17. Coronary Circulation

Question 1

Describe the coronary circulation. >

Answer 1

right coronary artery (RCA) and the left coronary artery (LCA),
which arise from the

anterior and
posterior aortic sinuses respectively.

> The RCA supplies the 
right atrium, 
right ventricle, 
sinoatrial node and, in
90% of people, also the atrioventricular node.

> The LCA divides into the
left anterior descending (LAD) artery
and the left circumflex (LCx) artery

and supplies the left atrium,
left ventricle and most
of the interventricular septum.

> In 30% of the population the
LCA and RCA supply equal proportions of

blood while in 50% the RCA is the dominant vessel.

Question 2

> Venous drainage

Answer 2

> Venous drainage occurs predominantly
via the coronary sinus.

receives blood from the great cardiac vein
draining the anterior aspect
of the heart)
and the middle cardiac vein
(draining the posterior aspect of the heart).

In addition, there are other vessels that drain directly into the heart chambers including the thebesian veins, which contribute towards true shunt.

Question 3

What is autoregulation? >

Answer 3

Autoregulation refers to the

intrinsic ability of an organ to maintain a

constant blood flow
despite a varying perfusion pressure.

The heart, kidney and brain are all examples of organs that exhibit this ability.
.

Question 4

How is coronary blood flow

autoregulated?

Answer 4

> The heart can autoregulate its blood supply at coronary perfusion pressures (CoPP) between 60 and 180 mmHg.

Outside this range, the coronary circulation becomes pressure dependent.

Autoregulation occurs via a combination of the following mechanisms:

1. Metabolic:
   During periods of increased myocardial activity, local tissue hypoxia and increased metabolic waste products such as H+, K+, adenosine and CO2 cause vasodilatation of the coronary vessels, thereby increasing coronary blood flow.
2. Myogenic:
   When the pressure within a small artery or arteriole is increased, the smooth muscle within these vessels automatically constricts, thereby reducing
   blood flow.

The reverse happens when the pressure within these vessels falls.

3. Endothelial:

Vascular endothelium produces various vasoactive substances including
nitric oxide (NO),
endothelium-derived relaxing factor (EDRF) and
prostacyclin (PGI2),
all of which produce vasodilatation;

conversely endothelin
and thromboxane A2 produce vasoconstriction.

When endothelium is damaged
(e.g. atherosclerotic plaques or ischaemia)

Production of these vasoactive substances is reduced,

making coronary vessels prone to vasospasm and platelet aggregation.

4. Autonomic:

ANS exerts a weak effect on the coronary circulation.

α-adrenergic receptor stimulation causes vasoconstriction
β-adrenergic and vagal stimulation leads to vasodilatation of coronary vessels.

5. Hormonal:
   Vasoactive hormones require an intact endothelium in order to produce their effect.
   Atrial natriuretic peptide causes vasodilatation while
   vasopressin and angiotensin II cause vasoconstriction

Question 5

What factors affect myocardial oxygen supply?

Answer 5

Myocardial oxygen supply is determined by coronary blood flow and the arterial oxygen content (CaO2).

Determinants of arterial oxygen content:
CaO2 = [Hb × Sats × 1.34] + [PaO2 × 0.023]
From this equation it can be seen that the only parameters we can
manipulate are the Hb and the PaO2 (see Chapter 4, ‘Oxygen transport’, for
full explanation of the above equation).

Question 6

Determinants of coronary blood flow:

x6

Answer 6

1. Coronary perfusion pressure (CoPP = Aortic pressure – Intraventricular pressure).

During systole the CoPP of the left ventricle can equal zero (or less) and therefore coronary blood flow only occurs during diastole.

In systole:
LVCoPP = [SBP – LVESP] = [120 mmHg – 120 mmHg] = 0 mmHg

In diastole:
LVCoPP = [DBP – LVEDP] = [70 mmHg – 10 mmHg] = 60 mmHg.

However, the coronary blood flow to both atria and the right ventricle occurs throughout the cardiac cycle.

In systole:
RVCoPP = [SBP – RVESP] = [120 – 25] = 95 mmHg

In diastole:
RVCoPP = [DBP – RVEDP] = [70 – 5] = 65 mmHg.

2. Perfusion time:
   As the heart rate increases, the diastolic time and therefore the coronary perfusion time, especially to the left ventricle, is reduced.
3. Coronary vessel patency:
   Atherosclerotic vessels are stenosed and have a reduced blood flow (as indicated by the Hagen–Poiseuille formula).
4. Coronary vessel diameter:
   The wider the diameter, the greater the blood flow (hence the administration of GTN in angina).
5. Blood viscosity:
   Haematocrit is a major determinant of blood viscosity and from the Hagen–Poiseuille formula it can be seen that as viscosity increases,
   flow decreases.
   However, as haematocrit decreases, so does the oxygen carrying capacity of blood.

6.
Determinants of arterial oxygen content:
CaO2 = [Hb × Sats × 1.34] + [PaO2 × 0.023]
From this equation it can be seen that the only parameters we can manipulate are the Hb and the PaO2

Question 7

What is the oxygen consumption of the heart at rest and exercise

How much of the cardiac output does the heart recieve

What is the CBF

What is the oxygen extraction ratio

What are the major determinants of myocardial oxygen
consumption?

Answer 7

The heart has the highest oxygen consumption per tissue mass of any organ in the body,
requiring 10 mL O2/min/100 g at rest and 70 mL/min/100 g during heavy exercise

(the kidney uses 5 mL/min/100 g while the brain uses
3 mL/min/100 g).

As a result, the heart receives 5% of the cardiac output,
giving it a coronary blood flow of 250 mL/min.

In order to support such high oxygen consumption, the heart at rest extracts approximately 70% of its coronary blood oxygen content (remember that the
rest of the body only extracts, on average 25%, of its arterial O2 content).

Therefore, during periods of increased mechanical activity the only way the heart can meet its increased oxygen consumption is by increasing its
coronary blood flow.

Factors that determine myocardial oxygen consumption include heart rate,
contractility, afterload, tissue mass and temperature (cold cardioplegic solutions are used during cardiopulmonary bypass surgery to reduce
myocardial oxygen consumption and minimise risk of ischaemia).