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Flashcards in Control of Cardiac Output Deck (21)
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Define stroke volume 

How is it calculated?

The volume of blood ejected from each ventricle during each ventricular contraction 


EDV= end diastolic volume

ESV= end systolic volume 


Define cardiac output

How is it calculated?

The volume of blood pumped out of each ventricle per minute 

CO = HR x SV

Units are L/min 


Define ejection fraction

How is it calculated?

Fraction of the end diastolic volume that is pumped out during ventricular systole

EF = SV / EDV x 100



What affects left ventricular stroke volume?

Preload (amount of volume that enters the ventricle before each contraction- end diastolic volume): 

  • Preload 1: systemic venous return
  • Preload 2: right heart function
  • Preload 3: pulmonary vascular resistance
  • Preload 4: mitral valve (stenosis=reduced preload)

Left ventricular contractility

Afterload (pressure against which the ventricle is ejecting blood):

  • Afterload 1: aortic valve diameter
  • Afterload 2: arterial blood pressure 
  • Afterload 3: arteriolar tone (r4)


What factors affect cardiac output?

Heart rate

  • ANS
    • Atrial reflex (influenced by venous return)
  • Hormones

Stroke volume

  • End diastolic volume; influenced by:
    • Preload
      • Filling time (HR)
      • Venous return (into R and L atria)
  • End systolic volume; influenced by:
    • Preload
    • Contractility 
      • Hormones
      • ANS
    • Afterload
      • Vasodilation/constriction 


How does the preload increase contractility?


Starling's Law: the more a muscle is stretched, the harder it will contract. 


When is ventricular filling most rapid?

How is this affected by increased HR?

How does this affect cardiac output?

How does mitral valve stenosis affect this?

The first half of ventricular filling (the rapid filling phase) is faster as the ventricle is more compliant. 

Raised HR will reduce ventricular filling time having most impact on early diastole- the ventricle will have less time to refill and cardiac output will drop. 

  • Mitral valve stenosis will worsen this as it provides obstruction to blood returning to the left atrium & ventricle from the lungs. 
  • Atrial fibrillation also worsens this as it reduces atrial systole (further reducing ventricular filling)


How does atrial fibrillation affect ventricular filling?

Atrial fibrillation causes a loss of atrial contraction which is required to overcome the resistance to filling by the ventricles towards the end of ventricular diastole (during diastasis) when the ventricles are less compliant and passive filling slows. 

Loss of atrial contraction reduces ventricular filling therefore reduces preload (cardiac output falls) 


How does the autonomic nervous system impact HR?

Parasympathetic stimulation (via vagus nerve):

  • Slows depolarisation of cardiac muscle (hyperpolarisation)
    • Cells take longer to reach the threshold for action potential firing.

Sympathetic stimulation:

  • Speeds up depolarisation so the cardiac muscle cells reach threshold for action potential firing much faster. 
  • Also reduces repolarisation 


Describe the baroreceptor reflex

  1. Baroreceptors in the aortic arch and carotic sinus detect stretch caused by increased blood pressure. 
  2. Increase afferent firing via vagus nerve and glossopharyngeal nerve to cardiac vasomotor centres in the medulla.
  3. Efferents:
    • Increase parasympathetic stimulation via vagus nerve to heart (SA and AV node- increase delay of conduction)
    • Decrease sympathetic stimulation of heart.
    • Decrease sympathetic stimulation of blood vessels (reducing vasoconstriction).
    • Increases nervous input to adrenal medulla to increase noradrenaline release and RAAS system. 
  4. Blood pressure falls 


Describe the chemoreceptor reflex

  1. Increased pCO2 and/or decreased pO2 detected by chemoreceptors in the carotid body and aortic arch. 
  2. Increased firing of chemoreceptors via vagus and glossopharyngeal nerves to chemoreceptors in the medulla.
  3. Chemoreceptors communicate with medullary cardiac and vasomotor centres which:
  • Increase firing via sympathetic nerves to increase heart rate 
  • Decrease vagal stimulation
  • Increase sympathetic stimulation of blood vessels- vasoconstriction 



What is the main way that the stroke volume can be increased?

Which law can be applied here?

Through increasing preload

Starling's Law: the more a muscle is stretched, the more powerfully it will contract. 




What affects preload?

Ventricular filling, which is influenced by:

  • Heart rate
  • Ventricular compliance
  • Central venous pressure, which is influenced by:
    • Venous compliance
    • Thoracic venous blood volume
      • Total blood volume
      • Venous return
        • Respiration
        • Muscle contraction
        • Gravity
  • Atrial contractility


What is the effect of ventricular hypertrophy on preload?

Reduced preload as hypertrophy reduces compliance as muscles fibres do not stretch as much. 


What is Starling's Law?

What occurs with ventricular overload/myocardial injury?

The more a muscle is stretched, the harder it will contract. 

  • Therefore: increased end diastolic volume (preload) will lead to increased stroke volume. 

Ventricular overload/ myocardial injury (from increased preload or afterload) = long term adverse remodelling:

  • Ventricular hypertrophy and decreased compliance = decreased end diastolic volume → decreased stroke volume


What is the effect of increased afterload on ventricular contractility?

What is a clinical example of this?

Increased afterload causes a higher pressure and a higher resistance against which the ventricles contract.

This reduces the contractility.

Clinical example: hypertension → ventricular hypertrophy


Define Contractility

What does it affect?


The force of contraction of the myocardium

  • Directly controls stroke volume (impacting end systolic volume)
  • Positive inotropes increase contractility, negative inotropes decrease



What is the effect on reducing afterload on ejection fraction?

Ejection fraction will increase (therefore end systolic volume will decrease) if afterload decreases as there is less resistance against the contraction of the ventricle. 


Describe indirect local control 

How does this affect preload?

Local control in each organ system contributes part of venous return. 

Total venous return is a main determinant of cardiac output therefore each organ system can alter preload by altering its blood flow. 


What is the Frank-Starling curve?

Describes the relationship between increasing myocardial stretch results in increase in cardiac contracility and increased cardiac output. 


What are the 4 phases of diastole?

Isovolumic relaxation

Rapid filling

Slow filling (diastasis)

Atrial systole