The Mechanisms Of Bronchopulmonary Dysplasis Induced By Hyperoxia And Mechanical Ventilation

Along with ventilation strategy changed, glucocorticosteroid used perinatal and lung surfactant applicated postnatal, more and more premature and extremely low birth weight infants can survive, but Bronchopulmonary dysplasis( BPD) still exists, BPD is one kind of chronic lung disease ( CLD). It is usually occured in the premature of hyaline membrane disease accepting oxygen and machinery ventilation treatment. At present most people have advocated to adopt BPD naming, thinking it can clearly discriminate many kinds of CLD in postnatal days. BPD is defined that premature infant, postnatal day 28 or week 36 (gestation week +postnatal week) need oxygen, this definition can accurately forecast prognosis of lung injury. BPD clinical manifestations are that infants depend on oxygen and still need machinery ventilation after primary affection basicly improved; have repeatedly lung infections which are not easy to controlled, have obvious dyspnea, easy to have CO₂ accumulated with hypoxia, their lung functional parameters obviously descend and some babies complicate pulmonary artery hypertension and cardia faiture. Some are serious to die of respiratory failure in postnatal 1 year. The survivals need to rely on oxygen or machine treatment for long-term (several months - several years) because of lung functional disturbance. It influences the survival quality of premature. Because BPD pathogenesis is complicated. At present no therapeutic strategy has been proven clinically effective. Therefore discussing the mechanisms of BPD induced by hyperoxia and mechanical ventilator has become a topic which will be resolved in infant domain.BPD may be divided into two kinds of pathological changes according to pathology characteristic. First, "traditional BPD", that inflammation is the main performance, displays leukocytic infiltrate, emphysema, pulmonary atelectasis and pulmonary fibrosis in lung organization. Second, "new BPD", that lung developmental arrest is the main performance, displays interalveolar septum to reduce, capillaries growth to disord, alveolar epithelium multiplication to be inhibited and so on. The research indicated that oxyradical derivate, inflammation reaction and machinery ventilation can damage lung epithelial cells, blood capillary endothelial cells, inhibit surface-active material function. It inhibits the development of pulmonary alveoli, small respiratory tract and small vessels, thus affects maturity of lung. The production of inflammation and oxyradical derivate may induce apoptosis. Ventilator induced lung injury (VILI) is a serious complication in mechnical ventilation process. Excessively high air pressure and large tidal volume can induce and aggravate lung injury. The mechanism of lung injury is : barotrauma; volutrauma; atelectrauma ;Biotrauma.Lung development results from the balance between stimulatory and inhibitory influences, and that two of the key regulators are glucocorticoids and TGF-β. In recent years along with transformation growth factor -β₁ (TGF-β₁) deeply studied, its role has been paid close attention in the pulmonary alveolus damage and impaired development of lung capillaries. TGF-β₁ is the major regulation that adjusts lung growth, cell division, extracellular matrix reconstruction, the respiratory tract reconstruction and inflammation. These commonly promote BPD occurring. TGF-β₁ participates in opposite two aspect mechanisms of lung damage and reparation, namely it induces apoptosis, infection, fibrosis and pulmonary alveolus remodeling. The interaction of TGF-β₁ and other cytokines regulates considerately the development of lung.Along with attention to apoptosis in the mechanisms of lung damage, plastiosome apoptosis way has become the hot point. It is a key point in apoptosis and other formal cell death. The releasing of cytochrome C ( Cytc) from plastiosome to ketoplasm is key point that induce apoptosis. It is confirmed that lung tissue apoptosis is correlated colsely with acute lung injury. Therefore, exploring the regular and control mechanism of plastiosome apoptosis of lung tissue in BPD not only for us to develop the mechanism of alveoli stunted, but also to find new therapeutic strategy.. We made a rabbit model of BPD, and viewed leucocyte count, the changes of lung alveoli and capillaries, the apoptosis course of alveolar endothelial cellâ…¡.At the same time, the lung were collected for immunohistochemistry and PCR. The purpose of this study was to investigate the effects of hyperoxia and mechanism ventilator on lung damage after BPD; dual effects of TGF-β₁ in mechanism of lung damage; the effects of apoptosis way of plastiosome /cytochrome C in the lung tissue cell apoptosis.Materials and Methods96 rabbits on postnatal day 3～5 were arranged randomly into four groups: three groups were subjected to hyperoxia (90 ml/L O₂) plus mechanical ventilation(MV), first received high peak inspiratory pressure(HPIP), second received moderate peak inspiratory pressure(MPIP), third received low peak inspiratory pressure(LPIP) and the last group with no mechanical ventilation