Preload and Afterload

Question 1

List factors which affect stroke volume

Answer 1

Preload
Contractility
Afterload

Question 2

What is preload?

Answer 2

this is the stretching of heart during diastole due to blood returning back to the heart. this increases SV → Starling’s law
initial stretching of the cardiac myocytes prior to contraction

Question 3

What is contractility?

Answer 3

strength of contraction at a given diastolic loading, due to sympathetic nerves and circulating adrenaline increasing [Ca2+]. sometimes it can contract strongly, sometimes more weakly

Question 4

what is afterload?

Answer 4

force that opposes ejection

this reduces SV -> Laplace’s law

Question 5

Define Starling’s law

Answer 5

energy of contraction is proportional to the length of muscle fibre at rest

Question 6

Define energy of contraction

Answer 6

the amount of work required to generate stroke volume

it depends on starling’s law and contractility

Question 7

What does preload do?

Answer 7

increases energy of contraction
this enhances stroke volume
makes the heart work better

Question 8

What does afterload do?

Answer 8

opposes stroke volume
this means a greater energy of contraction is required to eject blood from the heart
afterload is a force that needs to be overcome

Question 9

Explain starlings law

Answer 9

• ‘energy of contraction of cardiac muscle is proportional to the muscle fibre length at rest’
• this means that:
  
  • when the heart is filling, its relaxing (its in its diastolic phase)
  • the greater stretch of the ventricle in diastole = greater energy of contraction it has = so a greater stroke volume is achieved in systole (contracting phase)
  • this is an intrinsic property of cardiac muscle (nerves, hormones, etc. not involved)
• so, more that comes in, more it stretches, more goes out

Question 10

What does the ventricular function curve / starlings curve show

Answer 10

• if you fill the ventricles more = higher pressure = increase SV (because of the increased stretch). the ventricles will eventually reach a plateau phase → you can’t stretch it anymore
• the fundamental relationship between stretch and output stops at the plateau phase → nothing is elastic forever
• don’t give fluid to someone beyond the plateau point. it is potentially dangerous. it can cause congestion
• hence, you give fluid challenges. for example, if a patient is dehydrated, give them 1/2 litre, see what happens to their CO, SV, BP. if they respond, you might give some more