

RETROICOR successfully avoided false positives, still broadly affecting the estimated non-linear BOLD responses. Non-linearities in the model better explained CO₂-related BOLD signal fluctuations. The necessity to modify the standard procedure for physiological-noise correction in breath-hold studies was addressed, stating its crucial importance.

Air Hunger: A Primal Sensation and a Primary Element of Dyspnea

The sensation that develops as a long breath hold continues is what this article is about. We term this sensation of an urge to breathe "air hunger." Air hunger, a primal sensation, alerts us to a failure to meet an urgent homeostatic need maintaining gas exchange. Anxiety, frustration, and fear evoked by air hunger motivate behavioral actions to address the failure. The unpleasantness and emotional consequences of air hunger make it the most debilitating component of clinical dyspnea, a symptom associated with respiratory, cardiovascular, and metabolic diseases. In most clinical populations studied, air hunger is the predominant form of dyspnea (colloquially, shortness of breath). Most experimental subjects can reliably quantify air hunger using rating scales, that is, there is a consistent relationship between stimulus and rating. Stimuli that increase air hunger include hypercapnia, hypoxia, exercise, and acidosis; tidal expansion of the lungs reduces air hunger. Thus, the defining experimental paradigm to evoke air hunger is to elevate the drive to breathe while mechanically restricting ventilation. Functional brain imaging studies have shown that air hunger activates the insular cortex (an integration center for perceptions related to homeostasis, including pain, food hunger, and thirst), as well as limbic structures involved with anxiety and fear. Although much has been learned about air hunger in the past few decades, much remains to be discovered, such as an accepted method to quantify air hunger in nonhuman animals, fundamental questions about neural mechanisms, and adequate and safe methods to mitigate air hunger in clinical situations. © 2021 American Physiological Society. Compr Physiol 11:1449-1483, 2021.

Mechanisms underlying the sensation of dyspnea

Dyspnea is defined as a subjective experience of breathing discomfort that consists of qualitatively distinct sensations that vary in intensity. It is a common symptom among patients with respiratory diseases that reduces daily activities, induces deconditioning, and is self-perpetuating. Although clinical interventions are needed to reduce dyspnea, its underlying mechanism is poorly understood depending on the intertwined peripheral and central neural mechanisms as well as emotional factors. Nonetheless, experimental and clinical observations suggest that dyspnea results from dissociation or a mismatch between the intended respiratory motor output set caused by the respiratory neuronal network in the lower brainstem and the ventilatory output accomplished. The brain regions responsible for detecting the mismatch between the two are not established. The mechanism underlying the transmission of neural signals for dyspnea to higher sensory brain centers is not known. Further, information from central and peripheral chemoreceptors that control the milieu of body fluids is summated at higher brain centers, which modify dyspneic sensations. The mental status also affects the sensitivity to and the threshold of dyspnea perception. The currently used methods for relieving dyspnea are not necessarily fully effective. The search for more effective therapy requires further insights into the pathophysiology of dyspnea.

Dyspnea in COPD: New Mechanistic Insights and Management Implications

Dyspnea is the most common symptom experienced by patients with chronic obstructive pulmonary disease (COPD). To avoid exertional dyspnea, many patients adopt a sedentary lifestyle which predictably leads to extensive skeletal muscle deconditioning, social isolation, and its negative psychological sequalae. This "dyspnea spiral" is well documented and it is no surprise that alleviation of this distressing symptom has become a key objective highlighted across COPD guidelines. In reality, this important goal is often difficult to achieve, and successful symptom management awaits a clearer understanding of the underlying mechanisms of dyspnea and how these can be therapeutically manipulated for the patients' benefit. Current theoretical constructs of the origins of activity-related dyspnea generally endorse the classical demand-capacity imbalance theory. Thus, it is believed that disruption of the normally harmonious relationship between inspiratory neural drive (IND) to breathe and the simultaneous dynamic response of the respiratory system fundamentally shapes the expression of respiratory discomfort in COPD. Sadly, the symptom of dyspnea cannot be eliminated in patients with advanced COPD with relatively fixed pathophysiological impairment. However, there is evidence that effective symptom palliation is possible for many. Interventions that reduce IND, without compromising alveolar ventilation (VA), or that improve respiratory mechanics and muscle function, or that address the affective dimension, achieve measurable benefits. A common final pathway of dyspnea relief and improved exercise tolerance across the range of therapeutic interventions (bronchodilators, exercise training, ambulatory oxygen, inspiratory muscle training, and opiate medications) is reduced neuromechanical dissociation of the respiratory system. These interventions, singly and in combination, partially restore more harmonious matching of excessive IND to ventilatory output achieved. In this review we propose, on the basis of a thorough review of the recent literature, that effective dyspnea amelioration requires combined interventions and a structured multidisciplinary approach, carefully tailored to meet the specific needs of the individual.

