Coronary Circulation Physiology Notes

Coronary Arteries Distribution Of Coronary Blood Vessels

Coronary arteries

Heart muscle is supplied by two coronary arteries, the right and left coronary arteries, which are the first branches of the aorta.

• The arteries encircle the heart in the manner of a crown hence the name coronary arteries (Latin word corona = crown).
• The left coronary artery supplies mainly the anterior and lateral parts of the left ventricle.

Read And Learn More: Medical Physiology Notes

• The right coronary artery supplies the whole of the right ventricle and the posterior portion of the left ventricle.

There are many variations in the anatomy of coronary arteries.

1. In 50-60% of human beings, the right coronary artery is larger (right dominant) and supplies more blood to the heart than the left coronary artery
2. In 15-20% of humans, the left coronary artery is larger (left dominant)
3. In 20-30% of persons, both arteries supply almost equal amounts of blood.

Branches of Coronary Arteries

• The coronary arteries divide and subdivide into smaller branches, which run all along the surface of the heart.
• The smaller branches are called epicardiac arteries and give rise to further smaller branches known as final arteries or intramural vessels.
• The final arteries run at right angles through the heart muscle near the inner aspect of wall of the heart.

Venous Drainage

The venous drainage from the heart muscle is by three types of vessels.

1. Coronary sinus: It is the larger vein draining 75% of total coronary flow. It drains blood from the left side of the heart and opens into the right atrium near the tricuspid valve.
2. Anterior coronary veins: The anterior coronary veins drain blood from the right side of the heart and open directly into the right atrium
3. Thebesian veins: Thebesian veins drain deoxygenated- ted blood from the myocardium directly into the concerned chamber of the heart.

Physiological Shunt

• The physiological shunt is the diverted route (diversion) through which the venous blood is mixed with arterial blood.
• The deoxygenated blood flowing from the Asian veins into cardiac chambers makes up part of the normal physiological shunt.
• The other component of the physiological shunt is the drainage of deoxygenated blood wm bronchial circulation into the pulmonary vein without ts&ng oxygenated.

Coronary Blood Flow And Its Measurement

Normal Coronary Blood Flow

Normal blood flow through coronary circulation is about 200 mLVminute. It forms 4% of cardiac output. It is about 65-70 mL/minute/100 g of cardiac muscle.

Measurement Of Coronary Blood Flow

Direct Method

The coronary blood flow is measured by using an electromagnetic flow meter. It is directly placed around any coronary artery.

Indirect Method

1. By Fick’s principle

1. Coronary blood flow is measured by applying Fick’s principle using nitrous oxide (NO₂).
2. The subject is asked to inhale a known quantity of the gas with atmospheric air.
3. Then, blood samples are collected from an artery and from the coronary sinus by using a catheter.
4. The blood flow is determined by using the formula

2. By using a Doppler flowmeter

• Piezoelectric crystals are used in the Doppler flowmeter probe to transmit and receive the pulses of high-frequency sound waves.
• The Doppler flowmeter probe is mounted to a catheter and positioned at the ostium of the right or left coronary artery to measure the velocity of the phasic flow of blood.
• The cross-sectional area of the artery is determined by angiography. From the velocity of blood flow and the cross-sectional area, the volume of blood flow is calculated.

3. By video densitometry

This technique is used to measure both velocity of blood flow and the cross-sectional area of coronary arteries simultaneously.

From these two values, the coronary blood flow can be calculated.

Phasic Changes In Coronary Blood Flow

• The blood flow through coronary arteries is not constant. It decreases during systole and increases during diastole.
• The intramural vessels or final arteries supplying the myocardium are perpendicular to the cardiac muscles.
• So, during contraction, the intramural vessels are compressed and the flow of blood is reduced.
• During diastole, the compression is released and the blood vessels are distended.
• So, the blood flow is increased. The aortic pressure plays an important role in phasic changes in coronary blood flow.

Role Of Left Ventricle In Phasic Changes

• In the left ventricle, during the onset of isometric contraction, the flow declines sharply due to two reasons, namely an increase in myocardial tissue pressure and a decrease in aortic pressure.
• During the ejection period, a rise in aortic pressure causes a sharp rise in flow into the left coronary artery.
• However, the flow of blood through coronary capillaries is less. It is due to the high intramural myocardial pressure in the contracting ventricle.
• The decreased flow of blood is maintained until the closure of the aortic valve that is, till the end of systole.
• During the onset of diastole, the blood flow rises and it reaches the peak sharply. During the later part of the diastole, the flow is reduced slightly along with decreasing aortic pressure.
• Once again, there is s- s-r-arp fall in flow during the onset of systole.

