a research report from the chinese center for disease control and prevention showed that the incidence of paraquat poisoning increased by about 47.35% a year on average from 2002 to 2010 . although the survival rate of pq poisoning increased with the improvement of medical levels , there were still a considerable number of patients who died after large doses of oral pq . moderate and severe pq poisoning was mainly characterized by acute lung injury in the early stage and pulmonary fibrosis in the advanced stage . when the patients showed ali / ards or progressive pulmonary fibrosis , the conventional treatment was ineffective . used continuous positive airway pressure ( cpap ) to treat pq poisoning patients with respiratory failure and the patients died in 15 days . histopathology showed overexpansion of pulmonary parenchyma , suggesting that although cpap ventilation could increase lung volume , it might cause lung overexpansion and aggravate lung injury . lung protective strategies of ventilation ( lpvs ) has been proposed to treat ali / ards in recent years , which advocated small vt ventilation ( 6~8 ml / kg ) , permissive hypercapnia , and maintaining alveolus opening by using peep . at present , treating pq - induced acute lung injury by lpvs still lacks support from basic and clinical research . the present study was conducted to investigate the effects of different vt peep on pq - induced acute lung injury , blood gas analysis indexes , oxygenation index , and hemodynamics and aimed to provide theoretical guidance for the clinical application of mechanical ventilation . eighteen female piglets ( 6570 days , 25.02.1 kg ) were obtained from the experimental animal center of henan province , zhengzhou , china . the piglets were provided with water ad libitum and fasted 12 hours before the operation . after intramuscular injection of atropine ( 0.05 mg / kg ) ( harvest pharmaceutical co. ltd , shanghai , china ) and ketamine ( 15 mg / kg ) ( fujian gutian pharmaceutical co. ltd , gutian , china ) , the piglets were placed supine on a table with continuous ecg monitoring and ear - vein injection of ringer lactate solution ( 10 ml / kgh ) . the piglets received mechanical ventilation and the settings of volume controlled ventilation ( vcv ) were : vt=12 ml / kg , rr=30 breath / min , inspiratory / expiratory=1:2 , fraction of inspired oxygen ( fio2)=30% , peep=0 the central venous catheter was set by jugular vein and the picco catheter was set by femoral artery . each piglet received intraperitoneal injection of 20 ml 20% pq solution ( sigma , st . louis , mo , usa ) and the arterial blood gas analysis was measured every 30 minutes until pao2/fio2300 mmhg . the pao2/fio2300 mmhg in 30 minutes was considered to be successfully developed the ali / ards models . the piglets received intravenous injection of propofol ( 2.5 mgkgh ) ( pfizer , new york , ny , usa ) and sufentanil ( 0.025 gkgh ) ( humanwell pharmaceutical co. ltd . meanwhile , the piglets were given cis atracurium ( 0.1 mgkgh ) ( glaxosmithkline , london , uk ) intermittently to maintain muscle relaxation . the piglets were then randomly divided into three groups : small vt group ( vt=6 ml / kg , n=6 ) , middle vt group ( vt=10 ml / kg , n=6 ) , and large vt group ( vt=15 ml / kg , n=6 ) , with the peep set as 10 cmh2o . this study was performed in strict accordance with the recommendations in the guide for the care and use of laboratory animals of the national institutes of health ( bethesda , md , usa ) eighth edition , 2010 . the animal use protocol was reviewed and approved by the institutional animal care and use committee ( iacuc ) of the first affiliated hospital of zhengzhou university . the arterial blood was drawn at t2 ( 2 hours ) , t4 ( 4 hours ) , and t6 ( 6 hours ) after mechanical ventilation for blood gas analysis . the picco temperature sensor was connected with the jugular vein catheter and the picco detector was connected with the femoral artery catheter . we rapidly injected 10 ml ice saline ( 04c ) via the jugular vein catheter and recorded elwi , pvpi , heart rate ( hr ) , and mean arterial pressure ( map ) each time at baseline ( before ali / ards modeling ) , t0 , t2 , t4 , and t6 . tissues of the right lower lobe of the lung were fixed in 4% paraformaldehyde at room temperature , embedded in paraffin , sectioned serially at 10 m , and stained with hematoxylin and eosin ( he ) . the data were analyzed by unpaired student