Basics of ECG

Question 1

Describe the electrical conduction in the heart

Answer 1

The SA node generates action potentials:
These action potentials spread across the atria
They reach the AV node (conduction is delayed)
They spread rapidly through the bundle of His and its branches and the network of purkinje fibres to spread across the ventricles

Question 2

Physics of the ECG

Answer 2

Potential differences between different sites on the body surface are detected by electrodes placed on the sin coupled to a sensitive recording device
An ECG is a recording of the potential differences between time

Question 3

Describe how deflections are produced on an ECG

Answer 3

Lead one electrode acts as the recording (positive) electrode

• When depolarisation moves TOWARDS the recording electrode it will generate an UPWARD deflection on the ECG
• When depolarisation moves AWAY from the recording electrode it generates a DOWNWARD deflection on the ECG
• If there is no movement then there will be no deflections

Question 4

where is the lead one axis

Answer 4

From RA (-ve) to LA (+ve)

Question 5

Where is the lead two axis?

Answer 5

RA (-ve) to LL (+ve)

Question 6

Where is the lead 3 axis

Answer 6

LA (-ve) to LL (+ve)

Question 7

Which limb is the earthed limb

Answer 7

Right leg

Question 8

what does the P wave represent

Answer 8

atrial depolarisation

Question 9

what does the QRS complex represent and how long should it be

Answer 9

ventricular depolarisation

less than 3 small boxes

Question 10

what does the t wave represent

Answer 10

ventricular repolarisation

Question 11

When is the PR interval, what does it indicate, how long is it normally?

Answer 11

From the start of the P wave to the start of QRS interval

Time for SA node impulse to reach the ventricles, and indicates there is a delay in conduction of the AV node

0.12 - 0.2 seconds

Question 12

When is the ST segment and what does it represent?

Answer 12

From the end of the QRS complex to the start of the T wave

Represents ventricular contraction (systole)

Question 13

When is the QT interval, what does it indicate, what is the normal time for it?

Answer 13

Start of QRS complex to the end of T wave

Reflects time for ventricular depolarisation and repolarisation

Normally (depending on HR) 0.36 - 0.44s

Question 14

What does the TP segment represent

Answer 14

Relaxation of the ventricles (diastole)

Question 15

What leads are a 12 lead ECG comprised of

Answer 15

• 3 standard limb leads (I, II, III)
• 3 augmented voltage leads (aVR, aVL, aVF)
• 6 chest leads (V1 - 6) - precordial

Question 16

What does a 12 lead ECG allow you to look at?

Answer 16

To look at the heart from different directions which allows you to:

Determine axis of heart in thorax

Look for any ST segment or T wave changes in relation to specific heart regions

Look for any voltage changes

Question 17

What leads are the lateral leads, and which direction does it look at the heart from?

Answer 17

Leads I and aVL

Views heart from the left

Question 18

Which leads are the inferior leads and what direction are they looking at the heart from?

Answer 18

Leads II, III and aVF

Looks at heart from an inferior direction

Question 19

Where are each of the precordial leads placed?

Answer 19

V1 - 4th intercostal space on right of the sternum
V2 - 4th intercostal space on left of sternum
V3 - midway between V2 - 4
V4 - fifth intercostal space mid clavicular line
V5 - same horizontal level as V4, anterior axillary line
V6 - same horizontal level as V4, mid axillary line

Question 20

Which areas of the heart are each of the precordial leads viewing?

Answer 20

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

Question 21

Steps of analysing an ECG

Answer 21

1. Verify patient name and DOB
2. Check date and time ECG taken
3. Check calibration of ECG paper
   - usually 25mm/sec
4. determine axis if possible
5. Work out rate and rhythm
   - is electrical activity present
   - is rhythm regular or irregular
   - what is heart
   
   • 300/number of larger squares
     (regular)
   • count QRS complexes in 30
     large squares x 10 (irregular)
     
     • P waves present
     • Check PR interval (0.12 - 0.2)
     • if each P wave followed by QRS
     • If QRS duration normal (<0.10 sec)
6. Look at individual leads for ST or T wave changes

Question 22

Describe each of the waves in lead II (seen best in lead II)

Answer 22

P wave - atrial depolarisation

Q wave - left to right
depolarisation of the interventricular septum

R wave - depolarisation of the main ventricular mass

S - depolarisation of the ventricles at bases of heart

T wave - ventricular repolarisation

Question 23

When does systole and diastole happen?

Answer 23

Systole - ventricular contraction in the ST segment

Diastole - ventricles relaxing in the TP segment