Control of Cardiac Output Flashcards
As a recap, describe cardiac output, including the two equations involving it
Cardiac Output (CO) is the amount of blood ejected from the heart per minute. It’s proportional to how often the heart beats per minute (heart rate, HR) and how much blood is ejected per beat (stroke volume, SV). CO affects blood pressure and blood flow
CO = HR x SV
BP = CO x TPR (total peripheral resistance)
Define preload and afterload, including the laws governing them
PRELOAD: stretching of heart at rest, increases stroke volume, due to Starling’s law
AFTERLOAD: opposes ejection, reduces stroke volume, due to Laplace’s Law.
What is heart rate & contractility dependent on
Strength of contraction due to sympathetic nerves and circulating adrenaline increasing intracellular calcium
What does Starling’s Law state?
It states that the “energy of contraction of cardiac muscle is relative to the muscle fibre length at rest”
This means that the greater the stretch of the ventricles in diastole (blood entering), the greater the energy of contraction and the greater the stroke volume achieved in systole.
More blood in = more blood out
Intrinsic property of cardiac muscle
Starling’s Law can be represented by a graph showing SV against CVP (central venous pressure)
Describe what the graph presents and what the last part of the graph means
Throughout the graph, as the CVP increases, the SV increases. However, at the end of the graph, with an increasing CVP, the SV starts falling. This indicates the point at which the Laplace’s Law takes over, so the heart pumps less blood out.
This emphasises the importance of care with fluid replacement, as giving too much may have an opposing effect.
Why does Starling’s Law work (in the sense of contractile fibres)?
With an unstretched fibre, there is overlapping actin and myosin. This means that there is mechanical interference, so there is less cross-bridge formation available for contraction.
With a stretched fibre, there is less overlapping actin and myosin. This means that there is less mechanical interference, so there is potential for more cross-bridge formation. There is also an increased sensitivity to Ca2+ ions.
List some roles of Starling’s Law in cardiac physiology
- Balances outputs of the right and left ventricle (which is important = isovolumetric)
- It restores CO in response to intravenous fluid transfusions
- It is responsible for the fall in CO during a drop in blood volume or vasodilation (eg. haemorrhage, sepsis)
- Responsible for the fall in cardiac output during orthostasis (standing for a long time), leading to postural hypotension and dizziness as blood pools in legs
- It contributes to increased SV and CO during upright exercise
Define and describe the implications of Laplace’s Law
Afterload opposes the ejection of blood from the heart and is determined by wall stress directed through the heart wall.
Stress through the heart wall prevents muscle contraction. More energy of contraction is needed to overcome this wall stress to produce cell shortening and blood ejection.
Laplace’s Law describes the parameters that determine afterload
- Afterload is increased by increasing pressure & radius and reduced by increasing wall thickness
What is the equation associated with heart wall stress?
This describes the relationship between, wall stress (S), pressure (P), radius (r), wall thickness (w) and wall tension (T).
P = 2T/r combined with T = Sw gives us:
S = Pr / 2w
Why does the radius determine wall stress/afterload?
With a SMALLER ventricle RADIUS, there is:
- Greater wall curvature
- More wall stress is directed to the centre of the chamber
- Less afterload
- Better ejection
With a LARGER ventricular RADIUS, there is:
- Less wall curvature
- More wall stress is directed to the heart wall
- More afterload
- Less ejection
With a HUGE theoretical radius
- Negligible wall curvature
- Virtually all stress directed through wall
What is the importance of Laplace’s law
Opposes Starling’s law at rest:
Increased preload gives increased stretch of chamber (Starling’s law)
This increases chamber radius (decreases curvature) – increasing afterload
In a healthy heart, Starling’s Law overcomes Laplace’s – so ejection is OK.
Facilitates ejection during contraction:
Contraction reduces chamber radius so less afterload as the chamber empties.
This aids expulsion of last portion of blood and increases stroke volume.
Contributes to a failing heart at rest and during contraction:
In a failing heart the chambers are often dilated and radius is large - so increased
afterload opposing ejection.
Laplace’s Law means good ejection with small radius, bad with large radius.
Describe the afterload & ventricular pressure-volume loop
Hypertension – increased afterload
There is a longer time spent in isovolumetric contraction to increase pressure in the chamber above that in the aorta to open the valve.
This uses more energy and lowers the force of contraction reducing stroke volume and an increasing end-systolic volume (ESV).
Describe the preload & ventricular pressure-volume loop
Exercise – Increased preload
During exercise increased venous return leads to increased preload and higher end diastolic volume (EDV).
The ventricle will eject blood to the same end systolic volume (ESV) so there is an increase in stroke volume, shown by an increase in the width of the PV loop
What is the energy of contraction
The amount of work required to generate stroke volume - it depends on starling’s law and contractility
Preload increases the stroke volume and afterload opposes the stroke volume
Higher energy used in isovolumetric contraction reduces energy available for ejection
Stroke work carries out which two functions
1.Contracts until chamber pressure > aortic pressure (isovolumetric contraction)
2.Ejection from ventricle
