Control of cardiac output Flashcards

1
Q

What is cardiac output

A

The amount of blood ejected from the heart per minute

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2
Q

Equation for Cardiac output

A

CO = HR x SV

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3
Q

What does cardiac output affect

A

BP/TPR

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4
Q

What is preload

A

Stretching of heart at rest, increase stroke volume due to Starling’s law

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5
Q

What is afterload

A

Opposes ejection, reduces stroke volume due to Laplace’s law

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6
Q

Energy of contraction

A

Amount of work required to generate stroke volume

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7
Q

What are the 2 functions stroke work carries out

A

1) Contracts until chamber pressure > aortic pressure (isovolumetric contraction)
2) Ejection from ventricle

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8
Q

Define starling’s law

A

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

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9
Q

Roles and effects of starling’s law

A

1) Balance output of right ventricle and left ventricle
2) Responsible for fall in CO during a drop in blood volume
3) Restores CO in response to intravenous fluid transfusions
4) Responsible for fall in CO during orthostasis leading to postural hypotension and dizziness as blood pools in legs
5) Contributes to increased SV and CO during upright exercise

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10
Q

Define Laplace’s law

A

That increased blood pressure (P) will increase wall stress. This will increase afterload and reduce ejection.

Laplace’s law is good ejection with small radius, bad with large radius

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11
Q

How is afterload increased

A

Increasing pressure and radius

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12
Q

How is afterload reduced

A

Increase wall thickness

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13
Q

Afterload of a ventricle with a small radius

A

Less afterload
Better ejection
Greater wall curvature
More wall stress directed towards the centre of the chamber

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14
Q

Afterload of a large ventricle radius

A

More afterload
Less ejection
Less wall curvature
More wall stress directed through heart wall

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15
Q

Afterload of a huge theoretical radius

A

Negligible wall curvature

Virtually all stress directed through wall

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16
Q

Importance of Laplace’s law

A

1) Oppose Starling’s law
2) Facilitates ejection during contraction
3) Contributes to a failing heart at rest and during contraction

17
Q

Effects of chronic increase in arterial blood pressure

A

Increased energy expenditure attempt to maintain stroke volume but ultimately stroke volume will gradually decrease
Decrease in blood pressure would increase efficiency of the heart.

HIGH BP = REDUCE CO

18
Q

Starling’s law and ventricular pressure - volume loop

A

During exercise increased venous return leads to increased preload and more stretch
This causes a shorter isovolumetric contraction phase, and increase in SV due to Starling’s law
More blood back to the heart, more blood ejected from the heart

19
Q

Laplace’s law and ventricular pressure - volume loop

A

With high blood pressure, increased afterload
Longer time spent in isovolumetric contraction to increase pressure in the chamber above that in aorta to open the valve
This uses more energy and lowers the force of contraction reducing stroke volume