A heart attack happens when blood flow to part of the heart muscle gets blocked, starving that tissue of oxygen. In the vast majority of cases, the blockage starts with a fatty deposit inside an artery wall that cracks open, triggering a blood clot that seals off the vessel. In the United States, someone has a heart attack every 40 seconds, totaling about 805,000 per year.
How Plaque Builds Up Over Decades
The process that leads to a heart attack usually begins years or even decades before any symptoms appear. It starts with cholesterol particles (LDL, sometimes called “bad cholesterol”) slipping through the inner lining of a coronary artery and becoming trapped in the vessel wall. Once stuck there, these particles undergo chemical changes that trigger an immune response. White blood cells rush in to absorb the modified cholesterol, swelling into what scientists call foam cells. These foam cells pile up beneath the artery lining and form the earliest stage of a plaque: a fatty streak.
Over time, more cholesterol accumulates, more immune cells arrive, and a soft, lipid-rich core develops inside the artery wall. The body tries to wall off this mess by building a fibrous cap of tough tissue over the top. If the cap stays thick and intact, the plaque can sit quietly for years, gradually narrowing the artery but never causing a sudden event. These are considered stable plaques.
What Makes a Plaque Dangerous
Not all plaques are equally threatening, and the ones most likely to cause a heart attack aren’t necessarily the largest. What matters most is the structure. A “vulnerable” plaque has a thin fibrous cap (less than 65 micrometers thick, roughly the width of a human hair), a large pool of soft lipid material underneath, and heavy infiltration by inflammatory cells that weaken the cap from within. Histology studies show that up to 95% of plaque ruptures occur in plaques with these thin caps. The necrotic core inside a vulnerable plaque is roughly six times more likely to trigger clot formation than other plaque components.
A stable plaque, by contrast, has a thick cap, fewer inflammatory cells, more structural support from smooth muscle cells, and a smaller lipid core. It can still narrow an artery enough to cause chest pain during exertion, but it’s far less likely to rupture suddenly.
The Moment a Heart Attack Begins
A heart attack is set in motion when a vulnerable plaque cracks. Mechanical stress on the artery wall, a spike in blood pressure, or ongoing inflammation can all contribute to that moment. Once the cap tears, the soft, fatty interior of the plaque is exposed to flowing blood. The body treats this the same way it treats any wound: it rapidly forms a clot.
The clot can partially or completely block the artery. If it seals off the vessel entirely, the heart muscle downstream is cut off from oxygen. Heart cells can survive about 15 minutes of oxygen deprivation without permanent damage. After that, cells in the innermost layer of the heart wall begin to die. By 60 minutes, most of that inner layer is dead. After six hours of total blockage, the damage is essentially complete, and restoring blood flow can no longer save the affected tissue. This is why speed matters so much during a heart attack: every minute of delay means more muscle lost.
Heart Attacks Without Plaque Buildup
While atherosclerosis (plaque disease) causes the majority of heart attacks, it isn’t the only mechanism. In spontaneous coronary artery dissection, or SCAD, a tear develops in the inner wall of a heart artery. Blood seeps between the layers of the artery wall, and the resulting bulge or clot narrows the channel and blocks flow. People who experience SCAD often have none of the traditional risk factors like high blood pressure, high cholesterol, or diabetes. It disproportionately affects younger women.
Coronary microvascular disease is another pathway. Instead of blockages in the large coronary arteries, the tiny vessels that branch deep into the heart muscle become damaged and fail to dilate properly. This form is more common in women, particularly after menopause when estrogen levels drop. Symptoms tend to appear during routine daily activities or emotional stress rather than physical exercise, and standard heart tests often come back looking normal, which can delay diagnosis.
Risk Factors That Set the Stage
The factors that drive plaque formation and instability are well established. High LDL cholesterol is the most direct contributor, since LDL particles are the raw material that accumulates in artery walls. An LDL level of 160 mg/dL or higher is considered a significant independent risk factor. High blood pressure accelerates the process by damaging the artery lining, making it easier for cholesterol to infiltrate. A systolic reading consistently at or above 140, or a diastolic at or above 90, moves you into elevated cardiovascular risk territory. Readings above 160/100 are classified as a major risk factor.
Diabetes amplifies nearly every step of plaque development, from inflammation to clot formation. Smoking damages artery walls directly and promotes the chemical changes that make trapped LDL particles more inflammatory. Obesity, physical inactivity, and chronic stress all raise risk through overlapping pathways involving inflammation, blood sugar, and blood pressure.
Family history also plays a meaningful role. If a close relative had heart disease at a young age, your own risk is elevated regardless of how well you manage other factors. Genetic conditions that cause extremely high cholesterol from birth can lead to heart attacks as early as the 20s or 30s.
Two Types of Heart Attack
When someone arrives at the emergency room with heart attack symptoms, one of the first things doctors look at is the electrical pattern of the heart on an EKG. A STEMI (ST-elevation myocardial infarction) typically means a coronary artery is completely blocked, and the patient is rushed to a procedure to reopen the vessel as quickly as possible. An NSTEMI (non-ST-elevation myocardial infarction) usually indicates a partial blockage, though a meaningful number of NSTEMI patients turn out to have a fully blocked artery as well.
Both types cause real damage to heart muscle. The distinction matters primarily because it determines how urgently the blockage needs to be physically opened. STEMI patients are treated as the most time-critical cases.
Silent Heart Attacks
About 1 in 5 heart attacks are silent. The damage occurs, heart muscle dies, but the person doesn’t recognize what happened. They may have felt fatigue, mild discomfort, or nothing at all. Silent heart attacks are often discovered later during routine testing. They carry real consequences: the dead tissue is replaced by scar, which weakens the heart’s pumping ability and increases the risk of future events. People with diabetes are particularly prone to silent heart attacks because nerve damage can blunt the typical pain signals.