Session 6 ILOs - ECG

Question 1

Outline the components of the electrical conducting system and describe how excitation normally spreads through the myocardium

Answer 1

• Action potentials are initiated in the SAN
• Depolarisation spreads across the atrium, where they are picked up at the AVN and there is a slight delay
• Depolarisation then passes through into the bundle of His which splits into the left and right bundles is carried down through the interventricular septum
• Finally, depolarisation passes back up through towards the base of the heart via the Purkinje fibres

Question 2

Describe the correct positioning of recording electrodes for performing a 12 lead ECG (including colours)

Answer 2

Limb leads:
- RA limb lead (red)
- LA limb lead (yellow)
- LL limb lead (green)
- RL limb lead (black / grounding)
\+ augmented limb leads (make up other 3 leads)

Chest leads:

V1 - 4th intercostal space at right sternum edge
V2 - 4th intercostal space at left sternum edge

V3 - Midway between V2 and V4

V4 - 5th intercostal space at mid-clavical line (line down from clavicle)

V5 - Left anterior axillary line (horizontal with V4)
V6 - Left mid-axillary line (horizontal with V4 and V5)

Question 3

Explain what signal you would see on a a positive recording electrode when depolarisation and repolarisation spreads towards and away from that electrode

Answer 3

Depolarisation travelling towards the positive electrode = upwards deflection

Depolarisation travelling away from the positive electrode = downwards deflection

Repolarisation travelling away from the positive electrode = upwards deflection

Repolarisation travelling towards the positive electrode = downwards deflection

Question 4

State the meaning of sinus rhythm

Answer 4

Sinus rhythm: each QRS complex is preceded by a normal P wave

Question 5

Explain how you would calculate the heart rate from a rhythm strip for a regular and irregular heart rhythm

Answer 5

Regular heart rate:
Method 1: One big box is 0.2 seconds, so count the number of QRS complexes there are in the time period so 4 beats in 3.2 seconds (if 16 boxes), then use a ratio to get up to 60 seconds
Method 2 (squares method): count number of big squares between 2 QRS complexes then do 300 / number

Irregular method:
Count the number of QRS complexes in 6 seconds (30 big squares) then X 10

Question 6

Explain how you would calculate the following interval durations and their normal range:

• PR interval
• QRS interval
• QT interval

Answer 6

PR interval

• Measure from beginning of P wave, to the start of the QRS complex
• Normal range: 0.12 - 0.2 seconds (3-5 small boxes)

QRS interval

• Measure from beginning QRS complex to the end of the QRS complex
• Normal range: <0.12 seconds ( < 3 small boxes)

QT interval

• Measure from beginning of QRS complex to the end of the T wave
• Normal range: 0.42 seconds

Question 7

Describe each stage of the ECG trace for the Limb Lead II

Answer 7

P wave: atrial depolarisation
Isoelectric line: delay at AVN
Q part: depolarisation of interventricular septum
R part: depolarisation of apex and ventricular walls
S part: depolarisation up to the base of the ventricles
T wave: repolarisation of ventricles