Role Of Right Ventricle In Phasic- Changes

In the right ventricle, some flow occurs during systole. It is because the force of contraction is not as severe as in the case of the left ventricle. Still, it remains less than the flow during diastole.

Factors Regulating Coronary Blood Flow

Autoregulation

• Like any other organ, the heart also has the capacity to regulate its own blood flow.
• It is known as autoregulation. The coronary blood flow is not affected when mean arterial pressure varies between 60 and 150 mm Hg. Several factors are involved in the autoregulation mechanism.
• The coronary blood flow is regulated mostly by the local vascular response to the needs of cardiac muscle.

Coronary blood flow is regulated by four factors:

1. Need for oxygen
2. Metabolic factors
3. Coronary perfusion pressure
4. Nervous factors.

1. Need For Oxygen

• It is the most important factor in maintaining blood flow through the coronary blood vessels.
• The amount of blood passing through coronary circulation is directly proportional to the consumption of oxygen by cardiac muscle.
• Even in resting conditions, a large amount of oxygen, i.e. 70-80% is consumed from the blood by the heart muscle than by any other tissues.
• In conditions associated with increased cardiac activity, the need for oxygen increases enormously.
• Thus, the need for oxygen, i.e. hypoxia immediately causes coronary vasodilatation and increases the blood flow to the heart.

2. Metabolic Factors

• Coronary vasodilatation during hypoxic conditions occurs because of some metabolic products.
• The metabolic products, which increase the coronary blood flow by vasodilatation, are:

Adenosine

• During hypoxia, ATP in the muscle is degraded in large amounts forming ADP.
• Some ADP molecules are further degraded into adenosine, which is released into tissue fluids of the heart muscle.
• Adenosine is a potent vasodilator and it increases the blood flow to cardiac muscle. The adenosine is reabsorbed into the muscle cells and reused.

Other Substances

The other substances which cause vasodilatation are potassium ions, hydrogen ions, carbon dioxide, and adenosine phosphate compounds.

Reactive Hyperemia

Reactive hyperemia is the increase in blood flow due to the vasodilator effects of metabolites.

3. Coronary Perfusion Pressure

• Perfusion pressure is the balance between mean arterial pressure and venous pressure.
• Thus the coronary perfusion pressure is the balance between mean arterial pressure in the aorta and the right atrial pressure.
• Since the right material pressure is low, the mean arterial pressure becomes the major factor that maintains the coronary blood flow.
• The range of mean arterial pressure at which the coronary blood flow.

4. Nervous Factors

• The coronary blood vessels are innervated both by parasympathetic and sympathetic divisions of the autonomic nervous system.
• It is not known whether the autonomic nerves have a direct effect on blood flow in various conditions.
• However, these nerves influence the coronary blood flow indirectly by acting on the musculature of the heart.
• For example, stimulation of sympathetic nerves increases the rate and force of contraction of the heart.
• This, in turn, causes the liberation of more metabolites which dilate the blood vessels and increase the coronary blood flow.
• Similarly, when parasympathetic nerves are stimulated cardiac functions are inhibited and the production of metabolites is less. Coronary blood flow decreases.

Applied Physiology – Coronary Artery Disease

Coronary artery disease (CAD) is a heart disease that is caused by inadequate blood supply to cardiac muscle due to the occlusion of a coronary artery. It is also called coronary heart disease.

Coronary Occlusion

• Coronary occlusion refers to partial or complete obstruction of the coronary artery.
• The occlusion occurs because of atherosclerosis, a condition associated with the deposition of cholesterol on the walls of the artery, in due course, this part of the arterial wall becomes fibrotic and it is called atherosclerotic plague.
• The plaque is made up of cholesterol, calcium, and other substances from the blood.
• Because of atherosclerotic plaque, the lumen of the coronary artery becomes narrow. In severe conditions, the artery is completely occluded.
• The development of atherosclerotic plaque is common in coronary arteries near the origin of the aorta.
• This plague activates platelets resulting in thrombosis and the blood clot is called thrombus.
• When 3/4th of the lumen of the coronary artery is obstructed either by atherosclerotic plaque or thrombus, the blood flow to the myocardium is reduced.
• It results in ischemia of the myocardium. Coronary thrombosis is associated with spasms of the coronary artery.
• Smaller blood vessels are occluded by the thrombus or part of atherosclerotic plaque detached from the coronary artery. This thrombus or part of the plague is called embolus.

