Control Of Cardiac Output Flashcards
What is the definition of cardiac output (CO)? What is its normal value in a 70kg male?
The volume of blood pumped by each ventricle per minute (L/min)
OR
CO = SV x HR
SO SV (70ml) x HR (72bpm) = 5L/min typically
Define the ejection fraction (EF).
SV / EDV x 100
How much circulating blood is there in total and how much is in the heart and lungs?
Total: 5L
Heart & lungs: 1.5L
How much blood is in the heart when it is filled and empty?
Filled (EDV) = 140ml
Empty (ESV) = 50ml
Define stroke volume (SV). What is a typical value for a 70kg male?
The volume of blood ejected from each ventricle during each ventricular contraction
OR
SV = EDV - ESV
SO
EDV ~ 140ml - ESV ~ 50ml = SV = 90ml typically
What is End Systolic Volume (ESV)?
Filled volume of L ventricle prior to contraction + pushing of blood into the aorta
What is End Diastolic Volume (EDV)?
Residual volume of blood remaining in L ventricle after contraction + pushing of blood into aorta just before the L atrium empties into it
What is diastasis?
When volume of blood in the L ventricle is basically static just after the AV valves open
Define cardiac preload.
Initial stretching of cardiac myocytes prior to contraction i.e. volume load on L ventricle before contraction (directly measured via sarcomere length)
SO Higher EDV = higher preload
Define cardiac afterload.
Arterial pressure represents a ‘load’ OR resistance on the L ventricle
What factors influence stroke volume?
Preload 1: systemic venous return
Preload 2: R heart function
Preload 3: pulmonary vascular resistance
Preload 4: inlet i.e. mitral valve
Afterload 1: outlet i.e. aortic valve diameter
Afterload 2: arterial BP
Afterload 3: arteriolar tone
Contractility
What factors affect heart rate?
Venous return -> atrial reflex -> ANS innervation (e.g. SNS)
Venous return
Hormones
What factors affect preload and afterload?
Preload: venous return + filling time
Afterload: vasodilation or vasoconstriction
What factors affect contractility?
Hormones
ANS
What factors affect End Diastolic Volume (EDV) and End Systolic Volume (ESV)?
Contractility + afterload -> ESV
Preload -> EDV (+ ESV as it increases contractility)
When is filling time more rapid? Why?
First 1/2 of ventricular filling time as ventricle is more compliant
What effect does an increased heart rate (HR) have on cardiac output (CO)?
Reduced filling time impinges on early diastole so ventricular filling is reduced + CO is reduced (made worst by mitral stenosis obstruction/AF where atrial contraction removed)
What is the autonomic nervous system (ANS) impact on heart rate (HR)? Why?
PNS: vagus nerve decreases HR via hyperpolarization causing slower depolarization i.e. harder to reach AP threshold
SNS: sympathetic cardiac nerves increase HR + force of contraction via reduced repolarization causing faster depolarization i.e. easier to reach AP threshold
Where are the arterial baroreceptors in the body?
Aortic arch (afferent fibres of vagus nerve)
Carotid sinus R + L (afferent of glossopharyngeal nerve)
How does the baroreceptor reflex work?
- Increased BP stimulates baroreceptors increasing firing of glossopharyngeal + vagal afferent nerves
- Nerves go to cardiac + vasomotor centres in medulla of brain to SA node
- Increased efferent vagal nerve firing
- Decreased sympathetic nerve firing to veins, arteries + AV node
- Adrenal medulla releases NA + RAAS system
- BP decreased
How does the atrial receptor (Bainbridge) reflex work?
- Increased atrial pressure detected by atrial stretch receptors
- Increased vagal afferent nerve firing to cardiac + vasomotor centres in medulla of brain
- Decreased PS efferent firing to SA node
- Increased S efferent firing to SA node
- HR increased to deal with higher blood volume
How does the chemoreceptor reflex work?
- Increased pCO2 or decreased O2 stimulate chemoreceptors increasing firing of glossopharyngeal + vagal afferent nerves
- Nerves go to cardiac + vasomotor centres in medulla of brain
- Decreased efferent vagal nerve firing to SA node
- Increased sympathetic nerve firing to veins, arteries + AV node
- BP increased
Where are the chemoreceptors of the body?
Aortic arch (afferent fibres of vagus nerve)
Carotid sinus R + L (afferent of glossopharyngeal nerve)
Medulla of brain
Why is cardiac preload the major way in which stroke volume is increased?
Starling’s law: the more a muscle is stretched the more powerfully it will contract i.e. w/o other control mechanisms, heart’s intrinsic property results in what comes in will be pumped out response
What factors increase pre-load i.e. ventricular filling?
Increased aortic pressure which decreases HR
Increased atrial contractility
Increased ventricular compliance
Increased CVP
What factors increase CVP?
Decreased venous compliance
Increased thoracic venous BV: TBV, venous return (respiration, muscle contraction + gravity)
What is used as a surrogate for preload?
End-Diastolic Volume (EDV)
What happens to the stroke volume (SV) as End-Diastolic Volume (EDV) increases?
Increases + then drops
What will happen to the heart’s stroke volume (SV) in short-term and long-term increased pre-load?
Short-term: Increases SNS, contractility + HR
Long-term: contraction of heart muscle reduced due to myocardial injury or overload (increased pre-load or afterload) due to long-term adverse remodelling
What are the 2 reasons why preload may be increased in disease?
- Regional e.g. heart attack
2. Global e.g. dilated cardiomyopathy
What will to stroke volume (SV) if afterload is increased? Give an example of when this can occur.
The greater the load/resistance, the less muscle fibres can contract
E.G. hypertension
What can be used as a surrogate for afterload?
End Systolic Volume (ESV)
Define contractility.
Force of contraction of myocardium
What agents affect contractility?
Positive inotropic agents increase it
Negative inotropic agents decrease it
What factors affect total peripheral resistance?
Capacitance: vena cava, vein + venule
Exchange: capillaries
Resistance: small arteriole + small artery
Distribution: large artery + aorta
What is the main determinant of cardiac output (CO)?
Total venous return
What are indirect local control mechanisms of total venous return?
Local control in each organ system
Why is cardiac output controlled by indirect local control mechanisms?
As different organs need different amounts of blood when the body is doing different things e.g. rest + exercise
Where is most of the bodies blood being pumped to at rest?
Liver, kidneys + brain
Where is most of the bodies blood being pumped to during exercise?
Muscle
