L4 - Physiology of the Heart III Flashcards
What is the aorta?
Large artery away from the heart
Supplies all other arteries
At the base is the aorta valve
What are the coronary arteries?
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
What drives blood flow down the coronary arteries?
Pressure at top end of coronaries is the same pressure as in the aorta
Why does there need to be a pressure gradient in the coronary arteries?
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
When are the aortic valves open and closed?
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
How do you calculate coronary blood flow?
Perfusion pressure / resistance
How do coronary arteries change their blood flow?
Restrict and dilate
This changes their resistance
At what point in the cardiac cycle foes blood flow occur?
Diastole
Why does cardiac blood flow not occur in diastole?
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
What is the hearts oxygen consumption?
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
Which organs have the least oxygen consumption?
Brain, kidney and skin
Which organs have the highest oxygen consumption?
Heart and contracting skeletal muscle
How do you calculate oxygen delivery?
Arterial oxygen concentration X Coronary blood flow
What is the arterial oxygen concentration?
Relatively little dissolved in plasma
97-100%
What is arterial oxygen concentration determined by?
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
Why is the ordinarily little change in the oxygen content of arterial blood?
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
In the aortic pressure trace what is the dichroic notch?
When the aortic valve closes
After it closes some blood continues to leach out into the system and pressure falls further
How and what do you measure when monitoring blood pressure?
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
Left ventricular pressure trace - why does systolic BP - aortic pressure?
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
Left ventricular pressure trace - why does the aortic valve not influence diastolic pressure in the ventricles?
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
Is systolic BP always the same or always different?
The same
Is diastolic BP always the same or always different?
Different
- Arterial diastolic BP – 70-80 (supported by aortic valve)
- Ventricle diastolic BP – LVEDP
Does coronary perfusion occur during diastole or systole?
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
Overview of left ventricular pressure trace
Increases during systole
Falls back to nearly 0 during diastole
Overview of aortic pressure trace
Increases during systole
Falls back to arterial diastolic pressure – supported by aortic valve
The bigger the diastolic pressure the greater?
The perfusion
What are 3 physical factors that influence diastolic coronary flow?
Length of diastole
Raised LVEDP
Reduced diastolic pressure
How does raised LVEDP affect diastolic coronary flow?
Raised LVEDP –> decreases perfusion pressure
Failing hearts tend to run on high LVEDP
LVEDP stretches the muscle leading to more contraction
How does the length of diastole affect diastolic coronary flow?
Length of diastole –> tachycardia –> disproportionately reduces diastole
Systole is relatively fixed in terms of its duration
How does reduced diastolic pressure affect diastolic coronary flow?
Reduced diastolic pressure –>decreases perfusion pressure
What is auto regulation of coronary blood flow?
Ability of an organ to maintain a constant blood flow despite changes in perfusion pressure
- Protection mechanism
- Occurs by changing resistance
What can cause a fall in perfusion pressure?
Loss of blood –> coronary blood flow drops
When a fall in perfusion pressure occurs due to loss of blood how does the body respond?
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
What are the two ways coronary blood flow can be regulated?
Vascular control - metabolites and mediators
- Most important
Mechanical control
What does hypoxia cause in the coronary arteries?
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
What increases if metabolism in the heart is happening anaerobically?
Potassium ions Carbon dioxide Hydrogen ions Lactic acid These themselves cause coronary vasodilation
How does the sympathetic nervous system control coronary arteries?
Neural and humoral control
- Less important
- Large vessel α-adrenoceptor vasoconstriction
- Smaller vessel β2 vasodilatation
What is the main symptom of coronary artery disease?
Narrowing of the coronary artery –> cholesterol plaques
Overpowers all mechanisms coronary arteiers have to increase blood flow
How do patients with coronary disease know they aren’t getting enough blood flow to the heart?
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)
What two hormones does the heart release?
Heart is an endocrine organ - releases hormones which travel in the blood and act somewhere else
Atrial natriuretic peptide
B-natriuretic peptide
Where does atrial natriuretic peptide come from and cause?
Comes from the atria
Released during stretch / raised atrial pressure / volume overload
Where does B-natriuretic peptide come from and cause?
Comes from the ventricles
Released during stretch / raised ventricular pressure / volume overload
What are the main effects of atrial natriuretic peptide and B-natriuretic peptide?
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
What is the importance of atrial natriuretic peptide and B-natriuretic peptide inhibiting aldosterone, angiotensin II, endothelin and ADH?
These hormones all cause vasconstriction, water retention and salt retention
- Counter-regulatory system to the renin-angiotensin system
What are cardiac natriuretic peptide metabolised by?
Neutral Endopeptidase (NEP - neprilysin)
What can NEP be inhibited by?
A combination drug
- Sacubitril – neprilysin inhibitor
- Valsartan – angiotensin II blocker
Novel therapy for heart failure –> allows increased levels of cardiac natriuretic peptides
