True shunt in relation to venous admixture in an experimental porcine model of early ARDS

Using the multiple inert gas elimination technique, ventilation/perfusion (V/Q) relationships were studied in an experimental porcine model of the early Adult Respiratory Distress Syndrome (ARDS) to establish the nature of the increased venous admixture. Six animals served as controls and revealed no major changes apart from a 10% decrease in cardiac output during the 4-h observation period. All control animals showed a shift to a higher mean V/Q of perfusion (Qmean) and a maintained log standard deviation (Qsd) throughout the experiment. The distribution was unimodal and centered around a V/Q ratio of 1.0. The share of perfusion to V/Q ratio < 0.005 (i.e. true shunt, Q) remained unchanged at 6–7% of cardiac output. Nine animals, given a continuous infusion of E. coil endotoxin, showed a significant decrease of 53% in cardiac output (Q) at 4 h. Mean pulmonary artery pressure (MPAP) showed a 2-phase reaction with a peak level at 0.5 h, and a second gradual increase from 2 h onwards. Venous admixture doubled at 0.5 h, after which it declined but remained elevated throughout the observation period. All endotoxin animals showed a shift in perfusion to a higher Qmean with a significantly wider Qsd at 0.5, 2 and 4 h. The distribution was unimodal and centered around a V/Q ratio of 1.0. True shunt was unchanged at 6–7% of cardiac output throughout the study. The increase in venous admixture in this experimental ARDS model is consequently explained by the widening of the V/Q scatter and is due to a perfusion shift to lower ventilation/perfusion ratios rather than to an increase in true shunt.