Background

The most common cause of a heart attack — medically termed a myocardial infarction — is atherosclerosis. In this condition, plaque builds up within the walls of the coronary arteries, gradually compromising blood flow to the heart muscle. Heart muscle cells, known as cardiomyocytes, require a continuous supply of oxygen carried by the blood. When blood flow drops below what the tissue needs, the cells become oxygen-starved and eventually die. The term "myocardial infarction" breaks down into "myo" (muscle), "cardial" (heart), and "infarction" (tissue death due to lack of oxygen). While the general concept of a heart attack is widely known, the precise sequence of events inside the body — from plaque rupture to cell death and the resulting symptoms — is less commonly understood. This article describes the physiology of a myocardial infarction in a step-by-step manner, including the formation of a clot, the effects on cardiomyocytes, the development of referred pain, compensatory mechanisms, and the two main types of infarction based on the depth of heart wall involvement.

The Starting Point: Atherosclerotic Plaque

Atherosclerosis refers to the buildup of fatty, fibrous material within the inner lining of arteries. In the coronary arteries, these plaques can remain silent for years, causing only partial obstruction. Blood continues to flow past the plaque, and the heart muscle receives enough oxygen under resting conditions. However, such a plaque is vulnerable. During physical exertion — for example, running or playing a sport — the heart pumps faster, forcing blood to rush through the coronary arteries at higher speed and pressure. This turbulent flow can bombard the plaque, causing its fibrous cap to rupture.

Plaque Rupture and Thrombus Formation

When a plaque ruptures, the inner, soft, cheese-like material becomes exposed to the bloodstream. This material is highly thrombogenic, meaning it triggers rapid clot formation. Platelets adhere to the exposed surface and release chemicals that accelerate the clotting process. Within a short time, a large blood clot (thrombus) forms on the ruptured plaque. This clot physically obstructs the artery, dramatically reducing blood flow to the downstream portion of the vessel. The cardiomyocytes that depend on that section of the artery now receive far less oxygen than they need.

Early Oxygen Starvation and the Onset of Pain

As the cardiomyocytes become oxygen-starved (ischemic), they begin to send distress signals to the brain in the form of pain signals. Because the brain is not accustomed to receiving pain signals directly from the heart, it can misinterpret their origin. This phenomenon is called referred pain. Initially, the pain may feel like indigestion — a discomfort located just below the heart, above the stomach. This is often the very first sensation of a heart attack. As the clot continues to grow, now blocking two-thirds of the artery, the pain intensifies. By the same mechanism of referred pain, some individuals experience pain radiating to the left arm (most commonly) or even up to the jaw. This occurs because the nerves that supply the heart share the same spinal cord origins as nerves that supply the arm and jaw.

Adrenaline Surge and Compensatory Tachycardia

The brain, sensing both the pain signals and the abnormal beating pattern of the oxygen-starved heart, triggers a large release of adrenaline into the bloodstream. Adrenaline makes the heart beat faster (tachycardia) — an attempt to maintain overall cardiac output. However, adrenaline cannot remove or bypass the clot. The clot continues to enlarge, eventually filling nearly the entire lumen of the artery. At this point, blood flow to the affected region is minimal.

Cellular Consequences: Impaired Contraction and Membrane Rupture

Cardiomyocytes require a steady supply of oxygen to produce the energy (ATP) needed for contraction. Without adequate oxygen, they cannot generate enough energy, and their contractile function slows. Over time, the affected patch of heart muscle begins to beat less forcefully and may stop contracting altogether. The rest of the heart — still receiving blood — tries to compensate by beating even faster.

A further consequence of prolonged oxygen deprivation is the accumulation of toxic metabolic waste products inside the cardiomyocytes. Normally, blood flow carries away these wastes, but when flow is blocked, the toxins build up. This buildup damages the cell membranes. The membranes become leaky and eventually rupture. When the cells rupture, they release proteins that are unique to heart muscle cells — troponins (troponin I and troponin T). These proteins enter the bloodstream and become detectable, serving as key biomarkers for diagnosing a heart attack.

