The effect of mechanical ventilation on abdominal organs : Analysing the role of PEEP and perfusion

Abstract: Background: The effect of mechanical ventilation on abdominal organs is not well understood and investigated yet. Previous studies, using an animal sepsis-like model, found an association between mechanical ventilation (MV) and abdominal edema and inflammation.The presented thesis was aimed to investigate the role of perfusion in edema formation and inflammation, and to study the abdomen during mechanical ventilation in an ARDS model to reduce the confounding effect of inflammation related to sepsis.Methods: In the first paper presented, inflammation and edema in the abdomen were investigated in an endotoxin model. The study subjects were divided into two groups with different mean arterial pressures (MAP), another small group of healthy controls were studied as well. MRI analyses were used to measure perfusion in the different abdominal organs. In the second paper presented, differences in abdominal edema and inflammation were assessed in two groups of subjects, one group underwent MV and one group had spontaneously breathing.Results: In the first study, MRI analyses confirm that the group with higher MAP had better perfusion than the low MAP group. In the liver, perfusion was lower in LowMAP group compared to HighMAP group, but the HighMAP group had lower perfusion than the healthy controls. However, in the other studied organs HighMAP group and healthy controls had similar perfusion.Edema did not differ between the groups. Inflammation was globally higher in LowMAP group and correlated with hemodynamics. TNFα in liver tissue and portal vein serum correlated with intra-abdominal pressure (IAP).In the second study, the cytokine concentration was higher in serum in the MV group. MV did not increase abdominal edema or inflammation, compared to spontaneous breathing. Discussion and conclusion: Abdominal edema and inflammation are multifactorial phenomena, and many elements have to be included in the analysis. Perfusion plays an important role in determining inflammation and IAP. MV per se was not found to be related to increased edema and inflammation. In a previous study, the role of different levels of PEEP and different respiratory rate between mechanically ventilated and spontaneously breathing animals were not analyzed, but could have contributed to the results. The efforts made in this study to maintain similar respiratory rate and PEEP in both groups, could have contributed to the presented results.It is important to underline that, even if MV was not related to inflammation in abdomen, it was related to an increase in systemic inflammation, most probably because of an enhanced lung production of inflammatory mediators.Further studies, focusing on the role of respiratory rate and PEEP on abdomen, as well as the analysis of the inter-relations among inflammation, perfusion and edema, are needed to increase the pathophysiological understanding of these phenomena.