Size of microspheres passing through pulmonary circuit in the dog

After injection of microspheres into the pulmonary artery approximately 50% of spheres 2.8–4.0 µ in diameter can be found in the systemic circuit during the first circulation, but only 6% of those 8 µ or larger get through the pulmonary circuit when compared with simultaneously injected tagged erythrocytes. If vessels are impeding the flow of spheres and are circular in cross section, then more than half of the erythrocytes must be distorted while passing through the pulmonary circuit. A continuous infusion of norepinephrine (1–4 µg/kg/min) brings about a reduction in the percentage of the various sizes of spheres passing through the pulmonary vessels. This suggests vasoconstriction. Acetylcholine iodide (13–40 µg/kg/min) usually diminishes the percentage of spheres 2.8 µ and smaller, which can go through the pulmonary circuit, but increases the percentage of microspheres 5.7 µ and larger. During prolonged inspiration the percentage of microspheres passing through in each group studied was less than during expiration. If all microspheres injected are to pass through the pulmonary circuit in inspiration they must be 1.4 µ or smaller.