Loading and unloading breathing during exercise: respiratory responses and compensatory mechanisms

To characterize the ventilatory responses to resistive loading or unloading, we studied the effects of breathing 79% helium-21% oxygen (He-O⊂2), 79% argon-21% oxygen (Ar-O⊂2) and 79% SF⊂6-21% oxygen (SF⊂6-O⊂2) on the volume-time parameters, end-tidal partial pressure of CO⊂2 (PETCO⊂2), mouth pressure (P⊂Im), work of breathing (W⊂I), central inspiratory activity (dP/dt⊂I), and electromyographic activity of parasternal inspiratory muscles (EMGps) in 10 normal subjects at rest and during short-time steady-state exercise. There were no significant changes in tidal volume (V⊂T), breathing frequency (f), inspiratory (T⊂I) and expiratory (T⊂E) durations, minute ventilation (V⊂E), and P⊂ET CO⊂2 when air was replaced by He-O⊂2 or SF⊂6-O⊂2 at rest. V⊂E and P⊂ET CO⊂2 were not significantly different after replacement of air by He-O⊂2 or SF⊂6-O⊂2 during exercise. However, inhalation of He-O⊂2 decreased in V⊂T and increased in f, whereas inhalation of SF⊂6-O⊂2 led to the opposite effects compared with air during exercise. Both at rest and exercise, P⊂Im, W⊂I, dP/dt⊂I and EMGps were significantly less during He-O⊂2 breathing and higher during SF⊂6-O⊂2 breathing (P<0.01) from the first respiratory cycle after room air was replaced by He-O⊂2 or SF⊂6-O⊂2. Ar-O⊂2 breathing did not affect the time-volume parameters both at rest and during exercise compared with air. The increase in P⊂Im, W⊂I, and dP/dt⊂I was observed at Ar-O⊂2 inhalation during exercise relatively to air conditions (P<0.05). We conclude that internal resistive loaded (SF⊂6-O⊂2) or unloaded (He-O⊂2) breathing changes the neuromuscular output required to maintain constant ventilation. The mechanisms of load or unload compensation seem to be mediated by afferent information from lung and respiratory muscle receptors as well as by segmentary reflexes and properties of muscle fibers.