Effects of loading and unloading the respiratory system on respiratory control and muscle function

To determine the effects of internal respiratory loading and unloading on respiratory neuromuscular function, ventilatory (Vi), occlusion pressure (P0.15), transdiaphragmatic pressure (Pdi) and diaphragmatic electromyogram (EMGdi) responses to CO2 rebreathing were assessed in 6 normal volunteers rebreathing gas mixtures denser (63% SF6, 30% O2, 7% CO2) and less dense (63% He, 30% O2, 7% CO2) than air (63% N2, 30% O2, 7% CO2). Loading with SF6 decreased the Vi response to CO2 rebreathing and increased P0.15 and Pdi for a given EMGdi, while the greater the increase in pressure response the less was the decrease in Vi. Unloading with He had the opposite effect. The pattern of breathing was altered with SF6, with Ti and Te increasing and frequency decreasing, while there was no change in timing with He. Internal loading of inspiration and expiration with SF6 elicits compensatory responses that depend on changes of respiratory timing and enhanced diaphragm efficiency. Adjustments to unloading are generally opposite to that observed with loading.