A heart attack, or myocardial infarction, occurs when the blood supply to part of the heart muscle is suddenly and severely reduced or completely cut off, most often by a blood clot forming inside one of the coronary arteries — the vessels that supply the heart itself with oxygen-rich blood. Without that blood supply, the affected heart muscle begins to die within minutes, which is why a heart attack is one of the true medical emergencies where speed of treatment directly determines how much lasting damage occurs.
Not all heart attacks are identical, and understanding the distinction matters: some involve a complete blockage of a coronary artery, requiring the most urgent possible treatment, while others involve a partial blockage that, while still serious, is managed with a somewhat different, though still urgent, approach. This page covers how a heart attack actually happens, the key clinical distinction between the two main types, why speed of treatment is so critical, why heart attacks are frequently missed in women, symptoms, risk factors, how a heart attack is diagnosed, and the full range of treatment options available.
The coronary arteries can gradually narrow over years through a process called atherosclerosis, in which deposits of cholesterol, fat, and other substances build up within the artery wall, forming plaques. Many people live with this narrowing for years without any symptoms at all.
A heart attack occurs when one of these plaques ruptures or splits open. This exposes the plaque's contents to the bloodstream, triggering the body's clotting system to rapidly form a blood clot at the site — and it is this clot, rather than the underlying narrowing itself, that most often causes the sudden, complete or near-complete blockage responsible for a heart attack. Once blood flow is cut off, the affected area of heart muscle is starved of oxygen and begins to sustain damage within minutes, with the extent of permanent damage growing the longer blood flow remains blocked.
Heart attacks are classified based on findings on an ECG (electrocardiogram), which directly shapes how urgently and how they are treated.
The phrase cardiologists use — “time is muscle” — captures the central fact about heart attack treatment: heart muscle cells begin to die within minutes of losing their blood supply, and this damage accumulates continuously for as long as the blockage remains. For STEMI specifically, the guideline target is to open the blocked artery, most often with primary angioplasty (percutaneous coronary intervention, or PCI), within 90 minutes of arrival at a PCI-capable hospital — a benchmark called “door-to-balloon time” that is directly linked to survival and the amount of heart muscle ultimately preserved.
Importantly, the clock that matters most doesn't start at the hospital door — it starts when symptoms begin. The total time from symptom onset to treatment, not just the time within the hospital, is what most closely predicts outcomes. This is precisely why calling for an ambulance immediately at the first sign of possible heart attack symptoms, rather than waiting to see if they pass, driving yourself to hospital, or trying home remedies first, has such a direct bearing on how much heart muscle is ultimately saved.
Heart attack symptoms are frequently described using the classic picture of crushing central chest pain — but a significant number of people, particularly women, older adults, and people with diabetes, experience a heart attack without this classic presentation. Instead, symptoms may be more subtle: unusual fatigue, nausea, breathlessness, or pain in the back, jaw, or upper abdomen, sometimes with only mild or no chest discomfort at all.
This matters because atypical symptoms are consistently associated with delayed recognition, delayed treatment, and worse outcomes — both because patients themselves may not recognise what is happening and delay seeking help, and because atypical presentations can be harder to identify quickly in a medical setting. The practical takeaway is to take new, unexplained symptoms such as significant fatigue, nausea, or breathlessness seriously, particularly in the presence of known risk factors, rather than assuming a heart attack always announces itself with obvious, severe chest pain.
