CVS: Preload and Afterload Flashcards
Define cardiac output
Vol blood ejected per min
Proportional to HR + SV ∴ CO = HR + SV
CO from right side (via pulmonary artery) and left side (via aorta) are same
CO determine blood pressure + blood flow
What is preload?
Stretching of heart during diastole, increases SV → Regulated by Starling’s law
What is contractility?
Strength of contraction at given diastolic loading, due to sympathetic nerves + circulating adrenaline increasing Ca2_
What is energy of contraction?
Work required to generate stroke volume, depends on Starling’s law + contractility
What are the 2 functions of stroke work?
- Increases chamber pressure> aortic pressure (isovolumetric contraction
- Ejection
Define Starling’s law
Energy of contraction of cardiac muscle is proportional to the muscle fibre length at rest
This means:
- Greater stretch of ventricle in diastole (resting muscle)
- Greater energy of contraction
- Greater SV achieved in systole (contracting muscle)
Intrinsic property of cardiac muscle (nerves, hormones etc. not involved)
Describe preload in terms of the molecular basis of Starling’s law
Un-stretched fibre:
- Overlapping actin/myosin - Less mechanical interference, less cross-bridge formation available for contraction
Stretched fibre:
- Less overlapping actin/myosin - Less mechanical interference, potential for more cross-bridge formation, increased sensitivity to Ca2+ ions
What are the roles of Starling’s Law?
- Balances outputs of RV and LV → Prevents fluid congestion in heart
- Responsible for fall in CO following drop in blood volume (haemorrhage, sepsis), orthostasis (standing) leading to postural hypotension (dizziness, fainting)
- Contributes to increased CO during exercise
- Restores SV and CO in response to intravenous fluid transfusions
Breakdown of Starling’s law contributes to development of heart failure
What is afterload?
Force that opposes ejection, reduces SV → Regulated by Laplace’s law
Define Laplace’s law
Afterload opposes ejection of blood from the heart
Afterload is determined by Wall Stress → force through heart wall
More energy of contraction needed to overcome Wall Stress to produce ejection → Heart doesn’t function as efficiently with Wall Stress
Laplace’s law describes parameters that determine Afterload/Wall Stress (S):
- Pressure (P)
- Radius (r)
- Wall thickness (W)
Afterload (S) = P x r/2W
Afterload (S):
- Increased S- Produced by increasing Pressure and Radius
- Reduced S - Produced by increasing Wall Thickness
Therefore Laplace’s law states increased arterial blood pressure = Increased Afterload/Wall stress resulting in reduced ejection
How does afterload change when there’s smaller ventricular radius?
- Greater wall curvature
- More Wall Stress directed towards centre of chamber
- Less Wall stress directed through heart wall
- Better ejection - therefore less opposing force so decreased afterload
How does afterload change when there’s larger ventricular radius?
- Less wall curvature
- More wall stress directed through heart wall
- Greater afterload
- Less ejection
How does chronic high arterial blood pressure affect afterload?
- ⬆️Afterload/Wall Stress
- Increased energy expenditure
- Ultimately decreased SV/CO = Poor blood flow to end organs, poor perfusion of organs
How does Laplace’s law explain hypertrophy in heart failure?
- ⬆️r: Volume-overload heart failure
- E.g. MI causes poor stroke volume/ejection fraction
- Blood volume remains in heart
- ⬆️P: Pressure-overload heart failure
- E.g. Hypertension causes afterload which heart must work against
- S = P x r/ 2W therefore, increased r or P will increase afterload meaning less ejection
- To counteract this, the heart must compensate and it does this by:
- Increased Wall thickness (W) = hypertrophy (greater myocyte size)
- Same Wall stress but now over greater area (more sarcomeres)
- Less wall stress per sarcomere and less opposition to contraction of sarcomeres, greater SV/CO
- But requires more energy (as more sarcomeres used)
- This means greater O2 is needed, so ultimately contractility decreases, resulting in a circle of heart failure
What is the importance of Laplace’s law?
- Opposes Starling’s law at rest
- ⬆️Pre-load = ⬆️chamber radius
- Laplace’s law states that this will increase afterload, which will oppose ejection of blood from a ‘full’ chamber
- In a healthy heart, Starling’s law overcomes Laplace’s law to maintain good ejection
- Facilitates ejection during contraction
- Ventricular contraction = ⬇️chamber radius
- Laplace’s law states this will reduce afterload in ‘emptying’ chamber
- Aids ejection during reduced ventricular ejection phase of cardiac cycle
- Contributes to failing heart
- In failing heart, chamber often dilated, increasing chamber radius
- Reduction in ejection as Laplace’s law dictates that there is increased afterload opposing ejection
- Therefore, laplace’s law is good with small radii, but bad with large radii