Hypothesis: Pulmonary Afferent Activity Patterns During Slow, Deep Breathing Contribute to the Neural Induction of Physiological Relaxation

Control of respiration provides a powerful voluntary portal to entrain and modulate central autonomic networks. Slowing and deepening breathing as a relaxation technique has shown promise in a variety of cardiorespiratory and stress-related disorders, but few studies have investigated the physiological mechanisms conferring its benefits. Recent evidence suggests that breathing at a frequency near 0.1 Hz (6 breaths per minute) promotes behavioral relaxation and baroreflex resonance effects that maximize heart rate variability. Breathing around this frequency appears to elicit resonant and coherent features in neuro-mechanical interactions that optimize physiological function. Here we explore the neurophysiology of slow, deep breathing and propose that coincident features of respiratory and baroreceptor afferent activity cycling at 0.1 Hz entrain central autonomic networks. An important role is assigned to the preferential recruitment of slowly-adapting pulmonary afferents (SARs) during prolonged inhalations. These afferents project to discrete areas in the brainstem within the nucleus of the solitary tract (NTS) and initiate inhibitory actions on downstream targets. Conversely, deep exhalations terminate SAR activity and activate arterial baroreceptors via increases in blood pressure to stimulate, through NTS projections, parasympathetic outflow to the heart. Reciprocal SAR and baroreceptor afferent-evoked actions combine to enhance sympathetic activity during inhalation and parasympathetic activity during exhalation, respectively. This leads to pronounced heart rate variability in phase with the respiratory cycle (respiratory sinus arrhythmia) and improved ventilation-perfusion matching. NTS relay neurons project extensively to areas of the central autonomic network to encode important features of the breathing pattern that may modulate anxiety, arousal, and attention. In our model, pronounced respiratory rhythms during slow, deep breathing also support expression of slow cortical rhythms to induce a functional state of alert relaxation, and, via nasal respiration-based actions on olfactory signaling, recruit hippocampal pathways to boost memory consolidation. Collectively, we assert that the neurophysiological processes recruited during slow, deep breathing enhance the cognitive and behavioral therapeutic outcomes obtained through various mind-body practices. Future studies are required to better understand the physio-behavioral processes involved, including in animal models that control for confounding factors such as expectancy biases.

On the Use of Linear-Modelling-based Algorithms for Physiological Noise Correction in fMRI Studies of the Central Breathing Control

A full characterization of the physiological behavior of human central chemoreceptors through fMRI is crucial to understand the pathophysiology of central abnormal breathing patterns. In this scenario, physiological noise and activity of interest may be naturally correlated. Here, we examined the adequacy of linear-modelling-based retrospective physiological noise correction for studies of the central breathing control. We focused on the relationship between a nonlinear model of BOLD response, hypothesized to describe neuronal specific activity, and noise modelled by correction algorithms. Analyses were performed on fMRI acquisitions from healthy subjects during a breath hold task. A general linear model including static nonlinearities in the response to end-tidal CO₂ was applied to data preprocessed both with and without physiological noise correction. Relations between physiological noise and PETCO₂ were explored both with linear and nonlinear measures. Lastly, parametric maps of noise spatial distribution were extracted. Our results evidenced that correction algorithms based on linear modelling remove components that are both linearly and nonlinearly related to end-tidal CO₂, whereas uncorrected data showed spurious activations in regions outside gray matter. Thus, despite a correction step is fundamental, these algorithms are shown to be over-conservative approaches to noise correction and need to be adapted to the specific purpose.

Exploring the supra linear relationship between PetCO2 and fMRI signal change with ICA

The relationships between brain functions and the respiratory system are complex. Disentangling brain activity related to CO₂ changes from nonspecific vasoreactivity is a challenge when studying brain activity involved in the control of breathing with fMRI. In this work, we analyzed a dose dependent relationship between arterial CO₂ levels and brain response. To accomplish this goal, we developed a gas administration protocol, together with multi-subject ICA and specific nonlinear post-processing analysis. Our results highlighted a supra-linear response to CO₂ challenges in brainstem, thalamus and putamen. Results were discussed in the light of current knowledge about the central respiratory network.

