L4 - Physiology of the Heart III

Question 1

What is the aorta?

Answer 1

Large artery away from the heart
Supplies all other arteries
At the base is the aorta valve

Question 2

What are the coronary arteries?

Answer 2

First branch of the aorta
Left and right
- These then branch further to supply the heart muscle
- The main arteries run over the surface of the heart
- Allow surgeons to reach them and allow bypass grafts to be stitched onto arteries
Big arteries – sit on top of heart
Smaller arteries – perforate into heart muscle

Question 3

What drives blood flow down the coronary arteries?

Answer 3

Pressure at top end of coronaries is the same pressure as in the aorta

Question 4

Why does there need to be a pressure gradient in the coronary arteries?

Answer 4

Pressure at far end of coronaries arteries need to be less that at the top
- The end of the coronary tree is basically the inner surface of the ventricle
- Pressure in the ventricle will determine pressure at bottom end of coronary artery
Pressure difference = aorta pressure – ventricular pressure

Question 5

When are the aortic valves open and closed?

Answer 5

When the heart contracts the aortic valve opens in relation to flow
When the heart relaxes the aortic valve closes
- It then supports the blood above it
- Otherwise all the blood you have just ejected would rush back into the ventricle

Question 6

How do you calculate coronary blood flow?

Answer 6

Perfusion pressure / resistance

Question 7

How do coronary arteries change their blood flow?

Answer 7

Restrict and dilate

This changes their resistance

Question 8

At what point in the cardiac cycle foes blood flow occur?

Answer 8

Diastole

Question 9

Why does cardiac blood flow not occur in diastole?

Answer 9

When the heart muscle contracts it squeezes the smaller arteries

• Can’t get blood flow them down
• Every time heart contract it cuts of its own blood supply
• Coronary arteries on the surface still patent –> but nowhere for blood to go –> no blood flow

Question 10

What is the hearts oxygen consumption?

Answer 10

Heart is an active tissue – metabolically it takes up a lot of energy
One of the worst perfused organs
- No reserves / wasted blood flow
When we exercise the demand goes up and the flow goes up with it

Question 11

Which organs have the least oxygen consumption?

Answer 11

Brain, kidney and skin

Question 12

Which organs have the highest oxygen consumption?

Answer 12

Heart and contracting skeletal muscle

Question 13

How do you calculate oxygen delivery?

Answer 13

Arterial oxygen concentration X Coronary blood flow

Question 14

What is the arterial oxygen concentration?

Answer 14

Relatively little dissolved in plasma

97-100%

Question 15

What is arterial oxygen concentration determined by?

Answer 15

Mainly determined by oxygen bound to haemoglobin
- Haem contains iron atoms – each atom binds one oxygen molecules
- Each haemoglobin carries 4 oxygen molecules
Anaemia will cause reduced oxygen delivery

Question 16

Why is the ordinarily little change in the oxygen content of arterial blood?

Answer 16

97-100% normally
If already pretty much fully saturated - in exercise when hearts demand for oxygen goes up, you cannot carry more oxygen/change concentration
- Therefore primary determinant of oxygen delivery is coronary blood flow
- This is changed by pressure changes

Question 17

In the aortic pressure trace what is the dichroic notch?

Answer 17

When the aortic valve closes

After it closes some blood continues to leach out into the system and pressure falls further

Question 18

How and what do you measure when monitoring blood pressure?

Answer 18

Cuff around arm and monitor brachial artery pressure
Brachial artery is connected directly to aorta  measure of aortic pressure
Measure
- Systolic pressure - peak pressure generated by LV contraction
- Diastolic pressure - basal pressure after aortic valve is closed

Question 19

Left ventricular pressure trace - why does systolic BP - aortic pressure?

Answer 19

If aortic valve opens fully the pressure generated in the ventricle during systole is exactly the same as the pressure generated in the aorta/brachial artery

• Systolic pressures all the same
• Systolic BP = aortic pressure

Question 20

Left ventricular pressure trace - why does the aortic valve not influence diastolic pressure in the ventricles?

Answer 20

In the ventricle you are proximal to the aortic valve – the aortic valve is not influencing the diastolic pressure

• So when the ventricles stop contracting and starts relaxing the pressure drops to nearly 0
• Then starts to fill again in diastole ready for the next cycle
• End point of diastole –> LVEDP

Question 21

Is systolic BP always the same or always different?

Answer 21

The same

Question 22

Is diastolic BP always the same or always different?

Answer 22

Different

• Arterial diastolic BP – 70-80 (supported by aortic valve)
• Ventricle diastolic BP – LVEDP

Question 23

Does coronary perfusion occur during diastole or systole?

