We determined the role of expiratory flow limitation (EFL) on the ventilatory response to heavy exercise in six trained male cyclists [maximal O₂ uptake = 65 ± 8 (range 55-74) ml · kg¹ · min¹] with normal lung function. Each subject completed four progressive cycle ergometer tests to exhaustion in random order: two trials while breathing N₂O₂ (26% O₂-balance N₂), one with and one without added dead space, and two trials while breathing HeO₂ (26% O₂-balance He), one with and one without added dead space. EFL was defined by the proximity of the tidal to the maximal flow-volume loop. With N₂O₂ during heavy and maximal exercise, 1) EFL was present in all six subjects during heavy [19 ± 2% of tidal volume (VT) intersected the maximal flow-volume loop] and maximal exercise (43 ± 8% of VT), 2) the slopes of the ventilation (E) and peak esophageal pressure responses to added dead space (e.g., E/PET_CO2, where PET_CO2 is end-tidal PCO₂) were reduced relative to submaximal exercise, 3) end-expiratory lung volume (EELV) increased and end-inspiratory lung volume reached a plateau at 88-91% of total lung capacity, and 4) VT reached a plateau and then fell as work rate increased. With HeO₂ (compared with N₂O₂) breathing during heavy and maximal exercise, 1) HeO₂ increased maximal flow rates (from 20 to 38%) throughout the range of vital capacity, which reduced EFL in all subjects during tidal breathing, 2) the gains of the ventilatory and inspiratory esophageal pressure responses to added dead space increased over those during room air breathing and were similar at all exercise intensities, 3) EELV was lower and end-inspiratory lung volume remained near 90% of total lung capacity, and 4) VT was increased relative to room air breathing. We conclude that EFL or even impending EFL during heavy and maximal exercise and with added dead space in fit subjects causes EELV to increase, reduces the VT, and constrains the increase in respiratory motor output and ventilation.

dead space; helium-oxygen; feedback inhibition; respiratory muscle loading/unloading

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