| Abstract: | This study examined the relation between left ventricular (LV) function and the severity of acute myocardial ischemia in a conscious dog model. The LV ejection fraction (EF) was measured by multigated equilibrium radionuclide angiography, and regional myocardial blood flow was measured with radioactive microspheres before and 10 minutes after distal and then proximal occlusion of the left anterior descending (LAD, 13 dogs) or left circumflex (LC, 13 dogs) coronary artery. Two methods were used to evaluate the extent of ischemia. The first method determined the mass of myocardium that was ischemic based on different degrees of reduced blood flow. The second method estimated the severity of ischemia expressed as blood flow deficit resulting from each coronary occlusion. Global LV function was very sensitive to ischemia, and the relation between change in function and the degree of ischemia were described best by linear functions. The best linear correlation between mass of ischemic myocardium and percent reduction in EF resulted from the ischemic region defined as all tissue with 25% or greater reduction in blood flow, r = 0.84 for LAD (Y = 0.96X + 1.8) and r = 0.75 for LC (Y = 0.53X + 2.0) occlusions. Defining ischemic mass by more severe reduction in blood flow resulted in exclusion of ischemic myocardium that affected function. The myocardial blood flow deficit also correlated linearly with percent reduction in EF, r = 0.89 for LAD (Y = 1.31X + 2.7) and r = 0.81 for LC (Y = 0.83X - 0.1) occlusions. The slope of the regression lines using both analyses of ischemia were significantly greater (p less than 0.01) for LAD than LC occlusions, indicating that for comparable degrees of ischemia LAD as compared to LC occlusion decreased EF to a greater extent. Calculation of EF from attenuated corrected volumes resulted in small changes in LAD, but not LC, EF and did not account for the disproportionate effects of LAD and LC ischemia. In a separate group of studies (n = 18) EF measured by radionuclide angiography after LAD or LC occlusions correlated well with biplane contrast angiography r = 0.93, SEE 5.1. These data suggest that disproportionately greater effects of LAD compared to LC ischemia on global EF in the dog are due primarily to different pathophysiologic responses to ischemia. |
