CORONARY CIRCULATION

CORONARY CIRCULATION

The coronary circulation supplies the myocardium, a tissue that rivals the brain in terms of its nutritional demands and the critical importance of continued flow for normal function.

Anatomy

The myocardium is supplied by left and right coronary arteries that originate from the root of the ascending aorta immediately above the aortic valve.

The right coronary artery generally supplies the right heart, whereas the left coronary artery supplies the left.

The arteries course over the heart’s surface and then dive down through the muscle layers. The vasculature is notable for numerous collaterals connecting adjacent arteries and also for the presence of precapillary sphincters .

Coronary Circulation

• Only 1/10 mm of the endocardial surface can obtain

nutrition from the blood inside the cardiac chamber

• Left coronary artery supplies mainly anterior and lateral

portions of the left ventricle

• Right coronary artery supplies most of the right ventricle

and some posterior part of the left ventricle

• Most coronary venous blood returns to the right atrium by

coronary sinus (75% of total coronary flow)

• Anterior cardiac veins from right ventricle open directly into

the right atrium

• Thebesian veins empty directly into chambers of the heart

Regulation

At rest, the coronary circulation receives 5% of CO.

Cardiac muscle extracts 70% of available O2 from blood, and it has a very low capacity for anaerobic metabolism, much like the brain.

This O2 dependence means that any increase in

work must be matched by an increase in coronary flow, achieved entirely through local control mechanisms.

1. Local controls:

Coronary resistance vessels are exceptionally sensitive to adenosine. Local control mechanisms allow for a fourfold to fivefold increase in coronary flow when CO increases, a phenomenon called coronary reserve.

2. Central controls: Coronary resistance vessels are innervated by both

branches of the ANS, but their influence is overridden by local controls.

Coronary Circulation

• Right and left coronary arteries

External Heart: Anterior View

External Heart: Posterior View

Coronary Circulation: Arterial Supply

Coronary Circulation: Venous Supply

Coronary reserv

Precapillary sphincters

Precapillary sphincters comprise single smooth muscle cells wrapped around the inlets to individual capillaries . They contract and relax with changes in local metabolite concentrations and function as on/off switches to capillary flow.

When CO is minimal, most sphincters are contracted (“off”), and flow is inhibited. They relax intermittentlyas local metabolite levels rise but again contract when the increased flow washes the metabolites away.

At rest, only a small proportion (20%) of sphincters is relaxed, and capillaries are actively perfused, but the pattern of capillary flow shifts continually (vasomotion).

When cardiac workload increases, levels of metabolic waste products rise, and the sphincters spend a much greater percentage of time in the “on” position. At maximal levels of CO, all sphincters are open all the time, and coronary flow rises to maximal levels also.

Vasomotion and basis of coronary reserve

Extravascular compression

Blood flow through most systemic vascular beds follows the aortic pressure curve, rising during systole and falling during diastole.

Flow through the left coronary artery drops sharply during systole and then rises sharply with the onset of diastole. This unique flow pattern occurs because ventricular myocytes collapse the arterial supply vessels as they contract (extravascular compression).

The effect is felt strongest during early systole because aortic pressure, the main force maintaining vascular patency, is at a low point. During diastole, the compressive forces are removed, and blood surges through the musculature at peak rates.

Left coronary blood flow.

Normal Coronary Blood Flow

• Resting coronary blood flow (CBF) is about 225 ml/min

• CBF increases in proportion to exercise or work output

• Phasic changes in CBF during systole and diastole

Extravascular compression in the left ventricular wall.

LV= left ventricle;P= pressure.

Coronary Blood Flow

• Epicardial vs subendocardial CBF (intramyocardial

pressure)

• Epicardial arteries in the outer surface supply most of

the muscle

• Subendocardial arterial plexus is beneath the

endocardium

Aort kapağı

Sol koroner arter

Sağ koroner arter

Control of Coronary Blood Flow

• Local muscle metabolism is the primary controller of

CBF

• Oxygen demand as a major factor in local CBF

regulation

– Normally about 70% of O2 is removed as the

blood flows

• Role of adenosine in vasodilation

• And other substances

Nervous Control of Coronary Blood Flow

Autonomic nerves can affect the CBF both directly and

indirectly

Direct stimulation of coronary blood vessels

Indirect effects result from secondary changes in CBF caused

by increased or decreased activity of the heart

Direct effects of nervous stimuli on coronary vasculature

– Parasympathetic fiber distribution is not great

– There is more sympathetic innervation of coronary vessels

– Constrictor receptors are alpha adrenoreceptors (more epicardial)

– Beta receptors are vasodilatory (more in the intramuscular arteries)

Special Features of Cardiac Muscle Metabolism

• At rest, cardiac muscle normally consumes fatty acids

to supply most of its energy instead of carbohydrates

– About 70% of total energy from fatty acids

• However, under anaerobic or ischemic conditions,

glycolytic mechanism is required

• Glycolysis consumes tremendous amounts of blood

glucose and forms large amounts of lactic acid

• Hypoxia, release of adenosine and dilation of coronary

artery

Ischemic Heart Disease

• The most common cause of death

– Insufficient coronary blood flow

• Coronary ischemia, coronary occlusion and myocardial

infarction – congestive heart failure

• Atherosclerosis as a cause of ischemic heart disease

– Consumption of large amounts of cholesterol and lack of

mobility

– Development of atherosclerotic plaques in major coronary

arteries

Acute Coronary Occlusion

• Acute coronary occlusion occurs frequently in

atherosclerotic heart

• 1) Atherosclerotic plaque can cause a local blood clot

called a thrombus

– Unsmooth surface, adherence of blood platelets

• 2) Local muscular spasm of coronary arteries may

occur

– Spasm may result from irritation of smooth muscle

– Or from local nervous reflexes – plaque

• Spasm may lead to secondary thrombosis of the vessel

Value of Collateral Circulation in the Heart

• In normal heart, there is no communication between large

coronary arteries

• But many anastomoses do exist among the smaller arteries

(20-250 micrometre in diameter)

• This collateral circulation may delay appearance of

ischemic heart symptoms

Collaterals:Collaterals are vessels that connect adjacent arterioles.