and room air. All rabbits were sacrificed at 1h, 3h, 6h after trail respectively. White blood cell (WBC)counts and segmented neutrccyte counts in BALF were measured. The change of lung histopathology and radical alveolar counts (RAC) were assessed by hematoxylin-eosin (HE) staining and observed under light microscope, the change of lung ultrastructure by electron microscope at 6h. The lungs were collected for immunohistochemistry and PCR to explore the expressions of transforming growth factor-β₁ (TGF-β₁) and cytochromeC(Cytc) mRNA and the distribution of TGF-β₁ and Cytc. The apoptosis index of lung cell was identified by TUNEL.Results1 .The conditions of newborn rabbits: The rabbits were well and breathe regularly in control group, were bad and dyspneic in groups with MV. The most obvious expression was in LPIP group at 6h, second was in HPIP group at 6h, third was in MPIP group at 1h.2 .WBC counts and segmented neutrccyte counts in BALFWBC counts increased in groups with MV, the tendency was most significant in HPIP group at 6h, second was in LPIP, there was a descend tendency in MPIP group at 6h. Segmented neutrccyte counts increased to the highest tip in HPIP group at 3h, the same in LPIP group at 6h. Combinating thechanges of lung capillaries and fiber in pathological section, the beginning of lung capillaries injury was infections induced by inhaled hyperoxia and MV. Capillaries occlusion was a important cause of the decreasinging lung capillaries. The paraplasm of fibroblast began with the striking stage of inflammatory reaction.3.pathology consequencelight microscope: the shape of lung alveoli were regular, alveolar septum were not thick in controls. Inflammatory reactions of lung capillaries aggravated at 1h in groups with MV. Angiotelectasis and congestion appeared in interstitial substance, the hyperplasy of fibroblast produced more collagen fibers. Capillaries occlusion brought about QI-blood barrie thicking. The alveolar space enlarged, lung alveoli and lung capillaries decreased at 3h, dramatically at 6h. Lung alveoli were long and narrow, alveolar septum was widen, Capillaries occlusion appeard and interstitial cells displayed an increase. These expressions were significant in HPIP group, then LPIP group. The Lung alveoli collapse and pulmonary closure and asphyxial membrance were obviously in LPIP group.electron microscope: the plentiful lung capillaries and integrity blood vessel endothelium and ACE-â… ,â…¡and basal membrane were found in control group. The lung capillaries expanded , RBC and neutrophilic leukocytes appeard in alveolar space at 1h with MV, At 6h the lung capillaries reduced and Lung alveoli were long and narrow, interstitial cells increased, the plastiosome of ACE-â… ,â…¡swelled , lamelar body were destroyed and ACE-â…¡converted to ACE-1. The expressions at 6h were obviously in HPIP and LPIP groups, The Lung alveoli collapse and pulmonary closure were obviously in LPIP group. The changes of structure of plastiosome was correlated with cell apoptosis.4. TGF-β₁ protein and mRNA level expressionIn groups with MV, TGF-β₁ mRNA level rised at 1h and at peak at 3h ,difference was seen between HPIP group and others, p <0.05. TGF-β₁ protein level rised at 3h and at peak at 6h, the expression was significant in HPIP group, signficant difference was seen among four groups, p<0.01. The peak of TGF-β₁ protein level slightly delayed after the peak of TGF-β₁ mRNA level. TGF-β₁ protein level was positive-related to WBC, and negative to RAC.5. Cytc protein and mRNA level expressionIn groups with MV, Cytc mRNA level rised, most obviously in HPIP group,then in LPIP group. The peak was at 3h, The expression of Cytc protein level rised from 1h to 3h, at 3h signficant difference was seen between each group with MV to controls, p< 0.01, the level was less reducing from 3h to 6h. Cytc protein level rised most obviously in LPIP group at 6h.Conclusions;1. After hyperoxia and machinery ventilation, immature lung presents the alveolar septum and number diminuting, the pulmonary alveolus degree decreases, the blood capillary to reduce unenlightenedly, the lung fibrosis characteristic. The expression of cell stretch damage induced from high peak inspiratory Pressure is most serious.2. With machinery ventilation, The inflammation of cell stretch damage induced from High Peak Inspiratory Pressure reacts strongestly in immature lung, along with the time expand, the inflammation induced from atelectrauma of LPIP presents more serious.3. The oxidative stress result from hyperoxia and machinery ventilation causes TGF-β₁ to express highly, most obvious from HPIP. It promotes pulmonary alveolus and blood vessel to growth in initial period , then inhibites them.4. After hyperoxia and machinery ventilation, Cytc expression increases, most obvious from HPIP. Along with the time expand, the expression of atelectrauma of LPIP is remarkable, Cytc expression was related to the changes of structure mitochondria when ACE-â…¡apoptosing.5. Mitochondria composes and releases more Cytc to cytosolic with machinery ventilation, causing oxidative stress and Cytc results caspase activation cascade, at last apoptosis.