The midbrain periaqueductal gray as an integrative and interoceptive neural structure for breathing

The periaqueductal gray (PAG) plays a critical role in autonomic function and behavioural responses to threatening stimuli. Recent evidence has revealed the PAG's potential involvement in the perception of breathlessness, a highly threatening respiratory symptom. In this review, we outline the current evidence in animals and humans on the role of the PAG in respiratory control and in the perception of breathlessness. While recent work has unveiled dissociable brain activity within the lateral PAG during perception of breathlessness and ventrolateral PAG during conditioned anticipation in healthy humans, this is yet to be translated into diseases dominated by breathlessness symptomology, such as chronic obstructive pulmonary disease. Understanding how the sub-structures of the PAG differentially interact with interoceptive brain networks involved in the perception of breathlessness will help towards understanding discordant symptomology, and may reveal treatment targets for those debilitated by chronic and pervasive breathlessness.

Mirtazapine for chronic breathlessness? A review of mechanistic insights and therapeutic potential

INTRODUCTION

Chronic breathlessness is a common and distressing symptom of advanced disease with few effective treatments. Central nervous system mechanisms are important in respiratory sensation and control. Consequently, drugs which may modify processing and perception of afferent information in the brain may have a role. Antidepressants have been proposed; however, current evidence is limited. Of potentially suitable antidepressants, mirtazapine is an attractive option given its tolerability profile, low cost, and wide availability, along with additional potential benefits. Areas covered: The paper provides an overview of the physiology of breathlessness, with an emphasis on central mechanisms, particularly the role of fear circuits and the associated neurotransmitters. It provides a potential rationale for how mirtazapine may improve chronic breathlessness and quality of life in patients with advanced disease. The evidence was identified by a literature search performed in PubMed through to October 2018. Expert opinion: Currently, there is insufficient evidence to support the routine use of antidepressants for chronic breathlessness in advanced disease. Mirtazapine is a promising candidate to pursue, with definitive randomized controlled trials required to determine its efficacy and safety in this setting.

Breathing control center neurons that promote arousal in mice

Slow, controlled breathing has been used for centuries to promote mental calming, and it is used clinically to suppress excessive arousal such as panic attacks. However, the physiological and neural basis of the relationship between breathing and higher-order brain activity is unknown. We found a neuronal subpopulation in the mouse preBötzinger complex (preBötC), the primary breathing rhythm generator, which regulates the balance between calm and arousal behaviors. Conditional, bilateral genetic ablation of the ~175 Cdh9/Dbx1 double-positive preBötC neurons in adult mice left breathing intact but increased calm behaviors and decreased time in aroused states. These neurons project to, synapse on, and positively regulate noradrenergic neurons in the locus coeruleus, a brain center implicated in attention, arousal, and panic that projects throughout the brain.

Breathing control center neurons that promote arousal in mice

Slow, controlled breathing has been used for centuries to promote mental calming, and it is used clinically to suppress excessive arousal such as panic attacks. However, the physiological and neural basis of the relationship between breathing and higher-order brain activity is unknown. We found a neuronal subpopulation in the mouse preBötzinger complex (preBötC), the primary breathing rhythm generator, which regulates the balance between calm and arousal behaviors. Conditional, bilateral genetic ablation of the ~175 Cdh9/Dbx1 double-positive preBötC neurons in adult mice left breathing intact but increased calm behaviors and decreased time in aroused states. These neurons project to, synapse on, and positively regulate noradrenergic neurons in the locus coeruleus, a brain center implicated in attention, arousal, and panic that projects throughout the brain.

The nervous system of airways and its remodeling in inflammatory lung diseases

Inflammatory lung diseases are associated with bronchospasm, cough, dyspnea and airway hyperreactivity. The majority of these symptoms cannot be primarily explained by immune cell infiltration. Evidence has been provided that vagal efferent and afferent neurons play a pivotal role in this regard. Their functions can be altered by inflammatory mediators that induce long-lasting changes in vagal nerve activity and gene expression in both peripheral and central neurons, providing new targets for treatment of pulmonary inflammatory diseases.

Changes in cerebral vascular reactivity and structure following prolonged exposure to high altitude in humans

Although high‐altitude exposure can lead to neurocognitive impairment, even upon return to sea level, it remains unclear the extent to which brain volume and regional cerebral vascular reactivity (CVR) are altered following high‐altitude exposure. The purpose of this study was to simultaneously determine the effect of 3 weeks at 5050 m on: (1) structural brain alterations; and (2) regional CVR after returning to sea level for 1 week. Healthy human volunteers (n = 6) underwent baseline and follow‐up structural and functional magnetic resonance imaging (MRI) at rest and during a CVR protocol (end‐tidal PCO₂ reduced by −10, −5 and increased by +5, +10, and +15 mmHg from baseline). CVR maps (% mmHg⁻¹) were generated using BOLD MRI and brain volumes were estimated. Following return to sea level, whole‐brain volume and gray matter volume was reduced by 0.4 ± 0.3% (P < 0.01) and 2.6 ± 1.0% (P < 0.001), respectively; white matter was unchanged. Global gray matter CVR and white matter CVR were unchanged following return to sea level, but CVR was selectively increased (P < 0.05) in the brainstem (+30 ± 12%), hippocampus (+12 ± 3%), and thalamus (+10 ± 3%). These changes were the result of improvement and/or reversal of negative CVR to positive CVR in these regions. Three weeks of high‐altitude exposure is reflected in loss of gray matter volume and improvements in negative CVR.

