Preload and Afterload Flashcards
What is Cardiac Output?
What are the 2 equations used to work out CO and BP?
- Volume of blood ejected every minute.
- CO = SV x HR
BP = CO x TPR
What controls SV?
- Preload - stretching of heart during diastole, increases SV
- HR - sympathetic and parasympathetic nerves
- Contractility - strength of contraction due to sympathetic nerves
- Afterload - opposes ejection, decreases SV
What is the Energy of Contraction?
What does it depend on?
- The amount of work required to generate the SV
- Depends on Starling’s Law and contractility. The stroke work carries out isovolumetric contraction due to chamber pressure > aortic pressure for ejection:
Preload = ↑Energy of contraction and SV
Afterload requires a greater energy of contraction and opposes SV
What’s Starling’s Law?
Describe the main parts of Starling’s Curve of EDV against SV
How does stretching increase the energy of contraction?
- The energy of contraction of cardiac muscle is proportional to the muscle fibre length at rest.
↑Stretch of the ventricle in diastole = ↑Energy of contraction = ↑SV in systole.
- • Ascending part is during rest and changes in filling pressure/EDP largely affect SV.
• Plateau phase and then graph descends due to the excess filling of the ventricles = Overstretching of muscle = ↓SV - taken into consideration doing fluid replacement. - Un-stretched fibre:
Actin and myosin overlapping = ↓Cross-bridge formation for contraction
Stretched fibre:
↓Overlapping = ↑Sensitivity to Ca2+ = ↑Cross-bridges can form
Why is Starling’s Law important?
- Balances outputs of RV and LV
- Responsible for drop in CO after a drop in BV (e.g. haemorrhage)
- Responsible for drop in CO during Orthostasis (upright) → Postural hypotension (dizziness, fainting)
- Restores CO in response to IV fluid transfusions
- Increases CO during exercise
What is Afterload and what is it determined by?
What is Laplace’s Law?
What is the equation used?
How can you increase or decrease wall stress?
- The pressure the heart must work against to eject blood. It’s determined by Laplace’s Law and Wall stress, which is the force through the heart wall.
- It describes the parameters that determine Afterload, including wall stress (S), pressure (P), radius (r), and wall thickness (w).
- S = (P x r) / 2w
- Increased S - ↑Pressure and radius
Decreased S - ↑Wall thickness
Explain how Ventricular Radius affects Ejection
LOOK AT DIAGRAMS!
SMALL ventricle radius:
↑Wall curvature = ↑Wall Stress directed towards centre of chamber = ↓Afterload = Better Ejection
LARGE ventricle radius:
↓Wall curvature = ↑Wall Stress directed through heart wall = ↑Afterload = Poor Ejection
Why is Laplace’s Law important?
• Opposes Starling’s Law:
↑Preload = ↑Chamber radius. Laplaces Law says this will ↑Wall stress and Afterload = poor ejection
In a healthy heart, Starlings Law will overcome Laplaces Law to maintain good ejection.
• Facilitates ejection during contraction:
Ventricular contraction = ↓Chamber radius. Laplace’s says this will ↓Wall stress and Afterload = good ejection
• Contributes to a failing heart at rest and during contraction:
In a failing heart, chambers are often dilated = ↑Radius, which will ↑Wall stress and Afterload = poor ejection
*OVERALL, LAPLACE’S LAW IS GOOD WITH A SMALL RADIUS, BUT BAD WITH A LARGE RADIUS
What occurs during acute rises in BP?
What are the consequences of chronic high BP?
Why is BP kept constant during exercise?
- It is offset by Starling’s law, Intrinsic increase in contractility, Baroreflex
- ↑Afterload/Wall Stress = poor ejection:
↑Energy expenditure to maintain SV. Eventually causes a ↓SV/CO = Poor blood flow to end organs. - To prevent a ↓SV/CO - we need CO to increase instead.
How does heart failure oppose ejection?
Why does Hypertrophy occur in heart failure?
- In heart failure, the pressure and radius will increase, which ↑Wall stress and Afterload = Poor ejection.
- Heart compensates by ↑Wall thickness = Hypertrophy (bigger myocytes):
• Same wall stress over a ↑Surface area = Less wall stress per Sarcomere. There will be less opposition to contraction from sarcomeres = ↑SV/CO.
But, this requires a lot more energy and ↑O2 used = ↓Contractilty; heart failure as demands can’t be met.
How does Preload affect the LV pressure=-volume loop?
LOOK AT DIAGRAM!
↑EDV = ↑Starling’s law = ↑SV
Area of graph will be bigger (wider).
How does Afterload affect the LV pressure-volume loop?
LOOK AT DIAGRAM!
↑BP (Afterload) = ↑Isovolumetric contraction = ↓Energy for ejection = ↓SV
More energy used to eject less blood
Opposite occurs for ↓BP
