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Respiratory Investigation Unit, Department of Medicine, Queen's University, Kingston, Ontario, Canada K7L 2V7
We
mimicked important mechanical and ventilatory aspects of restrictive
lung disorders by employing chest wall strapping (CWS) and dead space
loading (DS) in normal subjects to gain mechanistic insights into
dyspnea causation and exercise limitation. We hypothesized that
thoracic restriction with increased ventilatory stimulation would evoke
exertional dyspnea that was similar in nature to that experienced in
such disorders. Twelve healthy young men [28 ± 2 (SE) yr of
age] completed pulmonary function tests and maximal cycle
exercise tests under four conditions, in randomized order: 1)
control, 2) CWS to 60% of vital capacity, 3) added DS
of 600 ml, and 4) CWS + DS. Measurements during exercise
included cardiorespiratory parameters, esophageal pressure, and Borg
scale ratings of dyspnea. Compared with control, CWS significantly
reduced the tidal volume response to exercise, increased dyspnea
intensity at any given work rate or ventilation, and thus limited
exercise performance. DS stimulated ventilation but had minimal effects
on dyspnea and exercise performance. Adding DS to CWS further increased
dyspnea by 1.7 ± 0.6 standardized Borg units (P = 0.012) and
decreased exercise performance (total work) by 21 ± 6% (P = 0.003) over CWS alone. Across conditions, increased dyspnea intensity
correlated best with decreased resting inspiratory reserve volume
(r = 
0.63, P < 0.0005). Dyspnea
during CWS was described primarily as "inspiratory difficulty"
and "unsatisfied inspiration," similar to restrictive disorders.
In conclusion, severe dyspnea and exercise intolerance were provoked in
healthy normal subjects when tidal volume responses were constrained in
the face of increased ventilatory drive during exercise.
dyspnea; exercise; mechanisms
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