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This study compares the effects of 60 minutes of ischemic arrest with profound topical hypothermia (10 dogs) on myocardial (1) blood flow and distribution (microspheres), (2) metabolism (oxygen and lactate), (3) water content (wet to dry weights), (4) compliance (intraventricular balloon), and (5) performance (isovolumetric function curves) with 180 minutes of cardiopulmonary bypass with the heart in the beating empty state (seven dogs). Studies performed before and 30 minutes after 1 hour of ischemic arrest with profound topical hypothermia showed: (1) total left ventricular blood flow increased 50 per cent but became redistributed away from the subendocardium (endocardial/epicardial flow ratio fell from 1.13 to 0.77,(2) left ventricular oxygen consumption fell 30 per cent while left ventricular oxygen extraction fell from 51 to 29 per cent; (3) lactate extraction fell from 15 to 4 per cent (two dogs produced lactate); (4) left ventricular endocardial (papillary muscle) water content rose 2.4 per cent; (5) left ventricular compliance decreased from 1.68 to 1.01 ml. H2O/mm. Hg (at 25 ml.); (6) left ventricular performance was depressed 49 per cent below control values. In contrast, 3 hours of cardiopulmonary bypass in the beating empty heart produced only minimal changes in these variables.