CVS 2022 - Coronary circulation.pdf functional anatomt,special features of cbf.regulation of cbf

What we will discuss today…
• Functional Anatomy
• Special features of CBF
• Measurement of CBF
• Regulation of CBF
• Insufficient CBF

Right coronary artery supply
• Right atrium
• Greater part of RV except area near ant. inter ventricular
groove
• Small part of LV near posterior inter ventricular groove
• Posterior part of inter ventricular septum
• Whole of conducting system of heart except a part of
left branch of AV bundle

Left coronary artery supply
• Left atrium
• Greater part of LV except area near post. inter ventricular
groove
• Small part of RV near anterior inter ventricular groove
• A part of left branch of the AV bundle

Coronary veins
• Coronary sinus
• Anterior cardiac veins
• Thebesian veins

Special features of Coronary Blood
Flow

Special features
• Normal coronary flow : 250 ml/min
• 70ml/100g tissue/min
• 5% of total cardiac output

Phasic changes of coronary flow
SYSTOLE
DIASTOLE
DIASTOLE

Phasic flow
• Intramural vessels are compressed in Iso Volumet. Contract.
• Aortic pressure will be very low during IVC
• So, no flow during IVC
• Variable flow during rest of systole
• Maximal flow occurs in diastole as vessels dilate→ 80%

Left coronary artery
• LV pressure is high, sharp fall in flow in IVC phase
• During rapid ejection phase→ aortic pressure rises → flow
increases rapidly
• During reduced ejection phase →aortic pressure falls →
flow decreases
• During diastole, maximum flow , remains high and falls
gradually

Right coronary artery
• Similar phasic changes occur
• RV systolic pressure is low : 25 mm Hg
• Reduction in flow during IVC
• Flow increases first and declines later in systole
• Flow increases during diastole

Subendocardial region
• Pressure in sub-endocardial region higher than outer portions
of heart muscle during systole
• Sub-endocardial region of LV receives blood flow only
during diastole
• More prone for ischemia Physiological
basis

Other regions
• Pressure differential between aorta & RV, and also
between aorta & atria, are more during systole
• So coronary flow in those parts of the heart is not
appreciably reduced during systole

Heart rate and CBF
• Tachycardia reduces diastole
• Blood flow reduced especially to LV

• Blood flow to left ventricle is decreased in AS patients
• Pressure in left ventricle must be much higher than
that in aorta to eject the blood
• So coronary vessels are severely compressed during
systole
Aortic stenosis

• Patients with AS are particularly prone to develop symptoms
of myocardial ischemia because of
– Compression of coronary vessels
– Myocardium requires more O2 to work more to expel blood
through stenotic aortic valve
Aortic stenosis

Measurement of Coronary Blood
Flow

Measurement of coronary blood flow
• Using Fick’s principle
– Kety’s method
• Using radioactive substances (Thallium-201 / 201Tl)
– Radioactivity detected with radiation detectors over the chest
– Used to study regional blood flow in heart
– To detect areas of ischemia and infarct
– To evaluate ventricular function

Regulation of Coronary Blood Flow

Autoregulation
• Local muscle metabolism is primary controller of coronary flow
• By local arteriolar vasodilation in response to nutritional needs of
cardiac muscle
• Whenever vigor of cardiac contraction is increased, the rate of
coronary blood flow also increases and vice versa
• It is regulated almost exactly in proportion to the need of the
cardiac musculature for oxygen

• At rest, heart extracts 70–80% of the O2 from each unit of blood
delivered to it
• So O2 consumption can only be increased significantly by
increasing blood flow
• Hypoxia in heart causes vasodilator substances to be released
from the muscle cells like adenosine

Chemical factors
• The products of metabolism cause coronary vasodilation
• Lack of O2
• Increased local concentrations of CO2, H+, K+, lactate,
prostaglandins, adenine nucleotides and adenosine

Reactive hyperemia
• If coronary artery is briefly occluded → release of obstruction
→ increase in blood flow
• Release of adenosine by hypoxia

Neural factors
• Coronary arterioles contain
– α-adrenergic receptors – mediate vasoconstriction
– β-adrenergic receptors – mediate vasodilation
• Direct effect of noradrenergic stimulation is constriction
• But activity in noradrenergic nerves to heart → coronary
vasodilation

• Norepinephrine increases the heart rate and the force of
contraction
• Vasodilation is due to production of vasodilator metabolites
in the myocardium secondary to the increase in its activity
• When BP falls, overall effect of reflex increase in noradrenergic
discharge is increased coronary blood flow

• Simultaneously cutaneous, renal, and splanchnic vessels are
constricted
• In this way circulation of the heart is preserved when flow to
other organs is compromised
• Direct effect of stimulation of vagal fibers to heart is coronary
vasodilation

Insufficient Coronary Blood Flow

Myocardial ischemia
• Commonest cause : atherosclerosis → reduced blood flow &
reduced O2
• Mild : symptomless
• Severe : angina pectoris & MI

Angina pectoris
• Periodic attacks of chest pain during muscular effort
• Severe and short duration
• Retrosternal region, referred to shoulder
• Coronary artery narrowing

Prinzmetal’s angina
• Refers to reversible myocardial ischemia that results from
coronary artery spasm
• During an episode of vasospasm, the patient develops ST
segment elevation in the affected territory

Role of Collateral Circulation in Heart
• Anastomoses exist among the smaller arteries sized 20 to 250
micrometers in diameter
• Damage to heart muscle is determined mainly by the degree
of collateral circulation
– that has already developed or
– that can open within minutes after the occlusion

Role of Collateral Circulation in Heart
• These developing collateral channels help in almost complete
recovery, when area of muscle involved is not too great
• Regular aerobic exercise promotes formation of collaterals

Myocardial infarction
• Rupture of an atheromatous plaque
• Coronary thrombosis → occlusion→ necrosis →death of
tissue
• Fibrosis and scar formation

Symptoms
• Angina pectoris
• Profuse sweating
• Cold extremities

Diagnosis
• ECG
• Serum enzymes
– CK-MB (MB isomer of creatine kinase)
– Troponin T
– Troponin I

Evolution of ECG changes in STEMI

Intensive coronary care unit
• Sedation : opioid analgesics
• Complete rest to prevent “coronary steal” syndrome
• If heart becomes excessively active, vessels of normal
musculature become greatly dilated
• This allows most of blood to flow through normal muscle
tissue
• So blood flow through collateral channels into ischemic area
decreases and ischemia worsens

• Nitrates like nitroglycerine
• Vasodilator used in the treatment of angina
• Acts by releasing NO

Thrombolysis
• Streptokinase : fibrinolysis by plasmin
• Tissue plasminogen activator ( t-PA)

Surgical procedures
• Coronary artery angioplasty
• Coronary artery bypass graft

Role of low dose aspirin
• Aspirin inhibits thromboxane A2
• Inhibits platelet aggregation
• Useful in preventing MI

Role of beta blockers
• Block sympathetic beta-adrenergic receptors
• So prevents sympathetic enhancement of heart rate &
cardiac metabolism during exercise or emotional episodes
• Helps in reducing the number of anginal attacks, as well as
their severity

Complications
• Cardiogenic shock
• Cardiac arrhythmias
• Left ventricular failure → Pulmonary edema
• Rupture of heart

CVS 2022 - Coronary circulation.pdf functional anatomt,special features of cbf.regulation of cbf

CVS 2022 - Coronary circulation.pdf functional anatomt,special features of cbf.regulation of cbf

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