Microembolization in pigs: effects on coronary blood flow and myocardial ischemic tolerance

Coronary microembolization has been reported to increase coronary blood flo w (CBF) through adenosine release. Because adenosine may increase ischemic tolerance against infarction, we tested the hypothesis that myocardial micr oembolization, a common finding in patients with ischemic heart disease, in duces cardioprotection. Additionally, because the use of microspheres is a common tool to measure tissue perfusion, the effects of small amounts of mi crospheres on CBF were examined. Using anesthetized pigs, we measured CBF w ith a transit time flow probe on the left anterior descending coronary arte ry (LAD). In six pigs the relationship between the amount of injected micro spheres (0-40 x 10(6), 15 mu m in diameter, left atrial injections) and the effect on CBF was examined. Coronary hyperemia occurred, which was linearl y related to the amount of microspheres injected: maximal increase in CBF ( %) = 2.8 +/- 1.5 (SE) + (5.8 + 0.7 x 10(-7) x number of injected microspher es). Because injection of 40 x 10(6) microspheres induced a long-lasting hy peremic response, which could be blocked by 8-p-sulfophenyl theophylline, i schemic tolerance was examined in five other pigs after two injections, eac h of 40 x 10(6) microspheres, at a 30-min interval. Six control pigs had no injections. Ischemic tolerance was evaluated by measuring infarct size (te trazolium stain) as the percentage of area at risk (fluorescent particles) after 45 min of LAD occlusion followed by 2 h of reperfusion. Pretreatment by microspheres increased infarct size from 60 +/- 3% of area at risk in co ntrol animals to 84 + 6% (P < 0.05). The injection of microspheres induced a significant hyperemic flow response without causing necrosis by itself. W e conclude that microembolization, evoking coronary hyperemia, does not imp rove but reduces myocardial ischemic tolerance against infarction in pigs.