CVS Session 7- The ECG and electrical activity of the heart

Question 0

What is the Electrocardiogram?

Answer 0

The electrical signal produced by the myocardium of the heart when electrical changes occur.
Is is detected by electrodes placed on the body surface.
It reflects the electrical activity of the heart.

Question 1

Which wave is absent in atrial fibrillation?

Answer 1

P wave

Question 2

What do electrodes record?

Answer 2

Changes in membrane potential

Question 3

What combination of effects explains the ECG?

Answer 3

• depolarisation and repolarisation

- their spread over the heart

Question 4

How many action potentials does one heartbeat produce?

Answer 4

One

Question 5

Describe conduction over the heart (5)

Answer 5

1. SA node fires action potential and depolarisation spreads over the atria to the AV node.
2. Delay of 120ms at the AV node.
3. Activity spreads down septum.
4. Spreads over ventricular myocardium from endo to epicardium.
5. Repolarisation occurs from epi to endocardium.

Question 6

Which way does depolarisation of ventricular myocytes occur?

Answer 6

From endocardium to epicardium

Question 7

Which way does repolarisation of ventricular myocytes occur?

Answer 7

From epi to endocardium

Question 8

What does the view of the electrode depend on?

Answer 8

It’s position relative to the spread of activity.

Question 9

Which way does the signal go if depolarisation is moving towards the electrode?

Answer 9

Upwards

Question 10

Which way does the signal go if depolarisation is moving away from the electrode?

Answer 10

Downwards

Question 11

Which way does the signal go if repolarisation is moving towards an electrode?

Answer 11

Downwards

Question 12

Which way does the signal go if repolarisation is moving away from the electrode?

Answer 12

Upwards

Question 13

What does the amplitude of the signal depend on? (2)

Answer 13

• how much heart muscle is depolarising

- how directly towards the electrode the excitation is moving

Question 14

Why is the area under the QRS wave and T wave the same?

Answer 14

Because it is the same heart muscle, producing the opposite action.

Question 15

What does each wave: P, Q, R, S and T represent?

Answer 15

P- atrial depolarisation
Q- septal depolarisation spreading to ventricles
R- main ventricular depolarisation 
S- end of depolarisation 
T- ventricular repolarisation

Question 16

Why does atrial repolarisation get lost in the QRS wave? (2)

Answer 16

• small amount of muscle therefore small signal

- happens at the same time as ventricular depolarisation

Question 17

What happens to the R wave if viewed end on?

Answer 17

Large downward signal

Question 18

How many inputs do amplifiers have?

Answer 18

2

One positive and one negative electrode.

Question 19

How do amplifiers work to turn two inputs into one?

Answer 19

Take the signal coming in on the negative electrode and invert it.
Then combine it with the positive input signal.
Then amplify the total.

Question 20

From which direction does lead I view the heart?

Answer 20

From the LHS

Question 21

From which direction does lead II view the heart?

Answer 21

Bottom left

Question 22

From which direction does lead III view the heart?

Answer 22

Bottom

Question 23

What is the QT interval?

Answer 23

The length of time taken for the ventricles to depolarise and repolarise.

Question 24

Where are the limb leads placed, and what are the corresponding colours?

Answer 24

Red- right upper
Yellow- left upper
Green- left lower
Blue- right lower

Question 25

What are the positions of the chest leads? What are their corresponding colours?

Answer 25

V1- red- RHS 4th intercostal space
V2- yellow- LHS 4th intercostal space
V3- green- LHS between V2 and V4
V4- brown- LHS 5th intercostal space, mid clavicular line
V5- black- LHS between V4 and V6
V6- purple- LHS 5th intercostal space, mid axillary line

Question 26

What is the ST segment representative of?

Answer 26

The time of ventricular contraction