Lung mechanics with relation to pulmonary haemodynamics, gas exchange and extravascular lung water in mechanically ventilated endotoxaemic pigs.

In a porcine model employing a continuous i.v. infusion of E. coli endotoxin the pathophysiology of early adult respiratory distress syndrome was studied with main emphasis on the early changes in lung mechanics and their relation to changes in pulmonary haemodynamics, gas exchange and extravascular lung water in intermittent positive pressure ventilated (IPPV) pigs under ketamine anaesthesia. Six animals served as controls and revealed no major physiological changes. Nine animals received endotoxin and developed significant changes in lung mechanics with increases in end-inspiratory pressure (32%), expiratory resistance (29%) and decrease in total dynamic lung compliance (27%). Changes in dynamic compliance and pulmonary haemodynamics displayed a 2-phase reaction. Venous admixture showed a rapid increase at with the increase in mean pulmonary arterial pressure (r = -0.8) and with the increase in venous admixture (r = -0.7). Extravascular lung water did not increase significantly. The decrease in dynamic compliance is most likely explained by peripheral airway constriction. A contributory factor might be pulmonary microvascular constriction with vascular stasis and mechanical compression of small airways. The increased venous admixture is best explained by a bronchiolar and microvascular constriction, i.e. a "dry" ventilation/perfusion inequality and not consequent to oedema. IPPV seems to counteract the increase in extravascular lung water.