Inspiratory muscles during exercise: a problem of supply and demand

Incremental Load Respiratory Muscle Training Improves Respiratory Muscle Strength and Pulmonary Function in Children with Bronchiectasis

Methods

Participants underwent respiratory muscle training for 24 weeks. The main results were changes in respiratory muscle strength and pulmonary function indices (forced expiratory volume in 1 s (FEV1), forced vital capacity (FVC), FEV1/FVC, peak expiratory flow rate (PEF), forced expiratory flow 25-75% (FEF25-75%), and maximal midexpiratory flow 75/25 (MMEF75/25)) before, 12 weeks after, and 24 weeks after the intervention. The secondary outcomes were changes in the exercise load and work rate, exercise work, Leicester Cough Questionnaire (LCQ) scale, and Fatigue Severity Scale (FSS).

Results

Compared with before the intervention, after 24 weeks of respiratory muscle training, the maximal inspiratory pressure (MIP) and maximal expiratory pressure (MEP) were significantly enhanced (P < 0.05), while FVC, FEV1, and PEF were significantly increased (P < 0.01). FEF25-75 and MMEF75/25 values showed significant improvement compared to those before training (P < 0.05). The exercise loading, work, and exercise work rate of expiratory muscle training were significantly improved compared to those before intervention (P < 0.05). The LCQ score increased significantly (P <  0.001), and the FSS score decreased significantly (P <  0.001).

Conclusion

Incremental load respiratory muscle training effectively improved children's lung function over the long term, improved the strength of their inspiratory and expiratory muscles, and improved their quality of life.

Physiological Predictors of Morbidity and Mortality in COPD: The Relative Importance of Reduced Inspiratory Capacity and Inspiratory Muscle Strength

Low resting inspiratory capacity (IC) and low maximal inspiratory pressure (MIP) have previously been linked to exertional dyspnea, exercise limitation and poor survival in chronic obstructive pulmonary disease (COPD). The interaction and relative contributions of these two related variables to important clinical outcomes are unknown. The objective of the current study was to examine the interaction between resting IC and MIP (both % predicted), exertional dyspnea, exercise capacity and long-term survival in patients with COPD. Two hundred and eighty-five patients with mild to advanced COPD completed standard lung function testing and a cycle cardiopulmonary exercise test. Multiple regression determined predictors of the exertional dyspnea-ventilation slope and peak oxygen uptake (V̇O_2peak). Cox regression determined predictors of 10-year mortality. IC was associated with the dyspnea-ventilation slope (standardized β=-0.44, p<0.001), while MIP was excluded from the regression model (p=0.713). IC and MIP were included in the final model to predict V̇O_2peak. However, the standardized β was greater for IC (0.49) than MIP (0.22). After adjusting for age, sex, body mass index, cardiovascular risk, airflow obstruction and diffusing capacity, resting IC was independently associated with 10-year all-cause mortality (hazard ratio=1.25, confidence interval_5-95%=1.16-1.34, p<0.001), while MIP was excluded from the final model (all p=0.829). Low resting IC was consistently linked to heightened dyspnea intensity, low V̇O_2peak and worse survival in COPD even after accounting for airway obstruction, inspiratory muscle strength, and diffusing capacity. These results support the use of resting IC as an important physiological biomarker closely linked to key clinical outcomes in COPD.

Exertional dyspnoea in patients with mild-to-severe chronic obstructive pulmonary disease (COPD): Neuromechanical mechanisms

KEY POINTS

Dyspnoea during exercise is a common and troublesome symptom reported by patients with chronic obstructive pulmonary disease (COPD) and is linked to an elevated inspiratory neural drive (IND). The precise mechanisms of elevated IND and dyspnoea across the continuum of airflow obstruction severity in COPD remains unclear. The present study sought to determine the mechanisms of elevated IND [by diaphragm EMG, EMGdi (%max)] and dyspnoea during cardiopulmonary exercise testing (CPET) across the continuum of COPD severity. There was a strong association between increasing dyspnoea intensity and EMGdi (%max) during CPET across the COPD continuum despite significant heterogeneity in underlying pulmonary gas exchange and respiratory mechanical impairments. Critical inspiratory constraints occurred at progressively lower ventilation during exercise with worsening severity of COPD. This was associated with the progressively lower resting inspiratory capacity with worsening disease severity. Earlier critical inspiratory constraint was associated with earlier neuromechanical dissociation and greater likelihood of reporting the sensation of 'unsatisfied inspiration'.

ABSTRACT

In patients with COPD, exertional dyspnoea generally arises when there is imbalance between ventilatory demand and capacity, but the neurophysiological mechanisms are unclear. We therefore determined if disparity between elevated inspiratory neural drive (IND) and tidal volume (V_T ) responses (neuromechanical dissociation) impacted dyspnoea intensity and quality during exercise, across the COPD severity spectrum. In this two-centre, cross-sectional observational study, 89 participants with COPD divided into tertiles of FEV₁ %predicted (Tertile 1 = FEV_1 =  87 ± 9%, Tertile 2 = 60 ± 9%, Tertile 3 = 32 ± 8%) and 18 non-smoking controls, completed a symptom-limited cardiopulmonary exercise tests (CPET) with measurement of IND by diaphragm electromyography [EMGdi (%max)]. The association between increasing dyspnoea intensity and EMGdi (%max) during CPET was strong (r = 0.730, P < 0.001) and not different between the four groups who showed marked heterogeneity in pulmonary gas exchange and mechanical abnormalities. Significant inspiratory constraints (tidal volume/inspiratory capacity (V_T /IC) ≥ 70%) and onset of neuromechanical dissociation (EMGdi (%max):V_T /IC > 0.75) occurred at progressively lower V̇_E from Control to Tertile 3. Lower resting IC meant earlier onset of neuromechanical dissociation, heightened dyspnoea intensity and greater propensity (93% in Tertile 3) to select qualitative descriptors of 'unsatisfied inspiration'. We concluded that, regardless of marked variation in mechanical and pulmonary gas exchange abnormalities in our study sample, exertional dyspnoea intensity was linked to the magnitude of EMGdi (%max). Moreover, onset of critical inspiratory constraints and attendant neuromechanical dissociation amplified dyspnoea intensity at higher exercise intensities. Simple measurements of IC and breathing pattern during CPET provide useful insights into mechanisms of dyspnoea and exercise intolerance in individuals with COPD. Abstract figure legend As chronic obstructive pulmonary disease severity increases, worsening gas exchange and respiratory mechanical impairment causes increased afferent receptor stimulation, increasing inspiratory neural drive at a given ventilation. The widening disparity between progressively greater inspiratory neural drive and reduced ventilatory output causes, 'neuromechanical dissociation'. This is strongly associated with a rapid increase in the intensity of dyspnea during exercise, and the onset of the sensation of 'unsatisfied inspiration'. This article is protected by copyright. All rights reserved.

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.

