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Coronary circulation in cyanotic congenital heart disease (CCHD) involves extramural coronaries, basal blood flow, flow reserve, the microcirculation, and atherogenesis.Dilated coronary arteries were examined histologically.Basal flow was determined with N-13 positron emission tomography.Hyperemic flow was induced by dipyridamole pharmacologic stress.Immunostaining against SM alpha-actin permitted morphometric analysis.Total cholesterols included cyanotic unoperated patients, acyanotic postoperative, acyanotic unoperated, and acyanotic pre and postoperative.Histologic examination disclosed loss of medial smooth muscle, increased medial collagen, and duplication ofintemal elastic lamina.Basal coronary flow was appreciably increased.Hyperemic flow was comparable to controls.Alterations in coronary arteriolar length, volume and surface densities indicated microcirculatory remodeling.Atherosclerosis was absent.Our data indicate that extramural coronary arteries dilate in CCHD in response to vasodilator substances coupled with mural attenuation.Basal flow was increased, but hyperemic flow was normal.The mechanism for preservation of flow reserve was microcirculatory remodeling.The coronary arteries were atheroma-free because of hypocholesterolemia, hypoxemia, upregulated nitric oxide, low platelet counts, and hyperbilirubinemia.