Control Of Cardiac Output

Question 1

What is cardiac output?

Answer 1

The amount of blood ejected from the heart per minute

Question 2

What is heart rate?

Answer 2

How often the heart beats per minute

Question 3

What is stroke volume?

Answer 3

How much blood is ejected from the heart per beat

Question 4

What does TPR stand for?

Answer 4

Total peripheral resistance

Question 5

What is preload of the heart?

Answer 5

LV wall stress at end diastolic volume - Stretching of the heart at rest, which increases stroke volume due to starlings law

Question 6

What is afterload in the heart?

Answer 6

LV wall stress during ejection - Opposes ejection and reduces stroke volume due to laplaces law.

Question 7

What is starlings law of the heart?

Answer 7

Energy of contraction of cardiac muscle is relative to heart rate and also the strength of contraction

Question 8

What does a greater stretch in the ventricle in diastole lead to?

Answer 8

Greater energy of contraction and subsequently greater stroke volume achieved in systole

Question 9

What do you do if patients are bleeding out and why does it work?

Answer 9

Give fluids to increase blood volume as this increases stroke volume

Question 10

What is the importance of starlings law?

Answer 10

• Balances outputs of the right and left ventricles
• responsible for a fall in cardiac output during a drop in blood volume or vasodilation
• restores cardiac output in response to IV fluid transfusions
• responsible for a fall in cardiac output during orthostasis, which leads to postural hypotension and dizziness
• contributes to an increased stroke volume and cardiac output during upright exercise

Question 11

How is afterload increased and reduced?

Answer 11

By increasing pressure and radius and reduced by increasing wall thickness

Question 12

What is the formula for wall stress?

Answer 12

Tension/ wall thickness

Question 13

Why is laplaces law important?

Answer 13

• opposes starlings law at rest - increased preload gives increased stretch or chamber. This increases the chamber radius (decreasing curvature) which increases offload
• facilitate ejection during contraction as it reduces chamber radius so there’s less afterload as the chamber empties. This aids expulsion of the last bit of blood and increases stroke volume
• contributes to failing heart at rest and during contraction - in a failing heart the chambers are often dilated and the radius is large, so there’s an increased load opposing ejection

Question 14

When is laplaces law good/bad?

Answer 14

Good with a small radius and bad with a large radius

Question 15

What are acute rises in blood pressure offset by?

Answer 15

Starlings law- increased stretch gives increased cofactors and stroke volume

Question 16

What is baroreflex?

Answer 16

Decreased sympathetic tone which decreases blood pressure

Question 17

What does a chronic increase in arterial blood pressure cause?

Answer 17

Increased energy spending attempts to maintain stroke volume but ultimately stroke volume will gradually decrease

Question 18

What is energy of contraction?

Answer 18

The amount of work required to generate stroke volume (depends on starlings law and contractility)

Question 19

What two functions does stroke work carry out?

Answer 19

• contracts until chamber pressure > aortic pressure (isovolumetric contraction)
• ejection from the ventricle

Question 20

What happens when there’s an increased radius in heart failure?

Answer 20

When the heart doesn’t contract properly, there is blood left in the ventricle leading to eventual volume overload

Question 21

What happens when there’s an increased pressure in heart failure?

Answer 21

Increases in either radius or pressure will increase the wall stress which opposes ejection

Question 22

How does the heart compensate for increased radius or pressure?

Answer 22

Ventricular hypertrophy (greater myocyte size and more sarcoma) which increases wall thickness. This decreases wall stress per sarcomere

Question 23

What happens to the heart load during exercise?

Answer 23

Increased venous return leads to increased preload and higher EDV.
The ventricle will eject volume to the same ESV so there is an inc in stroke volume

Question 24

What happens to the heart load in hypertension?

Answer 24

Longer time spent in isovolumetric contraction to increase pressure in the chamber.
This uses more energy and lowers the force of contraction reducing stroke volume and increasing ESV