The midbrain periaqueductal gray changes the eupneic respiratory rhythm into a breathing pattern necessary for survival of the individual and of the species

Modulation of respiration is a prerequisite for survival of the individual and of the species. For example, respiration has to be adjusted in case of speech, strenuous exercise, laughing, crying, or sudden escape from danger. Respiratory centers in pons and medulla generate the basic respiratory rhythm or eupnea, but they cannot modulate breathing in the context of emotional challenges, for which they need input from higher brain centers. In simple terms, the prefrontal cortex integrates visual, auditory, olfactory, and somatosensory information and informs subcortical structures such as amygdala, hypothalamus, and finally the midbrain periaqueductal gray (PAG) about the results. The PAG, in turn, generates the final motor output for basic survival, such as setting the level of all cells in the brain and spinal cord. Best known in this framework is determining the level of pain perception. The PAG also controls heart rate, blood pressure, micturition, sexual behavior, vocalization, and many other basic motor output systems. Within this context, the PAG also changes the eupneic respiratory rhythm into a breathing pattern necessary for basic survival. This review examines the latest developments regarding of how the PAG controls respiration.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Chemoreception and asphyxia-induced arousal

Arousal protects against the adverse and potentially fatal effects of asphyxia during sleep. Asphyxia stimulates the carotid bodies and central chemoreceptors but the sequence of events leading to arousal is uncertain. In this review, the theoretical mechanisms leading to arousal from sleep are briefly summarized and the issue of whether central respiratory chemoreceptors (CRCs) or other types of CO2-responsive CNS neurons contribute to asphyxia-induced arousal is discussed. We focus on the role of the retrotrapezoid nucleus, the raphe and the locus coeruleus and emphasize the anatomical and neurophysiological evidence which suggests that these putative central chemoreceptors could contribute to arousal independently of their effects on breathing. Finally, we describe recent attempts to test the contribution of specific brainstem pathways to asphyxia-induced arousal using optogenetic and other tools and the possible contribution of a group of hypoxia-sensitive brainstem neurons (the C1 cells) to breathing and arousal.

Lung/chest expansion contributes to generation of pleasantness associated with dyspnoea relief

Pleasantness associated with dyspnoea relief or 'respiratory pleasure' is considered as a particular sensory experience. The purpose of this study is to elucidate the mechanism of generation of this particular sensory experience. After taking deep breaths during normal breathing, 35 healthy subjects received three different magnitudes of inspiratory loads (light: 8.4; moderate: 23.4; severe: 70.5 cm H2O/L/s) to induce dyspnoeic sensation. We found that (1) deep breaths during normal breathing rarely induce 'respiratory pleasure', (2) a sudden removal of dyspnoea alone is not sufficient to produce 'respiratory pleasure', and (3) the generation of 'respiratory pleasure' can be observed when a sudden removal of dyspnoea accompanies a large increase in tidal volume (V(T)). In addition, qualitative assessment of 'respiratory pleasure' showed that this sensation is compatible with a strong, positively valenced sensation. These findings indicate that an increase in V(T) after removal of respiratory loading plays a crucial role in generation of 'respiratory pleasure' that is a specific sensory-emotional experience.

Respiratory Consequences of Mild-to-Moderate Obesity: Impact on Exercise Performance in Health and in Chronic Obstructive Pulmonary Disease

In many parts of the world, the prevalence of obesity is increasing at an alarming rate. The association between obesity, multiple comorbidities, and increased mortality is now firmly established in many epidemiological studies. However, the link between obesity and exercise intolerance is less well studied and is the focus of this paper. Although exercise limitation is likely to be multifactorial in obesity, it is widely believed that the respiratory mechanical constraints and the attendant dyspnea are important contributors. In this paper, we examined the evidence that critical ventilatory constraint is a proximate source of exercise limitation in individuals with mild-to-moderate obesity. We first reviewed existing information on exercise performance, including ventilatory and perceptual response patterns, in obese individuals who are otherwise healthy. We then considered the impact of obesity in patients with preexisting respiratory mechanical abnormalities due to chronic obstructive pulmonary disease (COPD), with particular reference to the effect on dyspnea and exercise performance. Our main conclusion, based on the existing and rather sparse literature on the subject, is that abnormalities of dynamic respiratory mechanics are not likely to be the dominant source of dyspnea and exercise intolerance in otherwise healthy individuals or in patients with COPD with mild-to-moderate obesity.

