The respiratory muscles in eucapnic obesity: Their role in dyspnea

Diaphragm and Lung Transplantation

Mutual interactions between the diaphragm and lung transplantation (LTx) are known to exist. Before LTx, many factors can exert notable impact on the diaphragmatic function, such as the underlying respiratory disease, the comorbidities, and the chronic treatments of the patient. In the post-LTx setting, even the surgical procedure itself can cause a stressful trauma to the diaphragm, potentially leading to morphological and functional alterations. Conversely, the diaphragm can significantly influence various aspects of the LTx process, ranging from graft-to-chest cavity size matching to the long-term postoperative respiratory performance of the recipient. Despite this, there are still no standard criteria for evaluating, defining, and managing diaphragmatic dysfunction in the context of LTx to date. This deficiency hampers the accurate assessment of those factors which affect the diaphragm and its reciprocal influence on LTx outcomes. The objective of this narrative review is to delve into the complex role the diaphragm plays in the different stages of LTx and into the modifications of this muscle following surgery.

Potential Therapeutic Role of Respiratory Muscle Training in Dyspnea Management of Cancer Survivors: A Narrative Review

Dyspnea is a disabling symptom presented in approximately half of all cancer survivors. From a clinical perspective, despite the availability of pharmacotherapies, evidence-based effective treatments are limited for relieving dyspnea in cancer survivors. Preliminary evidence supports the potential of respiratory muscle training to reduce dyspnea in cancer survivors, although large randomized controlled studies are warranted. The aims of this article were to review the relevant scientific literature on the potential therapeutic role of respiratory muscle training in dyspnea management of cancer survivor, and to identify possible mechanisms, strengths and limitations of the evidence as well as important gaps for future research directions.

Relationship Between Obesity and Youth Athletes' Physical Activity and Exercise-Related Cardiac Symptoms

Background: While studies have identified the negative cardiovascular effects of obesity, the effects of obesity on youth athletes are less studied. This study investigates the associations between obesity in youth athletes and exercise activity and exertional cardiac symptoms. Methods: The HeartBytes National Youth Database produced by Simon's Heart, a nonprofit that organizes adolescents' sports pre-participation examinations (PPEs), was used. This database contains demographic data, exercise-related symptoms, and electrocardiogram data obtained during PPEs. BMI was converted to percentiles, with obesity defined as BMI ≥95th percentile. Outcomes were evaluated using a chi-squared test with odds ratios (ORs) and 95% confidence intervals (CIs). Results: Of 7363 patients, there were 634 individuals with obesity. Youth athletes within normal weight ranges (5th-85th percentile) had higher exercise rates than those with underweight, overweight, or obesity (p < 0.001 for each). Athletes with obesity had higher odds of exertional symptoms overall (OR: 1.63 [CI: 1.36-1.96]; p < 0.001). However, for athletes who exercised >10 hours a week, there was no association between obesity and exertional symptoms. Athletes with obesity had higher odds of elevated blood pressure (OR: 5.35 [CI: 2.00-14.30]; p < 0.001) and hypercholesterolemia (OR: 3.84 [CI: 2.51-5.86]; p < 0.001). Conclusions: In this dataset, obesity in youth athletes is associated with decreased physical activity and increased exertional symptoms in general. Obesity is not associated with exertional symptoms in athletes who participated in higher weekly physical activity. Further studies are needed to elucidate the cause-effect relationship of these findings.

Thrombospondin-1 promotes fibro-adipogenic stromal expansion and contractile dysfunction of the diaphragm in obesity