s t test for comparison of same group at different time and  test for comparison of different groups at same time . eighteen female piglets ( 6570 days , 25.02.1 kg ) were obtained from the experimental animal center of henan province , zhengzhou , china . the piglets were provided with water ad libitum and fasted 12 hours before the operation . after intramuscular injection of atropine ( 0.05 mg / kg ) ( harvest pharmaceutical co. ltd , shanghai , china ) and ketamine ( 15 mg / kg ) ( fujian gutian pharmaceutical co. ltd , gutian , china ) , the piglets were placed supine on a table with continuous ecg monitoring and ear - vein injection of ringer lactate solution ( 10 ml / kgh ) . the piglets received mechanical ventilation and the settings of volume controlled ventilation ( vcv ) were : vt=12 ml / kg , rr=30 breath / min , inspiratory / expiratory=1:2 , fraction of inspired oxygen ( fio2)=30% , peep=0 the central venous catheter was set by jugular vein and the picco catheter was set by femoral artery . each piglet received intraperitoneal injection of 20 ml 20% pq solution ( sigma , st . louis , mo , usa ) and the arterial blood gas analysis was measured every 30 minutes until pao2/fio2300 mmhg . the pao2/fio2300 mmhg in 30 minutes was considered to be successfully developed the ali / ards models . the piglets received intravenous injection of propofol ( 2.5 mgkgh ) ( pfizer , new york , ny , usa ) and sufentanil ( 0.025 gkgh ) ( humanwell pharmaceutical co. ltd . meanwhile , the piglets were given cis atracurium ( 0.1 mgkgh ) ( glaxosmithkline , london , uk ) intermittently to maintain muscle relaxation . the piglets were then randomly divided into three groups : small vt group ( vt=6 ml / kg , n=6 ) , middle vt group ( vt=10 ml / kg , n=6 ) , and large vt group ( vt=15 ml / kg , n=6 ) , with the peep set as 10 cmh2o . this study was performed in strict accordance with the recommendations in the guide for the care and use of laboratory animals of the national institutes of health ( bethesda , md , usa ) eighth edition , 2010 . the animal use protocol was reviewed and approved by the institutional animal care and use committee ( iacuc ) of the first affiliated hospital of zhengzhou university . the arterial blood was drawn at t2 ( 2 hours ) , t4 ( 4 hours ) , and t6 ( 6 hours ) after mechanical ventilation for blood gas analysis . the picco temperature sensor was connected with the jugular vein catheter and the picco detector was connected with the femoral artery catheter . we rapidly injected 10 ml ice saline ( 04c ) via the jugular vein catheter and recorded elwi , pvpi , heart rate ( hr ) , and mean arterial pressure ( map ) each time at baseline ( before ali / ards modeling ) , t0 , t2 , t4 , and t6 . tissues of the right lower lobe of the lung were fixed in 4% paraformaldehyde at room temperature , embedded in paraffin , sectioned serially at 10 m , and stained with hematoxylin and eosin ( he ) . all data were analyzed using spss17.0 software ( spss inc . , chicago , il , usa ) . the data were analyzed by unpaired student s t test for comparison of same group at different time and  test for comparison of different groups at same time . as shown in table 1 , hr and map in the three groups increased after modeling success ( t0 ) compared with baseline , with significant difference ( p<0.05 ) . hr and map in the three groups all decreased after mechanical ventilation compared with t0 , with significant difference ( p<0.05 ) . there was no significant difference in hr in the three groups at the same time , but map in the three groups showed significant difference at the same time . as shown in figure 1 , ph , pao2 , and oxygenation index in the three groups all decreased after modeling success compared with baseline , and paco2 increased significantly ( p<0.05 ) . ph in the three groups decreased gradually after mechanical ventilation ( f=5.392 , p=0.000 ) and the small vt group decreased most obviously after ventilation for 6 hours ( figure 1a ) . as shown in figure 1b and c , pao2 and oxygenation index in the three groups increased after ventilation for 2 hours and then decreased gradually , with significant difference in the three groups ( pao2 : f=22.732 , p=0.000 ; oxygenation index : f=17.148 , p=0.000 ) . pao2 and oxygenation index in the small vt group showed the most obvious increase after ventilation for 2 hours and was much higher than the other two groups after ventilation for 6 hours . paco2 in the three groups increased gradually after mechanical ventilation and the small vt group showed the most obvious increase ( figure 1d ) . the elwi and pvpi in the three groups significantly increased after ali / ards compared with baseline ( table 2 ) . the elwi in the three groups increased obviously after ventilation for 2 hours and then the small vt group clearly decreased . pip and pplat in the three groups increased after modeling success compared with baseline without significant difference ( figure 2 ) . pip and pplat in the small vt group decreased gradually , and in the middle vt group and large vt group they increased after ventilation ( pip : f=13.787 , p=0.000 ; pplat : f=10.469 , p=0.000 ) . pip and pplat in the small vt group were much lower than in the other two groups after ventilation for 6 hours . the alveolar walls in normal lung tissues were thin and integral and there was no abnormal exudate in the pulmonary interstitium . in ali / ards models , the alveolar tissues showed obvious congestion and edema , swelling of capillary vessels , widened alveolar septum , formation of partially visible membrane , and extravasation of red blood cells in the pulmonary interstitium . after ventilation for 6 hours , the piglets in the three groups showed lung tissue fusion , further widened alveolar septum , further swelling of capillary vessels , and partial alveolar septum fracture . as shown in figure 3 , the large vt group had the most severe damage while the small vt group had only minimal damage . as shown in table 1 , hr and map in the three groups increased after modeling success ( t0 ) compared with baseline , with significant difference ( p<0.05 ) . hr and map in the three groups all decreased after mechanical ventilation compared with t0 , with significant difference ( p<0.05 ) . there was no significant difference in hr in the three groups at the same time , but map in the three groups showed significant difference at the same time . as shown in figure 1 , ph , pao2 , and oxygenation index in the three groups all decreased after modeling success compared with baseline , and paco2 increased significantly ( p<0.05 ) . ph in the three groups decreased gradually after mechanical ventilation ( f=5.392 , p=0.000 ) and the small vt group decreased most obviously after ventilation for 6 hours ( figure 1a ) . as shown in figure 1b and c , pao2 and oxygenation index in the three groups increased after ventilation for 2 hours and then decreased gradually , with significant difference in the three groups ( pao2 : f=22.732 , p=0.000 ; oxygenation index : f=17.148 , p=0.000 ) . pao2 and oxygenation index in the small vt group showed the most obvious increase after ventilation for 2 hours and was much higher than the other two groups after ventilation for 6 hours . paco2 in the three groups increased gradually after mechanical ventilation and the small vt group showed the most obvious increase ( figure 1d ) . the elwi and pvpi in the three groups significantly increased after ali / ards compared with baseline ( table 2 ) . the elwi in the three groups increased obviously after ventilation for 2 hours and then the small vt group clearly decreased . pip and pplat in the three groups increased after modeling success compared with baseline without significant difference ( figure 2 ) . pip and pplat in the small vt group decreased gradually , and in the middle vt group and large vt group they increased after ventilation ( pip : f=13.787 , p=0.000 ; pplat : f=10.469 , p=0.000 ) . pip and pplat in the small vt group were much lower than in the other two groups after ventilation for 6 hours . the alveolar walls in normal lung tissues were thin and integral and there was no abnormal exudate in the pulmonary interstitium . in ali / ards models , the alveolar tissues showed obvious congestion and edema , swelling of capillary vessels , widened alveolar septum , formation of partially visible membrane , and extravasation of red blood cells in the pulmonary interstitium . after ventilation for 6 hours , the piglets in the three groups showed lung tissue fusion , further widened alveolar septum , further swelling of capillary vessels , and partial alveolar septum fracture . as shown in figure 3 , the large vt group had the most severe damage while the small vt group had only minimal damage . pq - poisoning patients show acute lung injury , liver and kidney injury , and systemic inflammation . most patients