Exercise and its impact on dyspnea

Dyspnea is a subjective experience of breathing discomfort that can limit the ability and motivation to perform exercise or exertion. It is a common problem that affects specific groups of patients, such as, those suffering from chronic obstructive pulmonary disease, congestive heart failure, and interstitial lung disease, and in healthy humans during aging, pregnancy, and obesity. In this review, the current mechanistic model of exertional dyspnea is summarized and new research demonstrating how treatment strategies improve dyspnea by reducing central ventilatory drive, improving dynamic ventilatory mechanics, and improving respiratory muscle function is highlighted. Lastly, we review the effects of healthy aging and recent evidence for a male-female difference with respect to exertional-related dyspnea.

Voluntary and involuntary ventilation do not alter the human inspiratory muscle loading reflex

The reflex mechanism of the short-latency inhibitory reflex to transient loading of human inspiratory muscles is unresolved. Muscle afferents mediate this reflex, but they may act via pontomedullary inspiratory centers, other bulbar networks, or spinal circuits. We hypothesized that altered chemical drive to breathe would alter the initial inhibitory reflex if the neural pathways involve inspiratory medullary centers. Inspiration was transiently loaded in 11 subjects during spontaneous hypercapnic hyperpnea and matched voluntary hyperventilation. Electromyographic activity was recorded bilaterally from scalene muscles with surface electrodes. The latencies of the initial inhibitory response (IR) onset (32 ± 0.7 and 38 ± 1 ms for spontaneous and voluntary conditions respectively, P < 0.001) and subsequent excitatory response (ER) onset (80 ± 2.9 and 78 ± 2.6 ms, respectively, P = 0.46) and the normalized sizes of IR (65 ± 2 and 67 ± 3%, respectively, P = 0.50) and ER (51 ± 8 and 69 ± 6%, respectively, P = 0.005) were measured. Mean end-tidal Pco2 was 43 ± 1.5 Torr with dead space ventilation and was 14 ± 0.6 Torr with matched voluntary hyperventilation ( P < 0.001). A mean minute volume >30 liters was achieved in both conditions. The absence of significant difference in the size of the IR suggested that the IR reflex arc does not transit the brain stem inspiratory centers and that the reflex may be integrated at a spinal level. In voluntary hyperventilation, an initial excitation occurred more frequently and, consequently, the IR onset latency was significantly longer. The size of the later ER was also greater during voluntary hyperventilation, which is consistent with it being mediated via longer, presumably cortical, pathways, which are influenced by voluntary drive.

Mechanisms of dyspnea in healthy subjects

Dyspnea is a general term used to characterize a range of different descriptors; it varies in intensity, and is influenced by a wide variety of factors such as cultural expectations and the patient's experiences. Healthy subjects can experience dyspnea in different situations, e.g. at high altitude, after breath-holding, during stressful situations that cause anxiety or panic, and more commonly during strenuous exercise. Discussing the mechanisms of dyspnea we need to briefly take into account the physiological mechanisms underlying the sensation of dyspnea: the functional status of the respiratory muscles, the role of chemoreceptors and mechanoreceptors, and how the sense of respiratory motor output reaches a level of conscious awareness. We also need to take into account theories on the pathophysiological mechanisms of the sensation of dyspnea and the possibility that each pathophysiological mechanism produces a distinct quality of breathing discomfort. The terms used by subjects to identify different characteristics of breathing discomfort - dyspnea descriptors - may contribute to understanding the mechanisms of dyspnea and providing the rationale for a specific diagnosis.

Qualitative aspects of exertional dyspnea in patients with restrictive lung disease

Restrictive lung disease is a broad term encompassing a number of conditions in which lung volumes are reduced. Dyspnea is a common clinical manifestation of restrictive lung disease and frequently becomes a prominent and disabling symptom that undermines patients' ability to function and engage in activities of daily living (especially in those with more advanced restriction). Effective management of this disabling symptom awaits a better understanding of its underlying physiology. In recent decades, our understanding of the mechanisms of dyspnea in restrictive lung disease has been improved by a small, but significant, body of research. One approach to the study of dyspnea is to identify the major qualitative dimensions of the symptom in an attempt to uncover different underlying neurophysiologic mechanisms. This article will review the existing literature on the intensity and qualitative dimensions of dyspnea during exercise in patients with restrictive lung disease. The main focus will be on interstitial lung disease (ILD), since it is the prototypical restrictive disease.