Effect of end-inspiratory lung volume and breathing pattern on neural activation of the diaphragm and extra-diaphragmatic inspiratory muscles in healthy adults

This study examined the effect of changes in end-inspiratory lung volume (EILV) and breathing pattern on neural activation of the crural diaphragm (EMG_DIA) and of the sternocleidomastoid (EMG_SCM), scalene (EMG_SCA) and external intercostal muscles (EMG_INT) at constant ventilation (V̇_E). Twelve healthy adults performed a series of 30-sec breathing trials at a constant V̇_E corresponding to 15% of their maximum voluntary ventilation while (i) altering EILV at a constant breathing pattern and (ii) altering breathing pattern at a constant EILV. Using a real-time visual display of each participant's spirogram, EILV was voluntarily targeted at 65% (EILV_65%), 75% (EILV_75%), 85% (EILV_85%) and 95% (EILV_95%) of each participant's inspired vital capacity, while breathing frequency (f_R) was targeted at 15, 35 and 50 breaths/min using a metronome. The tidal volume needed for a participant to maintain V̇_E constant across trials was achieved via changes in end-expiratory lung volume. A multipair esophageal electrode catheter was used to record EMG_DIA, while surface electrodes were used to record EMG_SCM, EMG_SCA and EMG_INT. On average, EMG_DIA, EMG_SCM, EMG_SCA and EMG_INT increased as a function of increasing EILV at constant V̇_E, independent of changes in breathing pattern. The magnitudes of these increases were particularly notable in the transition from EILV_85% to EILV_95%, especially for EMG_SCM and EMG_SCA. In healthy adults, as EILV increases towards total lung capacity, progressive compensatory increases in neural activation of the diaphragm and extra-diaphragmatic inspiratory muscles are needed to support V̇_E, independent of changes in breathing pattern.

Thoracoabdominal asynchrony associates with exercise intolerance in fibrotic interstitial lung diseases

BACKGROUND AND OBJECTIVE

The precise coordination of respiratory muscles during exercise minimizes work of breathing and avoids exercise intolerance. Fibrotic interstitial lung disease (f-ILD) patients are exercise-intolerant. We assessed whether respiratory muscle incoordination and thoracoabdominal asynchrony (TAA) occur in f-ILD during exercise, and their relationship with pulmonary function and exercise performance.

METHODS

We compared breathing pattern, respiratory mechanics, TAA and respiratory muscle recruitment in 31 f-ILD patients and 31 healthy subjects at rest and during incremental cycle exercise. TAA was defined as phase angle (PhAng) >20°.

RESULTS

During exercise, when compared with controls, f-ILD patients presented increased and early recruitment of inspiratory rib cage muscle (p < 0.05), and an increase in PhAng, indicating TAA. TAA was more frequent in f-ILD patients than in controls, both at 50% of the maximum workload (42.3% vs. 10.7%, p = 0.01) and at the peak (53.8% vs. 23%, p = 0.02). Compared with f-ILD patients without TAA, f-ILD patients with TAA had lower lung volumes (forced vital capacity, p < 0.01), greater dyspnoea (Medical Research Council > 2 in 64.3%, p = 0.02), worse exercise performance (lower maximal work rate % predicted, p = 0.03; lower tidal volume, p = 0.03; greater desaturation and dyspnoea, p < 0.01) and presented higher oesophageal inspiratory pressures with lower gastric inspiratory pressures and higher recruitment of scalene (p < 0.05).

CONCLUSION

Exercise induces TAA and higher recruitment of inspiratory accessory muscle in ILD patients. TAA during exercise occurred in more severely restricted ILD patients and was associated with exertional dyspnoea, desaturation and limited exercise performance.

Pathophysiological mechanisms of exertional breathlessness in chronic obstructive pulmonary disease and interstitial lung disease

PURPOSE OF REVIEW

Breathlessness is a common and distressing symptom in patients with chronic obstructive pulmonary disease (COPD) and fibrotic interstitial lung disease (ILD), particularly during exercise. Effective medical management of exertional breathlessness in people living with COPD and fibrotic ILD is challenging for healthcare providers and requires an understanding of its mechanisms. Thus, in this brief review we summarize recent advances in our understanding of the pathophysiological mechanisms of exertional breathlessness in COPD and fibrotic ILD.

RECENT FINDINGS

The collective results of recent physiological and clinical trials suggest that higher intensity ratings of exertional breathlessness in both COPD and fibrotic ILD compared to healthy control individuals is mechanistically linked to the awareness of greater neural respiratory drive (quantified using inspiratory muscle electromyography) needed to compensate for pathophysiological abnormalities in respiratory mechanics and pulmonary gas exchange efficiency.

SUMMARY

Any therapeutic intervention capable of decreasing intrinsic mechanical loading of the respiratory system and/or increasing pulmonary gas exchange efficiency has the potential to decrease the prevalence and severity of activity-related breathlessness and improve related clinical and patient-reported outcomes (e.g., exercise tolerance and health-related quality of life) by decreasing neural respiratory drive in people with COPD and fibrotic ILD.

The Link between Reduced Inspiratory Capacity and Exercise Intolerance in Chronic Obstructive Pulmonary Disease

Low inspiratory capacity (IC), chronic dyspnea, and reduced exercise capacity are inextricably linked and are independent predictors of increased mortality in chronic obstructive pulmonary disease. It is no surprise, therefore, that a major goal of management is to improve IC by reducing lung hyperinflation to improve respiratory symptoms and health-related quality of life. The negative effects of lung hyperinflation on respiratory muscle and cardiocirculatory function during exercise are now well established. Moreover, there is growing appreciation that a key mechanism of exertional dyspnea in chronic obstructive pulmonary disease is critical mechanical constraints on tidal volume expansion during exercise when resting IC is reduced. Further evidence for the importance of lung hyperinflation comes from multiple studies, which have reported the clinical benefits of therapeutic interventions that reduce lung hyperinflation and increase IC. A reduced IC in obstructive pulmonary disease is further eroded by exercise and contributes to ventilatory limitation and dyspnea. It is an important outcome for both clinical and research studies.

Inspiratory muscle training reduces diaphragm activation and dyspnea during exercise in COPD