Myocardial Ischemia And Necrosis

Myocardial Ischemia

• Myocardial ischemia is the reaction of a part of the myocardium in response to hypoxia.
• Hypoxia develops when blood flow to a part of the myocardium decreases severely due to the occlusion of a coronary artery.
• When the quantum of myocardium affected by ischemia is very less due to obstruction of smaller blood vessels, the blood flow can be restored by the rapid development of coronary collateral arteries.

Necrosis

• When a large part of the myocardium is involved, or the occlusion is severe involving larger blood vessels, the ischemia leads to necrosis of the myocardium.
• Necrosis refers to the death of cells or tissues by injury or disease in a localized area. Necrosis is irreversible.

Myocardial Infarction – Heart Attack

• Myocardial infarction is the necrosis of the myocardium caused by insufficient blood flow due to embolus, thrombus, or vascular spasms. It is also called a heart attack.
• In myocardial infarction, death occurs rapidly due to ventricular fibrillation.

Myocardial Stunning

• Myocardial stunning is a type of transient mechanical dysfunction of the heart caused by a mild reduction in blood flow.
• A substantial reduction in coronary blood flow causes ischemia followed by necrosis.
• A mild reduction in blood flow causes only ischemia and it may not be sufficient to cause necrosis of myocardium.
• However, it produces some transient (short-lived) mechanical disturbances or dysfunction of the heart.
• Since it is short-lived, the heart recovers completely from this.

Symptoms of Myocardial Infarction

Common symptoms of myocardial infarction are:

1. Cardiac pain
2. Nausea
3. Vomiting
4. Palpitations
5. Difficulty in breathing
6. Extreme weakness
7. Sweating
8. Anxiety.

Cardiac Pain-Angina Pectoris

• Cardiac pain is the chest pain that is caused by myocardial ischemia. It is also called angina pectoris. It is the common manifestation of coronary artery disease.
• The pain starts beneath the sternum and radiates to the surface of the left arm and left shoulder.
• Cardiac pain is called referred pain since it is felt over the body away from the heart. It is because the heart and left arm develop from the same dermatomal segment in the embryo.

Cause for Cardiac Pain

• Ischemia is mainly due to hypoxia. During myocardial ischemia, there is an accumulation of anaerobic metabolic endproducts like uric acid.
• The metabolites and other pain-producing substances like substance P, histamine, and kinin stimulate the sensory nerve endings leading to pain.

Sensory Pathway

• The sensory pathway from the heart is as follows: The inferior cervical sympathetic nerve fibers carrying the sensations of pain (or stretch) from the heart reach the posterior gray horn of the first 4 thoracic segments of the spinal cord.
• Here, these fibers synapse with second-order neurons (substantial gelatinosa of Rolando) of the lateral spinothalamic tract. From here the sensations are carried to the sensory cortex via the thalamus.
• If hypoxia in the myocardium is relieved by coronary collateral circulation or by treatment, the pain-producing substances are washed away by blood flow.

Chronic Angina Pectoris

• In chronic angina pectoris, the patient does not feel the pain normally. The pain is felt only when the workload of the heart increases.
• The workload of the heart increases in conditions like exercise and emotional outbursts.
• When the frequency of angina attacks increases, the patient is prone to develop acute myocardial infarction.

Treatment for Angina Pectoris

1. By using drugs

• Vasodilator drugs: Vasodilator drugs like glycerol trinitrate or sodium nitrite relieve the pain by dilating coronary arteries.
• However, the main therapeutic effect of such drugs is to dilate splanchnic blood vessels which causes a reduction in venous return, cardiac output, the workload of the heart, and oxygen consumption in the myocardium so that, the release of pain-promoting substances is inhibited.
• Calcium channel blockers: These drugs block the influx of calcium into the cells. When calcium influx is blocked, the myocardial contractility and workload of the heart are decreased.
• Sympathetic blocking agents: Sympathetic blocking agents like propranolol (beta blocker) block the beta-adrenergic receptors.

This, in turn, inhibits cardiac activity resulting in a reduction in heart rate, stroke volume, the workload on the heart, oxygen consumption, and stoppage of production of nociceptive substances in the myocardium.

2. By thrombolysis

3. By surgical methods

1. Aortic-coronary artery bypass graft: The part of the myocardium affected by coronary occlusion is detected by angiography.
2. Then, the anastomosis is made between the aorta and the coronary artery beyond occlusion, by a technique called aortic-coronary artery bypass graft.
3. Mostly, a small vein from the lower limb is used for anastomosis. Though this method can relieve the pain, it is not useful if the myocardium is damaged extensively.
4. Percutaneous transluminal coronary angioplasty
5. Laser coronary angioplasty.