Endothelial damage caused by ischemia and reperfusion and different ventilatory strategies in the lung

Not all possible mediators of lung I/R injury that have been studied, such as cyclooxygenase and lipoxygenase products, have been presented in this review, but it is very clear that oxygen free radicals are the primary mediators of the damage, regardless of their origin. Oxygen radicals are generated by neutrophils, which are sequestered and activated in the ischemic-reperfused pulmonary tissue, and by xanthine oxidase, which is upregulated by ischemia and/or activated neutrophils. The contributions to lung injury by different species of oxygen radicals may very depending upon the lung model used to study I/R. Also, nitric oxide may be injurious or protective in lung I/R injury, depending upon some critical alveolar PO2 level present either during ischemia or at reperfusion. I/R-induced lung microvascular injury ultimately depends upon some balance between lung metabolic stress, the extent of the I/R-induced inflammatory response, endogenous antioxidant levels, and the timing, magnitude, and duration of oxygen free radical generation during both periods of ischemia and reperfusion. The final common pathway causing microvascular permeability to increase after lung I/R is the activation of the endothelial cell's contractile machinery. Particularly, endothelial contraction may occur in a MLCK-dependent fashion. Endothelial contraction may also be related to an intracellular Ca++ increase and subsequent calmodulin activation. The initiating event causing increased intracellular Ca++ is not known, but may be due to endothelial cell/leukocyte interactions, oxygen radical-mediated Ca++ transients, mobilization of intracellular Ca++ pools by various second messengers, or stimulation of Ca++ influx secondarily to changes in the activity of membrane ion pumps such as the Na+/H+ antiport. Increasing cAMP levels in the postischemic lung can prevent and actually reverse I/R-induced microvascular injury, by affecting MLCK, the endothelial cell cytoskeleton, and/or the function of sequestered leukocytes. Also, cAMP elevation aids the resolution of pulmonary edema by facilitating capillary fluid reabsorption. Whatever the mechanism, elevation of cAMP in the setting of lung I/R injury represents a potentially useful therapy for improving early lung function following lung transplantation. Finally, additional studies are necessary to elucidate the complete mechanisms responsible for producing microvascular injury during lung I/R. Specifically, a better understanding of the relationships between the many factors required to produce lung damage is needed. Many interventions into the lung I/R process provide protection against microvascular injury, suggesting that regulation of the endothelial barrier permeability to fluid, protein, and leukocytes is accomplished by several redundant systems. This situation may be similar to mechanisms reported to regulate the immune response mediated by T cells (62a), where T cell activation depends upon multiple signal inputs for the full immune response to occur. Thus, multiple signals in a correct sequence delivered to the endothelium may be necessary to produce the microvascular injury associated with lung ischemia and reperfusion.