Among patients with chronic obstructive pulmonary disease (COPD), those with the lowest maximal inspiratory pressures experience greater breathing discomfort (dyspnea) during exercise. In such individuals, inspiratory muscle training (IMT) may be associated with improvement of dyspnea, but the mechanisms for this are poorly understood. Therefore, we aimed to identify physiological mechanisms of improvement in dyspnea and exercise endurance following inspiratory muscle training (IMT) in patients with COPD and low maximal inspiratory pressure (Pi_max). The effects of 8 wk of controlled IMT on respiratory muscle function, dyspnea, respiratory mechanics, and diaphragm electromyography (EMGdi) during constant work rate cycle exercise were evaluated in patients with activity-related dyspnea (baseline dyspnea index <9). Subjects were randomized to either IMT or a sham training control group ( n = 10 each). Twenty subjects (FEV₁ = 47 ± 19% predicted; Pi_max  = -59 ± 14 cmH₂O; cycle ergometer peak work rate = 47 ± 21% predicted) completed the study; groups had comparable baseline lung function, respiratory muscle strength, activity-related dyspnea, and exercise capacity. IMT, compared with control, was associated with greater increases in inspiratory muscle strength and endurance, with attendant improvements in exertional dyspnea and exercise endurance time (all P < 0.05). After IMT, EMGdi expressed relative to its maximum (EMGdi/EMGdi_max) decreased ( P < 0.05) with no significant change in ventilation, tidal inspiratory pressures, breathing pattern, or operating lung volumes during exercise. In conclusion, IMT improved inspiratory muscle strength and endurance in mechanically compromised patients with COPD and low Pi_max. The attendant reduction in EMGdi/EMGdi_max helped explain the decrease in perceived respiratory discomfort despite sustained high ventilation and intrinsic mechanical loading over a longer exercise duration. NEW & NOTEWORTHY In patients with COPD and low maximal inspiratory pressures, inspiratory muscle training (IMT) may be associated with improvement of dyspnea, but the mechanisms for this are poorly understood. This study showed that 8 wk of home-based, partially supervised IMT improved respiratory muscle strength and endurance, dyspnea, and exercise endurance. Dyspnea relief occurred in conjunction with a reduced activation of the diaphragm relative to maximum in the absence of significant changes in ventilation, breathing pattern, and operating lung volumes.

Exertional dyspnoea in COPD: the clinical utility of cardiopulmonary exercise testing

Activity-related dyspnoea is often the most distressing symptom experienced by patients with chronic obstructive pulmonary disease (COPD) and can persist despite comprehensive medical management. It is now clear that dyspnoea during physical activity occurs across the spectrum of disease severity, even in those with mild airway obstruction. Our understanding of the nature and source of dyspnoea is incomplete, but current aetiological concepts emphasise the importance of increased central neural drive to breathe in the setting of a reduced ability of the respiratory system to appropriately respond. Since dyspnoea is provoked or aggravated by physical activity, its concurrent measurement during standardised laboratory exercise testing is clearly important. Combining measurement of perceptual and physiological responses during exercise can provide valuable insights into symptom severity and its pathophysiological underpinnings. This review summarises the abnormal physiological responses to exercise in COPD, as these form the basis for modern constructs of the neurobiology of exertional dyspnoea. The main objectives are: 1) to examine the role of cardiopulmonary exercise testing (CPET) in uncovering the physiological mechanisms of exertional dyspnoea in patients with mild-to-moderate COPD; 2) to examine the escalating negative sensory consequences of progressive respiratory impairment with disease advancement; and 3) to build a physiological rationale for individualised treatment optimisation based on CPET.

Advances in the Evaluation of Respiratory Pathophysiology during Exercise in Chronic Lung Diseases

Dyspnea and exercise limitation are among the most common symptoms experienced by patients with various chronic lung diseases and are linked to poor quality of life. Our understanding of the source and nature of perceived respiratory discomfort and exercise intolerance in chronic lung diseases has increased substantially in recent years. These new mechanistic insights are the primary focus of the current review. Cardiopulmonary exercise testing (CPET) provides a unique opportunity to objectively evaluate the ability of the respiratory system to respond to imposed incremental physiological stress. In addition to measuring aerobic capacity and quantifying an individual's cardiac and ventilatory reserves, we have expanded the role of CPET to include evaluation of symptom intensity, together with a simple "non-invasive" assessment of relevant ventilatory control parameters and dynamic respiratory mechanics during standardized incremental tests to tolerance. This review explores the application of the new advances in the clinical evaluation of the pathophysiology of exercise intolerance in chronic obstructive pulmonary disease (COPD), chronic asthma, interstitial lung disease (ILD) and pulmonary arterial hypertension (PAH). We hope to demonstrate how this novel approach to CPET interpretation, which includes a quantification of activity-related dyspnea and evaluation of its underlying mechanisms, enhances our ability to meaningfully intervene to improve quality of life in these pathologically-distinct conditions.

Mechanisms of exertional dyspnoea in symptomatic smokers without COPD

Dyspnoea and activity limitation can occur in smokers who do not meet spirometric criteria for chronic obstructive pulmonary disease (COPD) but the underlying mechanisms are unknown.

Detailed pulmonary function tests and sensory–mechanical relationships during incremental exercise with respiratory pressure measurements and diaphragmatic electromyography (EMGdi) were compared in 20 smokers without spirometric COPD and 20 age-matched healthy controls.

Smokers (mean±sd post-bronchodilator forced expiratory volume in 1 s (FEV1)/forced vital capacity 75±4%, mean±sd FEV1 104±14% predicted) had greater activity-related dyspnoea, poorer health status and lower physical activity than controls. Smokers had peripheral airway dysfunction: higher phase-III nitrogen slopes (3.8±1.8 versus 2.6±1.1%·L−1) and airway resistance (difference between airway resistance measured at 5 Hz and 20 Hz 19±11 versus 12±7% at 5 Hz) than controls (p<0.05). Smokers had significantly (p<0.05) lower peak oxygen uptake (78±40 versus 107±45% predicted) and ventilation (61±26 versus 97±29 L·min−1). Exercise ventilatory requirements, operating lung volumes and cardio-circulatory responses were similar. However, submaximal dyspnoea ratings, resistive and total work of breathing were increased in smokers compared with controls (p<0.05); diaphragmatic effort (transdiaphragmatic pressure/maximumal transdiaphragmatic pressure) and fractional inspiratory neural drive to the diaphragm (EMGdi/maximal EMGdi) were also increased (p<0.05) mainly reflecting the reduced denominator.

Symptomatic smokers at risk for COPD had greater exertional dyspnoea and lower exercise tolerance compared with healthy controls in association with greater airways resistance, contractile diaphragmatic effort and fractional inspiratory neural drive to the diaphragm.

Electrical activity of the diaphragm during progressive cycling exercise in endurance-trained men

The study aimed to investigate diaphragm respiratory drive modulation through electrical activity of the diaphragm (EADi) during progressive cycling in endurance-trained men (N=7) and to test day-to-day measurement reliability. Normalized EADi increased at exercise intensities from 40% workload (WL) to 70% and 85%WL but plateaued from 70% to 85% (p<0.05). V˙O2, V˙CO2, V˙E, increased at all exercise intensities, where Vt and BF increased from 40% to 55% WL and from 70% to 85% and RER increased at 70% and 85% (p<0.05). Bland-Altman plots of normalized EADi showed bias of 0.9% and -6.4% and limits of agreement of ±36.0% and ±30.4% for absolute measurements and relative changes from 40% WL, respectively. Within-day variability appeared constant indicating that measurements within a trial are reliable. Results suggest that diaphragm respiratory drive increases at moderate exercise intensities, but plateaus at high intensities where other respiratory muscles might contribute significantly to the breathing effort, perhaps to "protect" against diaphragm fatigue.

