ABSTRACTThe effect of a residual coronary artery stenosis on size of myocardial infarction was studied in an open-chest canine preparation of coronary occlusion and reperfusion. Eighteen male mongrel dogs (16 to 26 kg) underwent left thoracotomy under general anesthesia; the circumflex artery was instrumented with a hydraulic cuff occluder, a screw clamp, and an electromagnetic flow probe. Animals were randomized to one of three groups: group I (n = 6) had a 6 hr circumflex occlusion, group II (n = 6) had a 2 hr occlusion followed by 4 hr of partial reperfusion through a residual stenosis adjusted to 30% of baseline flow, and group III (n = 6) had full reperfusion for 4 hr after a 2 hr occlusion. Zones of risk, infarction, and no reflow were defined by staining with Evans blue, triphenyl tetrazolium chloride, and fluorescein, respectively. At 6 hr the hearts were excised and areas of risk, infarction, no reflow, and hemorrhage were determined by planimetry of serial transverse heart slices (5 mm thick). Infarction as a percent of the risk area was 96 + 1% in group I, 90 2% in group II, and 79 4% in group III (p < .001), and the differences between each of the groups were significant. Gross hemorrhage was seen in none of the six dogs in group I, two of the six in group II, and five of the six in group III, but did not affect infarct size. We conclude that residual stenoses may exert a deleterious effect on the outcome of coronary reperfusion. In the clinical setting of coronary thrombolysis they may deserve aggressive treatment to maximize salvage of myocardium.