EFFECT OF DIAPHRAGMATIC FATIGUE ON CONTROL OF RESPIRATORY MUSCLES ANDVENTILATION DURING CO2 REBREATHING

We studied the influence of diaphragmatic fatigue on the control of ve ntilation and respiratory muscle contribution to pressure swings in si x normal seated subjects. CO2 was rebreathed before and after diaphrag matic fatigue induced by breathing against an inspiratory resistance r equiring 60% maximal transdiaphragmatic pressure with each breath unti l exhaustion. After diaphragmatic fatigue for a given level of end-tid al Pco2, we found that tidal volume, breathing frequency, minute venti lation, duty cycle, and mean inspiratory flow did not change; esophage al pressure swings were the same, but gastric and transdiaphragmatic p ressure swings were decreased; and the slope of the transpulmonary pre ssure-gastric pressure relationship determined at zero flow points at end expiration and end inspiration was increased. End-expiratory trans pulmonary 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 th at 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 di aphragmatic contractility.