Delayed diaphragm injury and diaphragm force production

The present study was designed to examine the effect of delayed diaphragm injury produced by inspiratory resistive loading (IRL) on diaphragm force production. On Day 1, three groups of anesthetized and intubated NZW rabbits (n = 7 in each group) were subjected to moderate IRL (Pao approximately 30 cm H2O), high IRL (Pao approximately 45 cm H2O), or no load for 1.5 h. On Day 3, the baseline twitch transdiaphragmatic pressure (Pdi) and Pdi at 10 to 80 Hz were measured during bilateral phrenic stimulation and these measurements were repeated after another IRL (high level) in all three groups. Diaphragm injury was assessed by the point-counting technique. Marked diaphragm injury was observed in the high-IRL group (p < 0.01), but no significant diaphragm injury was observed in the moderate-IRL or control groups. The baseline twitch Pdi was maintained in both IRL groups, whereas the baseline Pdi-frequency values in the high-IRL group were significantly reduced at most frequencies (p < 0.05). The decreases in twitch and Pdi at different frequencies were more pronounced after the IRL on Day 3 in the high-IRL group compared with controls. Moderate IRL did not decrease diaphragm force either before or after the high IRL on Day 3. We conclude that the diaphragm injury induced by high IRL has a significant impact on diaphragm force production and the attendant force loss produced by IRL is dependent on the intensity of inspiratory loading.