Effect of diaphragmatic fatigue on control of respiratory muscles and ventilation during CO2 rebreathing.

We studied the influence of diaphragmatic fatigue on the control of ventilation and respiratory muscle contribution to pressure swings in six normal seated subjects. CO2 was rebreathed before and after diaphragmatic fatigue induced by breathing against an inspiratory resistance requiring 60% maximal transdiaphragmatic pressure with each breath until exhaustion. After diaphragmatic fatigue for a given level of end-tidal PCO2, we found that tidal volume, breathing frequency, minute ventilation, duty cycle, and mean inspiratory flow did not change; esophageal pressure swings were the same, but gastric and transdiaphragmatic pressure swings were decreased; and the slope of the transpulmonary pressure-gastric pressure relationship determined at zero flow points at end expiration and end inspiration was increased. End-expiratory transpulmonary pressure progressively decreased and end-expiratory gastric pressure progressively increased with increasing end-tidal PCO2 by the same magnitude before and after diaphragmatic fatigue. We conclude that diaphragmatic fatigue induces proportionately greater contributions of inspiratory rib cage muscles than of the diaphragm, which results in the preservation of ventilatory response to CO2 despite impaired diaphragmatic contractility.