Early shunting of 9 microns and 15 microns tracer microspheres from the acutely ischemic canine myocardium.

Recent reports have shown considerably differing results for myocardial shunting of 9 microns and 15 microns tracer microspheres (TMs) under various conditions. This could restrict the use of TMs for myocardial, especially collateral blood flow measurements. To determine the importance of coronary collateral blood flow and its early changes during the first 30 minutes after acute coronary artery occlusion (i.e. the 1st arrhythmic phase), we studied the shunting of 9 microns and 15 microns TMs from the ischemic myocardium during acute LAD ligation. In anesthetized dogs these TMs and subsequently Ringer solution were infused into the occluded coronary artery just distal to the ligation with constant low perfusion pressure. TM shunting (%S) into the lungs was then determined (%S = total lung radioactivity . 100/radioactivity infused). During a single LAD occlusion lasting 35 minutes (series I, n = 10) 9 microns TMs were infused immediately and 30 minutes after ligation, 15 microns TMs being infused after 15-20 minutes. In series II (n = 6) 9 microns TMs were infused immediately during the 1st, short (5 minutes) LAD occlusion. Following 90 minutes of reperfusion a 2nd LAD ligation (35 minutes) was performed with 9 microns TMs being infused immediately and 30 minutes after occlusion. During the first 30 minutes of acute coronary artery occlusion, TM shunting from the ischemic myocardium is negligible for 15 microns TMs (%S less than 0.5%; n = 5), whereas the mean 9 microns TM shunt of the early applied TM (i.e. A1, series I; n = 9; B2, series II; n = 6) amounts to a maximum of 1.21 +/- 0.2% (X +/- SEM). After 30 minutes of occlusion the mean 9 microns TM shunt amounts only to 0.71 +/- 0.15% (i.e. C1, series I; n = 4; C2, series II; n = 4). - In a coronary artery occlusion repeated once, 9 microns TM shunting, while increasing slightly due to the 90 minutes of reperfusion, still amounts to only 1.73 +/- 0.41% (n = 6). In three experiments 9 microns TMs were infused into the unoccluded, normally perfused LCX coronary artery during LAD occlusion. The mean LCX shunt value of 4% after a mean time of 25 minutes following TM infusion is in very good agreement with the 9 microns TM shunt values in the literature. These results clearly demonstrate that the TM technique with 9 microns microspheres is suitable for measuring changes in coronary collateral blood flow at least for a short time period after acute coronary artery occlusion.