Recording and reading an ECG
Get the leads right first. Then read every ECG the same way: rate, rhythm, axis, intervals, ST and T waves, and compare with the old one.
A nurse hands you an ECG: lead I shows negative P waves and negative QRS complexes, and aVR is positive. The patient is well, with no history of dextrocardia. An hour later another patient's ECG shows peaked T waves and a QRS of 140 ms.
What went wrong with the first, how do you read every ECG, and what do you do about the second?
- Patient flat, relaxed, warmReduces muscle artefact. Remove anything metal over the electrodes.
- Limb leadsRed right arm, yellow left arm, green left leg, black right leg.
- Chest leadsV1 4th space right sternal edge, V2 4th left, V4 5th midclavicular, V3 between, V5 anterior axillary, V6 midaxillary (same level as V4).
- Read in the same order every timeRate, rhythm, axis, P, PR, QRS, ST, T, QT.
- Act nowSTEMI, complete heart block, VT, hyperkalaemia, long QT with symptoms.
- CompareWith old ECGs.
- LabelName, date, time, symptoms at the time.
- ST elevation
- Complete heart block or Mobitz II
- Broad-complex tachycardia
- Peaked T waves with broad QRS
- QTc over 500 ms
- New AF with fast rate
- Lead reversal mimicking pathology
- Posterior MI (ST depression V1–V3 with tall R)
- Hyperkalaemia
- Wellens pattern (deep T inversion V2–V3)
- Pre-excitation (short PR, delta wave)
- Brugada pattern
| Paper | 25 mm/s, small square 40 ms, big square 200 ms |
| Rate (regular) | 300 ÷ big squares between R waves |
| Rate (irregular) | QRS complexes in 10 seconds × 6 |
| PR | 120–200 ms (3–5 small squares) |
| QRS | Under 120 ms (3 small squares) |
| QTc | Up to about 450 ms (men), 460–470 ms (women) |
| STEMI | New ST elevation in 2 contiguous leads (1 mm limb leads, 1.5–2.5 mm in V2–V3 by age and sex) or new LBBB with symptoms |
| Normal axis | Positive QRS in leads I and aVF |
Check every dose against a current drug reference and your hospital protocol.
On this page
Electrode placement
| Lead | Position |
|---|---|
| V1 | 4th intercostal space, right sternal edge |
| V2 | 4th intercostal space, left sternal edge |
| V3 | Midway between V2 and V4 |
| V4 | 5th intercostal space, midclavicular line |
| V5 | Anterior axillary line, same level as V4 |
| V6 | Midaxillary line, same level as V4 |
| Right arm (red) | Right wrist or shoulder |
| Left arm (yellow) | Left wrist or shoulder |
| Left leg (green) | Left ankle or lower abdomen |
| Right leg (black) | Right ankle (earth) |
Extra leads: right-sided (V4R) for right ventricular infarction in inferior STEMI; posterior (V7–V9) for suspected posterior MI.
Common recording errors
| Error | Clue |
|---|---|
| Left and right arm reversed | Lead I inverted (negative P and QRS), aVR positive |
| Chest leads too high | Poor R wave progression, pseudo-ischaemia |
| Muscle tremor | Fuzzy baseline (Parkinson's, cold, anxiety) |
| Loose electrode | Wandering baseline in one lead |
Reading routine
- DetailsName, date, time, symptoms, calibration (10 mm per mV), speed (25 mm/s).
- Rate and rhythmRegular? P before every QRS? Irregularly irregular (AF)?
- AxisLeads I and aVF positive: normal. Lead I positive, aVF negative: left axis. Lead I negative: right axis.
- P waves and PRLong PR: heart block. Short PR with delta wave: pre-excitation.
- QRSBroad: bundle branch block, ventricular rhythm, hyperkalaemia, drugs. Tall: hypertrophy.
- ST and TElevation, depression, inversion; which territory?
- QTCorrect for rate (QTc); long QT risk.
