How Filling Pressure and Inotropic State Govern Cardiac Output

Question 1

What are the equations for CO, BP and blood flow?

Answer 1

CO = HR x SV    
BP = CO x TPR 
Blood flow(CO) = BP/TPR

Question 2

What is preload and what does it do?

Answer 2

Stretching of the heart at rest and it controls stroke volume due to Starling’s Law. Energy of contraction of cardiac muscle is proportional to the muscle fibre length at rest. Greater stretch of ventricle in diastole (resting muscle), greater energy of contraction, greater SV achieved in systole (contracting muscle).

Question 3

What is contractility?

Answer 3

Strength of contraction at a given resting load due to sympathetic nerves and circulating adrenaline which increases intracellular calcium.

Question 4

What is afterload?

Answer 4

It is the pressure in the wall of the left ventricle during ejection. Opposes ejection and reduces SV due to Laplaces Law.

Question 5

What is energy of contraction?

Answer 5

The amount of work required to generate stroke volume. It depends on Starling’s Law and contractility.

Question 6

How do stretched and unstretched fibres affect preload?

Answer 6

Un-stretched fibre = overlapping actin/myosin – mechanical inference – less cross-bridge formation available for contraction.
Stretched fibre = less overlapping actin/myosin – less mechanical inference – potential for more cross-bridge formation and increases sensitivity to Ca ions.

Question 7

What is the role of Starling’s Law?

Answer 7

Balances outputs of the RV and LV, responsible for fall in CO during a drop in blood volume (e.g. haemorrhage, sepsis), restores CO in response to intravenous fluid transfusions, responsible for fall in CO during orthostasis (standing) –> postural hypotension & dizziness, contributes to increased stroke volume during upright exercise.

Question 8

What is afterload determined by and how do we overcome this factor?

Answer 8

Afterload is determined by Wall Stress which is a force directed through the heart wall that prevents sarcomeres contracting properly. More energy of contraction is need to overcome this Wall Stress to produce cell shortening and ejection.

Question 9

What are the equations in Laplaces Law? What do they mean?

Answer 9

This describes parameters that determine afterload: the relationship between - Wall Tension (T), Pressure (P), and Radius (r) in a chamber (ventricle): P = 2T / r
Value of 2 is because a chamber has 2 directions of curvature. Tension (T) is made up of Wall Stress (S) and Wall thickness (w):
P = 2Sw / r or S = P x r / 2w

Question 10

Explain how a small ventricle radius affects afterload.

Answer 10

A small ventricle radius means that there is greater wall curvature and so more wall stress directed towards the centre of the chamber. This produces less afterload and so better ejection. S = P x r / 2w

Question 11

Give 3 reasons why Laplaces Law is important?

Answer 11

1. It opposes Starling’s Law at rest - an increase in pre-load results in increased chamber radius and decreased curvature which increases afterload.
2. Facilitates ejection during contraction - ventricular contraction - decreased chamber radius and increases curvature so there will be less afterload in the emptying chamber which aids expulsion.
3. Contributes to a failing heart at rest and during contraction - in a failing heart the chambers are dilated - increased radius - less ejection as increased afterload.

Question 12

What does Laplace’s Law tell us a rise in BP will do?

Answer 12

It will increase wall stress which will increase afterload and reduce ejection.

Question 13

Explain how heart failure can cause an increase in chamber radius?

Answer 13

If you have a MI/mitral valve re-gurgitation/cardiomyopathies, the heart won’t contract as well so the ejection fraction is reduced and there is more blood in the chamber and you get volume overload which increases the radius.

Question 14

Explain how heart failure can increase pressure in the heart?

Answer 14

If you have hypertension/aortic stenosis you must increase the pressure/afterload in the chamber leading to pressure-overload HF.

Question 15

How does the heart respond to increased pressure and radius in the chambers during heart failure?

Answer 15

1. Wall thickness increases - to decrease wall stress
2. Afterload decreases - allows the same wall stress to be distributed over a greater area because there are more sarcomeres - maintains SV and CO. However this needs more energy and O2 so it decreases contractility and produces HF.

Question 16

How does exercise change a ventricular pressure-volume loop and why?

Answer 16

The loop’s right hand side stretches further right. This is because of the increased venous return, the increased end-diastolic volume and the increased SV.

Question 17

How does hypertension/hypotension change a ventricular pressure-volume loop and why?

Answer 17

Hypertension - increased arterial blood pressure - so increased isovolumetric contraction to increase pressure in the chamber above the aortic pressure to open the valves to start ejecting. Using more energy doing this than ejecting blood so SV goes down. The shape becomes narrower and taller.