Lecture 14: Contractility and Cardiac Output

Question 1

What factors affect cardiac output?

Answer 1

Heart rate
Contractility
Preload
afterload

Question 2

How is cardiac muscle contraction different from skeletal muscle contraction?

Answer 2

More responsive to adrenergic inputs and relies more heavily on Ca2+ since it allows heart to contract stronger

Question 3

How are cardiac glycosides used to treat heart failure?

Answer 3

Blocks the Na/K ATPase and Ca/Na exchanger to keep Na and Ca inside the cell for depolarization and stronger contraction

Question 4

What is the formula for cardiac output?

Answer 4

CO = HR x SV

Question 5

How does length tension (stretching) relate to contraction?

Answer 5

The higher the length tension the stronger the contraction

Question 6

What does preload mean? How is this measured?

Answer 6

How much blood is ready to be pumped into the system by the left ventricle (left ventricular end-diastolic volume). Measured by the tension (stretch) on the walls of the left ventricle

Question 7

What is the Frank Starling relationship?

Answer 7

The LVEDV determines the volume pumped by the left ventricle

Question 8

What is afterload?

Answer 8

Pressure needed to be overcome to open the valve and release the blood into the next vessel

Question 9

What are the equations for calculating these? What are the normal numbers?
Stroke volume
Ejection fraction
Cardiac output

Answer 9

SV (70 mL) = EDV (120 mL) - ESV (50 mL)
EF% (55%)= SV / EDV
CO (5 mL/min) = SV x HR

Question 10

If you increase preload, what happens to CO?

If you increase afterload, what happens to CO?

Answer 10

Increases (along with contractility)

Decreases (along with contractility)

Question 11

How does the heart overcome decreased CO?

Answer 11

Increasing contractility and HR

Question 12

How does HR affect contractility? What is this effect called?

Answer 12

Increased HR = increased Ca2+ stored in SR since there is no time to clear Ca2+ = stronger contraction.
Positive staircase effect

Question 13

What are the characteristics of the positive staircase effect?

Answer 13

Since there are more Ca2+ stored in cardiac myocyte, the next contraction is much stronger

Question 14

By which mechanism does activation of the sympathetic B-adrenergic receptors affect cardiac output?

Answer 14

Phosphorylation of Ca2+ channels, phospholamban and inhibitory troponin I produces a positive ionotropic effect

Question 15

How does activation of parasympathetic muscarinic receptors affect cardiac output? Which type of cardiac myocytes do they mostly affect?

Answer 15

• Decreases influx of Ca2+ and increases efflux of K+ via K+-Ach channels
• Atrial myocytes

Question 16

On the Ventricular pressure-volume loop, what happens at points 1-4? What represents the stroke volume?

Answer 16

1. Isovolumetric contraction (pressure rises rapidly)
   
   2. Ventricle overcomes the pressure and ejects the blood
   3. Isovolumetric relaxation
   4. Ventricular filling

Distance between the vertical lines

Question 17

What happens to the stroke volume (and thus the pressure-volume loop) when preload increases?

Answer 17

Increases EDV = increased stroke volume

Question 18

What happens to stroke volume and the ejection fraction of the afterload is increased? What medical conditions does this happen?

Answer 18

• Increased pressure needed to open the valve = decreased stroke volume and ejection fraction
• Aortic stenosis, HTN

Question 19

What happens to stroke volume and the ejection fraction if contractility is increased?

Answer 19

Increased SV and EF. Release more blood so less is left in the heart

Question 20

What does Volume work mean?
Pressure work?
Minute work?
Stroke work?

Answer 20

Cardiac output
Aortic pressure
CO x aortic pressure
SV x aortic pressure

Question 21

Why is increased aortic pressure (as in aortic stenosis or HTN) bad?

Answer 21

Higher pressure to overcome to pump the blood = decreases the stroke volume. Heart increases contractility and overworks itself to keep the stroke volume the same = hypertrophy of the heart

Question 22

What is the Fick Principle equation?

Answer 22

O2 consumption = CO x [O2] in PV - [O2] in PA

Question 23

What is the vascular function curve? What is the normal level?

Answer 23

Circulatory filling pressure when there is no blood flow (0 cardiac output)
+7-8 mmHg

Question 24

What happens to cardiac and vascular function curves if you increase or decrease contractility?

Answer 24

Cardiac function curve:
Increase: Increases CO (y axis) and decreases RAP (x axis)
Decrease: decreases CO (y axis) and increases RAP (x axis)

Vascular function curve: no effect

Question 25

What happens to cardiac and vascular function curves if you increase or decrease TPR (resistance?)

Answer 25

Cardiac function curve:
Increase: Decreases CO (y axis) and maintains RAP (x axis)
Decrease: Increases CO (y axis) and maintains RAP (x axis)

Vascular function curve:
Increase: Decreases CO (y axis) and maintains RAP (x axis)
Decrease: Increases CO (y axis) and maintains RAP (x axis)

Question 26

What happens to cardiac and vascular function curves if you increase or decrease blood volume?

Answer 26

Vascular function curve:
Increase: Increases CO (y axis) and increases RAP (x axis)
Decrease: decreases CO (y axis) and decreases RAP (x axis)

Cardiac function curve: no effect

Question 27

What happens during cardiac failure? What is increased and decreased?

Answer 27

Decreased inotropy and compliance
Increased blood volume and resistance

Body tries to increase the possible amount of blood to be pumped by holding on to fluids (like water) via kidneys - edema during CHF