BCR 6 - Angina and Valvular Heart Disease

What is the pathophysiological basis of cardiac chest pain?

Cardiac chest pain is caused by myocardial ischaemia, which occurs when coronary blood flow does not meet myocardial oxygen demand.

How does myocardial ischaemia usually develop?

It usually develops due to reduced coronary blood supply and less commonly due to excessive myocardial oxygen demand.

What form of ischaemic heart disease is the focus of stable chest pain physiology?

The focus is on stable angina rather than myocardial infarction.

What determines myocardial oxygen balance?

The relationship between coronary blood flow and myocardial oxygen demand.

What are epicardial coronary arteries?

Large coronary vessels that lie on the outer surface of the heart and act mainly as conductance vessels.

Why are epicardial arteries described as conductance vessels?

They provide minimal resistance to blood flow under normal conditions.

Which coronary vessels provide most vascular resistance?

Pre arterioles and arterioles provide the major resistance in the coronary circulation.

What is the role of pre arterioles and arterioles in coronary flow?

They change calibre to regulate resistance and control coronary blood flow.

Do capillaries significantly contribute to coronary resistance?

Capillaries and venous vessels make only a small contribution to overall resistance.

What is coronary flow reserve?

The ability of the coronary circulation to increase blood flow to meet increased myocardial demand.

How much can coronary blood flow increase during exercise?

Coronary blood flow can increase up to about five times resting levels during exercise.

What does coronary flow reserve represent physiologically?

It reflects the reserve capacity of the coronary circulation to augment flow.

What is the main role of coronary blood flow?

To deliver oxygen to the myocardium and remove metabolic waste.

What factors influence oxygen delivery to the myocardium?

Coronary blood flow, haemoglobin concentration and arterial oxygen tension.

Why is reduced oxygen carrying capacity rarely the primary cause of ischaemia?

Because coronary blood flow is usually the limiting factor rather than oxygen content.

How can anaemia or hypoxia affect myocardial ischaemia?

They can worsen existing ischaemia by reducing oxygen delivery.

How much oxygen does the heart extract at rest?

60% of available oxygen

How does myocardial oxygen extraction compare to skeletal muscle?

Skeletal muscle extracts only about 20 to 30%

Why must the heart increase blood flow during increased demand?

Because it cannot significantly increase oxygen extraction further.

How does Ohm’s law apply to coronary blood flow?

Flow equals pressure difference divided by resistance.

How can coronary blood flow be increased according to Ohm’s law?

By increasing pressure or decreasing resistance.

Which variable does the coronary circulation mainly adjust?

Resistance

What are the main regulators of coronary blood flow?

Metabolic control, autoregulation, extravascular compression, duration of diastole, hormonal and neural control, and the diastolic pressure gradient.

What is metabolic control of coronary blood flow?

It is regulation of flow by metabolites produced during myocardial metabolism.

Where are coronary arterioles located?

They lie within the metabolising myocardium.

How do metabolites influence coronary arterioles?

They diffuse into smooth muscle and cause vasodilatation.

Which metabolites regulate coronary blood flow?

Adenosine, potassium, pH and oxygen tension.

How is adenosine formed during ischaemia?

ATP is broken down to ADP, AMP and then adenosine.

Why is adenosine important in coronary physiology?

It is a potent vasodilator of coronary arterioles.

How is adenosine used clinically?

It is used during cardiac testing to provoke ischaemia.

What is autoregulation in the coronary circulation?

An intrinsic property of vascular smooth muscle that maintains constant flow across a range of pressures.

Over what pressure range is coronary autoregulation effective?

Approximately 60 to 120 mm/Hg

How does increased pressure affect vascular smooth muscle?

Increased pressure increases smooth muscle tone.

Why does autoregulation stabilise coronary flow?

Because resistance increases proportionally with pressure, keeping flow constant.

Does autoregulation cause exercise induced increases in flow?

No, it stabilises flow rather than increasing it during exercise.

Why is coronary blood flow mainly diastolic?

Because myocardial contraction during systole compresses intramyocardial vessels.

What happens to coronary vessels during systole?

Intramyocardial vessels are compressed, resulting in minimal systolic flow.

Are epicardial vessels affected by systolic compression?

They are largely unaffected.

Why is the sub endocardium particularly vulnerable to ischaemia?

It is furthest from epicardial vessels and fills slowest during diastole.

How does reduced perfusion pressure affect the sub endocardium?

It disproportionately reduces blood flow to this region.

How does tachycardia worsen myocardial ischaemia?

It shortens diastole, increases oxygen demand and maintains elevated left ventricular diastolic pressure.

Why does exercise increase susceptibility to ischaemia?

Because heart rate increases, diastole shortens and myocardial demand rises.

How are coronary resistance vessels innervated?

Pre arterioles and arterioles receive autonomic innervation.

What is the effect of alpha receptor activation on coronary vessels?

It causes vasoconstriction.

Where is alpha receptor activation greater?

It is greater in the epicardium.

Why is alpha mediated vasoconstriction beneficial?

It helps maintain homogeneous blood flow between epicardium and sub endocardium.

What is the effect of beta receptor activation on coronary vessels?

It causes vasodilatation.