Pulmonary disorders impact 40-80% of individuals with obesity. Respiratory muscle dysfunction is linked to these conditions; however, its pathophysiology remains largely undefined. Mice subjected to diet-induced obesity (DIO) develop diaphragmatic weakness. Increased intra-diaphragmatic adiposity and extracellular matrix (ECM) content correlate with reductions in contractile force. Thrombospondin-1 (THBS1) is an obesity-associated matricellular protein linked with muscular damage in genetic myopathies. THBS1 induces proliferation of fibro-adipogenic progenitors (FAPs)-mesenchymal cells that differentiate into adipocytes and fibroblasts. We hypothesized that THBS1 drives FAP-mediated diaphragm remodeling and contractile dysfunction in DIO. We tested this by comparing effects of dietary challenge on diaphragms of wild-type (WT) and Thbs1 knockout ( Thbs1 -/- ) mice. Bulk and single-cell transcriptomics demonstrated DIO-induced stromal expansion in WT diaphragms. Diaphragm FAPs displayed upregulation of ECM and TGFβ-related expression signatures, and augmentation of a Thy1 -expressing sub-population previously linked to type 2 diabetes. Despite similar weight gain, Thbs1 -/- mice were protected from these transcriptomic changes, and from obesity-induced increases in diaphragm adiposity and ECM deposition. Unlike WT controls, Thbs1 -/- diaphragms maintained normal contractile force and motion after DIO challenge. These findings establish THBS1 as a necessary mediator of diaphragm stromal remodeling and contractile dysfunction in overnutrition, and potential therapeutic target in obesity-associated respiratory dysfunction.

Effects of Exercise Mode on Improving Cardiovascular Function and Cardiorespiratory Fitness After Bariatric Surgery: A Narrative Review

ABSTRACT

Obesity affects 600 million people globally and increases the risk of developing cardiovascular disease, stroke, diabetes, and cancer. Bariatric surgery is an increasingly popular therapeutic intervention for morbid obesity to induce rapid weight loss and reduce obesity-related comorbidities. However, some bariatric surgery patients, after what is considered a successful surgical procedure, continue to manifest obesity-related health issues, including weight gain, reduced physical function, persistent elevations in blood pressure, and reduced cardiorespiratory fitness. Cardiorespiratory fitness is a strong predictor of mortality and several health outcomes and could be improved by an appropriate exercise prescription after bariatric surgery. This review provides a broad overview of exercise training for patients after bariatric surgery and discusses cardiorespiratory fitness and other potential physiological adaptations in response to exercise training.

The effect of trait self-control on dyspnoea and tolerance to a CO2 rebreathing challenge in healthy males and females

BACKGROUND

High trait self-control is associated with greater tolerance of unpleasant sensations including effort and pain. Dyspnoea and pain have several commonalities and this study aimed to investigate for the first time whether trait self-control influences responses to a hypercapnic rebreathing challenge designed to induce dyspnoea. As sex also influences tolerance to dyspnoea, we also sought to investigate whether this moderated the role of trait self-control.

METHODS

Participants (n = 65, 32 females) scoring high or low for trait self-control, performed a standardised rebreathing challenge, in which inspired carbon dioxide (CO₂) gradually increased over a period of 6 min or until an intolerable level of dyspnoea. Air hunger (AH) intensity - a distinctive quality of dyspnoea, was measured every 30 s. The multidimensional dyspnoea profile (MDP) was completed after the rebreathing challenge for a more complete overview of breathing discomfort.

RESULTS

Males high in trait self-control (SCHIGH) (302 ± 42 s), tolerated the rebreathing challenge for longer than males low in self-control (SCLOW) (252 ± 66 s, P = 0.021), experienced slower increases in AH intensity during the rebreathing challenge (0.03 ± 0.01 cm^.s ^- 1 vs. 0.04 ± 0.01 cm^.s ^- 1,P = 0.045) and reported lower perceived mental effort on the MDP (4.94 ± 2.46 vs. 7.06 ± 1.60, P = 0.007). There was no difference between SCHIGH and SCLOW females for challenge duration. However, SCHIGH females (9.29 ± 0.66 cm) reported greater air hunger at the end of the challenge than SCLOW females (7.75 ± 1.75 cm, P = 0.003). It is possible that SCLOW females were unwilling to tolerate the same perceptual intensity of AH as the SCHIGH females.

CONCLUSIONS

These results indicate that individuals high in trait self-control are more tolerant of dyspnoea during a CO₂ rebreathing challenge than low self-control individuals. Tolerance of the stimulus was moderated by the sex of the participant, presenting an interesting opportunity for future research.

