Role of vasoconstriction in gravity-nondependent central-peripheral gradient in pulmonary blood flow

To examine the effect of vasoconstriction on the gravity-nondependent distribution of pulmonary blood flow, albumin macroaggregates labeled with either 99mTc or 111In were injected at end expiration into dogs (anesthetized, supine, and breathing room air spontaneously). The first dose of macroaggregates was injected during baseline conditions and the second during infusion of serotonin or histamine. Five minutes after the second injection, the chest was opened and the lungs were removed, drained of blood, and dried while fully inflated. Single photon emission computed tomography was performed on the dry lungs to map the three-dimensional distribution of activity of the two isotopes. Images of coronal and sagittal slices were displayed. The results showed that changes in vascular muscle tone did not alter pulmonary blood flow distribution. The absolute flow in a central and peripheral region changed in proportion to cardiac output changes, indicating that during vasoconstriction, as during baseline conditions, there was a preferential perfusion to the interior of the lung. Examination of the average flow in the individual coronal slices showed that during serotonin or histamine infusion, the average flow in each slice changed in proportion to change in cardiac output but that the vertical average flow was not affected significantly. These findings are consistent with the notion that resistive properties (length, diameter, branching pattern) of the pulmonary vascular tree are responsible for the central-peripheral gradient.