What is an ECG?
An ECG records the heart's electrical activity using sticky sensors on the skin. It shows how fast the heart beats, whether the rhythm is regular, and gives clues about strain, reduced blood flow or past injury.
It does not send electricity into your body. A normal ECG also does not rule out every heart problem, so doctors may ask for more tests.
Getting an ECG: what to expect
- It is painless and usually takes about 5 to 10 minutes.
- Wear a top that is easy to open. You may be asked to remove metal jewelry.
- Avoid lotion or oil on the chest, arms and legs, as these can loosen the sensors.
- Lie still, breathe normally and do not talk during the recording.
- Tell the staff about your medicines and symptoms, and bring any old ECGs for comparison.
The waves of one heartbeat
| Part | What it means | Usual adult value |
|---|---|---|
| P wave | Upper chambers (atria) activate | Up to 120 ms |
| PR interval | Signal travels from atria to ventricles | 120 to 200 ms |
| QRS complex | Lower chambers (ventricles) activate | Under 120 ms |
| ST segment | Pause before the ventricles recover | Near the baseline |
| T wave | Ventricles recover | Upright in most leads |
| QTc | Total activation and recovery, adjusted for rate | About under 450 ms (men), 460 ms (women) |
Reading the ECG paper
Standard settings are 25 mm/s speed and 10 mm/mV height.
- 1 small box across = 0.04 seconds (40 ms)
- 1 large box across (5 small) = 0.20 seconds
- 5 large boxes = 1 second
- 10 small boxes tall = 1 millivolt
The 12 leads and heart regions
Ten sensors give 12 views of the heart.
| Leads | Region viewed | Usual artery |
|---|---|---|
| II, III, aVF | Inferior | Right coronary (often) |
| V1, V2 | Septal | Left anterior descending |
| V3, V4 | Anterior | Left anterior descending |
| I, aVL, V5, V6 | Lateral | Left circumflex or diagonal |
Chest sensor placement
- V1: 4th rib space, right edge of the breastbone
- V2: 4th rib space, left edge of the breastbone
- V3: midway between V2 and V4
- V4: 5th rib space, mid-collarbone line
- V5: same level as V4, front armpit line
- V6: same level as V4, middle armpit line
An 8-step reading method
Use the same order every time so that nothing is missed.
- Check the details. Right patient, date, time, paper speed and height settings.
- Rate. Normal adult resting rate is 60 to 100 per minute.
- Rhythm. Are the R waves evenly spaced? Is there a P wave before every QRS, and a QRS after every P?
- Axis. Look at leads I and aVF (table below).
- PR interval. Long, short or changing?
- QRS. Narrow or wide? Any deep Q waves?
- ST segment and T wave. Any elevation, depression or inversion?
- QT and comparison. Check the QTc, compare with old ECGs, and link it to the symptoms.
Three ways to get the rate
- 300 rule: 300 ÷ large boxes between two R waves (regular rhythm).
- 1500 rule: 1500 ÷ small boxes between two R waves (regular, more exact).
- 6-second rule: count QRS complexes in 30 large boxes, then multiply by 10 (best for irregular rhythm).
Quick axis check
| Lead I | Lead aVF | Axis |
|---|---|---|
| Positive | Positive | Normal |
| Positive | Negative | Possible left axis (check lead II) |
| Negative | Positive | Right axis |
| Negative | Negative | Extreme axis |
Common mistakes
- Reading only the machine's printed text. It can be wrong.
- Missing swapped arm sensors, which can make a normal ECG look abnormal.
- Calling shaking or muscle noise an arrhythmia.
- Reading the ECG without knowing the symptoms.
- Not comparing with an older ECG.
Rhythm library
Simplified drawings to show the main idea. Real ECGs vary a lot.
Heart rate from the strip
Rate: —
Corrected QT (QTc)
QTc: —
Where to get an ECG
A shared list built by the community. Add government hospitals, health centers, clinics and labs that offer ECG tests.
Fees are reported by visitors, and each entry shows the date it was checked. Figures may have changed, so always check with the facility. Ask your local health office or see the Department of Health for official information.
Community app. Not an official government service.