Introduction: Length-tension and force-velocity characteristics of respiratory muscles and hyperinflation are the likely determinants of dyspnea in subjects exercising under hyperbaric conditions. We hypothesize that hyperinflation plays a minor role and that the reduced velocity of shortening of the respiratory muscles modulates dyspnea for any given pleural pressure.
Methods: We studied five normal subjects who performed an incremental exercise test on a cycloergometer in both normobaric (SL) and hyperbaric (4 ATA) conditions. We measured breathing pattern, inspiratory pleural pressure swing (Pessw), ΔPes (i.e., the difference between the most and the less negative pleural pressures during tidal breathing), and dyspnea intensity (Borg score). End-expiratory lung volume (EELV) changes were evaluated by measuring changes in inspiratory capacity. Mean inspiratory flow (VT/TI) was used as an index of velocity of shortening of respiratory muscles.
Results: Compared with SL, at 4 ATA, peak exercise ventilation (VE) (84.5 vs 62.2 L·min−1) and VT/TI (2.99 vs 2.16 L·s−1) were lower, Pessw (30.9 vs 38.6 cm H2O) and ΔPes (43.8 vs 62.2 cm H2O) were higher, and Borg score was not different (7.60 vs 8.20 au). EELV decreased progressively during exercise but remained higher than at SL. Borg score was greater for a same VE and lower for a same ΔPes. VT/TI was lower for a same Pessw. The differences in EELV between SL and 4 ATA did not relate with the concurrent changes in Borg score.
Conclusion: The results confirm our hypothesis that during exercise in hyperbaric conditions, decreased velocity of shortening of respiratory muscles modulates pressure-induced increases in dyspnea, with hyperinflation playing a minor role.