ECG theory and practice

Question 1

What is detected by an ECG?

Answer 1

The extracellular current of the myocytes on the skin surface

Question 2

How many electrodes are used to perform a 12-lead ECG?

Answer 2

10

Question 3

What is an ECG lead?

Answer 3

An imaginary line (Lead axis) between 2 or more electrodes, it is NOT the wire that connects the electrode to the recording device

Question 4

Which electrode acts as a recording electrode?

Answer 4

The positive electrode

Question 5

What is represented by an upward deflection on an ECG?

Answer 5

The movement of depolarisation towards the recording electrode or depolarisation away from the recording electrode

Question 6

What is represented by a downward deflection on an ECG?

Answer 6

The movement of depolarisation away from the recording electrode

Question 7

What is represented by an isoelectric line?

Answer 7

No wave of depolarisation

Question 8

What are the 3 standard limb leads?

Answer 8

Lead I - RA (-ve) to LA (+ve)
Lead II - RA (-ve) to LL (+ve)
Lead III - LA (-ve) to LL (+ve)

Question 9

Which electrode is placed on the right leg?

Answer 9

A neutral, earthed electrode

Question 10

Which electrodes form Einthoven’s triangle?

Answer 10

Right Arm
Left Arm
Left Leg

Question 11

What causes the appearance of the P wave in a Lead II ECG?

Answer 11

Atrial depolarisation spreads from the SA node, inferiorly and to the left, so it moves towards the recording electrode (Left Leg) and so shows an upward deflection

Question 12

What causes the appearance of the Q wave in a Lead II ECG?

Answer 12

Left to right depolarisation of the intraventricular septum, which moves slightly away from the recording electrode (Left Leg) and so shows a slight downwards deflection

Question 13

What causes the appearance of the R wave in a Lead II ECG?

Answer 13

Depolarisation of the main ventricular mass, which moves towards the recording electrode (Left Leg) and so shows a large upwards deflection

Question 14

What causes the appearance of the S wave in a Lead II ECG?

Answer 14

Depolarisation of the ventricles at the base of the heart, moving slightly away from the recording electrode (Left Leg) and so shows a slight downwards deflection

Question 15

What causes the appearance of the T wave in a Lead II ECG?

Answer 15

Ventricular depolarisation, which moves away from the recording electrode. This has an opposite effect to depolarisation and so shows an upwards deflection on the ECG

Question 16

What is a normal QRS duration?

Answer 16

<0.1 seconds

Question 17

What is the normal P wave duration?

Answer 17

0.08 - 0.1 seconds

Question 18

What is the normal PR interval duration?

Answer 18

0.12 - 0.2 seconds

Question 19

What is the normal QT interval duration?

Answer 19

0.36 - 0.44 seconds at 60bpm

Question 20

What are the 3 types of leads?

Answer 20

Bipolar limb leads
Augmented limb voltage leads
Chest leads

Question 21

What are the 3 types of augmented voltage leads?

Answer 21

aVR (Right arm +ve)
aVL (Left arm +ve)
aVF (Left Food +ve)

Question 22

What are the 6 chest leads?

Answer 22

V1
V2
V3
V4
V5
V6

Question 23

What leads give a coronal plane view of the heart?

Answer 23

I
II
III
aVR
aVL
aVF

Question 24

What leads give a transverse plane view of the heart?

Answer 24

V1 - V6

Question 25

In what direction do the limb leads and augmented voltage leads record?

Answer 25

Question 26

What is the name given to the system comprising of the 3 bipolar limb leads and 3 augmented voltage leads?

Answer 26

Hexaxial reference system

Question 27

Which leads are lateral leads?

Answer 27

Lead I and aVL

Question 28

Which leads are inferior leads?

Answer 28

II, III and aVF

Question 29

Where should the 6 chest leads be placed?

Answer 29

V1 = 4th intercostal space, immediately right of sternum V2 = `4th intercostal space, immediately left of sternum V3 = Mid-way between V2 and V4 V4 = 5th intercostal space at mid-clavicular line V5 = Same horizontal level as V4 at anterior axillary line V6 = Same horizontal level as V4 at mid-axillary line

Question 30

What colour is the right arm electrode?

Answer 30

Red (R)

Question 31

What colour is the left arm electrode?

Answer 31

Yellow (L)

Question 32

What colour is the left leg electrode?

Answer 32

Green (F)

Question 33

What colour is the right leg electrode?

Answer 33

Black (N)

Question 34

To what level should an ECG be calibrated?

Answer 34

25mm/second on the time (x) axis 10mm/1mV on the amplitude (y) axis

Question 35

How can you calculate heart rate from an ECG?

Answer 35

HR = 300 ÷ Number of big squares between R waves

Question 36

What is the ECG rhythm strip?

Answer 36

A prolonged section of an ECG, usually from Lead II

Question 37

What are some examples of conditions that may not show up on an ECG?

Answer 37

Myocardial infarction Intermittent rhythm disturbances Stable angina

Question 38

What 7 questions should you ask yourself when looking at an ECG?

Answer 38

1. Is electrical activity present? 2. Is rhythm regular or irregular? 3. What is the heart rate? 4. Are P-waves present? 5. What is the PR interval? (Average 3-5 small squares) 6. Is each P-wave followed by a QRS complex? 7. Is the QRS duration normal? (< 2.5 small squares)