Preload + afterload Flashcards
Define CO
v of blood ejected from heart per minute
Equations w CO?
CO = HR x SV BP = CO x TPR
What’s resting CO?
70 bpm x 70 ml = 5 L/min
What’s exercising CO?
180 bpm x 120 ml = 22 L/min
What does CO determine?
BP + blood flow
What controls SV?
preload, HR, contractility, afterload
How does preload control SV?
Stretching of heart at rest, increases SV due to Starling’s law
How does HR control SV?
Sympathetic + parasympathetic nerves
How does contractility control SV?
Strength of contraction at given resting load, due to sympathetic nerves + A increasing [Ca2+ ]
How does afterload control SV?
Opposes ejection, reduces SV due to Laplaces law
Define energy of contraction
amount of work required to generate SV
What does energy of contraction depend on?
Starling’s Law + Contractility
Role of stroke work?
Increases chamber p > aorta (isovolumetric contraction)
Ejection
What’s Starling’s law?
‘Energy of contraction of cardiac muscle proportional to muscle fibre length at rest’
ie more stretch at rest –> greater contraction
What’s the intrinsic property of heart?
can increase strength of cardiac contraction
What happens if ↑CVP?
↑venous return to heart
↑EDV (end diastolic v)
↑preload
↑ejection
Features of Starling’s curve?
filling p mmHg (CVP or end distole p) vs SV (ml)
ascending limb - ↑filling p –> ↑SV
plateau - ↑filling p but no more work
Why do a fluid challenge?
give fluid, see if makes diff to CO + BP, see if they’re on ascending limb
Why does stretching increase energy of contraction?
stretched fibre has less overlapping actin/myosi
less mechanical inference
potential for more cross-bridge formation
↑ sensitivity to Ca2+
Roles of Starling’s law?
- Balances outputs of RV + LV
- ↓CO after drop in blood v (eg haemorrhage, sepsis)
- ↓CO during orthostasis (standing)–>postural hypotension 😵
- Restores CO in response to intravenous fluid transfusions
- ↑ CO during exercise
What’s afterload determined by?
Wall Stress directed via heart wall
What does wall stress do?
prevent actin-myosin
What does Laplaces law describe?
Wall Tension (T), Pressure (P), Radius (r) in ventricle P = 2T / r Tension (T) = Wall Stress (S) and Wall Thickness (w) so: P = 2Sw / r or S = P x r / 2w
Why value of 2?
chamber has 2 directions of curvature
Why does a smaller ventricle radius give better ejection?
greater wall curvature
Wall Stress towards centre of chamber not heart wall
less afterload
Why does a larger ventricle radius give better ejection?
less wall curvature
more Wall Stress directed via heart wall
more Afterload
Roles of Laplace’s law?
Opposes Starling’s law at rest
Facilitates ejection during contraction
Contributes to failing heart at rest + during contraction
How does Laplace’s oppose Starling’s law at rest?
↑ Pre-load –> ↑ chamber radius
Laplace’s says it will increase afterload +opposes ejection from full chamber
BUT Starling’s > Laplace’s
How does Laplace’s law facilitate ejection during contraction?
Ventricular contraction ↓chamber radius
Laplace’s says it will reduce afterload in emptying chamber so helps ejection during reduced ventricular ejection phase 4 of cardiac cycle :)
How does Laplace’s law contributes to failing heart at rest + during contraction?
failing heart has dilated chambers so ↑ radius
Laplace’s law says increase afterload opposing ejection
What does Laplace’s law say if there’s increased arterial BP?
↑ Wall Stress –> ↑ Afterload –> ↓ ejection
What happens if there’s an acute rise in arterial BP?
Starling’s law - ↑ stretch, ↑ contraction, ↑ SV
Intrinsic increase in contractility - Anrep response, local +ve inotropes
Baroreflex -↓ Sym NS, ↓ TPR, ↓ BP
How does Laplace’s law explain hypertrophy in heart failure?
S = P x r / 2w
↑r - Volume-overload (MI, cardiomyopathies, mitral valve re-gurgitation)
↑P - Pressure-overload (hypertension, aortic stenosis)
↑ afterload –> ↓ejection so heart compensates by increasing Wall Thickness (w) so:
↓ afterload –> ↑ SV/CO
same wall stress over greater area, less wall stress per sarcomere, less opposition to contraction of sarcomeres, greater O2 use, less contractility…
What’s the effect of increased afterload on ventricular p-v loop?
-↑ BP (Afterload)
-↑ isovolumetric contraction using more energy
-less energy for ejection so↓ SV
More energy used to eject less blood
What’s the effect of increased preload on ventricular p-v loop?
- ↑ venous return (exercise,intravenous fluids)
- ↑ end-diastolic v
- ↑ Starling’s law
- ↑ SV for little increase in energy used
