Experimental Study Of The Effect Of Controlled Mechanical Ventilation On Diaphragm Of Rat

Objective: To investigate the effect of controlled mechanical ventilation on diaphragm of SD rat, and hope to provide the theoretic basis for difficulties in discontinuing ventilatory support which were induced by the altered function and morphology in diaphragm with mechanical ventilation.Methods: Twenty-four adult male Sprague-Dawley rats were randomly assigned into three experimental groups: control, 18h-CMV, or 24h-CMV groups. Controlled mechanical ventilation was achieved using a small-animal ventilator (RES3020; AniRes2005) with respiratory rate 80/min,tidal volume about 5 ml/kg of body weight, inspiratory : expiratory 15:10, and positive end-expiratory pressure (PEEP) 1 cmH2O. By BL-420 measuring system, Trans-diaphragmatic pressure (Pdi), maximal trans- diaphragmatic pressure ( Pdimax ), diaphragm electromyogram (EMGdi ), and diaphragm muscle force were measured during controlled mechanical ventilation at various stimulation frequencies. Morphological changes of diaphragm myofibril were observed by transmission electron microscopy. Myosin heavy chain(MHC)isoforms expressed were analyzed with SDS-glycerol PAGE and Western blotting.Results 1 Compared with the control group, the Pdi was significantly reduced in both the 18h-CMV group and the 24h-CMV group (both P<0.01). The Pdimax decreased to 39.8% in the 18h-CMV group (P<0.01) and decreased further to 58.9% in the 24h-CMV group (P<0.01) compared with the control group [14.92±0.16 (SE) mmHg] in a time-dependent manner.2 Compared with the control group, the Fc and H/L was significantly reduced in the 18h-CMV group and the 24h-CMV group (P<0.01): the Fc decreased to 13.7% in the 18h-CMV group and decreased further to 22.6% in the 24h-CMV group; the H/L decreased to 47.6% in the 18h-CMV group and decreased further to 64.4% in the 24h-CMV group in a time-dependent manner.3 At all stimulation frequencies, diaphragm contractile force was significantly declined in the 18h-CMV group and the 24h-CMV group compared with the control group (P<0.05 or P<0.01). At the stimulation frequency of 100 Hz, force decreased to 14.3% in the 18h-CMV group (P<0.05) and decreased further to 46.3% in the 24h-CMV group (P<0.01) compared with the control group (98.13±0.50N). At a stimulation frequency of 75 Hz, contractile force decreased to 18.4% in the 18h-CMV group (P<0.05) and decreased further to 45.5% in the 24h-CMV group (P<0.01) compared with the control (93.55±0.48 N). It happened in a time-dependent manner also.4 The ultramicrostructure changes of rat diaphragm was found in 24h-CMV group, but not in 18h-CMV group and control group. The changes included disrupted myofibrils, increased numbers of lipid vacuoles in the sarcoplasm, and abnormally small mitochondria containing focal membrane disruptions.5 Compared with the control group, the percentage of MHCslow and MHC2A was significantly decreased in the 18h-CMV group and the 24h-CMV group (P<0.05). The MHC2A percentage was decreased further to 2.2%(27.7%±0.19% vs 29.9%±0.30%)and 4.3%(25.6%±0.17% vs 29.9%±0.30%)respectively compared to the control group.Conclusion1 CMV leads to Pdi,Pdimax,Fc,H/L,and diaphragm contractile force of the rat diaphragm decreasing. It can be found as early as 18h or 24h, and it will be worsening as CMV prolonging.2 CMV leads to structural injury and adaptively changes of MHC isoforms in the rat diaphragm with a manner of time-dependent.3 Short CMV could set up a fatigued diaphragm model in the SD rats, and alter the function and morphology of diaphragm. It means that mechanical ventilation, especially CMV could induce diaphragmatic dysfunction, which maybe one of the important reasons for difficult weaning in some patients.