As arteries to the heart become partially or fully blocked, the heart is starved of blood and oxygen.
A NSTEMI (where blood flow is partially blocked) results in a smaller section of the heart muscle being damaged, and can be considered less serious than a STEMI.
A STEMI is thought of as the most serious type of heart attack, according to the NHS, where there is a long interruption in blood flow to the heart.
This can cause extensive damage to the heart muscle. However, both STEMI and NSTEMI are serious medical emergencies and must be treated as so.