Pathophysiology of gas exchange and pulmonary perfusion in pneumococcal lobar pneumonia in dogs.

We placed an inoculum of Streptococcus pneumoniae type III into a left lower lobe bronchus of six dogs (group P), and in six other dogs (Group C) a sterile control inoculum was used. Measurements of shunt (Qs/Qt) and venous admixture (Qva/Qt) were made immediately before (day 1) and 48 h after (day 3) inoculation. All dogs in group P had extensive lobar pneumonia confirmed radiologically and at autopsy, whereas Group C had only small sterile lesions at the site of inoculation. In group P, mean Qs/Qt and Qva/Qt increased significantly to 0.15 and 0.21, respectively. Mean lobar Qs/Qt, calculated using blood samples from lobar veins at thoracotomy on day 3, was markedly increased in the pneumonia lobe (0.69) compared with the contralateral lower lobe (0.08), and alveolar ventilation of that lobe approached zero. Perfusion of the infected lobe determined by radioactive microspheres showed a variable and statistically nonsignificant decrease between control and infected states that was not affected by oxygen breathing. In group C there was no change between days 1 and 3 in gas exchange or in distribution of pulmonary perfusion. We conclude that hypoxemia in pneumonia was due to both increased shunt and venous admixture in the infected regions, and that local hypoxic vasoconstriction was in most instances ineffective in directing blood flow away from the consolidated lobe.