3. Determinants fo Cardiac Output

Question 1

List the determinants of cardiac output.

Answer 1

• VENOUS RETURN
• Preload
• Afterload
• Contractility
• HR

Question 2

Energetically, are HR and SV equivalent to increase cardiac output? Explain.

Answer 2

No. SV primarily drives cardiac output b/t it’s linked to venous return

Question 3

Equation for work done by heart

Answer 3

Work = force x distance

• force = directly proportional to pressurization of blood
• distance = ejection distance (how far fluid is squeezed)

Question 4

Define external work.

Answer 4

Movement of blood from the ventricles to the great vessels (stroke work)

Question 5

Equation for stroke work

Answer 5

Stroke work = arterial pressure x SV

Question 6

Define internal work.

Answer 6

• Work done against structural elements in the heart
• Mostly dissipated as heat

Question 7

What happens to internal and external work when you increase SV? Explain.

Answer 7

Increase SV –> filling more or empying more completely –> maximizes external work (based on stroke work equation) and minimizes internal work

Question 8

For a given SV, which is more energetically costly: an increase in volume or pressure? Why?

Answer 8

PRESSURE.

High BP –> heart has to keep squeezing during isovolumetric contraction –> additional work done on blood that pressurizes it to vessel-damaging pressures –> heart can’t contract as far –> lost stroke volume

Question 9

What happens to the external and internal work when you increase volume load?

Answer 9

External work increases more than internal work

Question 10

Define preload.

Answer 10

• Ventricular filling
• Central venous pressure

Question 11

Define afterload.

Answer 11

Increased aortic pressure

Question 12

What is the effect of preload on SV?

Answer 12

Increased SV

Question 13

What is the effect of afterload on SV?

Answer 13

Decreased SV

Question 14

List the 2 venous compartments.

Answer 14

• Central venous compartment
• Peripheral venous compartment

Question 15

Central vs. Peripheral venous compartment

Answer 15

CENTRAL:

• provides preload for ventricular filling
• always less than 1L

PERIPHERAL:

• 3.5-4L sitting in peripheral veins
• 4x as large as central venous compartment at rest

Question 16

What is the significance of veins’ high compliance?

Answer 16

Can have large changes in volume w/ little change in pressure

Question 17

Where is the majority of the blood volume located? What % of blood volume is this?

Answer 17

60% of blood volume is peripheralized in venous compartment

Question 18

Equation for flow of venous return.

Answer 18

Question 19

What pressure gradient b/t the venous compartments favors venous return?

Answer 19

• High peripheral venous pressure
• Low central venous pressure

Question 20

How does muscular contraction enhance venous return?

Answer 20

Contract a muscle —> presses on veins —> no backflow distally b/c of closed valves —> only direction of flow is back to the heart

Question 21

How does the venous muscle pump affect the volume and pressure of blood it pumps?

Answer 21

• Amount of blood that gets pumped is proportional to how much muscle you activated
• Pressure at which it’s pumped out is proportional to how hard you contract that muscle

Question 22

What is the relation b/t ventricular volume and tension?

Answer 22

Pre-loaded resting cardiac muscle length affects the magnitude of developed tension

Question 23

At steady state, venous return equals __?

Answer 23

Cardiac output

Question 24

How does central venous pressure affect venous return? Cardiac output?

Answer 24

• Decrease venous return
• Increase cardiac output

Question 25

What does the equilibrium values of central venous pressure tell us?

Answer 25

• CVP sufficient to stretch the heart for cardiac output
• CVP low enough against driving pressure of venules to get same amount of drainage from venules

Question 26

How does afterload affect blood flow?

Answer 26

Increases amount of time isovolumetric contraction lasts –> impedes forward flow –> decreases SV

Question 27

How does preload affect blood flow?

Answer 27

Question 28

Define contractility.

Answer 28

How much force of contraction generated for a given unit of preload

Question 29

How does increased contractility affect the heart and blood flow?

Answer 29

For same length and ventricular filling:

• greater peak tension
• greater shortening
• greater SV

Question 30

What are the short and long term ways to increase contractility?

Answer 30

• Short term: catecholamine (norepinephrine) stimulation
• Long term: cardiac hypertrophy (increase myofibrils per myocyte)

Question 31

What changes w/ contractile state?

Answer 31

End Systolic Pressure Volume Relation (ESPVR)