Preload and afterload Flashcards
What is preload?
Force of Stretching of heart during diastole, increases SV - Starling’s law
What is contractility and what is it due to?
Strength of contraction at a given diastolic loading, due to sympathetic nerves + circulating adrenaline increasing [Ca2+ ]i
(Heart rate and Contractility session)
What is afterload?
Force that opposes ejection, reduces SV - Laplaces law
What is energy of contraction?
Energy of contraction is the amount of work
required to generate stroke volume
What are the 2 functions stroke work carries out?
- Increases chamber pressure>aortic pressure(Isovolumetric contraction)
- Ejection
What is starling’s law of the heart?
Energy of contraction of cardiac muscle is
proportional to the muscle fibre length at rest
What is the meaning of starling’s law of the heart?
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)
What is involved in ventricular function curve?
- Normal resting filling pressure
-An increase or decrease in filling can result in considerable changes in stroke volume - Excess filling or an overstretched muscle will lead to a reduction in stroke volume
What is the molecular basis of starling’s law in un-stretched fibre?
Overlapping actin/myosin
- Mechanical inference -
Less cross-bridge formation
available for contraction
What is the molecular basis of starling’s law in stretched fibre?
Less overlapping actin/myosin
- Less mechanical inference -
Potential for more cross-bridge formation
Increased sensitivity to Ca2+ ions
What does starling’s law of the heart balance outputs of and what does this prevent?
Balances outputs of the RV and LV
Prevents fluid congestion in heart
What is starling’s law of the heart responsible for?
-Responsible for fall in cardiac output following
a drop in blood volume
-Responsible for fall in cardiac output during orthostasis (standing)
leading to postural hypotension (dizziness, fainting)
What does starling’s law of the heart contribute to?
Contributes to increased cardiac output during exercise
What does starling’s law of the heart restore in response to IV fluids?
Restores stroke volume and cardiac output in response
to intravenous fluid transfusions
What does the breakdown of starling’s law contribute to?
Breakdown of Starling’s law contributes
to development of cause heart failure
What is afterload?
Afterload opposes ejection of blood from the heart
What is afterload determined by?
Afterload is determined by Wall Stress - force through the heart wall
More energy of contraction needed to overcome Wall Stress to produce ejection
Heart doesn’t function as efficiently with Wall Stress
What does laplace’s law describe and what is the equation?
Laplace’s law describes parameters that determine Afterload/Wall Stress (S):
Pressure (P), Radius (r), wall thickness
S=P x r/2w
What is an increased afterload produced by?
produced by increasing
Pressure and Radius
What is reduced afterload produced by?
produced by increasing Wall
Thickness
How does a small ventricle radius impact afterload?
-Greater wall curvature
-More Wall Stress directed towards
centre of chamber
-Less wall stress directed through
heart wall
-Better ejection
How does a larger ventricle radius impact afterload?
-Less wall curvature
-More Wall Stress directed
through heart wall
-More Afterload
-Less Ejection
What does laplace’s law state in terms of pressure?
Laplaces law states that increased arterial blood pressure leads to Increased Afterload/Wall Stress – Reduced ejection
What are the consequences of chronic high arterial blood pressure?
- Increased afterload/Wall stress
-Increased energy expenditure to maintain stroke volume
-Ultimately decreased SV/CO-poor blood flow to end organs
What type of heart failure does an increased radius lead to?
Volume-overload heart failure
-MI causes poor stroke volume/ejection fraction and so blood volume remains in heart
What type of heart failure does an increased pressure lead to?
P : Pressure-overload heart failure
-hypertension causes increased afterload which heart must work against
How does the heart compensate to increased radius/pressure?
-Increased r/P will increase wall stress(afterload) - opposes ejection
-Increased Wall Thickness (w) leads to hypertrophy (greater myocyte size)
-Same Wall Stress but now over greater area (more sarcomeres)
-Less Wall stress per sarcomere
-Less opposition to contraction of sarcomeres
-Greater SV/CO
Why is the compensatory act by the heart due to increased radius/pressure not ideal?
This requires more energy (as more sarcomeres used)
Greater O2 use, ultimately decreases contractility - heart failure
How does laplace’s law oppose starling’s law at rest?
-Increased Pre-load leads to increased chamber radius
-Laplace’s law states that this will increase Afterload
-This will opposes ejection of blood from a ‘full’ chamber
-In healthy heart - Starling’s Law overcomes Laplace’s law to maintain good ejection
What does laplace’s law facilitate?
Facilitates ejection during contraction
-Ventricular contraction leads to reduced chamber radius
-Laplace’s law states this will reduce afterload/ in emptying chamber
-Aids ejection during reduced ventricular ejection phase of cardiac cycle
How does laplace’s law contribute to a failing heart?
In a failing heart - chambers often dilated - increased radius
Reduction in ejection as Laplace’s law dictates that there is increased Afterload opposing ejection
What does an increased preload have an effect on left ventricular pressure-volume loop?
e.g. events that increase venous return such as venoconstriction during exercise or administration of intravenous fluids
-Increased end-diastolic volume, Increased Starling’s law, Increased SV
What does a decreased preload have an effect on left ventricular pressure-volume loop?
Events causing a loss of blood volume, e.g. haemorrhage, dehydration
-Decrease in EDV, decreasing in Starling’s law, decreased SV
What does an increased afterload have an effect on left ventricular pressure-volume loop?
e.g. chronic hypertension
-Increased isovolumetric contraction to overcome greater aortic pressure and
open aortic valves for ejection, Less energy left for ejection, reduced SV
