The electrocardiogram (ECG)

Medical Physics | AQA A-Level Physics

Key Definition

Electrocardiogram (ECG): A recording of the electrical activity of the heart over time, measured using electrodes placed on the skin. The signal is typically about 1 mV in amplitude.

The standard ECG trace has three main features:
- P wave: a small upward deflection caused by depolarisationThe electrical discharge of heart muscle cells from their resting state, triggering contraction. A wave of depolarisation spreads across the heart in a coordinated sequence. of the atria (the atria contract).
- QRS complex: a large spike (about 1 mV) caused by depolarisation of the ventricles. This is the largest feature because the ventricles have much more muscle mass than the atria.
- T wave: a broader, smaller wave caused by repolarisationThe process by which heart muscle cells return to their resting electrical state after contraction, ready for the next depolarisation. of the ventricles (the ventricles reset for the next beat).
Electrodes are placed at specific positions on the chest and limbs. Conductive gel is used to reduce contact resistance and improve signal quality.
The amplifier must have high gain (the signal is only about 1 mV), low noise, and high input impedance so it does not draw significant current from the body.
Heart rate can be calculated from the time between successive R peaks (the tallest spikes in the QRS complex).