Territories
| Leads | Territory | Artery (usual) |
|---|---|---|
| II, III, aVF | Inferior | Right coronary |
| V1–V4 | Anterior and septal | Left anterior descending |
| I, aVL, V5–V6 | Lateral | Circumflex |
| V7–V9 (ST depression V1–V3) | Posterior | Right coronary or circumflex |
ECGs to act on now
| ECG | Action |
|---|---|
| ST elevation in a territory or new LBBB with symptoms | STEMI pathway |
| Complete heart block or Mobitz II | Atropine if unstable, pacing, senior |
| Regular broad-complex tachycardia | Treat as VT |
| Peaked T waves, broad QRS, absent P | Calcium gluconate, then insulin-dextrose |
| QTc over 500 ms | Stop QT-prolonging drugs, check potassium and magnesium |
| Fast AF with low BP | Senior; cardioversion |
| Short PR with delta wave and fast irregular broad rhythm | Pre-excited AF: avoid AV-node blockers |
Common mistakes
- Placing V1 and V2 in the 2nd space (too high).
- Reading the machine's interpretation instead of the ECG.
- Not comparing with old ECGs.
- Missing lead reversal and calling it dextrocardia or MI.
- Missing posterior MI because there is "no ST elevation".
- Not repeating the ECG with ongoing chest pain.
Frequently asked questions
Where do the chest ECG leads go?
V1 4th intercostal space right sternal edge, V2 4th space left sternal edge, V4 5th space midclavicular line, V3 between V2 and V4, V5 anterior axillary line and V6 midaxillary line at the level of V4.
What is a normal QRS duration?
Less than 120 ms (3 small squares).
How do you calculate heart rate on an ECG?
For a regular rhythm, 300 divided by the number of big squares between R waves. For an irregular rhythm, count the QRS complexes in 10 seconds and multiply by 6.
What does a reversed lead I mean?
Usually the arm leads have been swapped. Repeat the ECG before diagnosing dextrocardia.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. An ECG shows negative P waves and QRS in lead I with positive aVR in a well patient. What is the likely cause?
Answer: Arm leads reversed. Swapped red and yellow leads invert lead I.
- Hyperkalaemia: No.
- WPW: No.
- Inferior MI: No.
- Dextrocardia in all: Rare; check leads first.
2. Where is V1 placed?
Answer: 4th space, right sternal edge. V1 at the 4th intercostal space right of the sternum.
- 2nd space, right sternal edge: No.
- 5th space, midclavicular line: V4.
- Midaxillary line: V6.
- 4th space, left sternal edge: V2.
3. What is the upper limit of a normal QRS duration?
Answer: 120 ms. QRS under 120 ms (3 small squares).
- 440 ms: QT territory.
- 200 ms: That's the PR limit.
- 60 ms: No.
- 80 ms: Too low.
4. A woman with rheumatic mitral stenosis is in sinus rhythm. What is the typical ECG finding?
Answer: Broad, notched P waves. Mitral stenosis raises left atrial pressure, enlarging the left atrium. This gives a broad (more than 120 ms), bifid P wave in lead II and a deep negative terminal part in V1. Atrial fibrillation often develops later. Tall peaked P waves suggest right atrial enlargement.
- Tall, peaked P waves: Right atrial enlargement (pulmonary disease).
- Short PR with a delta wave: Wolff–Parkinson–White.
- ST elevation in V1–V4: Anterior STEMI.
- Low-voltage QRS complexes: Pericardial effusion, obesity.
5. A woman develops anaphylaxis after a bee sting: wheeze, hypotension and lip swelling. What is the first drug and route?
Answer: IM adrenaline 0.5 mg. IM adrenaline is first-line in anaphylaxis: α1 effects raise blood pressure and reduce oedema, β1 supports the heart and β2 opens airways and stabilises mast cells. Repeat after 5 minutes if needed. IV adrenaline boluses are for cardiac arrest or specialist use. Antihistamines and steroids are secondary.
- IV chlorphenamine: Secondary; does not reverse shock.
- IV hydrocortisone: Slow; secondary.
- Nebulised salbutamol: Helps wheeze but not shock or oedema.
- IV adrenaline 1 mg bolus: For cardiac arrest; dangerous otherwise.