die of respiratory failure caused by lung injury [ 79 ] and some survivors showed different degrees of pulmonary fibrosis after treatment . pq accumulated in the lungs after being absorbed into the blood , which might be related with the amine uptake system of the lungs . the mechanism of pq - induced ali / ards is that the cell membrane was damaged by massive reactive oxygen species , which further caused pulmonary hemorrhage , hyaline membrane degeneration , and necrocytosis . the patients showing acute lung injury ( e.g. , chest stuffiness , dyspnea ) develop progressive pulmonary fibrosis and finally die of lung failure . the main pathological changes of pq - induced ali were about alveolar capillary membrane damage , which caused pulmonary congestion and edema , reduction of lung volume and ventilation volume , lung compliance decrease , and imbalance of the ventilation - perfusion ratio . the increase of elwi is one of the most important features of ali / ards and is an important cause of refractory hypoxemia . pvpi is the ratio of pulmonary extravascular fluid / extravascular fluid and is important in determining the mechanism of pulmonary edema . pvpi>3 can distinguish between hydrostatic pressure and pulmonary edema [ 1618 ] . in this study , elwi increased significantly after modeling success compared with baseline , which reflected the degree of pulmonary edema to a certain extent . pvpi was much higher than the baseline after modeling success , which illustrated that pq - induced pulmonary edema was a permeability edema . lpvs has been proposed to treat ali / ards in recent years , which advocated small vt ventilation , permissive hypercapnia , and maintaining alveolus opening by using peep . since the currently recommended small vt ranged from 6 to 8 ml / kg , we used 6 ml / kg for the small vt in our study . the normal vt of pigs ranges from 10 to 12 ml / kg and we used 10 ml / kg for middle vt . the recommended large vt ranges from 1520 ml / kg and we used 15 ml / kg . peep is mainly set as 10 cmh2o in animal models , which had little effect on hemodynamics . lpvs combined with peep can increase alveolar pressure , promote the reabsorption of alveolar exudate , and reduce inflammation . application of appropriate peep could avoid alveolar collapse , improve hypoxemia , and increase the ventilation efficiency . as reported , the patients in the lpvs group had lower incidence of ventilator - associated lung injury ( vali ) and slower progression compared with the conventional mechanical ventilation ( cmv ) group . paco2 and pao2/fio2 in the lpvs group was much higher than in the cmv group , with a significant difference . in our study , pao2 and oxygenation index in the small vt group had a more obvious increase than in the other two groups . extravascular lung water in the small vt group was less than in the other two groups , which confirmed the treatment effects of small vt ventilation on pq - induced lung injury . small vt ventilation can cause co2 retention and paco2 rise , which can lead to acidosis . we found that paco2 rose and ph decreased after ventilation and the ph in the small vt group was the lowest of the three groups . it was considered that the permissive hypercapnia had protective effects on lungs , which might be related with enhanced antioxidant activity and attenuate lipid peroxidation of the lungs . our study showed that pip and pplat increased gradually after mechanical ventilation in the middle and large vt groups . lung histopathology showed obvious injuries in the middle and large vt groups , which might be associated with airway pressure increase and alveolar overinflation - induced lung injury . menendez et al . reported that overlarge vt ventilation could cause vasoconstriction , reduce endothelium dependent vascular relaxation , and promote hypoxic pulmonary vasoconstriction , which would aggravate lung injury and increase mortality by reducing the expression of -adrenalin . small tidal volume ventilation combined with appropriate peep can alleviate acute lung injury induced by paraquat , promote gas exchange , and improve oxygenation . lpvs may be a good choice to alleviate the degree of pulmonary edema and improve the oxygenation status after the pq poisoning patients showed acute lung injury and autonomous breathing was difficult to maintain . considering that our study was only a short - term experiment , further animal experiments and clinical practice