They are usually constricted in a healthy heart, but, if a supply vessel becomes occluded, they dilate in response to rising metabolite levels.

Flow through collaterals may prevent infarction if the occluded vessel is small. In time, these channels enlarge to provide near-normal flow to the ischemic area.

Flow interruption

Because the ventricular myocytes extract such high levels of O2 from the blood, a delicate balance exists between myocardial workload and coronary supply. If the balance is disturbed, then myocytes becomeischemic and infarcted. Most commonly, this occurs due to atherosclerosis and coronary artery disease.

Atherosclerosis:Atherosclerotic lesions appear at an early age in the

populations of most Western countries. They evolve to become complex plaques of lipids, hypertrophied myocytes, and fibrous material. Plaques enlarge at the expense of the vascular lumen and impair blood fl ow. This causes an imbalance between coronary supply and myocardial demand, resulting in ischemia.

Ischemic myocytes release large quantities of vasoactive compounds, such as adenosine, but vasodilators have no effect on plaque.

As the O2 deficit continues, the myocytes release lactic acid, which stimulates pain fibers within the myocardium and causes angina pectoris.

Coronary angiography

Coronary angioplasty

Myocardial Infarction

• After coronary occlusion, blood flow ceases beyond the

blockage

• Cardiac muscle has little or no blood flow

• The overall process is called myocardial infarction

• After the onset of MI, small amounts of collateral blood

begin to seep into the infarcted area

• Progressive dilation of local blood vessels

• In later stages, the vessel walls become highly permeable

and leak fluid

• Cardiac muscle tissue becomes edematous

• Subendocardial infarction and systolic contraction

Acute Myocardial Infarction Infarction

In the common heart attack a thrombus form in a coronary artery.

Death of more than 1/3 of the left ventricle will

lead to severe heart failure.

Causes of Death After Coronary Occlusion

• 1) Decreased cardiac output

• 2) Damming of blood in the pulmonary edema

• 3) Fibrillation of the heart

• 4) Rupture of the heart

Causes of Death After Coronary Occlusion

1) Decreased cardiac output (Systolic stretch and cardiac

shock)

• Systolic stretch

• Incapable heart to pump sufficient blood into the

peripheral arterial tree

• Coronary shock, cardiogenic shock, cardiac shock or

low cardiac output failure

• Cardiac shock occurs when >40% of the LV is

infarcted

• Death occurs in 85% of patients once they develop

cardiac shock

Causes of Death After Coronary Occlusion

1) Decreased cardiac output (Systolic stretch and cardiac

shock)

Causes of Death After Coronary Occlusion

2) Damming of blood in the body’s venous system

• Acutely reduced cardiac output leads to diminished

blood flow to the kidneys

• The kidneys fail to excrete enough urine

• This adds to progressively to the total blood volume

and congestive symptoms

• Development of pulmonary edema

Causes of Death After Coronary Occlusion

3) Fibrillation of the ventricles after myocardial infarction

• Sudden ventricular fibrillation

• Four factors into tendence for the heart to fibrillate:

a) Acute loos of blood supply to the cardiac muscle and increased

K ions in the extracellular space

* irritability of cardiac muscle

b) Ischemia of the muscle causes “injury current”

* ischemic muscle cannot completely repolarize

c) Powerful sympathetic reflexes develop after massive infarction –

irritability increases

d) Cardiac muscle weakness causes the ventricles to dilate

excessively.

* This increases the pathway length for impulse conduction

Causes of Death After Coronary Occlusion

4) Rupture of the infarcted area

• Dead heart muscle bulges outward with each

contraction

• Systolic stretch becomes greater and the heart may

rupture

• Loss of blood into the pericardial space and

development of cardiac tamponade

Stages of Recovery from Acute Myocardial Infarction

• Small or large ischemic area

• Replacement of dead muscle by scar tissue

• After a few days to three weeks, most of the nonfunctional muscle

becomes functional again or die

• Fibrous tissue begins developing among the dead fibers, ischemia

stimulates growth of fibroblasts

A heart slice with a fresh infarct

Prolonged ischemia of the heart leads to myocardial infarction. Cells begin to die approximately 20 min after the onset of a

coronary occlusion and killing is complete after 6 hr.

Recovery from MI• Value of rest in treating myocardial infarction

• Effect of exercise or emotional strain

• Function of the heart after recovery from MI

• Occasionally a heart that has recovered from a large

MI returns almost to full functional capability

• More frequently its pumping capability is permanently

decreased below that of a healthy heart

• Cardiac reserve – reduction to 100%

• Pain in coronary heart disease

– Feeling the heart

– Ischemic cardiac muscle often causes pain sensation

– Histamine, kinins, cellular proteolytic enzymes, lactic acid etc

Angina Pectoris• Progressive constriction of coronary arteries, cardiac

pain, angina pectoris

• It appears when the load on the heart increases

• Usually felt beneath the sternum and often referred to

left arm, shoulder and neck

• Exercise or emotional stress increases angina

• Treatment

• Vasodilator drugs

– Nitroglycerin and other nitrate drugs

• Beta adrenergic blockers – inhibition of sympathetic

activity of the heart

– Propranolol

Surgical treatment of coronary disease

• Coronary angiography

• Coronary angioplasty

• Aortic coronary bypass surgery

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