Mechanisms by which dopamine alters blood flow distribution during lobar collapse in dogs

Dopamine increases blood flow to a hypoxic left lower lobe in dogs. To elucidate possible mechanisms, left lower lobe collapse was induced in anesthetized dogs, and lobar (QLLL) and total (QT) pulmonary blood flow was measured by electromagnetic flow probes. Dopamine infusion increased mean pulmonary arterial pressure (Ppa), QT, and QLLL. However, the increase in QLLL was double that produced by a similar increase in Ppa without increase in QT (inflation of a Swan-Ganz balloon in right pulmonary artery) or by a similar increase in QT with smaller increase in Ppa (opening of arteriovenous fistulas). QLLL/QT was not changed by opening arteriovenous fistulas, but was increased by Swan-Ganz balloon inflation, and by infusion of dopamine. It is concluded that the increase in QLLL/QT produced by dopamine was due to a decrease in hypoxic vasoconstriction in the lobe secondary to an increase in mixed venous PO2 and to vasoconstriction in the oxygenated lung.     

CO2 and mechanical factors reduce blood flow in a collapsed lung lobe

Left lower lobe-to-total blood flow ratio (Ql/QT) was measured with electromagnetic flow probes in anesthetized open-chest dogs. There was a 66% reduction in Ql/QT during lobar collapse, a 53% reduction during lobar ventilation hypoxia with pulmonary venous PO2 and PCO2 equal to mixed venous tensions, and a 45% reduction during a similar degree of ventilation hypoxia but with normal end-tidal PCO2. We concluded that the reduction in blood flow during lobar collapse is due predominantly to hypoxic vasoconstriction, but that this mechanism is augmented by the raised PCO2 and mechanical factors present during collapse.