Location as a determinant of myocardial infarction in rabbits

Determinants of infarct size in the rabbit heart include risk zone size, re gional myocardial blood Row (RMBF), temperature and duration of ischemia. H owever, other factors might contribute, such as the location of the risk zo ne (apex to base), independent of known factors. Occlusion of a large margi nal branch of the circumflex artery in the rabbit produces a risk region th at typically comprises the entire apex of the left ventricle with decreasin g area involvement from apex to base. In a retrospective study of 65 rabbit hearts (subjected to 30 min of coronary artery occlusion) which had been s liced into six to eight cross-sectional slices, average area at risk (AR) c omprised 86 +/- 3% of the apical level, 68 +/- 2% of the middle level and 3 9 +/- 2% near the site of occlusion at the base of the heart. If necrosis w ere dependent on AR alone. then infarct size (area of necrosis/area at risk , AN/AR) would not vary by site. However, AN/AR in the apex was 54 +/- 3% w hile AN/AR near the base was 27 +/- 2%, P<0.0001. To lest if this salvage o f tissue at risk near the base was due to differences in regional myocardia l blood flow, we measured RMBF during occlusion in additional rabbits (n = 4). Average RMBF in the risk zone was 0.025 ml/min/g in the apex and 0.010 in the base, P = N.S. Nor was the salvage due to differences in temperature . During occlusion, temperature in the risk zone (n = 5) was 38.1 degrees C +/- 0.3 in the apex and 38.4 +/- 0.2 in the base (P = N.S,). When we exami ned this phenomenon in hearts that received a non-pharmacological intervent ion that decreases overall infarct size (ischemic preconditioning) and in h earts that received a pharmacological intervention that decreases overall i nfarct size, a similar pattern of decreasing infarct size as a percentage o f the area at risk from apex to base was observed. In conclusion, infarct s ize as a percentage of the AR depends on whether the AR is at the apex or b ase of the heart. A larger part of the AR undergoes necrosis toward the ape x of the heart. This phenomenon is independent of collateral flow or temper ature, and suggests that other as yet unknown factors contribute to infarct size. (C) 2000 Academic Press.