A heart attack happens when blood flow to part of the heart muscle gets suddenly blocked, usually by a blood clot that forms inside a coronary artery. About 805,000 people in the United States have a heart attack every year, and for roughly 605,000 of them, it’s their first. The underlying cause in about 80% of cases is coronary artery disease, a slow buildup of fatty deposits in the artery walls that can take decades before anything goes wrong.
How Plaque Builds Up in Your Arteries
The process starts with cholesterol. When LDL cholesterol (the “bad” kind) circulates in your blood, some of it seeps into the inner walls of your coronary arteries. Once trapped there, the cholesterol particles become chemically modified and trigger an immune response. Your body sends white blood cells to clean up the problem, but those cells gorge on the fatty deposits and become bloated “foam cells.” Layers of these foam cells form visible fatty streaks inside the artery wall.
Over time, some of these fatty deposits grow into larger, more complex structures. Foam cells die and release their contents, creating a soft, mushy core of lipids and cell debris deep inside the artery wall. The body tries to wall off this mess by building a fibrous cap over it, made mostly of collagen produced by smooth muscle cells. The result is what doctors call a plaque: a fatty core sealed beneath a tough outer shell, narrowing the artery from the inside.
What Makes a Plaque Rupture
Not all plaques are equally dangerous. The ones most likely to cause a heart attack tend to have a thin fibrous cap and a large, lipid-rich core. Two things thin the cap simultaneously. Smooth muscle cells, which produce the collagen holding the cap together, gradually die off. At the same time, immune cells infiltrating the plaque release enzymes that actively dissolve the remaining collagen. When the cap gets thin enough (research identifies the danger zone as less than 65 micrometers, thinner than a human hair), it can tear open.
Once a plaque ruptures, the soft core inside is exposed directly to the bloodstream. This core is loaded with substances that activate clotting. Within minutes, a blood clot forms at the rupture site. If the clot grows large enough to completely block the artery, blood flow to the heart muscle downstream stops. Without oxygen, heart muscle cells begin dying within 20 to 40 minutes. That’s a heart attack.
Chronic Risk Factors That Set the Stage
Heart attacks rarely come out of nowhere. They’re the end result of years of arterial damage driven by well-known risk factors.
High Blood Pressure
Blood pressure at or above 130/80 mmHg damages the inner lining of your arteries over time. That damage makes it easier for cholesterol to penetrate the artery wall and start the plaque-building process. Chronic high pressure also makes artery walls stiffer and less elastic, compounding the problem.
High LDL Cholesterol
LDL cholesterol is the raw material for plaque. The risk it poses depends heavily on whether plaque has already started forming. In a large study of over 23,000 patients, people with very high LDL levels (193 mg/dL or above) who already had calcium deposits in their coronary arteries were 3.5 times more likely to have a heart attack than those with lower LDL. Interestingly, people with the same high LDL but no existing plaque didn’t show increased risk, suggesting that LDL does its worst damage by feeding plaques that already exist.
Smoking
Cigarette smoke contains toxins that directly injure blood vessel walls and promote inflammation. About 75% of patients who experience coronary artery spasms are active smokers. Smoking also makes blood more prone to clotting and reduces the amount of oxygen your blood can carry.
Diabetes
Chronically elevated blood sugar damages artery walls and accelerates plaque formation. Even in people without detectable plaque buildup, diabetes is independently associated with future heart attacks.
Chronic Inflammation
Inflammation plays a role at every stage of the process, from the initial buildup of cholesterol in artery walls to the final rupture of a plaque. Your body produces an inflammatory marker called C-reactive protein, or CRP, which has been found embedded in atherosclerotic plaques themselves. CRP doesn’t just signal inflammation; it appears to actively worsen it by triggering part of the immune system called the complement cascade, which promotes further clotting and can damage artery walls and heart muscle cells directly.
Acute Triggers That Spark the Event
When someone already has vulnerable plaques in their arteries, certain short-term triggers can push one of those plaques to rupture. A major international study called INTERHEART tracked what patients were doing in the hours before their heart attacks and found two triggers that stood out.
Physical exertion more than doubled the odds of a heart attack in the hour it occurred (2.3 times higher risk). Anger or intense emotional upset carried a similar bump (2.4 times higher risk). When both happened together, the risk tripled. These triggers work by flooding the body with stress hormones, which raise heart rate and blood pressure, constrict blood vessels, and increase oxygen demand on the heart. All of that mechanical stress can crack open an already weakened plaque. The risk from these triggers was highest in the evening hours, between 6 p.m. and midnight.
Extreme cold, heavy meals, and sudden intense strain (like shoveling snow) act through similar mechanisms: they spike blood pressure and heart rate at a time when arteries may already be partially blocked.
Heart Attacks Without Typical Plaque Buildup
About 20% of heart attacks, particularly in younger adults, happen without classic coronary artery disease. Several distinct causes account for these cases.
Coronary Artery Dissection
Spontaneous coronary artery dissection, or SCAD, occurs when the inner wall of a coronary artery tears on its own. Blood seeps into the artery wall through the tear and forms a pocket that compresses the artery from the outside, choking off blood flow. SCAD occurs overwhelmingly in women and may account for up to 35% of heart attacks in women under 50. It’s also the most common cause of heart attacks during pregnancy and the postpartum period, responsible for about 43% of those cases. Most people who experience SCAD have few or none of the traditional risk factors like high cholesterol or high blood pressure.
Coronary Artery Spasm
Sometimes a coronary artery clamps down suddenly and severely, temporarily cutting off blood flow even without significant plaque. The underlying problem is that the muscle cells in the artery wall overreact to normal signals, contracting too forcefully. Smoking is the single biggest external trigger. Cocaine, amphetamines, and marijuana can also provoke spasms. These episodes often strike at night, when the nervous system shifts toward a state that can paradoxically cause constriction in people with hypersensitive arteries.
Blood Clots From Elsewhere
In roughly 5% of heart attacks in younger adults, a blood clot forms somewhere else in the body, travels through the bloodstream, and lodges in an otherwise healthy coronary artery. Another 5% are caused by inherited clotting disorders that make blood throughout the body too prone to clotting. About 4% trace back to congenital abnormalities in the shape or path of the coronary arteries themselves.
Why Speed Matters
Once a coronary artery is blocked, the damage is progressive. Heart muscle that goes without blood flow starts dying within minutes, and the longer the blockage lasts, the more muscle is permanently lost. That’s why the treatment goal is to reopen the artery as fast as possible, ideally within 90 minutes of arriving at the hospital. Every minute of delay means more heart tissue that can’t be saved, which directly affects how well the heart pumps for the rest of your life.