Introduction to the Electrocardiogram (ECG)

Question 1

What is an ECG?

Answer 1

• electrical currents produced during the cardiac cycle radiate through body fluids
• these mute currents can be detected by electrodes placed on the skin
• current transformed into waveforms
• ECG records these waveforms from different leads

Question 2

Why record an ECG?

Answer 2

• shows precise sequence of electrical events occurring in cardiac cells
• allows monitoring of phases of cardiac cycle
• can be used during general anaesthesia or IV sedation in hospital or outpatient setting (rare)

Question 3

What is telemetry?

Answer 3

Holter monitor
Mobile ECG
Allows patients on a cardiac ward to have 24 hour ECG monitoring but also be able to move around the ward freely

Question 4

What is a lead?

Answer 4

A pair of ECG electrodes defines a lead and each lead offers a different view of the heats electrical activity

Question 5

Where are electrodes placed in a 6-lead ECG?

Answer 5

LA/L - Left arm - YELLOW
RA/R - Right arm - RED
LL/F - Left leg - GREEN
RL/N - Right leg - BLACK

Question 6

What is Einthoven’s triangle?

Answer 6

Basis of recording stander bipolar limb leads

RA - LA - LL

Question 7

What is the ECG actually measuring?

Answer 7

The body is a good conductor of electricity
The heart contracts due to action potentials moving through the heart, resulting in a dipole (charge separation)
Creates an electrical field that can be recorded by electrodes placed on the skin

Question 8

What determines the shape and size of the deflection on the ECG?

Answer 8

The direction in which the depolarising current travels determines the shape.
The mass of tissue creating the dipole determines the size of the deflection.

Question 9

Which currents produce an upward/downward deflection

Answer 9

Depolarising + towards electrode = UPWARD
Depolarising + away from electrode = DOWNWARD
Repolarising + towards electrode = DOWNWARD
Repolarising + away from electrode = UPWARD

Question 10

Which electrodes form each of the bipolar leads in the 6 lead ECG?
What does each show?

Answer 10

• LEAD 1 - RA/LA shows depolarising current moving from right to left
• LEAD 2 - RA/LL shows current moving from right arm diagonally toward left left (base to apex)
• LEAD 3 - LA/LL shows current moving from left arm towards left leg

Question 11

What are the 3 augmented limb leads?

Answer 11

Unipolar

• aVR
• aVL
• aVF

Question 12

What does each of PQRST waves represent?

Answer 12

P = atrial depolarisation
Q = depolarisation down right bundle in Bundle of His
R = depolarisation of bottom of ventricles 
S = depolarisation of sides of ventricles
T = repolarisation of of ventricles

Question 13

What is the isoelectric line?

Answer 13

No net current flow means no ECG deflection

Question 14

What are the 3 important time intervals? What do they represent/what can they be used to calculate?

Answer 14

R-R interval = length of cardiac cycle, can calculate heart rate
P-R interval = time gap between atrial and ventricular depolarisation. Represents AVN delay
QRS duration = ventricular contradiction, should be tight

Question 15

What can we record from the chest electrodes?

Answer 15

larger ECG deflections
better spatial correlation to underlying regions
distinguishes between left and right ectopic beats