Regulation of Cardiac Output (B 2: W 2)

Question 1

How do you calculate cardiac output?

Answer 1

CO = Mean arterial pressure/Peripheral resistance

OR

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

Question 2

Why does cardiac output decrease with an increase in afterload?

Answer 2

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

Question 3

How do you calculate cardiac index?

Answer 3

CI = Cardiac Output/Body Surface

Question 4

Why do you want to increase cardiac output?

Answer 4

We need to consume more oxygen in exercise

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

Question 5

What is the relationship between cardiac index and age?

Answer 5

Cardiac index increases with age in years

Peaks when you are a teenager, decreases after that

Question 6

What is the effect of atrial pressure on cardiac output?

Answer 6

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

More contraction - higher cardiac output

Question 7

What is the effect of intrapleural pressure on cardiac function?

Answer 7

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

Question 8

What effect does sympathetic ANS stimulation have on cardiac output?

Answer 8

Increase in sympathetic activity causes

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

Question 9

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

Answer 9

Parasympathetic stimulation is dominant during rest

Equal to zero sympathetic stimulation

Question 10

What is the effect of heart rate on cardiac output?

Answer 10

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

Question 11

What is the relationship between venous return and atrial pressure?

Answer 11

• 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

Question 12

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

Answer 12

There is resistance to flow

Partial collapse of the veins due to excessive negative pressure

Question 13

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

Answer 13

• 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)

Question 14

How do changes in peripheral resistance affect the venous return?

Answer 14

A decrease in resistance leads to an elevated venous return

Increase resistance - lower venous return

Mean systemic filling pressure is the same

Question 15

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

Answer 15

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!

Question 16

What is the relationship between cardiac output and venous return?

Answer 16

Increase cardiac output - increase venous return

Question 17

What represents the steady state on this graph?

Answer 17

Intersection between two curves is the steady state point

Question 18

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

Answer 18

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

Question 19

How do changes in cardiac contractility alter venous return?

Answer 19

Increased contractility - increased venous retun

Decreased contractility - decreased venous return

Question 20

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

Answer 20

• 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)

Question 21

How does a decrease in compliance affect stroke volume?

Answer 21

Decreases stroke volume

Leads to a smaller pressure-volume loop

Question 22

What factors influence stroke volume?

Answer 22

• Diastolic filling
• Systolic ejection

Question 23

What influnces diastolic filling?

Answer 23

• Filling pressure
• Ventricular compliance
• Filling time

Question 24

What influences systolic ejection?

Answer 24

• Arterial pressure
• Myocardial contractility

Question 25

What is the direct method for measuring cardiac output?

Answer 25

Flow meter in vascular system

Use echo or Doppler ultrasound

Question 26

What is the oxygen method for indirectly measuring cardiac output?

Answer 26

Measure the amount of oxygen at a steady state

Calculate:

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

Question 27

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

Answer 27

Inject fixed amount of fluorescent dye, measure fluorescence over time

Calculate:

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