ECG

Question 1

What’s an ECG?

Answer 1

graphic representation of electrical pd changes against time of myocardium in cardiac cycle

Question 2

What needs to be considered when looking at QRS complex?

Answer 2

• orientation of recording leads
• concept of heart as an electrical dipole
• change in dipole size + orientation during ventricular excitation

Question 3

What are the different ECG leads?

Answer 3

6 coronal (attached to limbs)
6 transverse (attached to chest)

Question 4

What do the 6 frontal plane/ limb leads consist of?

Answer 4

3 bipolar leads (I, II, III)

Question 5

What are the 3 bipolar leads derived from?

Answer 5

Einthoven bipolar lead

Question 6

How are the bipolar leads designated?

Answer 6

each records difference in electrical potential between limbs

Question 7

What are the 3 unipolar leads?

Answer 7

derived leads: I, II, III, to form aVR, aVL, aVF
L–left wrist
R–right wrist
F–left ankle
N–right ankle

Question 8

What are the diff horizontal ‘Chest’ leads?

Answer 8

V1-6

Question 9

Where’s V1 placed?

Answer 9

4th IC, R sternal edge

Question 10

Where’s V2 placed?

Answer 10

4th IC, L sternal edge

Question 11

Where’s V3 placed?

Answer 11

equidistant V2-V4

Question 12

Where’s V4 placed?

Answer 12

5th IC, midclavicular line

Question 13

Where’s V5 placed?

Answer 13

L anterior axillary line in horizontal line w V4

Question 14

Where’s V6 placed?

Answer 14

mid axillary line, horizontal w V4 + V5

Question 15

Why P wave?

Answer 15

Hump before QRS
Atrial depolarisation
Electrically one structure
Starts at SAN (SR)
P wave morphology changes dependant on origin of depolarisation

Question 16

Why PR interval?

Answer 16

Onset of P wave to the onset of the QRS complex
Time taken from SAN to ventricles
Measures AVN conduction time
120-200ms

Question 17

Why QRS complex?

Answer 17

Ventricular depolarisation

Occurs after AVN/His/Purkinje system traversed

Question 18

What’s QRS interval?

Answer 18

Beginning of QRS to its end
1st deflection can be negative (Q) or positive (R)
Duration of ventricular depolarisation
80-120ms

Question 19

What’s T wave?

Answer 19

Follows the QRS complex
ventricular repolarisation
heart reverts back to normal

Question 20

What’s QT interval?

Answer 20

Onset of QRS to end of T wave
Ventricular depolarisation + repolarisation
Long QT interval syndrome–> sudden death
350-420ms

Question 21

What’s ST segment?

Answer 21

Flat segment between the end of QRS and start of T wave
Depression = ischaemia
Elevation = MI

Question 22

How’s an external electrical field is set up around the

dipole?

Answer 22

During flow of current along the cell surface

Question 23

Why will a pd occur between a part of a cardiac cell + another dipole?

Answer 23

When cardiac cell is depolarising or repolarising, diff currents flow across membrane at various points

Question 24

When is there no electrical field?

Answer 24

When cell is depolarised or repolarised + at RMP, there’s no diff in membrane potential at diff points along cell surface
(despite pd between inside + outside cell)

Question 25

Typical Voltage Scale?

Answer 25

10mm=1Mv

Question 26

Speed?

Answer 26

25mm/s

Question 27

1 small square?

Answer 27

0.04s