Control of Cardiac Output Flashcards
What is cardiac output?
Cardiac output (CO)- the amount of blood ejected from the heart per minute
How is cardiac output calculated? What influences cardiac output?
How often the heart beats per minute (heart rate, HR) and how much blood (ml) is ejected per beat (stroke volume, SV)
CO= HR x SV
Cardiac output changes according to demand:
Rest 70bpm x 70ml = approx. 5litres/min
Exercise 180bpm x 120ml= 22litres/min
Cardiac output affects blood pressure and blood flow
Blood flow (CO)= BP/TPR or BP= CO x TPR
(BP is blood pressure) (TPR is total peripheral resistance)
What are preload and afterload?
Preload and afterload are important in stroke volume
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 Starling’s law?
Energy of contraction of cardiac muscle is relative to the muscle fibre length at rest
Greater stretch of ventricle in diastole (blood entering)
…then greater energy of contraction
…and greater stroke volume achieved in systole
More blood in= more blood out
Intrinsic property of cardiac muscle (nerves, hormones etc. not involved)
How does the molecular basis of preload apply according to Starling’s law?
Un-stretched fibre Overlapping actin/myosin Mechanical inference Less cross-bridge formation available for contraction Stretched fibre Less overlapping actin/myosin Less mechanical inference Potential for more cross-bridge formation Increased sensitivity to Ca2+ ions
What are the roles and effect of Starling’s law in regards to preload?
Balances outputs of the right ventricle and left ventricle- important
Responsible for fall in cardiac output during a drop in blood volume or vasodilation (e.g. haemorrhage, sepsis)
Restores cardiac output in response to intravenous fluid transfusions
Responsible for fall in cardiac output during orthostasis (standing for a long time) leading to postural hypotension and dizziness as blood pools in legs
Contributes to increased stroke volume and cardiac output during upright exercise
How does Laplace’s law apply to afterload?
Afterload opposes the contraction that ejects blood from the heart and is determines by wall stress directed through the heart wall.
Stress through the wall of the heart prevents muscle contraction
More energy of contraction is needed to overcome this wall stress to produce cell shortening and ejection
Laplace’s law describes parameters that determine afterload:
Wall tension (T), pressure (P), and radius (r ) in a chamber (ventricle)
T is proportional to P r
Wall stress (S) is made up of tension (T) over wall thickness (w)
S= T/w so… S= Pr/2w divided by 2 because a chamber has 2 directions of curvature
Afterload is increased by increasing pressure and radius and reduced by increasing wall thickness
How does radius affect wall stress/afterload?
Small ventricle radius: Greater wall curvature More wall stress directed towards centre of chamber Less afterload Better ejection Larger ventricle radius: Less wall curvature More wall stress directed through heart wall More afterload Less ejection Huge theoretical radius: Negligible wall curvature Virtually all stress directed through wall S= P r/2w
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
How does arterial blood pressure change acutely and chronically?
Laplace’s law states that increased blood pressure (P) will increase wall stress which will increase afterload and reduce ejection.
Acute rises in blood pressure offset by…
Starling’s law- increased stretch give increased contraction and increased stroke volume
Local positive inotropes (noradrenaline)
Baroreflex- decreased sympathetic tone which decreases blood pressure
Chronic increase in arterial blood pressure…
Increased energy expenditure attempts to maintain stroke volume but ultimately stroke volume will gradually decrease
Decrease in blood pressure would increase efficiency of the heart
This is why blood pressure needs to be kept fairly constant during exercise, a high blood pressure will reduce cardiac output.
How does Laplace’s law apply to hypertrophy in heart failure?
Increased radius (r ) e.g. heart failure where heart does not contract properly (myocardial infarction, cardiomyopathies, mitral valve regurgitation) blood left in ventricle leading to eventual volume overload
Increased (P) e.g. pressure-overload heart failure due to increased pressure/afterload in chamber (hypertension, aortic stenosis)
Increases in either radius or pressure will increase wall stress (afterload) which opposes ejection
The heart compensates with ventricular hypertrophy (greater myocyte size and more sarcomeres), increasing wall thickness. This decreases wall stress per sarcomere and therefore afterload so maintains SV and CO.
But this requires more energy (more sarcomeres used) – greater O2. The amount of energy required continues to increase so ultimately contractility will decrease and produce more heart failure…it is a vicious circle.
What is energy of contraction and what does it depend on?
Energy of contraction is the amount of work required to generate stroke volume
Depends on Starling’s law and contractility
Stroke work carries out two functions:
1. Contracts until chamber pressure > aortic pressure (isovolumetric contraction)
2. Ejection from ventricle
Preload increase the stroke volume and afterload opposes the stroke volume
Higher energy used in isovolumetric contraction reduces energy available for ejection