Progressive Heart Failure and Respiratory Symptoms

As more cardiomyocytes lose their ability to contract, the heart's pumping efficiency declines. Blood begins to back up inside the heart. Because the heart normally pumps blood to the lungs for oxygenation, a failing pump causes blood to accumulate in the pulmonary circulation. This backflow can lead to fluid buildup in the lungs (pulmonary congestion), making it difficult to breathe. Shortness of breath (dyspnea) is a common symptom during a heart attack. Additionally, if the brain receives insufficient blood due to reduced cardiac output, the person may become dizzy or disoriented.

The Irreversible Damage: Rapid Cell Death

Approximately 15 to 18 minutes after the onset of a heart attack, the situation becomes critical. Cardiomyocytes begin to die, not just leak. Once the clot completely occludes the artery, the rate of cell loss is staggering — about 500 cardiomyocytes per second. Unlike skin cells or hair, cardiomyocytes have very limited regenerative capacity. Dead heart muscle cells are replaced by scar tissue, which does not contract. Therefore, the loss is permanent, and the heart will never beat normally again. This is why rapid medical intervention — to reopen the blocked artery — is essential to minimise the number of cells lost.

Two Main Types of Myocardial Infarction

Based on the depth of the heart wall that is affected, myocardial infarctions are classified into two major groups.

1. Transmural (Full-Thickness) Infarct

A transmural infarct involves the entire thickness of the heart wall, from the inner lining (endocardium) to the outer lining (epicardium). This type typically occurs when a large epicardial coronary artery — such as the left anterior descending (LAD) artery — is completely blocked. The LAD supplies a large portion of the left ventricular wall. A clot in this vessel therefore damages a full-thickness segment of the heart muscle. "Transmural" means across ("trans-") the wall ("mural").

2. Subendocardial (Partial-Thickness) Infarct

A subendocardial infarct involves only the inner part of the heart wall, usually the subendocardial region (the layer just beneath the inner lining). This occurs when a smaller penetrating artery — a branch off a major coronary vessel — becomes blocked. Such small arteries supply only a limited region of the heart muscle. The resulting infarct is therefore partial thickness. The word "subendocardial" indicates that the damage is below ("sub-") the endocardium. Partial-thickness infarcts are generally smaller in volume than transmural infarcts, but they still represent a serious medical event.

Summary of Key Events in a Heart Attack

  • A stable atherosclerotic plaque ruptures under increased blood flow or stress.
  • A thrombus forms on the ruptured plaque, obstructing the artery.
  • Downstream cardiomyocytes become oxygen-starved and send pain signals — often referred to the arm, jaw, or perceived as indigestion.
  • The brain releases adrenaline, making the heart race, but the clot continues to grow.
  • Oxygen-deprived cells accumulate toxic waste, their membranes leak, and they release troponin into the blood.
  • Cells begin to die at a rate of about 500 per second after 15–18 minutes.
  • The heart pumps less efficiently, causing shortness of breath and dizziness.
  • The location and depth of the infarct determine whether it is classified as transmural (full-thickness) or subendocardial (partial-thickness).

Understanding this sequence highlights why a heart attack is a time-sensitive emergency. Every minute of delay in restoring blood flow — through medications that dissolve clots or procedures that physically remove them — saves thousands of heart muscle cells and preserves cardiac function. The distinction between transmural and subendocardial infarcts is not merely academic; it helps clinicians predict the extent of damage and guide long-term management. However, regardless of the type, the underlying pathophysiology follows the same core principles: reduced oxygen supply, cellular energy failure, membrane rupture, and irreversible cell death. Recognition of symptoms — chest or arm pain, shortness of breath, indigestion-like discomfort — and immediate activation of emergency medical services remain the most important actions a person can take.

This article is for general educational purposes only and is not medical advice. It does not diagnose or treat any condition. A suspected heart attack is a medical emergency — call emergency services immediately. Anyone with chest pain, arm or jaw pain, shortness of breath, or other symptoms of a heart attack should seek emergency care without delay.