Why oxygen is non-negotiable
Cardiomyocytes are obligate aerobes: about 90% of their ATP comes from oxidative phosphorylation in mitochondria. Fatty acid beta-oxidation and pyruvate oxidation feed electrons into the electron transport chain, which drives ATP synthase. Oxygen is the terminal electron acceptor. Remove it, and the entire chain backs up.
The anaerobic substitute
With oxidative phosphorylation halted, the cell falls back on anaerobic glycolysis. Glycolysis yields a net 2 ATP per glucose, versus roughly 30 to 32 ATP from full oxidation. The cell compensates by increasing glucose uptake and mobilizing glycogen, but the yield per glucose is an order of magnitude lower.
Lactate production and acidosis
Pyruvate cannot enter mitochondria, so lactate dehydrogenase converts it to lactate: \[\text{pyruvate} + \text{NADH} + \text{H}^+ \rightarrow \text{lactate} + \text{NAD}^+\] This reaction regenerates NAD+ so glycolysis can continue, but it also releases protons. The accumulating lactate and H+ lower intracellular pH, producing ischemic acidosis.
The metabolic consequence
The switch to anaerobic glycolysis cannot meet the cell's ATP demand. The resulting ATP deficit and acidosis are the direct triggers for the ion pump failure and contractile failure that follow.