Ischemia is a condition in which the blood flow (and thus oxygen) is restricted or reduced in a part of the body. Cardiac ischemia is the name for decreased blood flow and oxygen to the heart muscle.Myocardial ischemia occurs when the blood flow through one or more of your coronary arteries is decreased. ... Or it can occur quickly when an artery becomes blocked suddenly. Conditions that can cause myocardial ischemia include: Coronary artery disease (atherosclerosis). Ischemia is caused by a decrease in blood supply to a tissue or organ. Blood flow can be blocked by a clot, an embolus, or constriction of an artery. It can occur due to gradual thickening of the artery wall and narrowing of the artery, as in atherosclerosis.