Lung hyperinflation in chronic obstructive pulmonary disease: mechanisms, clinical implications and treatment

Lung hyperinflation is highly prevalent in patients with chronic obstructive pulmonary disease and occurs across the continuum of the disease. A growing body of evidence suggests that lung hyperinflation contributes to dyspnea and activity limitation in chronic obstructive pulmonary disease and is an important independent risk factor for mortality. In this review, we will summarize the recent literature on pathogenesis and clinical implications of lung hyperinflation. We will outline the contribution of lung hyperinflation to exercise limitation and discuss its impact on symptoms and physical activity. Finally, we will examine the physiological rationale and efficacy of selected pharmacological and non-pharmacological 'lung deflating' interventions aimed at improving symptoms and physical functioning.

The role of spinal manipulation, soft-tissue therapy, and exercise in chronic obstructive pulmonary disease: a review of the literature and proposal of an anatomical explanation

The premise that lung function can regulate chest wall mobility is an accepted concept. Descriptions of the primary and accessory respiratory structures do not usually include spinal components as a part of these classifications. The case for including these components as a part of the respiratory mechanism and their role in the development of dyspnea and chest wall rigidity in chronic obstructive pulmonary disease (COPD) is reviewed. Mechanical impairment of the chest wall is a contributing factor in the prognosis of COPD. Reducing this impairment improves prognosis. Because spinal manipulation and soft-tissue therapy increase joint mobility and decrease muscle hypertonicity, respectively, applying these interventions to the chest wall in COPD could reduce chest wall rigidity, thereby improving breathing mechanics. Improvements in breathing mechanics reduce the work of the respiratory muscles and delay the onset of dyspnea. Exercise capacity is reliant on the ability to overcome activity-limiting dyspnea, which usually occurs prior to maximum exercise capacity being reached. Delaying the onset of dyspnea permits more exercise to be performed before dyspnea develops. Spinal manipulation and soft-tissue therapy have the potential to deliver such a delay. Because exercise tolerance is considered to be a strong predictor of quality of life and survival in COPD, any increase in exercise capacity would therefore improve prognosis for the disease.

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.

Effect of limb muscle fatigue on perception of respiratory effort in healthy subjects

The role of nonrespiratory peripheral afferents in dyspnea perception has not been fully elucidated yet. Our hypothesis is that fatigue-induced activation of limb muscle metaboreceptors served by group IV fine afferent fibers may impact on respiratory effort perception. We studied 12 healthy subjects breathing against progressive inspiratory resistive loads (10, 18, 30, 40, and 90 cmH2O·l−1·s) before and after inducing low-frequency fatigue of quadriceps muscle by repeating sustained contractions at ≥80% of maximal voluntary contraction. Subjects also underwent a sham protocol while performing two loaded breathing runs without muscle fatigue in between. During the loaded breathing, while subjects mimicked the quiet breathing pattern using a visual feedback, ventilation, tidal volume, respiratory frequency, pleural pressure swings, arterial oxygen saturation, end-tidal partial pressure of CO2, and dyspnea by a Borg scale were recorded. Compared with prefatigue, limb muscle fatigue resulted in a higher increase in respiratory effort perception for any given ventilation, tidal volume, respiratory frequency, pleural pressure swings, end-tidal partial pressure of CO2, and arterial oxygen saturation. No difference between the two runs was observed with the sham protocol. The present data support the hypothesis that fatigue of limb muscles increases respiratory effort perception associated with loaded breathing, likely by the activation of limb muscle metaboreceptors.

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.

Exercise Capacity in Chronic Obstructive Pulmonary Disease: Mechanisms of Limitation

Patients with chronic obstructive pulmonary disease (COPD) are often caught in a downward spiral that progresses from expiratory flow limitation to poor quality of life and invalidity. Within this downward spiral, exercise tolerance represents a key intermediate outcome. As recently stated by the GOLD initiative, improvement in exercise tolerance is now rec ognized as an important goal of COPD treatment. This objective will be achieved only by a comprehensive understanding of the mechanism of exercise limitation in this disease. The objective of this paper is to review the mechanisms of exercise limitation in COPD and discuss their relative contribution to exercise intolerance in patients suffering from this disease.

Optoelectronic Plethysmography has Improved our Knowledge of Respiratory Physiology and Pathophysiology

It is well known that the methods actually used to track thoraco-abdominal volume displacement have several limitations. This review evaluates the clinical usefulness of measuring chest wall kinematics by optoelectronic plethysmography [OEP]. OEP provides direct measurements (both absolute and its variations) of the volume of the chest wall and its compartments, according to the model of Ward and Macklem, without requiring calibration or subject cooperation. The system is non invasive and does not require a mouthpiece or nose-clip which may modify the pattern of breathing, making the subject aware of his breathing. Also, the precise assessment of compartmental changes in chest wall volumes, combined with pressure measurements, provides a detailed description of the action and control of the different respiratory muscle groups and assessment of chest wall dynamics in a number of physiological and clinical experimental conditions.

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.

Inspiratory muscle activity during incremental exercise in obese men

OBJECTIVE

The aim of this study was to assess overall inspiratory muscle activity during incremental exercise in obese men and healthy controls using the non-invasive, inspiratory muscle tension-time index (T(T0.1)). We studied 17 obese subjects (mean age+/-s.d.; 49+/-13 years) and 14 control subjects (42+/-16) during an incremental, maximal exercise test.

METHODS

Measurements included anthropometric parameters, spirometry, breathing patterns and inspiratory muscle activity. T(T0.1) was calculated using the equation T(T0.1)=P(0.1)/P(Imax) x T(I)/T(TOT) (where P(0.1) is mouth occlusion pressure, P(Imax) is maximal inspiratory pressure and T(I)/T(TOT) is the duty cycle).

RESULTS

At same levels of maximal exercise (%W(max)) (20, 40, 60, 80, 100% W(max)), obese subjects showed higher P(0.1) (P<0.001) and P(0.1)/P(Imax) (P<0.001) values than controls. T(T0.1) was thus higher in obese subjects for each workload increment and at maximal exercise (P<0.001).

CONCLUSIONS

During exercise, patients with obesity show alterations in inspiratory muscle activity as a result of both reduced inspiratory strength (as measured by maximal inspiratory pressure) and increased ventilatory drive (as reflected by mouth occlusion pressure), which prone obese subject to respiratory muscle weakness. Our results suggest that impaired respiratory muscle activity could contribute to a decrease in exercise capacity. T(T0.1) may be useful in our understanding concerning the benefits of endurance training.

Influence of diaphragm and rib cage muscle fatigue on breathing during endurance exercise

Inspiratory muscle fatigue (IMF) can develop during exhaustive exercise and cause tachypnea or rapid shallow breathing. We assessed the effects of rib cage muscle (RCM-F) and diaphragm fatigue (DIA-F) on breathing pattern and respiratory mechanics during high-intensity endurance exercise. Twelve healthy subjects performed a constant-load (85% maximal power) cycling test to exhaustion with prior IMF and a cycling test of similar intensity and duration without prior IMF (control). IMF was induced by resistive breathing and assessed by oesophageal and gastric twitch pressure measurements during cervical magnetic stimulation. Both RCM-F and DIA-F increased RCM and abdominal muscle force production during exercise compared to control. With RCM-F, tidal volume decreased while it increased with DIA-F. RCM-F was associated with a smaller increase in end-expiratory oesophageal pressure (i.e. decrease in lung volume) than DIA-F. These results suggest that RCM-F and not DIA-F is associated with rapid shallow breathing and that lowering the operating lung volume with DIA-F may help to preserve diaphragmatic function.

