Ventilation-perfusion distributions in different porcine lung injury models

Background: Acute lung injury is characterized by hypoxemia which may be ca used by hypoventilation, ventilation-perfusion ((V) over dot (A)/(Q) over d ot) mismatch, intrapulmonary shunting and oxygen diffusion impairment. The multiple inert gas elimination technique (MIGET) allows analysis of these f our causes of hypoxemia and is therefore the most comprehensive approach to investigate blood gas abnormalities. Using MIGET, we studied whether speci fic patterns of gas exchange abnormalities occur in different lung injury m odels and whether gas exchange abnormalities can be related to pathogenic a spects of lung injury. Methods: Lung injury was induced with oleic acid inj ection, endotoxin infusion or repeated lung lavage in groups of 6 mechanica lly ventilated pigs. Results: PaO2 decreased and PaCO2 increased significantly in all three lung injury models, but gas exchange was more impaired in the oleic add and lav age, as compared to the endotoxin group. Shunt was the major cause of hypox emia in our lung injury models, whereas (V) over dot (A)/(Q) over dot misma tch contributed to venous admixture only after oleic acid injection and lun g lavage. Oxygen diffusion limitation was not observed. Although alveolar v entilation was maintained after induction of lung injury, hypercapnia devel oped due to an increase of the ventilatory mean towards higher (V) over dot (A)/(Q) over dot ratios, increased shunt and increased carbon dioxide prod uction. Conclusions: Shunt and ventilation-perfusion mismatch fully explain the gas exchange disturbances observed in our lung injury models. Although (V) ove r dot (A)/(Q) over dot distributions can be related to pathogenic aspects o f the three study groups, we did not observe specific (V) over dot (A)/(Q) over dot patterns which allow diagnosis of the type of lung injury from a r ecovered (V) over dot (A)/(Q) over dot distribution.