Impact of obesity on pulmonary function: current understanding and knowledge gaps

PURPOSE OF REVIEW

Obesity is an increasing world-wide public health concern. Obesity both causes respiratory symptoms and contributes to many cardiorespiratory diseases. The effects of obesity on commonly used lung function tests are reviewed.

RECENT FINDINGS

The effects of obesity on lung function are attributed both to mechanical factors and to complex metabolic effects that contribute to a pro-inflammatory state. The effects of obesity on lung function correlate with BMI and correlate even better when the distribution of excess adipose tissue is taken into account, with central obesity associated with more prominent abnormalities. Obesity is associated with marked decreases in expiratory reserve volume and functional residual capacity. Total lung capacity, residual volume, and spirometry are less affected by obesity and are generally within the normal range except with severe obesity. Obesity decreases total respiratory system compliance primarily because of decreased lung compliance, with only mild effects on chest wall compliance. Obesity is associated with impaired gas transfer with decreases in oxygenation and varied but usually mild effects on diffusing capacity for carbon monoxide, while the carbon monoxide transfer coefficient is often increased.

SUMMARY

Obesity has significant effects on lung function. The relative contribution of the mechanical effects of obesity and the production of inflammatory cytokines by adipose tissue on lung function needs further study.

Effects of physical activity and smoking on cardio-ankle vascular index, respiratory muscle strength, and exercise performance in early normal weight adulthood: a cross-sectional study

Few studies have shown the impact of unhealthy habits on arterial stiffness, respiratory muscle strength and exercise performance in early normal weight adulthood. This study sought to determine the effects of physical activity and smoking on its parameters in normal weight male participants. Forty-eight participants were divided into four groups: physically inactive nonsmokers and smokers and physically active nonsmokers and smokers (n=12 in each group). All of the participants were measured for body composition, arterial stiffness, respiratory muscle strength and exercise performance. Two-way analysis of variance design was used to test the main and interaction effects of physical activity by group (smokers vs. nonsmokers). P-value of less than 0.05 was considered as a statistically significant difference. As a result, cardio-ankle vascular index and respiratory muscle strength were diminished in smokers and physically inactive participants, while body and visceral fat mass were increased in both those groups. Fat-free mass was lower in only physically inactive participants. This study also found the interaction effects on body fat and arterial stiffness. In conclusion, participants with healthy normal weight, but smoked and performed inappropriate physical activity, exhibited body composition imbalance, decreased respiratory muscle strength, exercise performance, and increased arterial stiffness. Therefore, smoking cessation and exercise in younger adults are appropriate ways of improving body composition, respiratory muscle strength, aerobic capacity and arterial stiffness instead of trying to control their weight by smoking.

Three weeks of respiratory muscle endurance training improve the O2 cost of walking and exercise tolerance in obese adolescents

Obese adolescents (OB) have an increased O2 cost of exercise, attributable in part to an increased O2 cost of breathing. In a previous work a short (3-week) program of respiratory muscle endurance training (RMET) slightly reduced in OB the O2 cost of high-intensity cycling and improved exercise tolerance. We hypothesized that during treadmill walking the effects of RMET would be more pronounced than those observed during cycling. Sixteen OB (age 16.0 ± 0.8 years; body mass [BM] 127.7 ± 14.2 kg; body mass index 40.7 ± 4.0 kg/m2 ) underwent to 3-week RMET (n = 8) superimposed to a multidisciplinary BM reduction program, or (CTRL, n = 8) only to the latter. Heart rate (HR) and pulmonary O2 uptake ( V ˙ O2 ) were measured during incremental exercise and 12-min constant work rate (CWR) walking at 60% (moderate-intensity, MOD) and 120% (heavy-intensity, HEAVY) of the gas exchange threshold (GET). The O2 cost of walking (aerobic energy expenditure per unit of covered distance) was calculated as V ˙ O2 /velocity. BM decreased (~4-5 kg) both in CTRL and in RMET. V ˙ O2 peak and GET were not affected by both interventions; the time to exhaustion increased following RMET. During MOD and HEAVY RMET decreased V ˙ O2, the O2 cost of walking (MOD: 0.130 ± 0.033 mL/kg/m [before] vs. 0.109 ± 0.027 [after], P = 0.03; HEAVY: 0.196 ± 0.031 [before] vs. 0.180 ± 0.025 [after], P = 0.02), HR and rates of perceived exertion; no significant changes were observed in CTRL. In OB a short RMET program lowered the O2 cost of MOD and HEAVY walking and improved exercise tolerance. RMET could represent a useful adjunct in the control of obesity.

