Lecture 55 - The Cardiac Cycle

Question 1

What prevents the atria and the ventricles from contracting at the same time?
What is this made of?

Answer 1

The fibrous skeleton of the heart

It is made of fibrous connective tissue

Question 2

Compare the pressure in the two sides of the heart

Answer 2

Right side: low pressure

Left side: high pressure

Question 3

Compare the right and left ventricles’ structure

Answer 3

Right: thin muscular wall
Left: thick muscular wall

Question 4

Compare the function of the right and left ventricles

Answer 4

Right: pumps deoxygenated blood to the lungs
Left: pumps oxygenated blood to the body

Question 5

Which circulation has greater resistance?

Answer 5

Systemic has greater resistance than pulmonary

Question 6

What generates the force in the heart?

Answer 6

The contraction of the muscular walls of the heart

Question 7

What are the stages of ventricular systole?

Answer 7

Isovolumetric contraction

Ventricular ejection

Question 8

What are the stages of ventricular diastole?

Answer 8

Isovolumetric relaxation

Ventricular filling

Question 9

Describe the mechanical events of the cardiac cycle

Answer 9

1. Late diastole: chambers filling passively
2. Atrial systole: blood forced into ventricles
3. Isovolumetric ventricular contraction
4. Ventricular ejection
5. Isovolumetric ventricular relaxation

Question 10

Describe the electrical events of the cardiac cycle

Answer 10

1. SA node depolarisation
2. P wave: atrial depolarisation
3. P-Q segment: conduction through AV node and bundle
4. Q wave: depolarisation through bundles of His
5. R wave: depolarisation at apex of heart
6. S wave: depolarisation through purkinje fibres through ventricle
7. S-T segment: ventricular contraction
8. T wave: ventricular repolarisation

Question 11

Describe what is happening at the SA node

Answer 11

Autorhythmic

70 bpm

Question 12

What controlls the path of conduction through the heart?

Answer 12

Insulation

Fibrous skeleton

Question 13

How is depolarisation conducted from the SA to the AV node?

Answer 13

Internodal pathways

Question 14

Compare depolarisation rates throughout the heart

Answer 14

Quick:
• along internodal pathways
• bundles of His

Slow:
• through atria

Question 15

Why would it be pointless to have sustained contraction of the heart?

How is this overcome?

Answer 15

This would not allow the heart to function properly, as the principal function of the heart is to pump blood. Sustained contraction would not allow the heart to refill

The refractory period ensures that the heart can refill between beats.

Question 16

Describe how an ECG is recorded

Answer 16

3 leads placed on the body
Einhoven’s Triangle

Leads detect the direction of flow of electric current.

Question 17

Describe what we see on an ECG recording.

Draw this

Answer 17

Slide 17

P wave
QRS complex
T wave

Question 18

What does the P wave represent?

Answer 18

Atrial depolarisation

Question 19

What does the Q wave represent?

Answer 19

Depolarisation of the Bundles of His

Question 20

What does the R wave represent?

Answer 20

Spread of depolarisation around the apex of the heart

Question 21

What does the S wave represent?

Answer 21

Spread of depolarisation up through the ventricles

Question 22

What does the QRS complex represent as a whole?

Answer 22

Ventricular depolarisation

Question 23

What does the T wave represent?

Answer 23

Ventricular repolarisation

Question 24

What does a reading above the baseline on an ECG recording indicate?

Answer 24

Net movement of charge towards the positive electrode

Question 25

What do we look for in a normal ECG?

Answer 25

* Is the rate regular? * Is rhythm regular * Are the waves regular? * One QRS compex per P wave? * Is P-R segment regular in length?

Question 26

What is regular heart rate?

Answer 26

60-100 bpm

Question 27

What is atrial fibrillation?

Answer 27

Erratic atrial activity; not contracting in a coordinated manner. Depolarisation does not reach the ventricle each time

Question 28

What is ventricular fibrillation?

Answer 28

Erratic electrical activity in the ventricle Ventricle beating on its own, under the ventricular pacemakers

Question 29

What causes ventricular fibrillation?

Answer 29

Damaged muscle of the heart

Question 30

How can ventricular fibrillation be overcome?

Answer 30

To restore the normal rhythm, the heart must be in the refractory period. This doesn't happen during ventricular fibrillation. De-fibrillator give a high voltage shock to synchronise all the muscle

Question 31

What causes the valves of the heart to open an close?

Answer 31

No muscle - it is a passive process. The pressure changes cause the valves to open and close

Question 32

When do the AV valves open?

Answer 32

When pressure in the atria is greater than in the ventricles

Question 33

When do the AV valves close?

Answer 33

When pressure in the ventricles is greater than in the atria

Question 34

When do the semilunar valves open?

Answer 34

When pressure in the ventricles is greater than in the vessel (aorta, pulmonary trunk)

Question 35

When do the semilunar valves close?

Answer 35

When pressure in the ventricles is less than in the vessels (aorta, pulmonary trunk)

Question 36

Which side of the heart has the tricupsid valve?

Answer 36

The right side of the heart

Question 37

What is happening during isovolumetric ventricular contraction?

Answer 37

The muscle of the ventricles is contracting However, the valves aren't open. The pressure in the ventricles is rising up to 120 mmHg

Question 38

Why does blood flow into the ventricles?

Answer 38

Blood has been ejected from the ventricles, and the muscle of the ventricles in relaxting, so the pressure in the ventricles drops very low. The pressure in the atria is higher than in the ventricles, so blood flows into the ventricles

Question 39

What happens in rheumatic heart disease?

Answer 39

AV valves aren't closing fully during ventricular contraction Blood flowing back into the atria Turbulent flow, engendering a heart murmur

Question 40

What are heart sounds due to? | What are they?

Answer 40

Closure of valves Sound 1: AV valves closing Sound 2: Semilunar valves closing

Question 41

Does blood flow cause heart sounds?

Answer 41

Laminar flow (ie normal blood flow) does not bring about heart sounds Turbulent flow through a leaky valve will bring about a murmur

Question 42

At which point in the ECG will the AV valves close?

Answer 42

QRS complex | ie ventricular contraction

Question 43

At which point during the ECG will semilunar valves close?

Answer 43

T wave

Question 44

What things does the Wigger's diagram show us?

Answer 44

``` 1/ ECG 2/ Pressure in: • aorta • left ventricle • left atrium 3/ Heart sounds 4/ Left ventricular volume ```