GRADUAL ONSET OF MYOCARDIAL-ISCHEMIA RESULTS IN REDUCED MYOCARDIAL-INFARCTION - ASSOCIATION WITH REDUCED CONTRACTILE FUNCTION AND METABOLIC DOWN-REGULATION

Background With moderate reductions in coronary blood flow, adjustments in myocardial metabolism can occur with a normalization of the imbalance between the decreased O-2 delivery and tissue O-2 demand. This state of ''downregulated'' metabolism is associated with reduced function and minimal irreversible injury and has been linked to myocardial hibernation. We hypothesized that (1) this process would occur when perfusion was reduced to severely ischemic levels, but only when flow declined at a slow rate rather than abruptly, and (2) this would result in blunted ischemia and reduced myocardial injury for a given period of low blood flow. Methods and Results The left anterior descending coronary artery in anesthetized open-chest pigs was cannulated and perfused with arterial blood by an extracorporeal perfusion pump. Regional function (percent segment shortening, %SS) was measured with sonomicrometry and a regional coronary vein cannulated for blood gas analysis and lactate measurements. Coronary blood flow (CBF) was reduced to 10% of control either in a step fashion (Fast Ischemia group) or gradually in a linear manner over 70 minutes (Ramp Ischemia group). In all animals, CBF was held for 60 minutes at this 10% level and then followed by 2 hours of reperfusion. In the Ramp Ischemia group, the linear fall in CBF resulted in an initial maintenance of both %SS and myocardial oxygen consumption (MV over dot o(2)) followed by linear decreases in both variables (r=.98 to .99) as flow fell to the 10% level. The relation of MV over dot o(2), to function was linear (r=.99) over the entire flow range. Although %SS, MV over dot o(2), CBF, coronary pressure, and hemodynamics during the 10% flow period were not different between groups, the increases in coronary venous lactate and PCO2 and fall in pH were blunted in the Ramp Ischemia group compared with the Fast Ischemia group. With reperfusion, a significant decrease in end-diastolic length was present only in the Fast Ischemia group. Additionally, although the region at risk was not different, infarction was markedly reduced in the Ramp Ischemia group (6.6+/-1.9%) compared with the Fast Ischemia group (31.4+/-6.9%). Conclusions These data are consistent with the hypothesis that the downregulation of myocardial metabolism with gradually decreased flow to severe levels results in reduced myocardial injury for a given period of low flow. We propose that the rate at which blood flow decreases with myocardial underperfusion is a novel determinant of infarct injury. This may have clinical implication in situations in which there is a time-dependent component to the decrease in coronary blood flow in acute ischemic events, ie, thrombus formation at a site of coronary stenosis.