Helium-Hyperoxia, Exercise, and Respiratory Mechanics in Chronic Obstructive Pulmonary Disease

RATIONALE

Hyperoxia and normoxic helium independently reduce dynamic hyperinflation and improve the exercise tolerance of patients with chronic obstructive pulmonary disease (COPD). Combining these gases could have an additive effect on dynamic hyperinflation and a greater impact on respiratory mechanics and exercise tolerance.

OBJECTIVE

To investigate whether helium-hyperoxia improves the exercise tolerance and respiratory mechanics of patients with COPD.

METHODS

Ten males with COPD (FEV(1) = 47 +/- 17%pred [mean +/- SD]) performed randomized constant-load cycling at 60% of maximal work rate breathing air, hyperoxia (40% O(2), 60% N(2)), normoxic helium (21% O(2), 79% He), or helium-hyperoxia (40% O(2), 60% He).

MEASUREMENTS

Exercise time, inspiratory capacity (IC), work of breathing, and exertional symptoms were measured with each gas.

RESULTS

Compared with air (9.4 +/- 5.2 min), exercise time was increased with hyperoxia (17.8 +/- 5.8 min) and normoxic helium (16.7 +/- 9.1 min) but the improvement with helium-hyperoxia (26.3 +/- 10.6 min) was greater than both these gases (p = 0.019 and p = 0.007, respectively). At an isotime during exercise, all three gases reduced dyspnea and both helium mixtures increased IC and tidal volume. Only helium-hyperoxia significantly reduced the resistive work of breathing (15.8 +/- 4.2 vs. 10.1 +/- 4.1 L . cm H(2)O(-1)) and the work to overcome intrinsic positive end-expiratory pressure (7.7 +/- 1.9 vs. 3.6 +/- 2.1 L . cm H(2)O(-1)). At symptom limitation, tidal volume remained augmented with both helium mixtures, but IC and the work of breathing were unchanged compared with air.

CONCLUSION

Combining helium and hyperoxia delays dynamic hyperinflation and improves respiratory mechanics, which translates into added improvements in exercise tolerance for patients with COPD.

Respiratory muscles and dyspnea in obese nonsmoking subjects

To our knowledge no data have been reported on the contribution to acute increase in dyspnea by the respiratory muscles in obese nonsmoking subjects. To better focus on this topic, we studied seven obese subjects and an age-matched normal control group, assessing baseline pulmonary function, breathing pattern, esophageal pressure (Pes), and gastric (Pga) and transdiaphragmatic (Pdi) pressures. Pes was also recorded during a sniff maneuver (Pessn). During a hypercapnic rebreathing test we recorded inspiratory swing in Pes (Pessw), expiratory changes in Pga, and inspiratory swings in Pdi (Pdisw). Change in inspiratory capacity was considered the mirror image of end-expiratory lung volume (EELV). Dyspnea was assessed by a modified Borg scale. Under control conditions, patients exhibited a reduced expiratory reserve volume and intrinsic positive end-expiratory pressure (PEEPi). At the end of hypercapnic stimulation, compared with controls our obese subjects exhibited greater respiratory frequency (Rf), shorter expiratory time, greater Pessw, and lower Pdisw. Increases in EELV and PEEPi were found in the obese subjects but not in controls. Changes in Borg correlated with changes in PETCO2, VE, Pessw (%Pessn), and Pdisw to a greater extent in patients than in controls. Stepwise regression analysis indicated the amount of variability in Borg that was predicted by both Pdisw (r2 = 0.31, p < 0.0004), and Pessw (%Pessn) (r2 = 0.09, p < 0.005) in controls, and by Pessw (%Pessn) (r2 = 0.40, p < 0.00001) in obese subjects. We conclude that the rib cage muscles contributed to dyspnea to a greater extent in this subset of obese subjects.

Arm exercise and hyperinflation in patients with COPD: effect of arm training

BACKGROUND

Unlike studies on leg exercise, reports on the regulation of dynamic hyperinflation during arm exercise are scanty. We ascertained the following in patients with COPD: (1) whether and to what extent upper-limb exercise results in dynamic hyperinflation, and (2) the mechanism whereby an arm-training program (ATP) reduces arm effort and dyspnea.

PATIENTS

Twelve patients with moderate-to-severe COPD were tested during incremental, symptom-limited arm exercise after a non-intervention control period (pre-ATP) and after ATP.

METHODS

Exercise testing (1-min increments of 5 W) was performed using an arm ergometer. Oxygen uptake (V(O2)), carbon dioxide output, minute ventilation (Ve), tidal volume, and respiratory rate (RR) were measured continuously during the tests. Inspiratory capacity (IC), exercise dyspnea, and arm effort using a Borg scale were assessed at each step of exercise.

RESULTS

Arm exercise resulted in a significant decrease in IC and significant positive relationships of IC with an increase in V(O2) and exercise dyspnea and arm effort. The results of ATP were as follows: (1) a significant increase in exercise capacity (p < 0.001); (2) no change in the relationships of exercise dyspnea and arm effort with Ve and IC, and of IC with V(O2); (3) at a standardized work rate, Ve, exercise dyspnea, and arm effort significantly decreased, while the decrease in IC was significantly less (p < 0.01) than before the ATP; the decrease in Ve was accomplished primarily by a decrease in RR; and (4) at standardized Ve, exercise dyspnea and arm effort decreased significantly.

CONCLUSIONS

Arm exercise results in the association of dynamic hyperinflation, exercise dyspnea, and arm effort in COPD patients. An ATP increases arm endurance, modulates dynamic hyperinflation, and reduces symptoms.

Pathophysiology of exercise dyspnea in healthy subjects and in patients with chronic obstructive pulmonary disease (COPD)

In patients with a number of cardio-respiratory disorders, breathlessness is the most common symptom limiting exercise capacity. Increased respiratory effort is frequently the chosen descriptor cluster both in normal subjects and in patients with chronic obstructive pulmonary disease (COPD) during exercise. The body of evidence indicates that dyspnea may be due to a central perception of an overall increase in central respiratory motor output directed preferentially to the rib cage muscles. On the other hand, the disparity between respiratory motor output and mechanical response of the system is also thought to play an important role in the increased perception of exercise in patients. The expiratory muscles also contribute to exercise dyspnea: a decrease in Borg scores is related to a decrease in end-expiratory lung volume and to a decrease in end-expiratory gastric pressure at isowork after lung volume reduction surgery. Changes in respiratory mechanics and intrathoracic pressure surrounding the heart can reduce cardiac output by affecting the return of blood to the heart from the periphery, or by interfering with the ability of the heart to eject blood into the peripheral circulation. Change in arterial blood gas content may affect breathlessness via direct or indirect effects. Old and more recent data have demonstrated that hypercapnia makes an independent contribution to breathlessness. In hypercapnic COPD patients an increase in PaCO2 seems to be the most important stimulus overriding all other inputs for dyspnea. Hypoxia may act indirectly by increasing ventilation (VE), and directly, independent of change in VE. Finally, chemical (metabolic) ventilatory stimuli do not have a specific effect on breathlessness other than via their stimulation of VE. We conclude that exercise provides a stimulus contributing to dyspnea, which can be applied to many diseases.

