Where the signal comes from
Heart muscle cells have a potential difference across their membrane: about −90 mV inside compared with outside (the resting potential). When a cell is triggered, ions flow and the inside quickly becomes positive (about +20 mV). This is depolarisation. Then the cell returns to its resting value: repolarisation. The full rise and fall is an action potential.
Each beat starts at the SA node (the heart's natural pacemaker) in the right atrium. The wave of depolarisation spreads over the atria, pauses at the AV node, then races down to the ventricles. So many cells change together that small currents flow through the body, and a voltage of about 1 mV appears between points on the skin.
A simple ECG machine
- Electrodes: metal (often silver/silver chloride) pads on the chest, wrists and ankles. Standard places give comparable traces.
- Conducting gel: removes air gaps and lowers the contact resistance with the skin. Skin is cleaned and hair may be shaved for good contact.
- Amplifier: it must have a very high input resistance, so it draws almost no current and the voltage is not lost across the skin resistance. It boosts 1 mV to about 1 V.
- Display / recorder: voltage against time on a screen or paper (often 25 mm s⁻¹).
The patient should lie still and relaxed: other muscles also make voltages that add noise to the trace.
The normal ECG waveform
- P wave: small bump, the atria depolarise and contract (about 0.08–0.1 s).
- QRS complex: tall sharp spike, the ventricles depolarise and contract (about 0.06–0.1 s). It is big because the ventricle muscle is large. Atrial repolarisation happens at the same time and is hidden.
- T wave: a rounded bump, the ventricles repolarise.
The R–R interval is the time for one beat. Heart rate (beats per minute) = 60 ÷ R–R (s). At rest an adult is about 60–100 beats per minute (R–R about 0.6–1.0 s). Doctors read the heights, shapes and gaps to spot problems such as a blocked artery or an irregular rhythm.
Try it: find your own heart rate
Put two fingers on your wrist and count beats for 15 s. Multiply by 4. Now work out your R–R time: 60 ÷ your rate. Jog on the spot for 1 minute and repeat. In the 3D free-play step, set the slider to both rates and watch the R peaks squeeze closer together.
Key formulas and definitions
- Heart rate (beats/min) = 60 ÷ R–R interval (s)
- Time on paper = distance ÷ paper speed (e.g. 25 mm s⁻¹)
- Gain of amplifier = output voltage ÷ input voltage
- Resting potential ≈ −90 mV (heart cell); skin signal ≈ 1 mV
Worked examples
1. The R–R interval is 0.80 s. Find the heart rate.
Rate = 60 ÷ 0.80 = 75 beats per minute.
2. A heart beats at 120 per minute. Find the R–R interval.
R–R = 60 ÷ 120 = 0.50 s.
3. On ECG paper running at 25 mm s⁻¹, two R peaks are 20 mm apart. Find the heart rate.
Time = 20 ÷ 25 = 0.80 s. Rate = 60 ÷ 0.80 = 75 per minute.
4. An ECG signal of 1.2 mV is amplified to 1.8 V. Find the gain.
Gain = 1.8 ÷ (1.2 × 10⁻³) = 1500.
5. Skin contact resistance is 20 kΩ. Why must the amplifier input resistance be about 10 MΩ, not 10 kΩ?
Skin and amplifier act as a potential divider. With 10 kΩ input only 10/(10 + 20) = one third of the signal reaches the amplifier. With 10 MΩ, 10 000/10 020 ≈ 99.8% reaches it.
6. In a trace, P waves appear regularly but are not always followed by a QRS complex. What does this suggest?
The atria are being triggered, but the signal is not always passing from the atria to the ventricles (a conduction block at the AV node). The ventricles miss beats.
Common mistakes
- Saying the T wave shows the atria. The T wave is ventricular repolarisation.
- Using 60 × R–R instead of 60 ÷ R–R for the heart rate.
- Thinking gel is there for comfort. It lowers contact resistance and removes air gaps.
- Saying the amplifier needs a low input resistance. It needs a very high one, so it draws almost no current.