EFFECTS OF MECHANICAL VENTILATION ON DIAPHRAGMATIC CONTRACTILE PROPERTIES IN RATS

We measured in rats the effects of 48 h of mechanical ventilation on t he weight, contractile properties, and enzymatic profile of the diaphr agm, the soleus and the extensor digitorium longus (EDL) muscles. Eigh teen animals were randomly divided into a mechanically ventilated (MV, n = 9) group or a control (C, n = 9) group. During the 48 h of mechan ical ventilation, animals in the MV group were anesthetized with sodiu m thiopental and enterally fed with a gastric catheter. Group C animal s were neither anesthetized nor mechanically ventilated during the 48- h experimental period, and they had access to food and water ad libitu m. Muscular contractile properties were measured in vitro by analysis of force-frequency curves and twitch characteristics. The weights of t he three muscles were significantly reduced in the MV group compared w ith those in the C group. This was accompanied in the diaphragm by a r eduction in the normalized force generated for all the frequencies of stimulation, except 20 Hz, whereas twitch characteristics were not mod ified. The forces generated by the soleus and EDL were not significant ly reduced in the MV group compared with those in the C group. Diaphra gm, soleus, and EDL citrate synthase and lactate dehydrogenase activit ies were not significantly different in the two groups. We conclude th at mechanical ventilation for 48 h in rats produces a selective force reduction in the diaphragm.