Effects of imposed pursed-lips breathing on respiratory mechanics and dyspnea at rest and during exercise in COPD

STUDY OBJECTIVES

To investigate the effect of volitional pursed-lips breathing (PLB) on breathing pattern, respiratory mechanics, operational lung volumes, and dyspnea in patients with COPD.

SUBJECTS

Eight COPD patients (6 male and 2 female) with a mean (+/-SD) age of 58 +/- 11 years and a mean FEV1 of 1.34 +/- 0.44 L (50 +/- 21% predicted).

METHODS

Wearing a tight-fitting transparent facemask, patients breathed for 8 min each, with and without PLB at rest and during constant-work-rate bicycle exercise (60% of maximum).

RESULTS

PLB promoted a slower and deeper breathing pattern both at rest and during exercise. Whereas patients had no dyspnea with or without PLB at rest, during exercise dyspnea was variably affected by PLB across patients. Changes in the individual dyspnea scores with PLB during exercise were significantly correlated with changes in the end-expiratory lung volume (EELV) values estimated from inspiratory capacity maneuvers (as a percentage of total lung capacity; r2 = 0.82, p = 0.002) and with changes in the mean inspiratory ratio of pleural pressure to the maximal static inspiratory pressure-generating capacity (PcapI) [r2 = 0.84; p = 0.001], measured using an esophageal balloon, where PcapI was determined over the range of inspiratory lung volumes and adjusted for flow.

CONCLUSION

PLB can have a variable effect on dyspnea when performed volitionally during exercise by patients with COPD. The effect of PLB on dyspnea is related to the combined change that it promotes in the tidal volume and EELV and their impact on the available capacity of the respiratory muscles to meet the demands placed on them in terms of pressure generation.

Dyspnoea in health and obstructive pulmonary disease : the role of respiratory muscle function and training

A consistent finding of recent research on respiratory muscle training (RMT) in healthy humans has been an attenuation of respiratory discomfort (dyspnoea) during exercise. We argue that the neurophysiology of dyspnoea can be explained in terms of Cambell's paradigm of length-tension inappropriateness. In the context of this paradigm, changes in the contractile properties of the respiratory muscles modify the intensity of dyspnoea predominantly by changing the required level of motor outflow to these respiratory muscles. Thus, factors that impair the contractile properties of the respiratory muscles (e.g. the pattern of tension development, functional weakening and fatigue) have the potential to increase the intensity of dyspnoea, while factors that improve the contractile properties of these respiratory muscles (e.g. RMT) have the potential to reduce the intensity of dyspnoea. In patients with obstructive pulmonary disease, functional weakening of the inspiratory muscles in response to dynamic lung hyperinflation appears to be a central component of dyspnoea. A decrease in the intensity of respiratory effort sensation (during exercise and loaded breathing) has been observed in both healthy individuals and patients with obstructive pulmonary disease after RMT. We conclude that RMT has the potential to reduce the severity of dyspnoea in healthy individuals and in patients with obstructive pulmonary disease, and that this probably occurs via a reduction in the level of motor outflow. Further work is required to clarify the role of RMT in the management of other disease conditions in which the function of the respiratory muscles is impaired, or the loads that they must overcome are elevated (e.g. cardiorespiratory and neuromuscular disorders).

Inter-test reliability for non-invasive measures of respiratory muscle function in healthy humans

The aims of this study were to quantify the inter-test reliability of several voluntary, non-invasive measures of respiratory muscle function and to determine the implications of these data for studies using a repeated-measures design. Systematic measurement differences were found for 50% of the variables ( P</=0.05, t-tests). Nevertheless, 95% ratio limits of agreement for most measures proved acceptable and similar to those reported elsewhere. The random error component of the agreement ratios ranged from 1.047 to 1.149 for measures of pulmonary function in healthy subjects ( n=46), 1.045 to 1.056 for maximum static respiratory pressures ( n=24), 1.062 to 1.173 for measures relating to the maximum pressure-flow-power relationship ( n=16-22), and 1.036 to 1.071 for measures relating to maximum incremental inspiratory muscle performance ( n=12). The judgement that the limits of agreement were acceptable is supported by the sample-size calculations. Estimated sample sizes based upon an alpha level of 0.05 and a statistical power of 0.9 were mostly </=11 for a repeated-measures experimental design, particularly for the larger effect sizes (>/=5%). However, peak expiratory flow, the maximum rate of pressure development and the time constant of relaxation, require larger sample sizes to detect small within-group changes. In conclusion, the described protocols provide reliable measurements for most parameters of respiratory muscle function in healthy subjects. Furthermore, experiments utilising a within-subjects design lasting up to 3 weeks can be conducted with feasible sample sizes (</=11 per group) where substantial (>/=5%) changes are expected.

Exercise training improves exertional dyspnea in patients with COPD: evidence of the role of mechanical factors

BACKGROUND

To our knowledge, no data have been reported on the effects of exercise training (EXT) on central respiratory motor output or neuromuscular coupling (NMC) of the ventilatory pump, and their potential association with exertional dyspnea. Accurate assessment of these important clinical outcomes is integral to effective management of breathlessness of patients with COPD.

MATERIAL AND METHODS

Twenty consecutive patients with stable moderate-to-severe COPD were tested at 6-week intervals at baseline, after a nonintervention control period (pre-EXT), and after EXT. Patients entered an outpatient pulmonary rehabilitation program involving regular exercise on a bicycle. Incremental symptom-limited exercise testing (1-min increments of 10 W) was performed on an electronically braked cycle ergometer. Oxygen uptake (O(2)), carbon dioxide output (CO(2)), minute ventilation (E), time, and volume components of the respiratory cycle and, in six patients, esophageal pressure swings (Pessw), both as actual values and as percentage of maximal (most negative in sign) esophageal pressure during sniff maneuver (Pessn), were measured continuously over the runs. Exertional dyspnea and leg effort were evaluated by administering a Borg scale.

RESULTS

Measurements at baseline and pre-EXT were similar. Significant increase in exercise capacity was found in response to EXT: (1) peak work rate (WR), O(2), CO(2), E, tidal volume (VT), and heart rate increased, while peak exertional dyspnea and leg effort did not significantly change; (2) exertional dyspnea/O(2) and exertional dyspnea/CO(2) decreased while E/O(2) and E/CO(2) remained unchanged. The slope of both exertional dyspnea and leg effort relative to E fell significantly after EXT; (3) at standardized WR, E, and CO(2), exertional dyspnea and leg effort decreased while inspiratory capacity (IC) increased. Decrease in E was accomplished primarily by decrease in respiratory rate (RR) and increase in both inspiratory time (TI) and expiratory time; VT slightly increased, while inspiratory drive (VT/TI) and duty cycle (TI/total time of the respiratory cycle) remained unchanged. The decrease in Pessw and the increase in VT were associated with lower exertional dyspnea after EXT; (4) at standardized E, VT, RR, and IC, Pessw and Pessw(%Pessn)/VT remained unchanged while exertional dyspnea and leg effort decreased with EXT.

