From a slow narrowing to a sudden blockage
Here is the crucial surprise: most heart attacks do not happen in the most narrowed arteries. They happen when a plaque that may have been only modestly narrowing suddenly cracks open. This is plaque rupture. The soft, greasy core that was safely sealed under its scar cap is now exposed to the bloodstream — and to blood, exposed plaque looks exactly like a wound that needs a clot.
Within minutes, platelets and clotting proteins pile onto the exposed core and form a clot — coronary thrombosis. If the clot blocks the artery enough, the muscle downstream is suddenly cut off, and what was a slow imbalance becomes an emergency. This whole family of sudden events is called acute coronary syndrome (ACS). The key contrast with stable angina is timing: stable angina is a chronic limitation; ACS is an acute crisis that can begin at rest, in the middle of the night, doing nothing at all.
Three flavors of acute coronary syndrome
Doctors sort ACS using two simple tools: a blood test and an electrical tracing. The blood test measures troponin, a protein that leaks out of heart muscle cells only when they are actually dying. If troponin rises, muscle is being lost — that defines a myocardial infarction (a true heart attack). If troponin stays normal but the symptoms fit a cracked plaque with only partial blockage, it is unstable angina — angina that has broken its polite rules, now striking at rest, but without (yet) killing muscle. Unstable angina is the urgent warning that a heart attack may be next.
When muscle is dying, the electrical tracing splits the heart attack into two types. If the artery is completely blocked, a large patch of muscle is cut off and the tracing shows a tell-tale ST elevation — this is a STEMI, the most time-critical emergency, demanding the artery be reopened as fast as possible. If the artery is only partly blocked or there is a working detour, muscle still dies but without that full pattern: this is an NSTEMI. Both are real heart attacks; the distinction drives how quickly and by what method the artery is reopened.
Sorting an acute coronary syndrome — two questions: Q1: Is troponin rising? (Is muscle actually dying?) Q2: Does the ECG show ST elevation? (Is the artery fully blocked?) +-----------------------+-------------------+-------------------------+ | | Troponin NORMAL | Troponin RISING | +-----------------------+-------------------+-------------------------+ | ECG: no ST elevation | UNSTABLE ANGINA | NSTEMI | | | (warning shot) | (partial-blockage MI) | +-----------------------+-------------------+-------------------------+ | ECG: ST ELEVATION | -- | STEMI | | | | (full-blockage MI, | | | | most time-critical) | +-----------------------+-------------------+-------------------------+ All three = acute coronary syndrome. All three are urgent.