The oxidation of substrates like free fatty acids (FFA), glucose, lactate and ketone bodies results in the energy of Adenosine triphosphate (ATP) and Creatine phosphate (CP). The substrate used depends on several factors like its level in the blood, level of insulin and catecholamine level. In the resting, fasting state 70-80 per cent of the oxygen consumption can come from the oxidation of FFA. With high glucose levels and resulting high insulin levels the fuel shifts to glucose. Glucose is phosphorylated to glucose 6 phosphate. It is either synthesized to glycogen or enters glycolysis with pyruvate as the end product. Lactate is used as the major fuel when its level increases with severe exercise. Lactate may leave the myocardium or if acetylated to acetyl coenzyme A enters the tricarboxylic acid cycle. Catecholamines increase lipolysis and thus increase the FFA levels. Normally free fatty acids are the main substrate. In ischaemia the glycolytic pathway is stimulated. This cannot substitute for oxidative phosphorylation and inevitably ischaemia thus leads to a fall in ATP levels. This impairs myocardial function.