ECG

Question 1

How are the electrical currents of the SAN detected?

Answer 1

The currents radiate through body fluids and so can be detected by electrodes on the skin.

Question 2

What is a lead?

Answer 2

A view of the heart

Question 3

What does an ECG measure?

Answer 3

The current flow of an electrical event through waveforms, doesn’t measure the AP. It monitors the phases of the cardiac cycle to be monitored.

Question 4

How many leads can an ECG have?

Answer 4

12 leads

• 6 chest/precordial leads
• 6 limb leads

Question 5

Where are the limb electrodes placed?

Answer 5

R (red) - Right arm
L (yellow) - Left arm
F (green) - Left leg
N (black) - Right leg
Placed on wrists and ankles away from bone

Question 6

What type of leads produced by the limb electrodes?

Answer 6

3 augumented unipolar leads - Measure current from the centre of the chest radiating out to one of the electrodes.
3 standard bipolar leads - Measure the current between two electrode.

Question 7

What is the purpose of N electrode?

Answer 7

To reduce and filter out any electrical interference

Question 8

Name the limb lead views and which electrodes are used to produce them.

Answer 8

Bipolar:
I = R to L 
II = R to F
III = L to F
(number of L's from the limbs = number of the lead)

Unipolar:
avR = centre to R
avL = centre to L
avF = centre to F

Question 9

What is the difference between the recordings produced by chest leads and limb leads?

Answer 9

Chest leads have larger peaks as they are closer to the heart and is able to determine beats from the L or R side of the heart.

Question 10

Where are the chest electrodes placed?

Answer 10

V1 - R 4th ICS (adjacent to sternum)
V2 - L 4th ICS (adjacent to sternum - tricuspid area)
V3 - Between V2 and V4
V4 - L 5th ICS midclavicular line (apex - mitral area)
V5 - L 4th ICS anterior axillary line
V6 - L 4th ICS midaxillary line

Question 11

What do the electrodes measure?

Answer 11

The dipole seperation between depolarising and resting cells that is generated as the heart contracts.

Question 12

What determines the shape of the recording produced?

Answer 12

The direction of current determines the shape of the deflection:
Towards the electrode = Upward
Away from electrode = Downward

Question 13

What determines the size of the deflection?

Answer 13

Size is proportional to tissue mass.

Question 14

What reading is produced if the flow travels at 90 degrees

Answer 14

No reading

Question 15

What happens to the deflections during repolarisation?

Answer 15

The deflections are opposite:
Towards the heart = Downwards
Away from the heart = Upwards

Question 16

What recording results from no net flow?

Answer 16

Isoelectric / flat line

Question 17

Why is atrial repolarisation not shown on an ECG?

Answer 17

Overlaps with ventricular depolarisation

Question 18

Which lead is normally used to read the ECG?

Answer 18

Lead II

Question 19

What does the P wave represent?

Answer 19

Atrial depolarisation - SAN depolarises and spreads through atrium from R to F (upward deflection)

Question 20

What does the Q wave represent?

Answer 20

Travels down bundle of his and spreads to R ventricle, moving away from F (Downward deflection)

Question 21

Is it normal to have an absent Q?

Answer 21

Can be due to anatomical variation

Question 22

What does the R wave represent?

Answer 22

Spread through ventricles from R to F (upwards)

Question 23

What determines the size of R?

Answer 23

The muscle mass of the ventricles

Question 24

What does S wave represent?

Answer 24

Spread up through ventricles away from F (downwards)

Question 25

What does the QRS complex show?

Answer 25

Ventricular depolarisation

Question 26

What does the T wave represent?

Answer 26

Repolarisation - It repolarises from outside to inside form F to R (upward as travelling away from F but is opposite in repolarisation)

Question 27

How can HR be calculated from an ECG?

Answer 27

R to R peak = 0.6-1 seconds

Question 28

How can the AVN delay be calculated?

Answer 28

P to R peak = 0.12-0.2 seconds

Question 29

What is the average speed for ventricular depolarisation?

Answer 29

0.06-0.1 seconds