4. Myocardial Blood Flow and Oxygen Consumption

Question 1

What arteries supply perfusion to the heart?

Answer 1

Left and Right Coronary Arteries

Question 2

What is the equation for coronary blood blow?

Answer 2

Coronary blood Flow = Perfusion Pressure / Resistance

Question 3

What is perfusion pressure?

How is coronary blood flow related to perfusion pressure?

Answer 3

• The difference between top and bottom end pressures of coronary circulation
• Top end - diastolic arterial blood pressure = 80mmHg
• Bottom end - LV EDP = 8mmHg
• Perfusion Pressure = Diastolic Arterial BP - LV EDP
• Coronary blood flow is directly proportional to Perfusion Pressure

Question 4

What is resistance of vessels determined by?

How is coronary blood flow related to resistance?

Answer 4

• Length of vessel (longer vessel = increased resistance)
• Viscosity of blood (more viscous = increased resistance)
• Size of arterial lumen (as r4, small change in radius gives large change in resistance)
• Coronary blood flow is indirectly proportional to resistance

Question 5

What happens to the coronary arteries when the heart contracts?
What does this mean for coronary perfusion?

Answer 5

• Resistance of coronary arteries increases as they are squeezed until they are obstructed
• Coronary perfusion occurs in diastole

Question 6

How does the oxygen demand of the heart change with exercise?
What implication does this have for coronary blood flow?

Answer 6

• Oxygen demand of the heart dramatically increases with exercise
• Require a way to increase coronary blood flow to increase oxygen supply during exercise

Question 7

What is different about the heart’s blood supply compared to other organs?
How does the heart ensure it receives enough oxygen?

Answer 7

• It does not receive an excess blood supply i.e. only receives what it requires
• It extracts more oxygen from the blood it receives

Question 8

What is the equation for Oxygen Delivery?

Answer 8

Oxygen Delivery = Arterial Oxygen Concentration x Coronary Blood Flow

Question 9

What determines arterial oxygen concentration?

Answer 9

• Relatively little dissolved in the plasma

- Mainly determined by Oxygen bound to Haemoglobin (1 molecule can bind 4 Oxygen molecules)

Question 10

What is a condition that can decrease Arterial Oxygen Concentration?

Answer 10

Anaemia due to decreased haemoglobin

Question 11

What is the primary determinant of Oxygen Delivery?

Answer 11

As arterial oxygen concentration is normally constant, coronary blood flow is the primary determinant of Oxygen Delivery

Question 12

What 4 effects regulate coronary blood flow?

Answer 12

• Mechanical effects (pressures in heart)
• Local effects (vasoconstrictor/dilator metabolites)
• Neural effects (ANS)
• Humoral effects (adrenaline)

Question 13

Describe an aortic pressure trace.

Answer 13

• When the heart contracts and aortic valve opens, generate a peak pressure - Systolic BP (120mmHg)
• When heart begins to relax blood wants to flow back into heart but closure of aortic valve supports blood pressure - Diastolic BP (80mmHg)

Question 14

Describe a left ventricular pressure trace.

Answer 14

• When the ventricle contracts, generate a peak pressure - Systolic pressure (120mmHg)
• When ventricle begins to relax there is no aortic valve to support pressure therefore pressure drops - LV EDP (8mmHg)

Question 15

Describe a left ventricle to aorta pressure trace.

Answer 15

• Providing aortic valve opens normally, systolic pressures are the same
• Diastolic pressures are different as closure of aortic valve supports pressure in aorta
• Difference between Diastolic BP and LV EDP is the Coronary Perfusion Pressure

Question 16

Where is the window for coronary flow?

What 3 things can change the window for coronary flow?

Answer 16

• Between diastolic BP and LV EDP

1. Tachycardia disproportionately reduces diastole and therefore time for coronary perfusion
2. Increased LV EDP (hypertension/heart failure) reduces perfusion pressure
3. Decreased diastolic BP reduces perfusion pressure

Question 17

Define autoregulation.

Answer 17

The ability of an organ to maintain a constant blood flow despite changes in perfusion pressure

Question 18

How does autoregulation maintain coronary blood flow?

Answer 18

• If perfusion pressure decreases (blood loss) the heart responds by decreasing resistance of coronary arteries to maintain constant blood flow
• i.e. coronary blood flow is maintained despite continued reduction in perfusion pressure

Question 19

What mediates the fall in resistance in autoregulation?

Answer 19

Local vasodilator metabolites

Question 20

What is the evidence that local metabolites mediate regulation of coronary blood flow?

Answer 20

Hypoxia induces marked vasodilation in situ but not in isolated coronary artery, therefore must be local metabolites causing vasodilation

Question 21

How does the heart increase coronary blood flow in hypoxia?

Answer 21

Produces metabolites (Adenosine, K+ ions, H+ ions, CO2, lactic acid) that stimulate vasodilation to decrease resistance and increase coronary blood flow

Question 22

What receptors mediate coronary blood flow in larger and smaller coronary vessels?

Answer 22

• Larger have α-adrenoceptors that cause vasoconstriction

- Smaller vessels have β2-adrrnoceptors that cause vasodilation

Question 23

What hormones can the heart release?

Answer 23

• Atria - Atrial Natriuretic Peptide (ANP)

- Ventricles - Brain Natriuretic Peptide (BNP)

Question 24

What stimulates release of ANP and BNP?

Answer 24

• Stretching of myocardial fibres
• Increased atrial/ventricular pressures
• Volume overload

Question 25

What are the main effects of ANP and BNP oon the kidney and vasculature?

Answer 25

• Increases renal excretion of Na+ (natriuresis) and water (diuresis)
• Decreases VSMC contraction (except in efferent arteriole to maintain GFR)
• Increases vascular permeability

Question 26

What do ANP and BNP block the release/actions of?

Answer 26

• Aldosterone (normally increases Na+ and water retention
• Angiotensin II (normally causes vasoconstriction)
• Endothelin (normally causes vasoconstriction)
• ADH (normally increases water retention)

Question 27

What are cardiac natriuretic peptides metabolised by?

Answer 27

Nepralysin (neutral endopeptidase)