CONCLUSION

In conclusion, increases in NMC, aerobic capacity, and tolerance to dyspnogenic stimuli and possibly breathing retraining are likely to contribute to the relief of both exertional dyspnea and leg effort after EXT.

Diaphragm activation during exercise in chronic obstructive pulmonary disease

Although it has been postulated that central inhibition of respiratory drive may prevent development of diaphragm fatigue in patients with chronic obstructive pulmonary disease (COPD) during exercise, this premise has not been validated. We evaluated diaphragm electrical activation (EAdi) relative to maximum in 10 patients with moderately severe COPD at rest and during incremental exhaustive bicycle exercise. Flow was measured with a pneumotachograph and volume by integration of flow. EAdi and transdiaphragmatic pressures (Pdi) were measured using an esophageal catheter. End-expiratory lung volume (EELV) was assessed by inspiratory capacity (IC) maneuvers, and maximal voluntary EAdi was obtained during these maneuvers. Minute ventilation (V E) was 12.2 +/- 1.9 L/min (mean +/- SD) at rest, and increased progressively (p < 0.001) to 31.0 +/- 7.8 L/min at end-exercise. EELV increased during exercise (p < 0.001) causing end-inspiratory lung volume to attain 97 +/- 3% of TLC at end-exercise. Pdi at rest was 9.4 +/- 3.2 cm H(2)O and increased during the first two thirds of exercise (p < 0.001) to plateau at about 13 cm H(2)O. EAdi was 24 +/- 6% of voluntary maximal at rest and increased progressively during exercise (p < 0.001) to reach 81 +/- 7% at end-exercise. In conclusion, dynamic hyperinflation during exhaustive exercise in patients with COPD reduces diaphragm pressure-generating capacity, promoting high levels of diaphragm activation.

Symptom perception during acute bronchoconstriction

The hypothesis underlying the present study was that some of the variability in symptom intensity seen during acute bronchoconstriction may result from varying intensities of several stimuli yielding several sensations that can be identified by specific descriptive expressions (symptoms). A total of 232 subjects inhaled methacholine in doubling concentrations to a 20% decrease in FEV(1), or 64 mg/ml. The study identified the prevalence of dyspnea, nonspecific discomfort associated with the act of breathing, and 10 specific symptom expressions. Each symptom intensity was rated in Borg scale units. The contribution of the specific symptoms to the intensity of dyspnea is illustrated in the following equation (r = 0. 84): Dyspnea = 0.44 + 0.19 Difficult breathing + 0.41 Chest tightness + 0.20 Breathlessness + 0.14 Labored breathing + 0.11 Chest pain. Dyspnea was more intense with broncho-constriction, baseline pulmonary impairment, weight, and sex (being female). Dyspnea was less intense with age (being older) and as airway responsiveness to methacholine increased (p < 0.05 for all factors). Chest tightness and chest pain were at polar extremes on the discrimination scale, i.e., easily discriminated; chest tightness, difficult and labored breathing were not easily discriminated.

Evolution of inspiratory and expiratory muscle pressures during endurance exercise

We investigated the relationship between minute ventilation (VE) and net respiratory muscle pressure (Pmus) throughout the breathing cycle [Total Pmus = mean Pmus, I (inspiratory) + mean Pmus, E (expiratory)] in six normal subjects performing constant-work heavy exercise (CWHE, at approximately 80% maximum) to exhaustion on a cycle ergometer. Pmus was calculated as the sum of chest wall pressure (elastic + resistive) and pleural pressure, and all mean Pmus variables were averaged over the total breath duration. Pmus, I was also expressed as a fraction of volume-matched, flow-corrected dynamic capacity of the inspiratory muscles (P(cap, I)). VE increased significantly from 3 min to the end of CWHE and was the result of a significantly linear increase in Total Pmus (Delta = 43 +/- 9% from 3 min to end exercise, P < 0.005) in all subjects (r = 0. 81-0.99). Although mean Pmus, I during inspiratory flow increased significantly (Delta = 35 +/- 10%), postinspiratory Pmus, I fell (Delta = -54 +/- 10%) and postexpiratory expiratory activity was negligible or absent throughout CWHE. There was a greater increase in mean Pmus, E (Delta = 168 +/- 48%), which served to increase VE throughout CWHE. In five of six subjects, there were significant linear relationships between VE and mean Pmus, I (r = 0.50-0.97) and mean Pmus, E (r = 0.82-0.93) during CWHE. The subjects generated a wide range of Pmus, I/P(cap, I) values (25-80%), and mean Pmus, I/P(cap, I) increased significantly (Delta = 42 +/- 16%) and in a linear fashion (r = 0.69-0.99) with VE throughout CWHE. The progressive increase in VE during CWHE is due to 1) a linear increase in Total Pmus, 2) a linear increase in inspiratory muscle load, and 3) a progressive fall in postinspiratory inspiratory activity. We conclude that the relationship between respiratory muscle pressure and VE during exercise is linear and not curvilinear.

Exercise-induced arterial hypoxemia

Exercise-induced arterial hypoxemia (EIAH) at or near sea level is now recognized to occur in a significant number of fit, healthy subjects of both genders and of varying ages. Our review aims to define EIAH and to critically analyze what we currently understand, and do not understand, about its underlying mechanisms and its consequences to exercise performance. Based on the effects on maximal O(2) uptake of preventing EIAH, we suggest that mild EIAH be defined as an arterial O(2) saturation of 93-95% (or 3-4% <rest), moderate EIAH as 88-93%, and severe EIAH as <88%. Both an excessive alveolar-to-arterial PO(2) difference (A-a DO(2)) (>25-30 Torr) and inadequate compensatory hyperventilation (arterial PCO(2) >35 Torr) commonly contribute to EIAH, as do acid- and temperature-induced shifts in O(2) dissociation at any given arterial PO(2). In turn, expiratory flow limitation presents a significant mechanical constraint to exercise hyperpnea, whereas ventilation-perfusion ratio maldistribution and diffusion limitation contribute about equally to the excessive A-a DO(2). Exactly how diffusion limitation is incurred or how ventilation-perfusion ratio becomes maldistributed with heavy exercise remains unknown and controversial. Hypotheses linked to extravascular lung water accumulation or inflammatory changes in the "silent" zone of the lung's peripheral airways are in the early stages of exploration. Indirect evidence suggests that an inadequate hyperventilatory response is attributable to feedback inhibition triggered by mechanical constraints and/or reduced sensitivity to existing stimuli; but these mechanisms cannot be verified without a sensitive measure of central neural respiratory motor output. Finally, EIAH has detrimental effects on maximal O(2) uptake, but we have not yet determined the cause or even precisely identified which organ system, involved directly or indirectly with O(2) transport to muscle, is responsible for this limitation.

