Transmission fatigue of the rabbit diaphragm.

This study evaluates the role of transmission fatigue of the diaphragm in rabbits subjected to inspiratory resistive loading (IRL) sufficiently severe to increase peak tidal airway pressure to about 50% of that elicited by 100 Hz phrenic nerve stimulation. After 58 +/- 14 min of IRL, the transdiaphragmatic pressure (Pdi) responses to phrenic nerve stimulation at 20, 60, and 100 Hz were reduced by approximately one third. In contrast, IRL induced no significant change in the response to direct diaphragm stimulation (in the presence of transient neuromuscular blockade). Although respiratory acidosis occurred during IRL (pH 7.04 +/- 0.04, PCO2 90 +/- 10, PO2 131 +/- 38), it was not sufficient to explain the reduced contractility. In a separate series of experiments, the diaphragm compound action potential elicited by unilateral phrenic nerve stimuli was recorded by implanted diaphragm electrodes and the Pdi elicited by contralateral phrenic nerve stimulation at 100 Hz was measured. Both action potential amplitude and Pdi declined during IRL and both improved after 10 min of recovery. These findings demonstrate that transmission fatigue plays a major role in rabbit diaphragm fatigue induced by spontaneous breathing against inspiratory resistance.