Electrocardiogram

Question 1

What is an ECG?

Answer 1

A recording of potential changes, detected by electrodes positioned on the body surface, that allows the electrical activity of the heart to be monitored

Question 2

What information can an ECG provide?

Answer 2

Cardiac rate and rhythm, chamber size, electrical axis of the heart and assess myocardial ischaemia and infarction

Question 3

What determines magnitude and direction?

Answer 3

Magnitude determined by mass of cardiac muscle involved in generation of signal
Direction is determined by the overall activity of the heart at any instant in time

Question 4

What is an ECG lead?

Answer 4

An imaginary line between 2 or more electrodes

Question 5

Isopotential

Answer 5

Equal magnitude potentials which are recorded on the body’s surface

Question 6

3 standard limb leads

Answer 6

Lead I (right arm -ve to left arm +ve)
Lead II (right arm -ve to left leg +ve)
Lead III (left arm -ve to left leg +ve)

Question 7

P wave:

• What does it represent?
• Normal duration

Answer 7

• Depolarisation of atrial muscle

- 0.08-0.10 seconds

Question 8

QRS complex:

• Q wave
• R wave
• S wave

Answer 8

Q = downward deflection preceding R wave
R = deflection upwards
S = downward negative reflection following an R wave

Question 9

QRS complex:

• What does it represent?
• Normal duration

Answer 9

• Ventricular depolarisation

- <0.10 seconds

Question 10

T wave:

- What does it represent?

Answer 10

Ventricular repolarisation

Question 11

PR interval:

• What does it represent?
• Normal duration

Answer 11

• AV nodal delay

- 0.12-0.2 seconds (3-5 small boxes)

Question 12

ST segment:

- What does it represent?

Answer 12

Ventricular contraction (systole)

Question 13

TP interval:

- What does it represent?

Answer 13

Ventricular relaxation (diastole)

Question 14

Chest leads - where do the following leads look?

• V1 and V2
• V3 and V4
• V5 and V6

Answer 14

• V1 and V2 - come from right look at interventricular septum
• V3 and V4 - looking at anterior of the heart
• V5 and V6 - look at lateral aspect (left ventricle) of the heart

Question 15

Positions at which chest electrodes are placed

Answer 15

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 mid-clavicular line
V5 = same horizontal level as V4 in anterior axillary line
V6 = same horizontal level as V4 in mid axillary line

Question 16

If the paper speed is 25mm/sec:
- What does one large box represent?
- What does one small box represent?
(time and amplitude)

Answer 16

• 0.2 seconds of time, 5mm amplitude

- 0.04 seconds of time, 1mm amplitude

Question 17

What is an ECG rhythm strip?

Answer 17

Prolonged recording of one lead (usually lead II), and this allows you to determine rate and rhythm

Question 18

In regular rhythm, how is heart rate measured from ECG?

Answer 18

300/number of large squares in R-R interval

Question 19

In irregular rhythm, how is heart rate measured from ECG?

Answer 19

Count the number of QRS complexes in 30 boxes and multiply by 10

Question 20

Steps when analysing ECG (6)

Answer 20

1 - Verify patient details
2 - Check date and time of when ECG was taken
3 - Check calibration of ECG paper
4 - Determine the axis, if possible
5 - Work out the rate and rhythm
6 - Look at individual leads for voltage criteria changes any ST or T wave changes

Question 21

Steps in working out rate and rhythm in ECG

Answer 21

• Is electrical activity present?
• Is the rhythm regular or irregular?
• What is the heart rate?
• Are there P-waves present?
• What is the PR interval?
• Is each P wave followed by QRS complex?
• Is the QRS duration normal?