Diaphragm and phrenic nerve activities during inspiratory loading in anesthetized rabbits.

To determine whether decreases in neural activation and/or neuromuscular transmission to the diaphragm contribute to ventilatory failure observed during inspiratory resistive loaded breathing, peak integrated activity of the intact phrenic nerve (ENGdi) and costal diaphragm (EMGdi), transdiaphragmatic pressure swings (Pdi) and evoked diaphragm compound action potentials (M-wave) were measured in anesthetized rabbits subjected to inspiratory resistive loads of varying intensities and duration breathing supplemental oxygen. Sustainable loads were studied for 4 h. Severe loads were applied in increments and studied for 50 min each. Loading resulted in parallel increases in ENGdi, EMGdi and Pdi that stabilized within 20 min. With severe loading, peak inspiratory pressure (Pao = -55 +/- 4 cm H2O) was maintained for 30 min after which there was a significant drop in inspiratory pressure (task failure). Despite hypoventilation and profound changes in blood gases, both activation (ENGdi) and neuromuscular transmission (ENGdi/EMGdi ratio or M-wave) were maintained throughout all loads even at task failure when a critical level of PaO2 (27 +/- 1 mm Hg) was reached. We conclude that neural activation and neuromuscular transmission to the diaphragm do not fail during inspiratory resistive breathing and discuss our findings in terms of current concepts of diaphragm fatigue and control of breathing in this model.