Control of cardiac output Flashcards
What is cardiac output
The amount of blood ejected from the heart per minute
Equation for Cardiac output
CO = HR x SV
What does cardiac output affect
BP/TPR
What is preload
Stretching of heart at rest, increase stroke volume due to Starling’s law
What is afterload
Opposes ejection, reduces stroke volume due to Laplace’s law
Energy of contraction
Amount of work required to generate stroke volume
What are the 2 functions stroke work carries out
1) Contracts until chamber pressure > aortic pressure (isovolumetric contraction)
2) Ejection from ventricle
Define starling’s law
Energy of contraction of cardiac muscle is relative to the muscle fibre length at rest
Roles and effects of starling’s law
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
Define Laplace’s law
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
How is afterload increased
Increasing pressure and radius
How is afterload reduced
Increase wall thickness
Afterload of a ventricle with a small radius
Less afterload
Better ejection
Greater wall curvature
More wall stress directed towards the centre of the chamber
Afterload of a large ventricle radius
More afterload
Less ejection
Less wall curvature
More wall stress directed through heart wall
Afterload of a huge theoretical radius
Negligible wall curvature
Virtually all stress directed through wall
Importance of Laplace’s law
1) Oppose Starling’s law
2) Facilitates ejection during contraction
3) Contributes to a failing heart at rest and during contraction
Effects of chronic increase in arterial blood pressure
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
Starling’s law and ventricular pressure - volume loop
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
Laplace’s law and ventricular pressure - volume loop
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
