Myocardial Infarction: Everything You Need to Know About a Heart Attack
A heart attack can begin with something microscopic: a tiny disruption in the surface of an atherosclerotic plaque inside a coronary artery. Within minutes, platelets can accumulate, a blood clot can grow, blood flow can fall dramatically, and heart muscle that has been receiving oxygen every second of a person's life can suddenly become ischemic.
If blood flow is not restored, cells begin to die. This is myocardial infarction (MI). Despite enormous advances in cardiovascular medicine, myocardial infarction remains one of the most important medical emergencies worldwide. At the same time, our understanding of it has changed dramatically. A century ago, a major myocardial infarction was frequently fatal and treatment options were extremely limited. Today, a blocked coronary artery can sometimes be identified and reopened within hours—or even minutes—of a patient's arrival at hospital.
Understanding myocardial infarction therefore requires understanding two very different processes.
1) The first develops silently over years or decades: atherosclerosis.
2) The second can occur within minutes: acute coronary thrombosis and myocardial ischemia.
To understand a heart attack properly, we need to understand both.
What Is a Myocardial Infarction?
The term sounds complicated, but its meaning is relatively simple.
Myocardium means heart muscle.
Infarction means tissue death caused by inadequate blood supply.
A myocardial infarction therefore describes injury and death of heart muscle caused by ischemia—insufficient blood flow and oxygen delivery relative to the needs of the myocardium.
Most classic heart attacks occur because blood flow through a coronary artery is suddenly reduced or interrupted. Frequently, this happens when an atherosclerotic plaque becomes disrupted and a thrombus—a blood clot—forms over it.
But this is an important distinction:
Not every myocardial infarction is caused by an atherosclerotic plaque rupture or even by a coronary thrombus.
Anything that produces sufficiently severe myocardial ischemia can potentially cause infarction. This is why modern medicine distinguishes myocardial infarction from the broader concept of myocardial injury.
Heart attack is not the same as cardiac arrest
These terms are often used interchangeably in everyday conversation, but they describe different events.
A myocardial infarction is primarily a problem of blood flow to heart muscle.
A cardiac arrest is the sudden loss of effective cardiac mechanical activity and circulation.
A myocardial infarction can trigger a dangerous arrhythmia such as ventricular fibrillation and thereby cause cardiac arrest. But cardiac arrest can also occur for many other reasons.
Someone having a heart attack may therefore be completely conscious and talking. Someone in cardiac arrest is unresponsive and has no effective circulation.
PART I — HOW THE HEART RECEIVES BLOOD
The Heart Has Its Own Blood Supply
The heart pumps blood to almost every organ in the body, but the blood inside its chambers cannot adequately supply the thick muscular walls of the heart itself.
The myocardium therefore receives blood through the coronary arteries, which originate near the beginning of the aorta.
The left main coronary artery generally divides into two major branches:
the left anterior descending artery (LAD), supplying much of the anterior wall and interventricular septum;
the left circumflex artery (LCx), supplying much of the lateral and, depending on coronary anatomy, posterior myocardium.
The right coronary artery (RCA) supplies much of the right ventricle and inferior myocardium and commonly supplies the atrioventricular node. The exact territories vary between individuals because coronary anatomy and dominance differ.
This becomes clinically important because the location of a coronary obstruction influences the region of myocardium affected, the ECG abnormalities produced, the complications that may develop, and sometimes the patient's symptoms.
The Fundamental Equation: Oxygen Supply Must Meet Oxygen Demand
Heart muscle is metabolically active tissue. Cardiomyocytes continuously require oxygen to generate ATP and maintain contraction, ion gradients, and cellular integrity.
Think of myocardial oxygen balance as a continuously negotiated equation.
On one side is oxygen supply. This depends on factors including coronary blood flow, arterial oxygen content and hemoglobin concentration.
On the other side is oxygen demand. Demand rises with factors such as heart rate, contractility and ventricular wall stress.
Normally, supply is sufficient to meet demand. But imagine someone with a significantly narrowed coronary artery who develops severe anemia and a heart rate of 170 beats per minute. Oxygen-carrying capacity has fallen while myocardial oxygen consumption has risen dramatically. The coronary artery does not necessarily need to become completely blocked for ischemia to occur.
This concept—myocardial oxygen supply versus demand—is fundamental to understanding why not every MI has the same mechanism.
PART II — THE DISEASE THAT OFTEN STARTS DECADES BEFORE THE HEART ATTACK
