EFFECT OF GLOBAL INSPIRATORY MUSCLE FATIGUE ON VENTILATORY AND RESPIRATORY MUSCLE RESPONSES TO CO2

We evaluated the effect of global inspiratory muscle fatigue on ventil ation and respiratory muscle control during CO2 rebreathing in normal subjects. Fatigue was induced by breathing against a high inspiratory resistance until exhaustion. CO2 response curves were measured before and after fatigue. During CO2 rebreathing, global fatigue caused a dec reased tidal volume (VT) and an increased breathing frequency but did not change minute ventilation, duty cycle, or mean inspiratory flow. B oth esophageal and transdiaphragmatic pressure swings were significant ly reduced after global fatigue, suggesting decreased contributions of both rib cage muscles and diaphragm to breathing. End-expiratory tran spulmonary pressure for a given CO2 was lower after fatigue, indicatin g an additional decrease in end-expiratory lung volume due to expirato ry muscle recruitment, which leads to a greater initial portion of ins piration being passive. This, combined with the reduction in VT, decre ased the fraction Of VT attributable to inspiratory muscle contributio n; therefore the inspiratory muscle elastic work and power per breath were significantly reduced. We conclude that respiratory control mecha nisms are plastic and that the respiratory centers alter their output in a manner appropriate to the contractile state of the respiratory mu scles to conserve the ventilatory response to CO2.