In-vivo myocardial substrate alteration during perfused ventricular fibrillation

Objectives: Earlier work suggests the in-vivo heart alters its substrate utilization as a function of cardiac work. Previous work has also demonstrated the high oxygen requirements of the heart during ventricular fibrillation (VF). The authors hypothesized that myocardial substrate utilization during VF with perfusion is similar to the normal beating heart under conditions of increased workload. Methods: Myocardial substrate selection was studied in the in-vivo porcine myocardium using ¹³carbon nuclear magnetic resonance (¹³C NMR) under conditions of increased cardiac work (dobutamine group) and VF with extracorporeal perfusion (VF group). Once the animal preparation was completed, metabolic steady state was achieved with the infusion of unlabeled acetate into the left anterior descending (LAD) coronary artery. The infused substrate was then changed to [2¹³C] acetate and glutamate pool labeling was monitored by ¹³C NMR. The glutamate C4 resonance areas at baseline and after intervention of either increased workload (dobutamine group) or perfused VF (VF group) were compared within groups using paired t-tests. Results: Baseline aortic and great cardiac vein lactates, glucose levels, blood gases, hemoglobin levels, and temperatures were similar between groups. In both groups, there was a significant decrease from baseline in the labeling of C4 glutamate peaks (dobutamine group: 20.2 ± 14.9 vs 84.7 ± 32.7, p = 0.002; and VF group: 49.8 ± 24.4 vs 83.9 ± 24.4, p = 0.02), indicating selection against acetate oxidation in favor of other endogenous substrates. Conclusions: In the in-vivo heart, despite the absence of functional contractions, changes in substrate utilization during perfused VF are similar to changes that occur with increased workload in the normal beating heart.