Preload and afterload

Question 1

What is preload?

Answer 1

Force of Stretching of heart during diastole, increases SV - Starling’s law

Question 2

What is contractility and what is it due to?

Answer 2

Strength of contraction at a given diastolic loading, due to sympathetic nerves + circulating adrenaline increasing [Ca2+ ]i
(Heart rate and Contractility session)

Question 3

What is afterload?

Answer 3

Force that opposes ejection, reduces SV - Laplaces law

Question 4

What is energy of contraction?

Answer 4

Energy of contraction is the amount of work
required to generate stroke volume

Question 5

What are the 2 functions stroke work carries out?

Answer 5

1. Increases chamber pressure>aortic pressure(Isovolumetric contraction)
2. Ejection

Question 6

What is starling’s law of the heart?

Answer 6

Energy of contraction of cardiac muscle is
proportional to the muscle fibre length at rest

Question 7

What is the meaning of starling’s law of the heart?

Answer 7

Greater stretch of ventricle in diastole (resting muscle)
Greater energy of contraction
Greater SV achieved in systole (contracting muscle)
Intrinsic property of cardiac muscle (nerves, hormones etc. not involved)

Question 8

What is involved in ventricular function curve?

Answer 8

1. Normal resting filling pressure
   -An increase or decrease in filling can result in considerable changes in stroke volume
2. Excess filling or an overstretched muscle will lead to a reduction in stroke volume

Question 9

What is the molecular basis of starling’s law in un-stretched fibre?

Answer 9

Overlapping actin/myosin
- Mechanical inference -
Less cross-bridge formation
available for contraction

Question 10

What is the molecular basis of starling’s law in stretched fibre?

Answer 10

Less overlapping actin/myosin
- Less mechanical inference -
Potential for more cross-bridge formation
Increased sensitivity to Ca2+ ions

Question 11

What does starling’s law of the heart balance outputs of and what does this prevent?

Answer 11

Balances outputs of the RV and LV
Prevents fluid congestion in heart

Question 12

What is starling’s law of the heart responsible for?

Answer 12

-Responsible for fall in cardiac output following
a drop in blood volume
-Responsible for fall in cardiac output during orthostasis (standing)
leading to postural hypotension (dizziness, fainting)

Question 13

What does starling’s law of the heart contribute to?

Answer 13

Contributes to increased cardiac output during exercise

Question 14

What does starling’s law of the heart restore in response to IV fluids?

Answer 14

Restores stroke volume and cardiac output in response
to intravenous fluid transfusions

Question 15

What does the breakdown of starling’s law contribute to?

Answer 15

Breakdown of Starling’s law contributes
to development of cause heart failure

Question 16

What is afterload?

Answer 16

Afterload opposes ejection of blood from the heart

Question 17

What is afterload determined by?

Answer 17

Afterload is determined by Wall Stress - force through the heart wall
More energy of contraction needed to overcome Wall Stress to produce ejection
Heart doesn’t function as efficiently with Wall Stress

Question 18

What does laplace’s law describe and what is the equation?

Answer 18

Laplace’s law describes parameters that determine Afterload/Wall Stress (S):
Pressure (P), Radius (r), wall thickness
S=P x r/2w

Question 19

What is an increased afterload produced by?

Answer 19

produced by increasing
Pressure and Radius

Question 20

What is reduced afterload produced by?

Answer 20

produced by increasing Wall
Thickness

Question 21

How does a small ventricle radius impact afterload?

Answer 21

-Greater wall curvature
-More Wall Stress directed towards
centre of chamber
-Less wall stress directed through
heart wall
-Better ejection

Question 22

How does a larger ventricle radius impact afterload?

Answer 22

-Less wall curvature
-More Wall Stress directed
through heart wall
-More Afterload
-Less Ejection

Question 23

What does laplace’s law state in terms of pressure?

Answer 23

Laplaces law states that increased arterial blood pressure leads to Increased Afterload/Wall Stress – Reduced ejection

Question 24

What are the consequences of chronic high arterial blood pressure?

Answer 24

1. Increased afterload/Wall stress
   -Increased energy expenditure to maintain stroke volume
   -Ultimately decreased SV/CO-poor blood flow to end organs

Question 25

What type of heart failure does an increased radius lead to?

Answer 25

Volume-overload heart failure
-MI causes poor stroke volume/ejection fraction and so blood volume remains in heart

Question 26

What type of heart failure does an increased pressure lead to?

Answer 26

P : Pressure-overload heart failure
-hypertension causes increased afterload which heart must work against

Question 27

How does the heart compensate to increased radius/pressure?

Answer 27

-Increased r/P will increase wall stress(afterload) - opposes ejection
-Increased Wall Thickness (w) leads to hypertrophy (greater myocyte size)
-Same Wall Stress but now over greater area (more sarcomeres)
-Less Wall stress per sarcomere
-Less opposition to contraction of sarcomeres
-Greater SV/CO

Question 28

Why is the compensatory act by the heart due to increased radius/pressure not ideal?

Answer 28

This requires more energy (as more sarcomeres used)
Greater O2 use, ultimately decreases contractility - heart failure

Question 29

How does laplace’s law oppose starling’s law at rest?

Answer 29

-Increased Pre-load leads to increased chamber radius
-Laplace’s law states that this will increase Afterload
-This will opposes ejection of blood from a ‘full’ chamber
-In healthy heart - Starling’s Law overcomes Laplace’s law to maintain good ejection

Question 30

What does laplace’s law facilitate?

Answer 30

Facilitates ejection during contraction
-Ventricular contraction leads to reduced chamber radius
-Laplace’s law states this will reduce afterload/ in emptying chamber
-Aids ejection during reduced ventricular ejection phase of cardiac cycle

Question 31

How does laplace’s law contribute to a failing heart?

Answer 31

In a failing heart - chambers often dilated - increased radius
Reduction in ejection as Laplace’s law dictates that there is increased Afterload opposing ejection

Question 32

What does an increased preload have an effect on left ventricular pressure-volume loop?

Answer 32

e.g. events that increase venous return such as venoconstriction during exercise or administration of intravenous fluids
-Increased end-diastolic volume, Increased Starling’s law, Increased SV

Question 33

What does a decreased preload have an effect on left ventricular pressure-volume loop?

Answer 33

Events causing a loss of blood volume, e.g. haemorrhage, dehydration
-Decrease in EDV, decreasing in Starling’s law, decreased SV

Question 34

What does an increased afterload have an effect on left ventricular pressure-volume loop?

Answer 34

e.g. chronic hypertension
-Increased isovolumetric contraction to overcome greater aortic pressure and
open aortic valves for ejection, Less energy left for ejection, reduced SV