Central fatigue of the rabbit diaphragm

This study evaluates the importance of central fatigue of the diaphragm in rabbits subjected to inspiratory muscle resistive loading (IRL). Ten rabbits were subjected to constant IRL while unanesthetized and breathing supplemental oxygen. During 10-20 minutes of spontaneous breathing against IRL, there were no significant changes in arterial oxygen saturation or in diaphragm contractility, measured by the quasi-static transdiaphragmatic pressure response to a 0.3-sec train of 100 Hz supramaximal phrenic nerve stimuli. After an initial decrease due to application of the load, the minute ventilation decreased further, by an average of 15%, while arterial pCO₂ increased to an average of 59 mmHg (p < 0.05). The normalized diaphragm pressure-time index initially increased from 0.02 to 0.18 during IRL, then decreased an average of 29% (p < 0.05). These results show that severe IRL causes a decrease in the level of diaphragmatic effort over time despite increased chemical drive and despite a preserved ability of the muscle to respond to phrenic nerve stimuli. This adaptation may help to prevent peripheral diaphragm fatigue.