The Human heart has one of the highest energy demands per gram of tissue. The oxygen requirement averages 6-8 ml/min/ 100 g at rest.
Approximately 80% of the heart's oxygen consumption is related to its mechanical work. Only 20% is used to maintain the basic cellular metabolism. Myocardial blood flow must be closely matched to energy demand in the normal heart. Because of this cardiac energy metabolism is extremely sensitive to reductions in perfusion caused by coronary artery disease.
The heart muscle differs from other tissues in its ability
to utilize a number of different substrates as the basis of
energy production. Depending on a number of factors the heart can
metabolize substrates such as free-fatty acids, glucose, lactate,
pyruvate, ketone bodies, and amino acids. In the heart during a
fasting state, 
fatty acid metabolism is the primary
source for
energy production providing approximately 70% of the heart's
energy requirements. Carbohydrates account for the remaining 30%
of the energy. The increase of plasma lactate levels during
exercise provide an important energy source for cardiac
metabolism.
Long-chain fatty acids enter the cell and are 
activated to
long-chain-acetyl-coenzyme-A which enters the tricarboxylic acid
cycle. Free fatty acids can also enter the tricarboxylic acid
cycle directly.
Glucose is transported into the cell and phosphorylated to
glucose-6-phosphate. At this point, glucose metabolism branches
into glycogen synthesis and glycolysis. Pyruvate, the end product
of glycolysis, enters the mitochondria and is 
converted to
acetyl-CoA, which then enters the tricarboxylic acid cycle.