Lecture 14: Contractility and Cardiac Output

Question 1

What is cardiac output?

Answer 1

Amount of blood ejected by each ventricle in one minute

Question 2

How do you calculate cardiac output?

Answer 2

Cardiac Output = Heart Rate x Stroke Volume

Question 3

What are the four factors that can affect cardiac output?

Answer 3

Heart Rate
Myocardial Contractility
Preload
Afterload

Question 4

What is preload?

Answer 4

Easy definition: Amount of blood in ventricles before start of contraction
Proper definition: Amount of stretch in muscle fibers of heart at beginning of left ventricular systole

Question 5

Describe the Frank-Starling relationship.

Answer 5

Volume of blood ejected by ventricle depends on how much blood was in left ventricle at end of diastole

Question 6

What is afterload?

Answer 6

Resistance ventricles must overcome to eject blood to open aortic/pulmonary valve

Question 7

If there is a greater amount of afterload, what happens to the contraction?

Answer 7

Contraction is slower

-more force needs to be generated and length tension curve is not ideal

Question 8

What is contractility?

Answer 8

How hard myocardium contracts for a given preload

Question 9

What is contractility generally proportional to?

Answer 9

Amount of calcium available to troponin on actin filaments of contractile apparatus

Question 10

If heart rate is increased, what happens to contractility?

Answer 10

Increase in contractility (inotropic effect)

-more calcium enters cell and taken up by SR

Question 11

How do cardiac glycosides work?

Answer 11

Goal: Increase sarcoplasmic Ca2+ concentration inside cells to boost muscle contraction and treat heart failure

Cardiotonic drugs inhibit Na+/K+ pump (more Na+ inside cell). This inhibits Na+/Ca2+ exchanger, causing increased sarcoplasmic Ca2+ concentration

Question 12

How do you calculate stroke volume?

Answer 12

Stroke Volume = End Diastolic Volume - End Systolic Volume

*should be around 70 mL

Question 13

How do you calculate ejection fraction?

Answer 13

Ejection Fraction % = Stroke Volume/End Diastolic Volume

Question 14

If preload increases, what happens to cardiac output and contractility?

Answer 14

Both increase

Question 15

If afterload increases, what happens to cardiac output and contractility?

Answer 15

Cardiac output decreases. Therefore, heart rate and contractility must increase to compensate

Question 16

Describe what happens with a sympathetic, positive inotropic effect?

Answer 16

Contractility and Cardiac Output increase

• β adrenergic activation
• phosphorylation of SR calcium channels
• phosphorylation of (inhibitory) troponin I

Question 17

Describe what happens with a parasympathetic, negative inotropic effect?

Answer 17

Atria only affected

-less calcium enters the cell and more potassium leaves the cell

Question 18

Describe what is going on with the ventricular pressure volume loop.

Answer 18

1. Left Ventricle is filling with blood (minimal change in pressure)
2. Mitral Valve closes and pressure builds (no chance in volume)
3. Aortic valve opens and pressure continues to increase but volume starts to go down because of ejection.
4. Aortic valve closes and left ventricle relaxes. Pressure decreases but there is no change in volume.

Question 19

What is isovolumetric contraction?

Answer 19

When volume stays the same, but pressure increases

Question 20

What is isovolumetric relaxation?

Answer 20

When volume stays the same, but pressure decreases

Question 21

What changes in the ventricular pressure volume loop if there is an increased preload?

Answer 21

Left ventricular volume increases but pressure levels stay same

Question 22

What compensation occurs when there is an increased preload?

Answer 22

Increased stroke volume

Increased force of contraction

Question 23

What changes in the ventricular pressure volume loop if there is an increased afterload?

Answer 23

Left ventricular pressure increases

Starting volume is increased (not as much blood is ejected)

Question 24

What compensation occurs when there is an increased afterload?

Answer 24

Reduced stroke volume

Reduced ejection fraction

Question 25

In what conditions do we see an increased afterload?

Answer 25

Aortic stenosis

Hypertension

Question 26

What changes in the ventricular pressure volume loop if there is increased contractility?

Answer 26

Left ventricular pressure increases

Starting volume is decreased (more blood is ejected)

Question 27

What compensation occurs when there is increased contractility?

Answer 27

Increased stroke volume

Increased ejection fraction