Dyspnea and Activity Limitation in COPD: Mechanical Factors

Dyspnea and activity limitation are the primary symptoms of chronic obstructive pulmonary disease and progress relentlessly as the disease advances. In COPD, dyspnea is multifactorial but abnormal dynamic ventilatory mechanics are believed to be important. Dynamic lung hyperinflation occurs during exercise in the majority of flow-limited patients with chronic obstructive pulmonary disease and may have serious sensory and mechanical consequences. This proposition is supported by several studies, which have shown a close correlation between indices of dynamic lung hyperinflation and measures of both exertional dyspnea and exercise performance. The strength of this association has been further confirmed by studies that have therapeutically manipulated this dependent variable. Relief of exertional dyspnea and improved exercise endurance following bronchodilator therapy correlate well with reduced lung hyperinflation. The mechanisms by which dynamic lung hyperinflation give rise to exertional dyspnea and exercise intolerance are complex. However, recent mechanistic studies suggest that dynamic lung hyperinflation-induced volume restriction and consequent neuromechanical uncoupling of the respiratory system are key mechanisms. This review examines, in some detail, the derangements of ventilatory mechanics that are peculiar to chronic obstructive pulmonary disease and attempts to provide a mechanistic rationale for the attendant respiratory discomfort and activity limitation.

Role of chemoreceptors in mediating dyspnea

Dyspnea, or the uncomfortable awareness of respiratory distress, is a common symptom experienced by most people at some point during their lifetime. It is commonly encountered in individuals with pulmonary disease, such as chronic obstructive pulmonary disease (COPD), but can also be seen in healthy individuals after strenuous exercise, at altitude or in response to psychological stress. Dyspnea is a multifactorial sensation involving the brainstem, cortex, and limbic system, as well as mechanoreceptors, irritant receptors and chemoreceptors. Chemoreceptors appear to contribute to the sensation of dyspnea in two ways. They stimulate the respiratory control system in response to hypoxia and/or hypercapnia, and the resultant increase respiratory motor output can be consciously perceived as unpleasant. They also can induce the sensation of dyspnea through an as yet undetermined mechanism-potentially via direct ascending connections to the limbic system and cortex. The goal of this article is to briefly review how changes in blood gases reach conscious awareness and how chemoreceptors are involved in dyspnea.

Pathophysiology of dyspnea evaluated by breath-holding test: studies of furosemide treatment

Breath-holding is one of the most powerful methods to induce the dyspneic sensation, and the breath-holding test gives us much information on the onset and endurance of dyspnea. In conscious subjects, immediately after the start of breath-holding at functional residual capacity (FRC), there is a certain period of no particular respiratory sensation lasting for 20-30s, which is designated "no respiratory sensation period". This period is terminated by the onset of dyspnea and followed by a progressive increase in the intensity of dyspnea until the breaking point of breath-holding. The measurement of the period of no respiratory sensation provides us with information about the threshold of dyspneic sensation whereas the measurement of the total breath-holding time is a behavioral measure of the tolerable limit of dyspneic sensation. The behavioral measure of tolerable limit of dyspnea can permit the study of dyspnea even in anesthetized animals while observing escape behavior in response to airway occlusion. Inhaled furosemide causes prolongation of both the period of no respiratory sensation and total breath-holding time in conscious subjects, indicating that inhaled furosemide alleviates experimentally induced dyspnea. Alleviation of dyspnea with inhaled furosemide in conscious subjects is also consistent with the result of animal studies in which inhaled furosemide suppresses the escape behavior in the lightly anesthetized condition. The purpose of this article is to emphasize the usefulness of breath-holding test as a tool for evaluation of dyspnea. Furthermore, the possible mechanisms of alleviation of dyspnea with inhaled furosemide are highlighted.

Mechanical ventilatory constraints during incremental cycle exercise in human pregnancy: implications for respiratory sensation

