Recording a 12-lead ECG

What it is

A 12-lead electrocardiogram uses ten electrodes, four on the limbs and six on the chest, to build twelve views of the heart's electrical activity: six limb leads (I, II, III, aVR, aVL, aVF) that look at the heart in the frontal plane and six chest leads (V1–V6) that look at it in the horizontal plane. The tracing is only as good as the recording: a misplaced chest electrode can fake or hide an infarct pattern, swapped limb electrodes can mimic dextrocardia or a change in axis, and a tense or cold patient adds muscle artefact. The standard recording runs at a paper speed of 25 mm/s with a calibration of 10 mm/mV, which the machine prints on the strip so the reader can check it. The patient lies semi-recumbent and relaxed, with skin cleaned and dry under each electrode. The chest electrodes follow fixed bony landmarks counted from the sternal angle, and the limb electrodes go on the limbs themselves, not on the trunk, unless the trace is labelled. In suspected acute coronary syndrome the ECG is recorded and read within minutes of first contact, and extra right-sided or posterior leads are added when the question calls for them. Every tracing is labelled with the patient's details, the date, the time and the symptoms at the moment it was taken, so it can be compared with earlier and later ones.

Indications

• Chest pain, breathlessness, palpitations, syncope or collapse.
• Suspected acute coronary syndrome, pericarditis, pulmonary embolism or an arrhythmia.
• Electrolyte disturbances (potassium, calcium) and drugs or overdoses that affect conduction or the QT interval.
• Before surgery or starting drugs that prolong the QT interval, and as a baseline in heart disease.
• Follow-up after a change in rhythm, treatment or symptoms, compared with the previous tracing.

Contraindications and precautions

• No absolute contraindication; never delay defibrillation or resuscitation to record a 12-lead ECG.
• Allergy to the electrode adhesive: use a hypoallergenic electrode.
• Broken skin, burns or dressings at a site: move the electrode slightly and note it on the tracing.
• Amputation or a cast: place the limb electrode on the stump or trunk end of the limb and label the trace.
• Respect privacy and dignity when exposing the chest; offer a chaperone and cover the patient between steps.

Equipment

• Electrocardiograph with patient cable and ten lead wires, checked and charged.
• Single-use adhesive electrodes in date (dry electrodes give a noisy trace).
• Gauze, skin cleanser and a razor or clippers for hairy skin.
• A sheet or gown for privacy, and the patient's identification details.
• Previous ECGs for comparison when available.

Step by step (and why)

1. Identify the patient, explain and get verbal consent — a relaxed, informed patient keeps still and gives a clean trace.
2. Lie the patient semi-recumbent, arms by the sides, legs uncrossed — muscle tension and movement cause artefact.
3. Clean, dry and, if needed, clip the skin at each site — oil, sweat and hair raise skin impedance and cause noise.
4. Place the four limb electrodes on the wrists or forearms and ankles — trunk placement changes the limb leads.
5. Find the sternal angle, slide to the 2nd space and count down to the 4th: V1 right, V2 left of the sternum — high V1–V2 fakes patterns.
6. Place V4 in the 5th space at the midclavicular line, then V3 halfway between V2 and V4 — V4 anchors the rest.
7. Place V5 at the anterior axillary line and V6 at the midaxillary line, level with V4 — not following the rib upward.
8. In women, place V4–V6 under the breast, not on it — breast tissue displaces and damps the signal.
9. Connect the leads, ask the patient to breathe normally and keep still, check speed and calibration, then record.
10. Label name, ID, date, time and symptoms, and show it to the clinician at once if abnormal — the timing guides care.

Electrode landmarks (colour codes vary by standard: check the cable)

Complications

• Limb lead reversal: arms swapped invert lead I and make aVR upright; it mimics dextrocardia or an axis change.
• Chest electrodes too high: V1–V2 one space up can produce an rSr' pattern, T-wave inversion or poor R-wave progression.
• Muscle tremor, shivering or movement artefact that hides P waves or mimics an arrhythmia.
• Mains interference (a regular fine oscillation) from nearby equipment or a poor electrode contact.
• Minor skin irritation from the adhesive; rarely, a delay in treatment when recording is prioritised over a shock.

Next step

• Read the tracing with a systematic method, or hand it at once to the clinician responsible.
• Suspected acute coronary syndrome: call a senior immediately if there is ST elevation or a new conduction block.
• Inferior ST elevation: add right-sided leads (V4R); tall R and ST depression in V1–V3: add posterior leads V7–V9.
• Compare with any previous ECG and repeat the recording when symptoms change or after treatment.
• File the labelled tracing in the record; remove the electrodes gently and clean the skin.

Test yourself

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Elsewhere in DurgiPedia

• ECG basics: waves, intervals and leads
• The cardiac electrical axis
• Recording a 12-lead ECG
• Cardiac monitoring at the bedside: leads, alarms and rhythm changes to report

Connections

← All clinical skills

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Electrocardiogram (StatPearls (NCBI Bookshelf), 2023).

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Representative model, for educational purposes only.