Physiological mechanisms of dyspnea during exercise in the presence of external thoracic restriction: role of increased neural respiratory motor drive

Background & rationale. "Dyspnea" refers to the awareness of breathing discomfort that accompanies an increase in physical activity in health and across various diseases. It is arguably the most severe and burdensome symptom experienced by patients with chronic pulmonary disorders and is an important contributor to physical activity-limitation and adverse health outcomes, including hospitalization and death. Nevertheless, the mechanisms of dyspnea on exertion in health and disease remain partially understood. Accumulating evidence implicates neuromechanical uncoupling of the respiratory system as a likely mechanism of activity-related dyspnea, particularly in patients with chronic pulmonary diseases. According to this hypothesis, sensory intensity and unpleasantness ratings of dyspnea increase as a function of the widening disparity (as exercise progresses) between neural respiratory drive and the simultaneous response of the respiratory system, particularly as it relates to tidal volume (VT) expansion. An alternative and largely untested hypothesis states that the increased perception of dyspnea during exercise may reflect the awareness of increased neural respiratory drive needed achieve any given ventilation (V· E) in the presence of "abnormal" restrictive constraints on VT expansion. To date, the contribution of pathophysiological abnormalities in neural respiratory drive, dynamic respiratory mechanics and their interaction to the symptom of dyspnea during exercise in patients with chronic pulmonary disorders has proved difficult to study (beyond correlation) due to the presence of multiple co-morbidities that may independently contribute to the perception of dyspnea. Research Objectives. In light of the information cited above, the objectives of this research project were to better understand the physiological mechanisms of exertional dyspnea. Methods. This randomized cross-over study examined the acute effects of external thoracic restriction by chest wall strapping (CWS) – an accepted model of the "abnormal" restrictive constraints on VT expansion typical of patients with chronic pulmonary disorders - on detailed assessments of V· E, breathing pattern, dynamic respiratory mechanics, neural respiratory drive (as assessed by changes in the diaphragm electromyogram; EMGdi), and sensory intensity and unpleasantness ratings of dyspnea during symptom-limited incremental cycle exercise testing in 20 healthy, young men with normal lung function and cardiorespiratory fitness. Results. The key findings of this study include: [1] relatively greater dynamic mechanical constraints on VT expansion were evident during exercise with vs. without CWS; [2] EMGdi was consistently higher during exercise with vs. without CWS; [3] CWS had no effect on neuromechanical coupling of the respiratory system, as evidenced by relative preservation of the relationship between increasing EMGdi and VT expansion (adjusted for CWS-induced reductions in vital capacity) during exercise; [4] sensory intensity and unpleasantness ratings of dyspnea were significantly higher during exercise with vs. without CWS; and [5] CWS had no effect on the relationship between increasing EMGdi and each of the intensity and unpleasantness of dyspnea during progressive exercise. Conclusions & implications. We concluded that the increased perception of dyspnea during exercise with CWS could not be readily explained by increased neuromechanical uncoupling of the respiratory system, but that it likely reflected the awareness of increased neural respiratory drive needed to overcome the "abnormal" restrictive constraints on VT expansion. These findings may have implications for our understanding of the pathophysiological mechanisms of exertional dyspnea causation in patients with chronic restrictive lung disorders. This information, in turn, may aid in the development of more effective dyspnea relieving interventions for use in these patients.

Récupéré en direct depuis OpenAlex et désinversé. Les résumés ne sont pas conservés dans cette base de données : les index inversés représentent 8,6 Go des 9,3 Go de texte de la base, et le serveur dispose de 13 Go libres.