The aim of this study was to identify the physiological mechanisms of exertional respiratory discomfort (breathlessness) in pregnancy by comparing ventilatory (breathing pattern, airway function, operating lung volumes, oesophageal pressure (P(oes))-derived indices of respiratory mechanics) and perceptual (breathlessness intensity) responses to incremental cycle exercise in 15 young, healthy women in the third trimester (TM(3); between 34 and 38 weeks gestation) and again 4-5 months postpartum (PP). During pregnancy, resting inspiratory capacity (IC) increased (P < 0.01) and end-expiratory lung volume decreased (P < 0.001), with no associated change in total lung capacity (TLC) or static respiratory muscle strength. This permitted greater tidal volume (V(T)) expansion throughout exercise in TM(3), while preserving the relationship between contractile respiratory muscle effort (tidal P(oes) swing expressed as a percentage of maximum inspiratory pressure (P(Imax))) and thoracic volume displacement (V(T) expressed as a percentage of vital capacity) and between breathlessness and ventilation . At the highest equivalent work rate (HEWR = 128 +/- 5 W) in TM(3) compared with PP: , tidal P(oes)/P(Imax) and breathlessness intensity ratings increased by 10.2 l min(-1) (P < 0.001), 8.8%P(Imax) (P < 0.05) and 0.9 Borg units (P < 0.05), respectively. Pulmonary resistance was not increased at rest or during exercise at the HEWR in TM(3), despite marked increases in mean tidal inspiratory and expiratory flow rates, suggesting increased bronchodilatation. Dynamic mechanical constraints on V(T) expansion (P < 0.05) with associated increased breathlessness intensity ratings (P < 0.05) were observed near peak exercise in TM(3) compared with PP.

IN CONCLUSION

(1) pregnancy-induced increases in exertional breathlessness reflected the normal awareness of increased and contractile respiratory muscle effort; (2) mechanical adaptations of the respiratory system, including recruitment of resting IC and increased bronchodilatation, accommodated the increased V(T) while preserving effort-displacement and breathlessness-VE relationships; and (3) dynamic mechanical ventilatory constraints contributed to respiratory discomfort near the limits of tolerance in late gestation.

The multiple dimensions of dyspnea: review and hypotheses

Although dyspnea is a common and troubling symptom, our understanding of the neurophysiology of dyspnea is woefully incomplete. Most measurements of dyspnea treat it as a single entity. Although the multidimensional dyspnea concept has been mentioned for many decades, only recently has the concept been the subject of experimental tests. Emerging evidence has begun to favor the hypothesis that dyspnea comprises multiple dimensions or components that can be measured as different entities. Most recently, studies have begun to show that there is a separable 'affective dimension' (i.e. unpleasantness and emotional impact). Understanding of the multidimensional measurement of pain is far in advance of dyspnea, and has enabled progress in the neurophysiology of pain, including identification of separate neural structures subserving various elements of pain perception. We propose here a multidimensional model of dyspnea based on a state-of-the-art pain model, and review existing evidence in the light of this model.

Ventilatory constraints and dyspnea during exercise in chronic obstructive pulmonary disease

Dyspnea (respiratory difficulty) and activity limitation are the primary symptoms of chronic obstructive pulmonary disease (COPD) and progress relentlessly as the disease advances, contributing to reduced quality of life. In COPD, the mechanisms of dyspnea are multifactorial, but abnormal dynamic ventilatory mechanics are believed to play a central role. In flow-limited patients with COPD, dynamic lung hyperinflation (DH) occurs during exercise and has serious sensory and mechanical consequences. In several studies, indices of DH strongly correlate with ratings of dyspnea intensity during exercise, and strategies that reduce resting hyperinflation (either pharmacological or surgical) consistently result in reduced exertional dyspnea. The mechanisms by which DH gives rise to exertional dyspnea and exercise intolerance are complex, but recent mechanistic studies suggest that DH-induced inspiratory muscle loading, restriction of tidal volume expansion during exercise, and consequent neuromechanical uncoupling of the respiratory system are key components. This review examines the specific derangements of ventilatory mechanics that occur in COPD during exercise and attempts to provide a mechanistic rationale for the attendant respiratory discomfort and activity limitation.

The clinical importance of dynamic lung hyperinflation in COPD

Lung hyperinflation commonly accompanies expiratory flow-limitation in patients with Chronic Obstructive Pulmonary Disease (COPD) and contributes importantly to dyspnea and activity limitation. It is not surprising, therefore, that lung hyperinflation has become an important therapeutic target in symptomatic COPD patients. There is increasing evidence that acute dynamic increases in lung hyperinflation, under conditions of worsening expiratory flow-limitation and increased ventilatory demand (or both) can seriously stress cardiopulmonary reserves, particularly in patients with more advanced disease. Our understanding of the physiological mechanisms of dynamic lung hyperinflation during both physical activity and exacerbations in COPD continues to grow, together with an appreciation of its serious negative mechanical and sensory consequences. In this review, we will discuss the basic pathophysiology of COPD during rest, exercise and exacerbation so as to better understand how this can be pharmacologically manipulated for the patient's benefit. Finally, we will review current concepts of the mechanisms of symptom relief and improved exercise endurance following pharmacological lung volume reduction.