Exertional dyspnoea in obesity

The purpose of cardiopulmonary exercise testing (CPET) in the obese person, as in any cardiopulmonary exercise test, is to determine the patient's exercise tolerance, and to help identify and/or distinguish between the various physiological factors that could contribute to exercise intolerance. Unexplained dyspnoea on exertion is a common reason for CPET, but it is an extremely complex symptom to explain. Sometimes obesity is the simple answer by elimination of other possibilities. Thus, distinguishing among multiple clinical causes for exertional dyspnoea depends on the ability to eliminate possibilities while recognising response patterns that are unique to the obese patient. This includes the otherwise healthy obese patient, as well as the obese patient with potentially multiple cardiopulmonary limitations. Despite obvious limitations in lung function, metabolic disease and/or cardiovascular dysfunction, obesity may be the most likely reason for exertional dyspnoea. In this article, we will review the more common cardiopulmonary responses to exercise in the otherwise healthy obese adult with special emphasis on dyspnoea on exertion.

When obesity and chronic obstructive pulmonary disease collide. Physiological and clinical consequences

In many parts of the world, the prevalence of both chronic obstructive pulmonary disease (COPD) and obesity is increasing at an alarming rate. Such patients tend to have greater respiratory symptoms, more severe restriction of daily activities, poorer health-related quality of life, and greater health care use than their nonobese counterparts. Physiologically, increasing weight gain is associated with lung volume reduction effects in both health and disease, and this should be considered when interpreting common pulmonary function tests where lung volume is the denominator, such as FEV1/FVC and the ratio of diffusing capacity of carbon monoxide to alveolar volume, or indeed when evaluating the physiological consequences of emphysema in obese individuals. Contrary to expectation, the presence of mild to moderate obesity in COPD appears to have little deleterious effect on respiratory mechanics and muscle function, exertional dyspnea, and peak symptom-limited oxygen uptake during cardiopulmonary exercise testing. Thus, in evaluating obese patients with COPD reporting activity restriction, additional nonpulmonary factors, such as increased metabolic loading, cardiocirculatory impairment, and musculoskeletal abnormalities, should be considered. Care should be taken to recognize the presence of obstructive sleep apnea in obese patients with COPD, as effective treatment of the former condition likely conveys an important survival advantage. Finally, morbid obesity in COPD presents significant challenges to effective management, given the combined effects of erosion of the ventilatory reserve and serious metabolic and cardiovascular comorbidities that collectively predispose to an increased risk of death from respiratory failure.

Evaluation of cardiorespiratory fitness and respiratory muscle function in the obese population

Cardiorespiratory fitness (CRF) is one of the most important health metrics in apparently healthy individuals, those at increased risk for cardiovascular (CV) disease and virtually all patient populations. In addition to CRF, a host of other variables obtained from aerobic exercise testing provides clinically valuable information. Individuals classified as obese (i.e. a body mass index ≥30 kg/m(2)) have varying degrees of CV, pulmonary and skeletal muscle dysfunction that impact CRF and other key aerobic exercise testing variables. Moreover, there is now evidence indicating inspiratory and expiratory respiratory muscle function, even in the absence of interstitial lung disease, is potentially compromised as a result of obesity. When obesity-induced respiratory muscle dysfunction is present, it has the potential to contribute to the limitations in CRF. The current review will discuss aerobic exercise testing and the assessment of respiratory muscle function in the obese population.

