Pulmonary embolization causes hypoxemia by redistributing regional blood flow without changing ventilation

To explore mechanisms of hypoxemia after acute pulmonary embolism, we measu red regional pulmonary blood flow and alveolar ventilation before and after embolization with 780-mu m beads in five anesthetized, mechanically ventil ated pigs. Regional ventilation and perfusion were determined in similar to 2.0-cm(3) lung volumes by using 1-mu m-diameter aerosolized and 15-mu m-di ameter injected fluorescent microspheres. Hypoxemia after embolization resu lted from increased perfusion to regions with low ventilation-to-perfusion ratios. Embolization caused an increase in perfusion heterogeneity and a fa ll in the correlation between ventilation and perfusion. Correlation betwee n regional ventilation pre- and postembolization was greater than correlati on between regional perfusion pre- and postembolization. The majority of re gional ventilation-to-perfusion ratio heterogeneity was attributable to cha nges in regional perfusion. Regional perfusion redistribution without compe nsatory changes in regional ventilation is responsible for hypoxemia after pulmonary vascular embolization in pigs.