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The effects of isoflurane on the transmural distribution of myocardial blood flow distal to an acute critical coronary stenosis and the relationship between the changes in regional blood flow and function were studied to determine whether isoflurane can produce a transmural “steal” phenomenon and to assess the role of this phenomenon in producing changes in regional myocardial function.After production of acute critical coronary stenosis under baseline chloralose and fentanyl anesthesia, the animals were exposed to increasing end-tidal concentrations of isoflurane (0.796, 1.4%, and 2.1%) without control of the hemody-namic parameters. At 2.1% isoflurane, the blood pressure then was restored to the baseline level by administration of phen-ylephrlne. Changes in the following parameters were assessed: global contractility (measured by changes in pressure with time), regional myocardial function (assessed by systolic wall thickening and measured by sonomicrometers), transmural distribution of myocardial perfusion (measured by the radioactive mlcrosphere method), and regional oxygen consumption and extraction.Distal to the critical stenosis, a transmural redistribution of myocardial blood flow (endocardial-epicardial ratio < 1) occurred with all concentrations of isoflurane. With higher concentrations (1.4% and 2.1%), a significant decrease in subendocardial blood flow occurred only in the presence of hemodynamic changes and was restored by phenylephrlne. In this area, changes in regional myocardial function correlated most strongly with changes in subendocardlal perfusion (y = −0.17 + 1.70x - 0.58x2, r2 = 0.90). In the stenotic region, oxygen extraction remained stable, but oxygen consumption decreased in parallel with reductions in regional myocardial function. In the normal region, oxygen consumption did not change, but oxygen extraction decreased with increasing isoflurane concentrations.These results show that isoflurane is a coronary vasodilator able to induce a transmural redistribution of myocardial blood flow distal to an acute critical coronary stenosis. A true transmural steal, however, was not produced reliably in the absence of hemodynamic changes, suggesting that isoflurane either is only a moderate vasodilator, or that the decrease in subendocardlal blood flow is offset by the negative inotroplc action of the drug. When regional myocardial dysfunction distal to a severe coronary stenosis occurs, this correlates with decreasing subendocardlal blood flow during isoflurane anesthesia, suggesting ischemia as the cause.