Ischaemia means a “reduced blood supply”.
The coronary arteries supply blood to the heart muscle and no alternative blood supply exists, so a blockage in the coronary arteries reduces the supply of blood to heart muscle.
Most ischaemic heart disease is caused by atherosclerosis, usually present even when the artery lumens appear normal by angiography.
Initially there is sudden severe narrowing or closure of either the large coronary arteries and/or of coronary artery end branches by debris showering downstream in the flowing blood.
It is usually felt as angina, especially if a large area is affected.
The narrowing or closure is predominantly caused by the covering of atheromatous plaques within the wall of the artery rupturing, in turn leading to a heart attack (Heart attacks caused by just artery narrowing are rare).
A heart attack causes damage to heart muscle by cutting off its blood supply.