The Normal ECG

Question 1

Describe the electrophysiological basis for impulse generation, conduction and the ECG.

Answer 1

Two adjacent cells with different charges creates a dipole. This dipole has a direction (ie. a vector) so the orientation of the dipole matters, as does the position of the electrodes.

Placing a lead with a reference end and a sensor end can read the direction and magnitude of this dipole.

The body is a good conductor. Depolarisation of the heart involves many more cells than just 2 adjacent myocytes. The spread of depolarisation involves many dipoles that are oriented in different directions simultaneously. The voltage that is detected at any instant results from the net dipole that = sum of all of these vectors.

Remember that the position (i.e. direction) of the electrodes matters.

• Depolarisation heading in the same direction as the orientation of a lead produces a positive voltage deflection.
• Depolarisation heading in the opposite direction as the orientation of a lead produces a negative voltage deflection.
• Depolarisation heading perpendicular to the orientation of a lead produces zero voltage deflection.* *

Question 2

Describe the basic components of the ECG waveform and normal values of the major intervals.

Answer 2

P wave: atrial depolarisation.

QRS complex: collectively ventricular depolarisation

T wave: ventricular repolarisation

PR interval (0.12-0.2s)

• Start of atrial depolarisation to start of ventricular depolarisation
• Atrial contraction occurs here
• Also includes AV node delay

RR interval

• Time between successive ventricular depolarisations
• Used for measuring ventricular HR

QT interval (<0.44s men, <0.46s women)

• Start of ventricular depolarisation to start of ventricular repolarisation
• Ventricular contraction occurs here

QRS complex duration (<0.1s)

• Time taken to coplete ventricular depolarisation

ST segment

• Period during hich ventricle is fully depolarised
• Useful in detecting MI and infarction

Question 3

Relate the orientation of the ECG leads to the electrical events that occur in the heart

Answer 3

We are able to superimpose the limb leads over the heart to give an axial reference system.

• The leads measure the same electical events simultaneously, but observe them from different angles.
• The axial reference system explains the differences in the ECG waveform for each of the leads.
• The chest leads look at the heart only in 2 dimensions

There are an extra 6 chest leads that allow us to look at the heart in three dimensions

Question 4

Basic 3 lead ECG

Answer 4

Forms a triangle called Einthoven’s triangle.

Question 5

Augmented limb leads

Answer 5

• Lead aVF
• Lead aVR
• Lead aVL

Question 6

Inferior leads

Answer 6

aVF, II and III

Question 7

Lateral leads

Answer 7

I, aVL, V5 and V^

Question 8

Anterior leads

Answer 8

V1-V4