Breathlessness during activity is increasingly recognized as a common symptom in the elderly, in older individuals with minor airway obstruction due to tobacco smoking, and in those with obesity. The underlying mechanisms of perceived respiratory difficulty in each of these populations remain unknown and are largely unstudied. Objective: The main purpose of this original research was to elucidate the alterations in central ventilatory drive and in dynamic ventilatory mechanics that might explain the origin of increased breathlessness during exercise in these three populations. General Hypothesis: In individuals with largely preserved resting pulmonary function, increased intensity of breathlessness during activity will arise as a result of measurable abnormalities of dynamic ventilatory mechanics, central respiratory drive, or both in combination. General Methods: We examined sensory-mechanical relationships during cardio-pulmonary exercise testing in each population (total n=146). We compared indices of ventilatory control (ventilation, pulmonary gas exchange and metabolic load), dynamic mechanics (breathing pattern and operating lung volumes) and ratings of dyspnea intensity in three well characterized cohorts: elderly versus younger; smokers with minor airway obstruction versus age-matched non-smokers, and obese versus lean participants. Results: Individuals across all three study populations had preserved resting pulmonary function. Compared with their respective control groups all three symptomatic groups demonstrated consistent abnormalities in dynamic airway function during exercise [expiratory flow limitation (EFL), dynamic increases in end expiratory lung volume, and restricted tidal volume response]; all had greater ventilatory requirements reflecting variable ventilation-perfusion and metabolic abnormalities. In all three groups, intensity of breathlessness increased as ventilatory demand approached capacity. In the elderly (with or without airway obstruction), breathlessness intensity ratings at a standardized ventilation during exercise correlated with indices of mechanical volume restriction secondary to EFL. In obese individuals, increased ventilatory drive secondary to increased metabolic loading (and not mechanical abnormalities) was the primary factor contributing to exertional breathlessness. Conclusion: Although the origin of breathlessness during physical exertion in the elderly (with or without minor airway obstruction) and in obese individuals is multi-factorial, we identified the central etiological importance of the combination of increased ventilatory drive and restrictive dynamic mechanical constraints to increasing ventilation. / Thesis (Ph.D, Physiology) -- Queen's University, 2008-07-02 14:17:41.602