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Negrini, Daniela, Anna Candiani, Federica Boschetti, Beatrice Crisafulli, Massimo Del Fabbro, Dario Bettinelli, and Giuseppe Miserocchi. Pulmonary microvascular and perivascular interstitial geometry during development of mild hydraulic edema. Am J Physiol Lung Cell Mol Physiol 281: L1464-L1471, 2001.--To study pulmonary arteriolar vasomotion in control conditions and in the transition to hydraulic edema, changes in subpleural pulmonary arteriolar diameter and perivascular interstitial volume were evaluated in anesthetized spontaneously breathing rabbits. Images of subpleural pulmonary microvessels were recorded in control conditions and for up to 180 min during a 0.5 ml.kg-1.min-1 intravenous saline infusion through an intact parietal pleural window. Images were digitized and analyzed with a semiautomatic procedure to determine vessel diameter and perivascular interstitial thickness from which interstitial fluid volume was derived. In control yessels, the diameter of-30-, -50-, and -80-[mu]m arterioles and the perivascular interstitial thickness were fairly stable. During infusion, the diameter increased maximally by 20% in -30-[mu]m vessels, was unchanged in -50-[mu]m vessels, and decreased by 25% in -80-[mu]m arterioles; the perivascular interstitial volume increased by 54% only around 30-[mu]m microvessels. In papaverine-treated rabbits, all arterioles dilated and a larger increase in perivascular interstitial thickness was observed. The data suggest that the opposite vasomotor behavior of 30- and 80-[mu]m arterioles during development of mild edema may represent a local specific response of the pulmonary microcirculation to reduce capillary pressure in the face of an increased transendothelial fluid filtration, thus counteracting progression toward severe edema. pulmonary arteriolar vasomotion; perivascular interstitial volume; mild interstitial edema