Lecture 9 – The Electrocardiogram

The Physical Principles of Electrocardiography

an ECG is a non-invasive technique that records the sum of all cardiac electrical activity from the body surface


Dipole Concept in a Volume Conductor

  • the body tissues acts as a volume conductor because they contain ions that can carry electrical current

  • at any instant, the electrical activity of the heart can be represented as a single dipole

    • a dipole is a pair of equal but opposite charges ( + and - ) separated by a small distance

    • the dipole represents the wavefront of depolarisation. the -ve end is the resting tissue, and the +ve end is the depolarised tissue. the arrow points form negative to positive (direction of wavefront movement)


What Determines the Measured Voltage on an ECG

the voltage recorded by any two electrodes depends on three factors:

  • vector — the dipole is represented as a vector (an arrow)

    • direction of the vector: direction of the dipole

    • length of the vector: strength (voltage) of the dipole


  • projection — an ECG lead records a projection of the cardiac vector onto its specific lead line

    • rule for a positive deflection: a positive deflection is recorded when the positive end of the dipole is facing the positive electrode of the lead

    • rule for a negative deflection: a negative deflection is recorded when the positive end of the dipole is facing the negative electrode of the lead


Key Principle: the ECG doesn’t directly ‘see’ the AP of a single cell. Instead, it sees the net, instantaneous vector sum of all the electrical activity occurring in the heart at that moment, projected onto the different lead axes


ECG Waveforms and Cardiac Cycle

the standard ECG trace is a voltage vs. time recording with three main deflections: the P wave, QRS complex, and T wave

**Important Relationship: the ECG doesn’t directly show contraction or relaxation. It shows the electrical activation that causes contraction. Mechanical systole occurs shortly after the QRS, and diastole occurs after the T wave


ECG Recording Process

a standard clinical 12-lead ECG records the heart’s electrical activity from 12 different perspectives (leads). there are 10 physical electrodes placed on the body


  • lead systems

leads aren’t just ‘wires’; each lead is a specific view or projection of the cardiac vector

  • Einthoven’s Triangle and the Hexaxial System

Einthoven’s Triangle

an equilateral triangle formed by the three limb leads with the heart at the centre

Hexaxial System

  • the six frontal plane leads arranged every 30° around the heart.

    • this allows calculation of the mean QRS axis


  • The 12-Lead ECG Layout

a standard 12-lead ECG includes:

→ 6 frontal plane leads: I, II, III, aVR, aVL, aVF

→ 6 horizontal plane leads: V1 – V6

→ 1 rhythm strip: often a long recording of lead II to assess rhythm continuously


Detecting ECG Abnormalities

by analysing the shape, size, and timing of the deflections, many abnormalities can be detected

  • abnormalities in rate and rhythm

  • abnormalities in conduction

  • abnormalities related to ischemia and infarction

myocardial ischemia/infarction alters the normal sequence of repolarisation and depolarisation, causing characteristic changes

  • calculating the mean QRS axis

the mean QRS axis is the average direction of ventricular depolarisation in the frontal plane

it’s normally between -30° and +110°