INTRODUCTION
Chronic respiratory diseases (CRD) are associated with abnormalities in the airways and other structures of the lung, with asthma being the most common CRD in the pediatric age range, and cystic fibrosis (CF) being the most frequent genetic disease in Caucasians 1. Asthma is a chronic inflammatory disease characterized by variable airway obstruction, leading to hyperresponsiveness, inflammation, and respiratory symptoms, representing a major cause of pediatric hospitalization worldwide 2. On the other hand, CF is a hereditary, autosomal recessive disease, caused by the mutation of a gene that encodes the transmembrane conductance regulator protein (CFTR). The absence or dysfunction of the protein leads to a multisystem disease, inducing obstruction in secretory glands and a pro-inflammatory state, especially in the lungs 3.
In patients with CRD, exercise intolerance is common and is usually considered as the inability of individuals to perform exercise at the same levels that would be expected for an age-matched control4. Patients with asthma use to report exercise‐associated symptoms which are related to multiple factors, including the degree of airway obstruction, decreased ventilatory capacity, a greater sensation of dyspnea, exercise-induced bronchoconstriction (EIB), or low exercise capacity 5. Despite this, there is no clear consensus on their exercise capacity. Some studies reported no differences between healthy and asthmatic patients 6,7, while others showed lower respiratory capacity in those with a diagnosis of asthma 8–10. For children and adolescents with CF, evidence reports a reduction in exercise capacity compared to healthy controls 11.
The forced expiratory volume in the first second (FEV1) is one of the most used clinical parameters for monitoring CRD, including asthma 2 and CF 3. Evidence indicates that FEV1 correlates with clinical worsening and EIB in children and adolescents with asthma12, but implications of lung function on reduced exercise capacity are still unclear 13,14. In patients with CF, evidence suggests that only a part of the variability in exercise capacity can be explained by FEV115. In general, the mechanisms responsible for exercise limitation in CRD are still poorly understood. In individuals with asthma, exercise intolerance may result from a combination of complex interactions between mechanical, physiological, and psychological mechanisms, including bronchial smooth muscle contraction due to increased breathing, loss of heat, and moisture in the respiratory tract 5. On the other hand, there are controversial data on mechanisms underlying low exercise capacity in CF, which may be related to poor nutritional status, peripheral muscle dysfunction, dysfunctional gas exchange, and exercise-induced ventilatory dysfunction 15.
During progressive exercise, minute ventilation (VE) must increase through a combination of a rapid increase in tidal volume to a maximum of approximately 50% of forced vital capacity (FVC) and a progressive but steady increase in respiratory rate 16. The most typical feature of CRD is progressive expiratory airflow obstruction and the development of expiratory flow limitation. As exercise ventilatory demands increase, the combination of high respiratory rates and decreased expiratory flows may result in an insufficient expiratory time to completely exhale the inspired breath 17. Ventilatory limitation in CRD can be reflected in different parameters during CPET, such as ventilatory efficiency or breathing reserve (BR). Ventilatory efficiency is represented by ventilatory equivalents for oxygen consumption (VE/VO2), and for carbon dioxide production (VE/VCO2)18. The increase in ventilatory demand due to abnormal ventilatory control in CRD can lead to poor ventilatory efficiency, with a need for greater minute ventilation (VE) to eliminate the same amount of carbon dioxide as compared to healthy children7,19. On the other hand, BR compares how closely VE achieved in peak exercise approaches the maximal voluntary ventilation (MVV) 20. The ratio of peak exercise minute ventilation to MVV (BRI), ranges from 0.40 to 0.75 in untrained healthy individuals 21. In patients with CRD the BRI is elevated, suggesting reduced BR at peak exercise11,22. BR has been considered a powerful predictor of mortality in CF patients awaiting lung transplantation23, although it has not been reported in patients with asthma 24.
A better understanding of how CRD may affect aerobic fitness and the identification of the main mechanisms leading to exercise intolerance may help researchers and health professionals to better monitor and treat those patients. Thus, the aim of this study was to evaluate exercise capacity and its association with lung function, ventilatory limitation, and ventilatory efficiency in children and adolescents with mild‐to‐moderate CF and asthma when compared to healthy controls.