Answer 23

Only occurs during diastole
Perfusion pressure in diastole is dependent on aortic diastolic pressure and LVEDP
- The difference between the two is the pressure gradient - determines blood flow

Question 24

Overview of left ventricular pressure trace

Answer 24

Increases during systole

Falls back to nearly 0 during diastole

Question 25

Overview of aortic pressure trace

Answer 25

Increases during systole | Falls back to arterial diastolic pressure – supported by aortic valve

Question 26

The bigger the diastolic pressure the greater?

Answer 26

The perfusion

Question 27

What are 3 physical factors that influence diastolic coronary flow?

Answer 27

Length of diastole Raised LVEDP Reduced diastolic pressure

Question 28

How does raised LVEDP affect diastolic coronary flow?

Answer 28

Raised LVEDP --> decreases perfusion pressure Failing hearts tend to run on high LVEDP LVEDP stretches the muscle leading to more contraction

Question 29

How does the length of diastole affect diastolic coronary flow?

Answer 29

Length of diastole --> tachycardia --> disproportionately reduces diastole Systole is relatively fixed in terms of its duration

Question 30

How does reduced diastolic pressure affect diastolic coronary flow?

Answer 30

Reduced diastolic pressure -->decreases perfusion pressure

Question 31

What is auto regulation of coronary blood flow?

Answer 31

Ability of an organ to maintain a constant blood flow despite changes in perfusion pressure - Protection mechanism - Occurs by changing resistance

Question 32

What can cause a fall in perfusion pressure?

Answer 32

Loss of blood --> coronary blood flow drops

Question 33

When a fall in perfusion pressure occurs due to loss of blood how does the body respond?

Answer 33

Pressure stays low Blood flow would also stay low if nothing else happened - Autoregulation kicks in and resistance in the tissue drops -- Due to local metabolite effect - Allows greater blood flow

Question 34

What are the two ways coronary blood flow can be regulated?

Answer 34

Vascular control - metabolites and mediators - Most important Mechanical control

Question 35

What does hypoxia cause in the coronary arteries?

Answer 35

Causes coronary vasodilatation in situ but not in isolated coronary artery - Coronaries being present in whole organ needed for local effects - Suggests caused by local metabolite – adenosine

Question 36

What increases if metabolism in the heart is happening anaerobically?

Answer 36

``` Potassium ions Carbon dioxide Hydrogen ions Lactic acid These themselves cause coronary vasodilation ```

Question 37

How does the sympathetic nervous system control coronary arteries?

Answer 37

Neural and humoral control - Less important - Large vessel α-adrenoceptor vasoconstriction - Smaller vessel β2 vasodilatation

Question 38

What is the main symptom of coronary artery disease?

Answer 38

Narrowing of the coronary artery --> cholesterol plaques | Overpowers all mechanisms coronary arteiers have to increase blood flow

Question 39

How do patients with coronary disease know they aren't getting enough blood flow to the heart?

Answer 39

Get accumulation of metabolites --> angina/pain in chest May be fine at rest - heart demands not too high On exercise --> heart rate increases --> blood pressure increases --> when narrowing of arteries becomes an issue (don’t have the oxygen reserve)

Question 40

What two hormones does the heart release?

Answer 40

Heart is an endocrine organ - releases hormones which travel in the blood and act somewhere else Atrial natriuretic peptide B-natriuretic peptide

Question 41

Where does atrial natriuretic peptide come from and cause?

Answer 41

Comes from the atria | Released during stretch / raised atrial pressure / volume overload

Question 42

Where does B-natriuretic peptide come from and cause?

Answer 42

Comes from the ventricles | Released during stretch / raised ventricular pressure / volume overload

Question 43

What are the main effects of atrial natriuretic peptide and B-natriuretic peptide?

Answer 43

Increase renal excretion of sodium and water (diuresis) - Try to get rid of fluid to lower the pressure Relax vascular smooth muscle (except efferent arterioles of renal glomeruli) - Still increase perfusion pressure in glomeruli - Increase Na and water filtration into kidney Increased vascular permeability Inhibit the release or actions of - Aldosterone - Angiotensin II - Endothelin - Anti-diuretic hormone

Question 44

What is the importance of atrial natriuretic peptide and B-natriuretic peptide inhibiting aldosterone, angiotensin II, endothelin and ADH?

Answer 44

These hormones all cause vasconstriction, water retention and salt retention - Counter-regulatory system to the renin-angiotensin system

Question 45

What are cardiac natriuretic peptide metabolised by?

Answer 45

Neutral Endopeptidase (NEP - neprilysin)

Question 46

What can NEP be inhibited by?

Answer 46

A combination drug - Sacubitril – neprilysin inhibitor - Valsartan – angiotensin II blocker Novel therapy for heart failure --> allows increased levels of cardiac natriuretic peptides