When is beta mediated vasodilatation particularly important?

During exercise.

What happens to coronary control after heart transplantation?

Neural connections are lost.

How does loss of innervation affect vasodilator responses?

There is reduced vasodilator response to adenosine.

Why do transplant patients have resting tachycardia?

Due to loss of parasympathetic control.

Why is maximal exercise heart rate reduced after transplantation?

Because of abnormal sympathetic responses.

What role does the endothelium play in coronary flow reserve?

It releases endothelium derived relaxing factor which causes vasodilatation.

What is endothelium derived relaxing factor?

Nitric oxide released by endothelial cells.

How does nitric oxide affect coronary resistance?

It reduces resistance by causing vasodilatation.

What coronary flow reserve is seen in healthy arteries?

Flow can increase up to five times resting levels.

How does early atherosclerosis affect coronary flow reserve?

It impairs nitric oxide production despite minimal mechanical obstruction.

Why can early atheroma reduce flow reserve?

Because endothelial dysfunction limits vasodilatation.

What happens to coronary flow in progressive atherosclerotic lesions?

A trans stenotic pressure gradient develops.

How do distal arterioles respond in moderate coronary disease?

They dilate at rest to maintain blood flow.

What happens to coronary flow reserve in moderate disease?

Flow reserve is reduced.

At what level of stenosis can resting ischaemia occur?

85% diameter reduction

Why does severe stenosis cause resting ischaemia?

Because resistance is increased even at rest.

What is fractional flow reserve?

A minimally invasive measurement of the pressure drop across a coronary stenosis during maximal blood flow.

Why is fractional flow reserve needed?

Because angiography alone does not assess functional significance.

Why do normal epicardial vessels contribute little resistance?

They are large conductance vessels under physiological conditions.

How is maximal coronary flow achieved during FFR measurement?

By inducing distal vasodilatation with adenosine.

What does fractional flow reserve compare?

Distal coronary pressure to proximal pressure across a stenosis.

What FFR value indicates a haemodynamically significant lesion?

An FFR less than 0.80

What does FFR assess clinically?

The functional significance of a coronary lesion rather than its appearance.

Why can patients have angina with normal coronary angiograms?

Because disease may exist in the microvasculature.

What does reduced coronary flow reserve with normal angiography suggest?

Microvascular disease.

Why cannot microvascular disease be seen on angiography?

Because it affects pre arterioles and arterioles rather than large epicardial vessels.

What functional abnormalities occur in microvascular disease?

Reduced vasodilator response and abnormal endothelial function.

What abnormal vascular responses are seen in microvascular disease?

Excessive vasoconstrictor responses.

Which conditions are commonly associated with microvascular disease?

Hypertension and diabetes.

What symptoms can microvascular disease cause?

Angina despite absence of large vessel disease.

What are the long term consequences of microvascular disease?

Increased risk of myocardial infarction, heart failure and death.

Why does myocardial ischaemia cause chest pain? (not cause but function)

Because metabolites stimulate sensory nerve fibres around coronary microvessels.

Where are pain sensing nerve fibres located in the heart?

Around coronary microvessels.

Which metabolite is a key mediator of anginal pain?

Adenosine.

How is the role of adenosine in pain demonstrated clinically?

Intracoronary adenosine infusion can reproduce chest pain.

How are anginal pain signals transmitted to the brain?

Via the spinal cord, thalamus and cerebral cortex.

What is angina pectoris?

Chest pain or discomfort caused by myocardial ischaemia due to an imbalance between oxygen supply and demand.

Why does angina occur?

When myocardial oxygen supply is insufficient to meet myocardial oxygen demand.

Where are anginal symptoms typically felt?

Symptoms are usually felt in the chest, reflected by the term pectoris.

How is anginal pain commonly described?

It is described as tightness, heaviness, pressure, band like or burning discomfort.

How is anginal pain transmitted?

Pain is likely transmitted via sympathetic nerve fibres and is often poorly localised.

What characterises typical anginal pain?

It is predictable, triggered by exertion and relieved by rest within minutes.

What is the most common trigger for angina?

Physical exercise.

Why does rest relieve angina?

Rest reduces myocardial oxygen demand, restoring the balance between supply and demand.

What is the most common cause of angina?

Obstructive epicardial coronary artery disease.

How does epicardial coronary artery disease cause angina?

Narrowing of arteries reduces coronary flow reserve and limits increased blood flow during demand.

At what degree of narrowing does angina commonly occur?

Usually when luminal narrowing is around seventy to eighty five percent or more.

Why is reduced coronary flow reserve important in angina?

It prevents the heart from increasing blood flow during stress or exercise.

What other mechanisms can cause angina besides epicardial disease?

Microvascular disease, coronary artery spasm and increased demand states.

What is microvascular angina?

Angina caused by disease of pre arterioles and arterioles that cannot be visualised on angiography.

What is coronary artery spasm?

Sudden transient narrowing of a normal or mildly diseased coronary artery.

Which conditions increase myocardial oxygen demand and can cause angina?

Hypertrophic cardiomyopathy and severe hypertension.

Why is angina described as a symptom rather than a disease?

Because it represents myocardial ischaemia from multiple possible causes.