The Effect of Exercise Test Modality on Dyspnea Perception in Obese Patients with COPD
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Background: Obesity is becoming increasingly prevalent in patients with chronic obstructive pulmonary disease (COPD) but the physiological and clinical consequences of their combination remain poorly understood. In particular, the impact of obesity on dyspnea and exercise intolerance in COPD is clinically pertinent but little studied and is the main focus of this thesis. Previous studies utilizing cycle ergometry have concluded that obesity does not convey either mechanical or sensory disadvantages during physical activity in COPD. However, it remains to be seen whether such advantages persist in COPD during weight-bearing walking when metabolic requirements are greater than during cycling Aim: To examine contributors to dyspnea in obese COPD by comparing physiological and perceptual responses at similar work rates during the two exercise conditions where metabolic loading and respiratory muscle activity are distinctly different. Methods: Obese (body mass index >30 kg/m2) patients were recruited with moderate to severe airflow obstruction (post-bronchodilator FEV1 30-79% predicted). We compared metabolic, ventilatory (breathing pattern, operating lung volumes, respiratory muscle function and electromyography of the diaphragm) and dyspnea (intensity and quality) during symptom-limited cycle and treadmill exercise protocols using a matched linearized incremental 10 W step rise in work rate. Results: Cycle exercise was associated with reduced oxygen uptake, greater arterial oxygen saturation, earlier ventilatory threshold, greater neuromuscular efficiency and activity of the diaphragm, and less expiratory muscle activity compared to treadmill walking (p<0.01). However, ventilation, breathing pattern, operating lung volumes, global respiratory effort and electrical activation of the diaphragm were similar at comparable work rates across modalities. Accordingly, dyspnea intensity and quality were not different. Conclusions: Our results indicate that, when the rise in work rate is standardized, dyspnea intensity and quality are independent of small inter-modality difference in metabolic acidosis and arterial oxygen saturation that could potentially influence efferent output from the central respiratory controller. Moreover, altered afferent inputs associated with inter-modality differences in diaphragmatic function and expiratory muscle activity do not directly influence dyspnea at least when respiratory neural drive, breathing pattern and operating lung volumes are similar.