Effect of CPAP treatment on inspiratory arousal threshold during NREM sleep in OSAS

The maximal inspiratory effort recorded at the end of apnea has been considered as an index of arousal threshold in obstructive sleep apnea syndrome (OSAS). Previous investigations have shown that the arousal threshold is higher in patients with OSAS than in normal subjects. The aim of the present study was to investigate the effect of continuous positive airway pressure (CPAP) treatment on the inspiratory-effort-related arousal threshold in patients with OSAS. In ten male patients, 40 episodes of apnea during stage 2 non-REM (NREM) sleep were analyzed. Apnea duration (t), esophageal pressure (Pes) at the first occluded breath (Pes1), the minimum of the three initial Pes swings (Pes min), the maximum of the three final Pes swings (Pes Max), DeltaPes (Pes Max-Pes min), RPes (rate of increase of intrathoracic pressure, DeltaPes/t), n (number of occluded breaths during apnea), DeltaPes/n, n/t, and SaO(2) were determined before and after occlusion. These apneic episodes were compared to ten episodes of apnea provoked by a mask occlusion device after 1, 7, 30, and 90 days of CPAP treatment. The therapy resulted in a decrease in the inspiratory-effort-related arousal threshold, as measured by a reduction of Pes Max, without significant changes in apnea duration and apnea-related hypoxemia. Pes1 and DeltaPes/n, which are markers of respiratory drive, significantly decreased between observations. CPAP treatment decreases the inspiratory-effort-related arousal threshold and induces a decrease in ventilatory drive in response to upper airway occlusion.

Cortical substrates for the perception of dyspnea

Dyspnea is a common, unpleasant, and impairing symptom in various respiratory diseases and other diseases. Despite growing understanding of the multiple peripheral mechanisms giving rise to dyspnea, little is known about the cortical mechanisms underlying its perception. The results of neuroimaging studies have shown that distinct brain areas process the dyspneic sensation, among which the anterior insular seems to be the most important. Based on the findings of the first relevant neuroimaging studies, this review describes the cortical structures associated with the perception of dyspnea. Moreover, similarities to the perception of pain are discussed, and implications for future research are provided.

Theta oscillations and sensorimotor performance

Performance and cognitive effort in humans have recently been related to amplitude and multisite coherence of alpha (7-12 Hz) and theta (4-7 Hz) band electroencephalogram oscillations. I examined this phenomenon in rats by using theta band oscillations of the local field potential to signify sniffing as a sensorimotor process. Olfactory bulb (OB) theta oscillations are coherent with those in the dorsal hippocampus (HPC) during odor sniffing in a two-odor olfactory discrimination task. Coherence is restricted to the high-frequency theta band (6-12 Hz) associated with directed sniffing in the OB and type 1 theta in the HPC. Coherence and performance fluctuate on a time scale of several minutes. Coherence magnitude is positively correlated with performance in the two-odor condition but not in extended runs of single odor conditional-stimulus-positive trials. Simultaneous with enhanced OB-HPC theta band coherence during odor sniffing is a significant decrease in lateral entorhinal cortex (EC)-HPC and OB-EC coherence, suggesting that linkage of the olfactory and hippocampal theta rhythms is not through the synaptic relay from OB to HPC in the lateral EC. OB-HPC coupling at the sniffing frequency is proposed as a mechanism underlying olfactory sensorimotor effort as a cognitive process.

Role in the inspiratory off-switch of vagal inputs to rostral pontine inspiratory-modulated neurons

Neurons of the pontine respiratory group (PRG) in the region of the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus are believed to play an important role in promoting the inspiratory (I) off-switch (IOS). In decerebrate gallamine-paralyzed cats ventilated with a cycle-triggered pump system (lung inflation during the neural I phase), we studied the effects of vagal (V) afferent inputs on firing of I-modulated neurons (the most numerous population in PRG) and on I duration. The predominant V effect on unit activity was inhibitory, as shown by two types of test: (a) withholding of inflation during an I phase, which produced increase of unit firing and of its respiratory modulation (58/66 neurons in 14 cats), indicating disinhibition due to removal of phasic V input; (b) delivery of afferent V stimulus trains during a no-inflation I phase, which produced decrease of unit firing and of its respiratory modulation (20 neurons). In addition, application of V input during the neural expiratory (E) phase, which lengthened E phase duration, produced no effect on the neurons' firing, suggesting that the inhibition during I was presynaptic in origin. The results may be interpreted by the hypothesis that the medullary late-I presumptive IOS neurons receive excitatory inputs from the PRG I-modulated neurons as well as from V afferents.. With relatively strong V input, this pontine excitatory output is reduced by inhibition, whereas with relatively weak V input that excitatory output is increased due to reduction of inhibition. Thus the pontine and the V influences on the IOS can operate in a complementary manner dependent on state.