Ischaemia occurs when blood flow to a muscle is insufficient to meet the oxygen requirements of that muscle. When this muscle is the heart, it is termed myocardial ischaemia.
Myocardial ischaemia generally occurs as a result of narrowed coronary arteries. If blood flow is soon increased, or oxygen requirements are lowered, ischaemia will resolve without permanent muscle damage.
During myocardial ischaemia, the chemical and electrical processes of the heart are affected. These changes may present as a depression of the ST segment in one of more leads of the ECG. In some cases no effect is visible at all.
A myocardial infarction (or heart attack) occurs if an adequate blood flow is not restored and the heart muscle dies. Initially symptoms similar to an ischaemic attack will occur as the muscle cells operate anaerobically, however, the pain is sudden and not stress related.
In the following example, the ST segment (shaded) is depressed below the isoelectric PR segment (arrowed).