Lactate extraction fails to accurately reflect regional lactate production in ischemic myocardium

Lactate extraction (defined as arteriovenous lactate concentration difference divided by arterial concentration and expressed as a percent) is often reported as the indicator of anaerobic cardiac metabolism in studies dealing with myocardial ischemia. However, lactate extraction ignores the effect of regional blood flow and, therefore, fails to consider the total mass of lactate consumed or produced (lactate flux). This study examined the relationship between lactate flux and calculated lactate extraction. Fourteen anesthetized dogs were instrumented to allow sampling of blood from the left anterior descending coronary artery (LADa) and vein (LADv) and a circumflex coronary vein (CFXv), as well as measurement of regional myocardial blood flow (RMBF) using microspheres, and measurement of systemic hemodynamic variables. Complete data sets (before LADa occlusion, after 15 minutes of LAD occlusion, and after 1 hour of reperfusion) were obtained in nine dogs. Only minor systemic hemodynamic changes occurred during LADa occlusion when compared with “before” and “after” values. Likewise, LADa occlusion produced only minor alterations in blood gas tensions, pH, concentrations of glucose, lactate, and RMBF in samples from the CFX perfusion zone. In contrast, LAD occlusion decreased RMBF in the LADa perfusion zone and produced significant hypercarbia and acidemia, as well as an increased LADv lactate concentration. In the LAD zone, lactate extraction decreased significantly from 15.9% ± 7.0% before LAD occlusion to −77.4% ± 21.8% during LAD occlusion (P < 0.05). However, lactate flux (arteriovenous concentration difference × RMBF) in the LAD zone before and during LAD occlusion was not statistically significantly different (1.3 ± 0.8 mg/min/100 g and −1.5 ± 0.8 mg/min/ 100 9, respectively). Thus, whereas lactate production is an indicator of anaerobic myocardial metabolism, lactate extraction is an inflated index of the extent of anaerobic metabolism because it accounts for the arteriovenous concentration difference only. Inclusion of RMBF to calculate actual lactate flux is more appropriate, and the change in this value may be much less than calculated lactate extraction.