Preload + afterload

Question 1

Define CO

Answer 1

v of blood ejected from heart per minute

Question 2

Equations w CO?

Answer 2

CO = HR x SV
BP = CO x TPR

Question 3

What’s resting CO?

Answer 3

70 bpm x 70 ml = 5 L/min

Question 4

What’s exercising CO?

Answer 4

180 bpm x 120 ml = 22 L/min

Question 5

What does CO determine?

Answer 5

BP + blood flow

Question 6

What controls SV?

Answer 6

preload, HR, contractility, afterload

Question 7

How does preload control SV?

Answer 7

Stretching of heart at rest, increases SV due to Starling’s law

Question 8

How does HR control SV?

Answer 8

Sympathetic + parasympathetic nerves

Question 9

How does contractility control SV?

Answer 9

Strength of contraction at given resting load, due to sympathetic nerves + A increasing [Ca2+ ]

Question 10

How does afterload control SV?

Answer 10

Opposes ejection, reduces SV due to Laplaces law

Question 11

Define energy of contraction

Answer 11

amount of work required to generate SV

Question 12

What does energy of contraction depend on?

Answer 12

Starling’s Law + Contractility

Question 13

Role of stroke work?

Answer 13

Increases chamber p > aorta (isovolumetric contraction)

Ejection

Question 14

What’s Starling’s law?

Answer 14

‘Energy of contraction of cardiac muscle proportional to muscle fibre length at rest’
ie more stretch at rest –> greater contraction

Question 15

What’s the intrinsic property of heart?

Answer 15

can increase strength of cardiac contraction

Question 16

What happens if ↑CVP?

Answer 16

↑venous return to heart
↑EDV (end diastolic v)
↑preload
↑ejection

Question 17

Features of Starling’s curve?

Answer 17

filling p mmHg (CVP or end distole p) vs SV (ml)
ascending limb - ↑filling p –> ↑SV
plateau - ↑filling p but no more work

Question 18

Why do a fluid challenge?

Answer 18

give fluid, see if makes diff to CO + BP, see if they’re on ascending limb

Question 19

Why does stretching increase energy of contraction?

Answer 19

stretched fibre has less overlapping actin/myosi
less mechanical inference
potential for more cross-bridge formation
↑ sensitivity to Ca2+

Question 20

Roles of Starling’s law?

Answer 20

• Balances outputs of RV + LV
• ↓CO after drop in blood v (eg haemorrhage, sepsis)
• ↓CO during orthostasis (standing)–>postural hypotension 😵
• Restores CO in response to intravenous fluid transfusions
• ↑ CO during exercise

Question 21

What’s afterload determined by?

Answer 21

Wall Stress directed via heart wall

Question 22

What does wall stress do?

Answer 22

prevent actin-myosin

Question 23

What does Laplaces law describe?

Answer 23

Wall Tension (T),  Pressure (P), Radius (r) in ventricle
P = 2T / r 
Tension (T) = Wall Stress (S) and Wall Thickness (w) so:
P = 2Sw / r     or      S = P x r / 2w

Question 24

Why value of 2?

Answer 24

chamber has 2 directions of curvature

Question 25

Why does a smaller ventricle radius give better ejection?

Answer 25

greater wall curvature
Wall Stress towards centre of chamber not heart wall
less afterload

Question 26

Why does a larger ventricle radius give better ejection?

Answer 26

less wall curvature
more Wall Stress directed via heart wall
more Afterload

Question 27

Roles of Laplace’s law?

Answer 27

Opposes Starling’s law at rest
Facilitates ejection during contraction
Contributes to failing heart at rest + during contraction

Question 28

How does Laplace’s oppose Starling’s law at rest?

Answer 28

↑ Pre-load –> ↑ chamber radius
Laplace’s says it will increase afterload +opposes ejection from full chamber
BUT Starling’s > Laplace’s

Question 29

How does Laplace’s law facilitate ejection during contraction?

Answer 29

Ventricular contraction ↓chamber radius
Laplace’s says it will reduce afterload in emptying chamber so helps ejection during reduced ventricular ejection phase 4 of cardiac cycle :)

Question 30

How does Laplace’s law contributes to failing heart at rest + during contraction?

Answer 30

failing heart has dilated chambers so ↑ radius

Laplace’s law says increase afterload opposing ejection

Question 31

What does Laplace’s law say if there’s increased arterial BP?

Answer 31

↑ Wall Stress –> ↑ Afterload –> ↓ ejection

Question 32

What happens if there’s an acute rise in arterial BP?

Answer 32

Starling’s law - ↑ stretch, ↑ contraction, ↑ SV
Intrinsic increase in contractility - Anrep response, local +ve inotropes
Baroreflex -↓ Sym NS, ↓ TPR, ↓ BP

Question 33

How does Laplace’s law explain hypertrophy in heart failure?

Answer 33

S = P x r / 2w
↑r - Volume-overload (MI, cardiomyopathies, mitral valve re-gurgitation)
↑P - Pressure-overload (hypertension, aortic stenosis)

↑ afterload –> ↓ejection so heart compensates by increasing Wall Thickness (w) so:
↓ afterload –> ↑ SV/CO
same wall stress over greater area, less wall stress per sarcomere, less opposition to contraction of sarcomeres, greater O2 use, less contractility…

Question 34

What’s the effect of increased afterload on ventricular p-v loop?

Answer 34

-↑ BP (Afterload)
-↑ isovolumetric contraction using more energy
-less energy for ejection so↓ SV
More energy used to eject less blood

Question 35

What’s the effect of increased preload on ventricular p-v loop?

Answer 35

• ↑ venous return (exercise,intravenous fluids)
• ↑ end-diastolic v
• ↑ Starling’s law
• ↑ SV for little increase in energy used