CARDIO- Preload and Afterload

Question 1

how is stroke volume controlled

Answer 1

preload, contractility, afterload

Question 2

what is preload

Answer 2

stretching of the heart during diastole

Question 3

what is contractility

Answer 3

strength of contraction at a given diastolic load- due to sympathetic nerves and circulating adrenaline increases calcium

Question 4

what is afterload

Answer 4

force that opposes ejection, reduces SV

Question 5

what is energy of contraction

Answer 5

amount of work required to generate stroke volume - depends on starlings law and contractility

Question 6

what are the two functions of stroke work

Answer 6

increase chamber pressure > aortic pressure and ejection

Question 7

what is starlings law

Answer 7

that the stroke volume increases as volume in the heart increases

based on the principle that contraction of cardiac muscle is proportional to the muscle fibre at rest

Question 8

what does starlings law mean

Answer 8

greater stretch of ventricle in diastole
greater energy of contraction
greater SV achieved in systole

Question 9

how is starlings curve relevant

Answer 9

whilst increase filling = increase in SV it eventually reaches a plateau phase
and then excess filling leads to an overstretched muscle and therefore a decrease in SV

this is an important consideration when giving fluids

Question 10

explain preload under the molecular basis of starlings law (unstretched and stretched)

Answer 10

unstretched fibre - there are a lot of overlapping actin / myosin
therefore a lot of mechanical inferences
meaning there are less cross-bridge formations available for contraction

stretched fibre
less overlapping of actin / myosin
less mechanical inferences
potential for more cross-bridge formation
increased sensitivity to calcium ions (levels do not rise, calcium just works better)

Question 11

what are the roles of starlings law

Answer 11

balances outputs of the RV/LV - prevents fluid congestion in the heart
responsible for the fall in cardiac output during blood loss
responsible for postural hypotension
contributes to CO during excercise - increased return to the heart
restores SV and CO after IV transfusion

Question 12

what is afterload determined by

Answer 12

wall stress - force through the heart wall

Question 13

what is Laplace’s law

Answer 13

describe parameters that determine afterload / wall stress (s), pressure (p), radius (r), Wall thickness (w)

s = p x r / 2w

Question 14

how does radius affect wall stress and therefore ejection

Answer 14

small ventricular radius:
greater wall curvature, more wall stress directed towards the centre of the chamber, less directed through the heart wall, better ejection

larger ventricular radius:
less wall curvature, more wall stress directed through the heart wall, more afterload, less ejection

Question 15

what does Laplace’s law mean in relation to arterial blood pressure

Answer 15

increased arterial blood pressure leads to increased afterload / wall stress = reduced ejection

Question 16

what are the consequences of chronic high arterial blood pressure

Answer 16

increased afterload / wall stress
increased energy expenditure to maintain SV
ultimately decreased SV/ CO - poor blood flow to end organs
chronic high blood pressure is bad for the heart

Question 17

how does Laplace’s law explain hypertrophy in heart failure

Answer 17

Increase in volume - overload heart failure - MI causes poor stroke volume /ejection fraction so blood volume remain in the heart

this causes hypertension which causes afterload which the heart must work against

therefore:
the heart wall thickness increases in thickness
wall stress becomes greater
this increases SV/CO

but this requires more energy, greater O2 use which causes more heart failure

Question 18

what is the importance of Laplace’s law

Answer 18

-opposes starlings law
in a healthy heart - starlings law overcomes Laplace’s law to maintain good ejection
- facilitates ejection during contraction - aids ejection during the reduced ventricular ejection phase of the cardiac cycle
- contributes to a failing heart

Question 19

outline the importance of Starling’s law during IV infusion of fluids to regulate BV and BP and Laplace’s law in reducing CO in HF

Answer 19

starlings law - infusion with IV fluids = increase in BV = increase in stretch of fibres = increased force of ejection = increased BP

Laplace’s law = increase in BP = increased afterload and wall stress = reduced ejection (as wall stress opposes ejection)