Cardiac Function

Question 1

How to calculate cardiac output (and units)

Answer 1

Cardiac output = Stroke volume (litres/beat) x heart rate (beat/min)

Litres/min

Question 2

Cardiac output, stroke volume, heart rate at rest

Answer 2

5 L/min; 70ml; 70bpm

Question 3

Effect of sympathetic nervous system on heart rate

Answer 3

Activation of B1-adrenoceptors causes an increase in heart rate

Question 4

Effect of parasympathetic nervous system on heart rate

Answer 4

Activation of muscarinic receptors causes a decrease in heart rate

Question 5

Effect of hormones on heart rate

Answer 5

Adrenaline acting on beta-adrenoceptors causing an increase in heart rate

Question 6

Effect of intra/extracellular ions on heart rate

Answer 6

Alterations in membrane potential - e.g. K+

Question 7

Stroke Volume

Answer 7

Volume of blood ejected by Left ventricle in a single heart beat

Question 8

End Diastolic Volume vs End Systolic Volume

Answer 8

The volume of blood in the heart during the respective phases of contraction

Stroke volume is 70ml; difference

**note that ESV is not 0

Question 9

Factors affecting stroke volume

Answer 9

Pre-load (If more blood enters, more will leave)
Cardiac Contractility (ability of heart to contract)
After-load (pressure against which blood is pushed)

Question 10

Pre-load and factors influencing it

Answer 10

Amount of blood is returned to the heart during a cardiac cycle

Primarily influenced by venous return

Also affected by filling time & atrial contractility

Question 11

Why does an increase in LV volume cause an increase in stroke volume (mechanically)

Answer 11

If there is more stretch on cardiac muscle fibres, their contraction will have more force and can push out more blood (length-tension relationship)

Question 12

Cardiac contractility

Answer 12

The amount of force generated by cardiac muscle fibers. It can vary independently from the degree of stretch i.e. ventricular contractility can vary at any given end-diastolic volume depending upon other influences

Question 13

Factors cause increases in stroke volume by affecting cardiac contractility

Answer 13

Increased SNS activity
Calcium
Positive inotropic drugs
e.g. digoxin

Question 14

Factors cause decreases in stroke volume by affecting cardiac contractility

Answer 14

Reduced SNS activity
Hypoxia
Acidosis
Heart failure!!

Question 15

Afterload

Answer 15

Amount of pressure that the heart needs to exert to eject the blood during ventricular contraction

An increase in aortic pressure will increase afterload as more pressure needs to build up in the LV to allow valve opening

Question 16

What does an increase in afterload correspond to in terms of stroke volume at constant pre-load

Answer 16

An increase in afterload would reduce stroke volume at constant preload

Question 17

Discuss the changes that occur on LV volume against LV pressure during a contraction

Answer 17

Question 18

Effect of increased afterload on stroke volume and ESV

Answer 18

Increased afterload decreases SV by increasing ESV

Question 19

Effect of increased preload on stroke volume and ESV

Answer 19

Increased SV but same ESV

Question 20

Effect of increased cardiac contractility on stroke volume and ESV

Answer 20

Increased SV and decreased ESV