A heart attack happens when blood flow to part of the heart muscle gets blocked long enough to cause permanent damage. In roughly 80% of cases, the underlying cause is coronary artery disease, a condition where fatty deposits called plaques build up inside the arteries that feed the heart. But the blockage itself usually isn’t a slow, gradual closing off. It’s a sudden event triggered by a plaque rupturing and forming a blood clot.
How Plaque Builds Up in Your Arteries
The process starts with damage to the inner lining of a coronary artery. High blood pressure, smoking, high blood sugar, and high cholesterol can all injure this lining over time. Once damaged, the artery wall becomes sticky. Cholesterol particles (LDL, the “bad” cholesterol) slip through the damaged lining and get trapped underneath it. White blood cells follow, trying to clean up the cholesterol, but they become overwhelmed and engorged, forming what’s essentially a pool of fatty, inflamed debris inside the artery wall.
The body tries to contain this mess by building a fibrous cap over it, like a scab forming over a wound. As long as that cap stays thick and intact, blood flows over it without issue. You might live for years, even decades, with plaques in your arteries and never know it. The plaque narrows the artery somewhat, but that narrowing alone isn’t what causes most heart attacks.
The Moment a Heart Attack Starts
The real danger comes when a plaque becomes unstable. Inside the plaque, immune cells called macrophages release enzymes that slowly digest the fibrous cap from within, particularly at its thinnest edges. At the same time, the cells that would normally repair and reinforce the cap aren’t keeping up. The cap gets thinner and weaker until it cracks open.
When the cap ruptures, the plaque’s inner contents are suddenly exposed to the bloodstream. This material is intensely clot-promoting. A protein called tissue factor, concentrated in the most inflamed areas of the plaque, triggers the blood’s clotting system almost immediately. Platelets rush to the site and a blood clot (thrombus) forms rapidly, sometimes large enough to completely seal off the artery within minutes.
Once the artery is blocked, the section of heart muscle it supplies is starved of oxygen. If that blockage lasts more than 20 to 40 minutes, heart muscle cells begin to die. The longer the artery stays closed, the more muscle is lost, which is why speed of treatment matters so much.
Complete vs. Partial Blockage
Not every heart attack involves a fully blocked artery. When the clot completely seals the vessel, it typically causes what’s called a STEMI, the more severe type of heart attack that produces a distinctive pattern on an electrocardiogram. The entire thickness of the heart wall in that area is at risk.
When the clot only partially blocks the artery, or blocks it briefly before partially dissolving, the result is an NSTEMI. This type is generally considered less immediately dangerous, but it’s not harmless. About 25% of NSTEMI cases actually do involve a completely occluded artery on imaging, putting them in a high-risk category comparable to a STEMI. Either type causes real damage and requires urgent treatment.
Heart Attacks Without Plaque Rupture
While coronary artery disease accounts for the majority of heart attacks, several other mechanisms can cut off blood flow to the heart.
Coronary Artery Spasm
Sometimes a coronary artery suddenly clamps down on itself, temporarily choking off blood flow even when there’s no significant plaque buildup. This is called vasospastic angina (or Prinzmetal angina). The spasm can be triggered by cocaine or amphetamine use, cold weather exposure, cigarette smoking, or certain medications like some decongestants and migraine drugs. An imbalance in the nervous system’s control over blood vessel tone can also make people prone to these spasms. The episodes often strike at rest, particularly in the early morning hours.
Spontaneous Coronary Artery Dissection
In spontaneous coronary artery dissection (SCAD), the wall of a coronary artery tears on its own, without any trauma or underlying plaque disease. Blood seeps into the artery wall and collects there, forming a pocket that bulges inward and compresses the channel where blood normally flows. The artery essentially pinches itself shut from the inside. SCAD disproportionately affects younger women and is a recognized cause of heart attacks in people who don’t fit the typical risk profile.
Microvascular Disease
Some heart attacks occur even though the large coronary arteries appear completely clear on imaging. In these cases, the problem lies in the tiny resistance vessels, smaller than a fraction of a millimeter, that branch off deep into the heart muscle. When these microscopic vessels can’t dilate properly in response to demand, the heart doesn’t get enough blood during stress or exertion. This condition, called microvascular coronary dysfunction, is the predominant cause of ischemia in women who have chest pain and abnormal stress tests but no visible blockages. It carries a 2.5% annual rate of major events including heart attack, stroke, and heart failure.
What Drives Plaque Buildup in the First Place
Since coronary artery disease is behind most heart attacks, the factors that accelerate plaque formation are effectively the root causes. These aren’t mysteries. They’re the same risk factors cardiologists have tracked for decades, but understanding how each one contributes makes them feel less like an abstract checklist.
High LDL cholesterol is the raw material. Without excess LDL circulating in the blood, plaques don’t form in meaningful amounts. Current guidelines recommend keeping LDL below 100 mg/dL for people at high risk of heart disease, and below 70 mg/dL for those at very high risk (such as people who’ve already had a cardiac event). For otherwise healthy adults at low risk, staying under 130 mg/dL is the target.
High blood pressure damages artery walls through sheer mechanical force, creating the entry points that let cholesterol in. Smoking accelerates the process on multiple fronts: it damages the artery lining directly, promotes inflammation, makes blood more prone to clotting, and lowers HDL (the protective cholesterol). Diabetes raises blood sugar levels that injure artery walls and promote inflammation, which is why people with diabetes face heart attack risk comparable to people who already have known heart disease.
Obesity, physical inactivity, and chronic stress feed into these same pathways. They raise blood pressure, worsen cholesterol profiles, increase inflammation, and promote insulin resistance. Family history also plays a significant role. If a close relative had a heart attack at a young age (before 55 for men, before 65 for women), your own risk is meaningfully elevated regardless of your lifestyle choices.
Why Heart Attacks Happen Differently in Women
Women tend to develop coronary artery disease about a decade later than men, partly due to the protective effects of estrogen before menopause. But the differences go beyond timing. Women are more likely to have heart attacks driven by microvascular disease or coronary artery spasm rather than a single large blocked artery. They’re also more likely to experience SCAD.
This matters because standard cardiac tests are designed to detect blockages in large arteries. A woman with microvascular dysfunction may have a normal-looking angiogram and still be at significant risk. Data from the NHLBI-sponsored WISE study showed that women with no obstructive coronary artery disease but evidence of reduced blood flow to the heart had notably worse outcomes than women without those flow abnormalities. The condition is real, carries real risk, and requires specific testing to identify.