Ischaemic heart disease (IHD), also referred to as coronary artery disease (CAD), describes a state in which myocardial oxygen supply is insufficient to meet myocardial metabolic demand.
At face value, that is a simple supply–demand mismatch. In reality, it reflects a complex, progressive, inflammatory disease of the coronary arteries that evolves over decades before becoming clinically apparent.
The heart extracts a very high proportion of oxygen from coronary blood at rest. Unlike skeletal muscle, it cannot simply extract much more when demand rises. When the myocardium needs more oxygen — during exercise, stress, tachycardia — the only way to meet that demand is to increase coronary blood flow.
Anything that limits the ability of coronary arteries to increase flow becomes clinically important.
To understand IHD properly, we need to follow the biology.
Atherosclerosis: A Chronic Inflammatory Disease
It is a chronic inflammatory process involving:
- Endothelial dysfunction
- Lipoprotein retention and oxidation
- Innate immune activation
- Smooth muscle cell migration and proliferation
- Extracellular matrix remodelling
The earliest and most important shift is loss of endothelial homeostasis.
The Process Begins with Endothelial Dysfunction
The coronary endothelium is an active, regulatory surface. It produces nitric oxide (NO), which promotes vasodilation and inhibits platelet aggregation. It suppresses inflammation. It maintains barrier integrity.
When exposed chronically to hypertension, hyperglycaemia, smoking toxins or elevated LDL, the endothelial cells change their behaviour.
Nitric oxide production falls. Oxidative stress increases. Adhesion molecules are expressed on the endothelial surface. Damaged endothelium allows LDL to more readily cross into the intima.
The vessel wall becomes permissive to lipid retention and leukocyte recruitment, allowing inflammation within the intima to become established.
LDL Retention and the Inflammatory Response
Once LDL enters the intima, it becomes oxidised.
Oxidised LDL is biologically active. It attracts monocytes from the circulation. Those monocytes migrate into the intima and differentiate into macrophages.
Macrophages ingest oxidised LDL via scavenger receptors. Unlike normal LDL receptors, these do not downregulate when intracellular cholesterol rises. The macrophage continues to accumulate lipid and becomes a foam cell.
Foam cells cluster together, forming fatty streaks.
At this stage, the lumen may still appear relatively preserved. There may be no symptoms. But the inflammatory process is established within the vessel wall.
Structural Change: Plaque Formation
Persistent inflammation alters the architecture of the artery.
Smooth muscle cells migrate from the media into the intima. They proliferate and synthesise extracellular matrix proteins, including collagen. A fibrous cap forms over a lipid-rich necrotic core composed of foam cells, cholesterol crystals and cellular debris.
Now there is a defined atherosclerotic plaque.
Two features determine how that plaque behaves:
- How much it narrows the lumen.
- How stable its fibrous cap is.
These are related but not identical.
Understanding that distinction is essential for clinical reasoning later.
Coronary Flow Reserve and Stable Angina
Under resting conditions, even moderately narrowed coronary arteries may provide adequate perfusion. The problem arises when demand increases.
During exertion, heart rate rises and myocardial contractility increases. Wall tension increases. All of this increases oxygen consumption.
In a healthy coronary artery, flow increases proportionally.
In an artery with fixed stenosis, flow reserve is limited. The vessel cannot dilate sufficiently to match the rise in demand.
The myocardium becomes relatively underperfused.
At a cellular level, ATP production decreases. Anaerobic metabolism increases. Lactate accumulates. Transient contractile dysfunction develops.
If perfusion is restored, these changes reverse.
This is stable angina — predictable, exertion-related and reversible.
The limitation of flow is fixed rather than dynamic.
Plaque Instability and Acute Coronary Syndromes
In some plaques, ongoing inflammation weakens the fibrous cap.
Macrophages release matrix metalloproteinases that degrade collagen. The cap thins. Mechanical stress during normal cardiac cycles can precipitate rupture.
When rupture occurs, subendothelial collagen and tissue factor are exposed to circulating blood. Platelets adhere, activate and aggregate. The coagulation cascade is triggered. A thrombus forms on top of the disrupted plaque.
Now the limitation of flow is dynamic rather than fixed.
If the thrombus partially occludes the artery, perfusion becomes unstable. This produces unstable angina or NSTEMI.
If occlusion becomes complete and sustained, myocardial cells downstream are deprived of oxygen beyond the threshold for survival. Irreversible necrosis begins within 20–30 minutes.
This progression represents myocardial infarction — a later stage of the same disease process, distinguished by a dynamic thrombotic obstruction rather than fixed stenosis.
Downstream Effects on the Myocardium
Infarcted myocardium undergoes coagulative necrosis. Inflammatory cells infiltrate. Over weeks, scar tissue replaces viable myocytes.
Scar tissue does not contract. It does not conduct electrical impulses normally. Over time, ventricular remodelling may occur. Systolic function may decline.
Chronic ischaemia without infarction can also produce hibernating myocardium and progressive systolic dysfunction.
These outcomes are consequences of prolonged mismatch between supply and demand
🩻 Clinical Consequences of IHD
Clinically, this single disease process presents in different ways depending on stage: predictable exertional angina in the setting of fixed stenosis, unstable angina when thrombosis is transient, myocardial infarction when occlusion is sustained, and chronic ischaemic cardiomyopathy when repeated or prolonged ischaemia leads to ventricular dysfunction.
🤔Think of IHD in sequence
Endothelial dysfunction
→ Lipid retention and oxidation
→ Chronic inflammation
→ Plaque formation
→ Either fixed stenosis or cap instability
→ Impaired flow
→ Reversible ischaemia or irreversible necrosis
That sequence links molecular biology to patient presentation.
🎯 Why Does This Matter?
When you encounter a patient with chest pain, the question is not simply “Is this IHD?”
Instead you should consider:
- Where are they in the process?
- Is coronary flow limited by fixed stenosis?
- Has a plaque become unstable?
- Is thrombosis transient or sustained?
Understanding the sequence — from endothelial dysfunction to thrombosis — allows you to reason forward from mechanism to presentation.
Build the physiology carefully — the clinical reasoning will follow.
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