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Learn7408Paper 3B · Option: Medical physicsBiological measurementThe electrocardiogram
Biological measurement · Subtopic 1 of 1

The Electrocardiogram

Heart muscle contracts when its cells depolarise, creating small potential differences (about 11 mV) across the body. An ECG records them using electrodes on the skin, with conducting gel to reduce contact resistance and a high-impedance amplifier. The P wave shows the atria depolarising, the QRS complex the ventricles depolarising, and the T wave the ventricles repolarising.

AQA 7408 Paper 3B≈ 30 minRevision 1 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Explain how heart muscle produces electrical signals.
  • Describe how an ECG is recorded.
  • Identify P, QRS and T and find heart rate from a trace.
Pause and think

Why is the ECG signal recorded at the wrists and chest, when the heart is deep inside the body?

Section 2 of 14

Core idea

Start hereElectricity of a heartbeat

Every heartbeat starts with an electrical wave sweeping through the heart muscle. The signal is only about a millivolt by the time it reaches your skin, but a sensitive amplifier can trace it, and the shape tells a doctor how the heart is working.

The big idea

Depolarisation of heart muscle makes small p.d.s on the skin. P = atria depolarise, QRS = ventricles depolarise, T = ventricles repolarise. Heart rate =60T=\frac{60}T.

1. Recording

Why high input impedance
  1. Vmeasured=εRinRin+RbodyV_{\text{measured}}=\varepsilon\frac{R_{\text{in}}}{R_{\text{in}}+R_{\text{body}}}
    Amplifier input and body form a potential divider.
  2. Rin≫Rbody⇒V≈εR_{\text{in}}\gg R_{\text{body}}\Rightarrow V\approx\varepsilon
    Almost the whole signal appears across the amplifier.
Gel and clean skin lower RbodyR_{\text{body}} too.
Worked exampleGain needed

A 1.01.0 mV heart signal must drive a 1.01.0 V display.

Goal
Gain.
Strategy
G=VoutVinG=\frac{V_{\text{out}}}{V_{\text{in}}}.
  1. G=1.01.0×10−3G=\frac{1.0}{1.0\times10^{-3}}
    Ratio.
  2. =1000=1000
    Calculate.
AnswerVoltage gain 10001000.
✓ CheckMains hum (50 Hz) is amplified too, so filtering is needed ✓.

2. Reading a trace

Worked exampleHeart rate from paper

Paper speed 2525 mm s⁻¹; R peaks 2020 mm apart.

Goal
Heart rate.
Strategy
Time between beats, then 60T\frac{60}T.
  1. T=2025=0.80 sT=\frac{20}{25}=0.80\text{ s}
    Distance ÷ speed.
  2. rate=600.80=75 bpm\text{rate}=\frac{60}{0.80}=75\text{ bpm}
    Beats per minute.
Answer7575 bpm.
✓ CheckNormal resting range is 6060–100100 ✓.
ECG waveform

One heartbeat, three features.

Common misconceptionHidden wave Atrial repolarisation happens during the QRS complex and is hidden by it.
Key takeawayKeep still Skeletal muscle signals swamp the trace, so the patient lies relaxed.
Pause and think

Why is the QRS spike much larger than the P wave?

Summary

  • P, QRS, T.
  • Gel, electrodes, high-impedance amplifier.
  • Rate =60T=\frac{60}T.
Key words
Depolarisation
Change in cell potential that triggers contraction.
ECG
Electrocardiogram: trace of heart p.d.s.
HistoryEinthoven. Willem Einthoven recorded the first practical ECG in 1903 using a string galvanometer weighing about 270 kg. He won the Nobel Prize in 1924 and named the P, Q, R, S and T waves.
Fun factSmartwatch ECGs. Some smartwatches record a single-lead ECG between the watch back and a fingertip on the crown.
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