Tidal volume inflection and its sensory consequences during exercise in patients with stable asthma.

Sixteen patients with stable asthma performed a symptom-limited constant work-rate CWR cycle exercise during which breathing pattern, operating lung volumes, dyspnea intensity and its qualitative descriptors were measured. An inflection in the relation between tidal volume (V-T) and ventilation (V-E) was observed in each subject. The sense of "work/effort" was the dominant dyspnea descriptor selected up to the V-T/(V) over dot(E) inflection, whereas after it dyspnea intensity and the selection frequency of "unsatisfied inspiration" rose steeply in 37.5% of subjects in whom inspiratory reserve volume (IRV) had decreased to a critical level of 0.6 L at the V-T inflection point. In contrast, dyspnea increased linearly with exercise time and (V) over dot(E), and "work/effort" was the dominant descriptor selected throughout exercise in 62.5% of subjects in whom the V-T/(V) over dot(E) inflection occurred at a preserved IRV. The V-T inflection during exercise in patients with stable asthma marked a mechanical event with important sensory consequences only when it occurred at a critical reduced IRV. (C) 2012 Elsevier B.V. All rights reserved.