Emerging concepts in the evaluation of ventilatory limitation during exercise: the exercise tidal flow-volume loop

Traditionally, ventilatory limitation (constraint) during exercise has been determined by measuring the ventilatory reserve or how close the minute ventilation (VE) achieved during exercise (i.e., ventilatory demand) approaches the maximal voluntary ventilation (MVV) or some estimate of the MVV (i.e., ventilatory capacity). More recently, it has become clear that rarely is the MVV breathing pattern adopted during exercise and that the VE/MVV relationship tells little about the specific reason(s) for ventilatory constraint. Although it is not a new concept, by measuring the tidal exercise flow-volume (FV) loops (extFVLs) obtained during exercise and plotting them according to a measured end-expiratory lung volume (EELV) within the maximal FV envelope (MFVL), more specific information is provided on the sources (and degree) of ventilatory constraint. This includes the extent of expiratory flow limitation, inspiratory flow reserve, alterations in the regulation of EELV (dynamic hyperinflation), end-inspiratory lung volume relative to total lung capacity (or tidal volume/inspiratory capacity), and a proposed estimate of ventilatory capacity based on the shape of the MFVL and the breathing pattern adopted during exercise. By assessing these types of changes, the degree of ventilatory constraint can be quantified and a more thorough interpretation of the cardiopulmonary exercise response is possible. This review will focus on the potential role of plotting the extFVL within the MFVL for determination of ventilatory constraint during exercise in the clinical setting. Important physiologic concepts, measurements, and limitations obtained from this type of analysis will be defined and discussed.

Role of expiratory flow limitation in determining lung volumes and ventilation during exercise

We determined the role of expiratory flow limitation (EFL) on the ventilatory response to heavy exercise in six trained male cyclists [maximal O2 uptake = 65 +/- 8 (range 55-74) ml. kg-1. min-1] with normal lung function. Each subject completed four progressive cycle ergometer tests to exhaustion in random order: two trials while breathing N2O2 (26% O2-balance N2), one with and one without added dead space, and two trials while breathing HeO2 (26% O2-balance He), one with and one without added dead space. EFL was defined by the proximity of the tidal to the maximal flow-volume loop. With N2O2 during heavy and maximal exercise, 1) EFL was present in all six subjects during heavy [19 +/- 2% of tidal volume (VT) intersected the maximal flow-volume loop] and maximal exercise (43 +/- 8% of VT), 2) the slopes of the ventilation (DeltaVE) and peak esophageal pressure responses to added dead space (e.g., DeltaVE/DeltaPETCO2, where PETCO2 is end-tidal PCO2) were reduced relative to submaximal exercise, 3) end-expiratory lung volume (EELV) increased and end-inspiratory lung volume reached a plateau at 88-91% of total lung capacity, and 4) VT reached a plateau and then fell as work rate increased. With HeO2 (compared with N2O2) breathing during heavy and maximal exercise, 1) HeO2 increased maximal flow rates (from 20 to 38%) throughout the range of vital capacity, which reduced EFL in all subjects during tidal breathing, 2) the gains of the ventilatory and inspiratory esophageal pressure responses to added dead space increased over those during room air breathing and were similar at all exercise intensities, 3) EELV was lower and end-inspiratory lung volume remained near 90% of total lung capacity, and 4) VT was increased relative to room air breathing. We conclude that EFL or even impending EFL during heavy and maximal exercise and with added dead space in fit subjects causes EELV to increase, reduces the VT, and constrains the increase in respiratory motor output and ventilation.

Crural diaphragm activation during dynamic contractions at various inspiratory flow rates

The purpose of this study was to evaluate the influence of velocity of shortening on the relationship between diaphragm activation and pressure generation in humans. This was achieved by relating the root mean square (RMS) of the diaphragm electromyogram to the transdiaphragmatic pressure (Pdi) generated during dynamic contractions at different inspiratory flow rates. Five healthy subjects inspired from functional residual capacity to total lung capacity at different flow rates while reproducing identical Pdi and chest wall configuration profiles. To change the inspiratory flow rate, subjects performed the inspirations while breathing across two different inspiratory resistances (10 and 100 cmH2O . l-1 . s), at mouth pressure targets of -10, -20, -40, and -60 cmH2O. The diaphragm electromyogram was recorded and analyzed with control of signal contamination and electrode positioning. RMS values obtained for inspirations with identical Pdi and chest wall configuration profiles were compared at the same percentage of inspiratory duration. At inspiratory flows ranging between 0.1 and 1.4 l/s, there was no difference in the RMS for the inspirations from functional residual capacity to total lung capacity when Pdi and chest wall configuration profiles were reproduced (n = 4). At higher inspiratory flow rates, subjects were not able to reproduce their chest wall displacements and adopted different recruitment patterns. In conclusion, there was no evidence for increased demand of diaphragm activation when healthy subjects breathe with similar chest wall configuration and Pdi profiles, at increasing flow rates up to 1.4 l/s.

Respiratory muscle dysfunction in mechanically-ventilated patients

The interaction between a patient and a ventilator is the major determinant of the amount of respiratory muscle rest achieved by the machine. We are beginning to acquire a better understanding of the mechanisms that underlie this complex interaction, but this information has yet to be integrated into the routine clinical management of ventilator-supported patients. To achieve that goal, we need better techniques of detecting and monitoring patient-ventilation asynchrony, and the development of simple algorithms that can minimize its occurrence. Finally, research is needed to determine the occurrence and importance of respiratory muscle fatigue during failed weaning attempts so as to better guide the timing and pace of the weaning process in problematic patients.

Contractility of the ventilatory pump muscles

The ventilatory muscles are striated skeletal muscles, and their in situ function is governed by the same relationships that determine the contractile force of muscles in vitro. The ventilatory muscles, however, are functionally distinct from limb skeletal muscles in several aspects, the most notable being that the ventilatory muscles are the only skeletal muscles upon which life depends. Among the muscles that participate in ventilation, the diaphragm is closest to its optimal resting length at functional residual capacity (FRC) and has the greatest capacity for shortening and volume displacement, making it the primary muscle of inspiration. All inspiratory muscles shorten when the lung is inflated above FRC, but interactions among the various inspiratory muscles make for a wider range of high force output than could be achieved by any one muscle group acting in isolation. The velocity of inspiratory muscle shortening, especially diaphragmatic shortening, causes maximal dynamic inspiratory pressures to be substantially lower than maximal static pressures. This effect is especially pronounced during maximal voluntary ventilation, maximal exercise, and maximal inspiratory flow, volume maneuvers over the full vital capacity. During quiet breathing, the ventilatory muscles operate well below the limits of their neural activation and contractile performance. During intense activity, however, the diaphragmatic excursion approaches its limits over the entire vital capacity, and respiratory pressures may near their dynamic maximum. Because the system may operate near its available capacities during increased ventilatory demands, multiple strategies are available to compensate for deficits. For example, if the diaphragm is acutely shortened, it can still generate the required respiratory pressure if it receives more neural drive. Alternatively, other muscles can be recruited to take over for an impaired diaphragm. Thus, the whole system is highly versatile.