Mechanical ventilation results in progressive contractile dysfunction in the diaphragm

These experiments tested the hypothesis that a relatively short duration of controlled mechanical ventilation (MV) will im-pair diaphragmatic maximal speci?c force generation (specific Po) and that this force de?cit will be exacerbated with increased time on the ventilator. To test this postulate, adult Sprague-Dawley rats were randomly divided into one of six experimental groups: 1) control (n = 12); 2)12hofMV(n = 4); 3)18hofMV(n = 4); 4) 18 h of anesthesia and sponta-neous breathing (n = 4); 5)24hofMV(n = 7); and 6)24hof anesthesia and spontaneous breathing (n = 4). MV animals were anesthetized, tracheostomized, and ventilated with room air. Animals in the control group were acutely anesthe-tized but were not exposed to MV. Animals in two spontane-ous breathing groups were anesthetized and breathed spon-taneously for either 18 or 24 h. No differences (P = 0.05) existed in diaphragmatic speci?c Po between control and the two spontaneous breathing groups. In contrast, compared with control, all durations of MV resulted in a reduction (P = 0.05) in diaphragmatic speci?c tension at stimulation fre-quencies ranging from 15 to 160 Hz. Furthermore, the MV-induced decrease in diaphragmatic speci?c Po was time dependent, with speci?c Po being =18 and =46% lower (P = 0.05) in animals mechanically ventilated for 12 and 24 h, respectively. These data support the hypothesis that relatively short-term MV impairs diaphragmatic contractile function and that the magnitude of MV-induced force de?cit increases with time on the ventilator