The term ‘acute coronary syndrome’ covers a range of disorders, including a heart attack (myocardial infarction) and unstable angina, that are caused by the same underlying problem.
The underlying problem is a sudden reduction of blood flow to part of the heart muscle. This is usually caused by a blood clot that forms on a patch of atheroma within a coronary artery.

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The types of problems range from unstable angina to an actual myocardial infarction. In unstable angina a blood clot causes reduced blood flow but not a total blockage.

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Therefore, the heart muscle supplied by the affected artery does not die (infarct). The location of the blockage, the length of time that blood flow is blocked and the amount of damage that occurs determine the type of ACS.