The modified Medical Research Council scale for the assessment of dyspnea in daily living in obesity: a pilot study

Background

Dyspnea is very frequent in obese subjects. However, its assessment is complex in clinical practice. The modified Medical Research Council scale (mMRC scale) is largely used in the assessment of dyspnea in chronic respiratory diseases, but has not been validated in obesity. The objectives of this study were to evaluate the use of the mMRC scale in the assessment of dyspnea in obese subjects and to analyze its relationships with the 6-minute walk test (6MWT), lung function and biological parameters.

Methods

Forty-five obese subjects (17 M/28 F, BMI: 43 ± 9 kg/m²) were included in this pilot study. Dyspnea in daily living was evaluated by the mMRC scale and exertional dyspnea was evaluated by the Borg scale after 6MWT. Pulmonary function tests included spirometry, plethysmography, diffusing capacity of carbon monoxide and arterial blood gases. Fasting blood glucose, total cholesterol, triglyceride, N-terminal pro brain natriuretic peptide, C-reactive protein and hemoglobin levels were analyzed.

Results

Eighty-four percent of patients had a mMRC ≥ 1 and 40% a mMRC ≥ 2. Compared to subjects with no dyspnea (mMRC = 0), a mMRC ≥ 1 was associated with a higher BMI (44 ± 9 vs 36 ± 5 kg/m², p = 0.01), and a lower expiratory reserve volume (ERV) (50 ± 31 vs 91 ± 32%, p = 0.004), forced expiratory volume in one second (FEV₁) (86 ± 17 vs 101 ± 16%, p = 0.04) and distance covered in 6MWT (401 ± 107 vs 524 ± 72 m, p = 0.007). A mMRC ≥ 2 was associated with a higher Borg score after the 6MWT (4.7 ± 2.5 vs 6.5 ± 1.5, p < 0.05).

Conclusion

This study confirms that dyspnea is very frequent in obese subjects. The differences between the “dyspneic” and the “non dyspneic” groups assessed by the mMRC scale for BMI, ERV, FEV₁ and distance covered in 6MWT suggests that the mMRC scale might be an useful and easy-to-use tool to assess dyspnea in daily living in obese subjects.

Respiratory muscle endurance training in obese patients

OBJECTIVE

Increased respiratory muscle work is associated with dyspnea and poor exercise tolerance in obese patients. We evaluated the effect of respiratory muscle endurance training (RMET) on respiratory muscle capacities, symptoms and exercise capacity in obese patients.

DESIGN

A total of 20 obese patients hospitalized for 26 ± 6 days to follow a low-calorie diet and a physical activity program were included in this case-control study. Of them, 10 patients performed RMET (30-min isocapnic hyperpnea at 60-80% maximum voluntary ventilation, 3-4 times per week during the whole hospitalization period: RMET group), while the other 10 patients performed no respiratory training (control (CON) group). RMET and CON groups were matched for body mass index (BMI) (45 ± 7 kg m(-2)) and age (42 ± 12 years). Lung function, respiratory muscle strength and endurance, 6-min walking distance, dyspnea (Medical Research Council scale) and quality of life (short-form health survey 36 questionnaire) were assessed before and after intervention.

RESULTS

Similar BMI reduction was observed after hospitalization in the RMET and CON groups (-2 ± 1 kg m(-2), P < 0.001). No significant change in lung function and respiratory muscle strength was observed except for vital capacity, which increased in the RMET group (+0.20 ± 0.26 l, P = 0.039). Respiratory muscle endurance increased in the RMET group only (+52 ± 27%, P < 0.001). Compared with the CON group, the RMET group had greater improvement in 6MWT (+54 ± 35 versus +1 ± 7 m, P = 0.007), dyspnea score (-2 ± 1 versus -1 ± 1 points, P = 0.047) and quality of life (total score: +251 ± 132 versus +84 ± 152 points, P = 0.018) after hospitalization. A significant correlation between the increase in respiratory muscle endurance and improvement in 6MWT distance was observed (r (2) = 0.36, P = 0.005).

CONCLUSIONS

The present study indicates that RMET is feasible in obese patients and can induce significant improvement in dyspnea and exercise capacity. RMET may be a promising tool to improve functional capacity and adherence to physical activities in this population, but further studies are needed to confirm these results.