Regulation of Cardiac Output (B 2: W 2) Flashcards Preview

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How do you calculate cardiac output?

CO = Mean arterial pressure/Peripheral resistance


CO = Stroke Volume (SV) x Heart Rate (HR)


Why does cardiac output decrease with an increase in afterload?

Afterload is so hight that ejection of blood is dramatically reduced due to an increase in the afterload until the heart is no longer able to generate pressure to overcome the afterload


How do you calculate cardiac index?

CI = Cardiac Output/Body Surface


Why do you want to increase cardiac output?

We need to consume more oxygen in exercise

Cardiac index increases as you increase work during exercise - linear relationship


What is the relationship between cardiac index and age?

Cardiac index increases with age in years 

Peaks when you are a teenager, decreases after that


What is the effect of atrial pressure on cardiac output?

Increased mean atrial pressure causes an increase in filling of the ventricles

More contraction - higher cardiac output 


What is the effect of intrapleural pressure on cardiac function?

Inspiration shifts cardiac function curve to the left

  • By inspiring, cardiac output increases 
    • Lower atrial pressure allows for greater venous return
  • By expiring, cardiac output decreases
    • Higher atrial pressure leads to less venous return 


What effect does sympathetic ANS stimulation have on cardiac output? 

Increase in sympathetic activity causes 

  • More force of contraction
  • Elevated end systolic pressure
  • Increased cardiac output 


What is the effect of parasympathetic ANS stimulation on the heart?

Parasympathetic stimulation is dominant during rest

Equal to zero sympathetic stimulation


What is the effect of heart rate on cardiac output?

The curve is bell-shaped

  • Cardiac output increases until ~170 bpm
    • At constant stroke volume, more beats per minute pump more blood 
  • Cardiac output decreases at much higher heart rate
    • Not enough filling time
    • Decreased stroke volume


What is the relationship between venous return and atrial pressure?

  • Venous return is high at low pressures below 0
  • As you increase atrial pressure, there is a transitional zone and a down slope
  • There is a point of zero venous return = mean systemic filling pressure 


What is the reason for the plateau on the venous return curve?

There is resistance to flow

Partial collapse of the veins due to excessive negative pressure


What happns to the venous return curve when you chage the mean systemic filling pressure?

  • An increase in fluid elevates the vascular function curve, allowing for more venous return to the heart 
    • Increases cardiac output by increasing preload
  • A decrease in blood volume leads to a lower venous return
    • Heart is not pumping as well (e.g. dehydrated patient)



How do changes in peripheral resistance affect the venous return?

A decrease in resistance leads to an elevated venous return

Increase resistance - lower venous return

Mean systemic filling pressure is the same 


What happens to venous return when you alter resistance and mean filling pressure?

Changing blood volume and resistance leads to vascular function curves with different slopes

Best case: lower resistance, increase volume --> increase cardiac output

Cardiac output must match venous return!


What is the relationship between cardiac output and venous return?

Increase cardiac output - increase venous return 


What represents the steady state on this graph?

Intersection between two curves is the steady state point


What led to an increase in the steady state from point A to point B?

Increased mean systemic filling pressure - increased cardiac output and venous return 


How do changes in cardiac contractility alter venous return?

Increased contractility - increased venous retun

Decreased contractility - decreased venous return


How do changes in compliance (stretch) impact end-diastolic pressure?

  • Decreased compliance (increased mass)
    • End diastolic pressure increases 
    • End diastolic volume decreases 
  • Increased compliance (late stage CHF)
    • End diastolic pressure decreases
    • Large resting volume (EDV)


How does a decrease in compliance affect stroke volume?

Decreases stroke volume

Leads to a smaller pressure-volume loop


What factors influence stroke volume?

  • Diastolic filling
  • Systolic ejection


What influnces diastolic filling?

  • Filling pressure
  • Ventricular compliance
  • Filling time


What influences systolic ejection?

  • Arterial pressure
  • Myocardial contractility 


What is the direct method for measuring cardiac output?

Flow meter in vascular system

Use echo or Doppler ultrasound 


What is the oxygen method for indirectly measuring cardiac output?

Measure the amount of oxygen at a steady state


CO = O2 uptake by lungs/(Arterial O2 - Venous O2)


What is the indicator (cardio-green) dilution method for indirectly measuring cardiac ouptut?

Inject fixed amount of fluorescent dye, measure fluorescence over time


CO = mg of dye/(Average dy concentration x duration of first passage)