12-Lead ECG
The 12-lead electrocardiogram records the electrical activity of the heart from ten electrodes on the limbs and chest. It shows 12 views of the heart: six limb leads (I, II, III, aVR, aVL, aVF) and six chest leads (V1 to V6).
It gives the heart rate, rhythm, conduction, axis, chamber size, and signs of ischaemia, infarction, electrolyte disturbance and drug effects. It is quick (about 5 minutes), painless, cheap and has no risk.
Paper speed is 25 mm/s and the gain is 10 mm/mV. One small square is 40 ms and 0.1 mV, and one large square is 200 ms and 0.5 mV.
The ECG is the first test in chest pain, palpitations, syncope, breathlessness and suspected electrolyte disturbance. It is also the main screening test in many cardiac conditions.
Cardiovascular disease causes about a third of deaths worldwide. A first ECG detects about 50 to 60% of acute myocardial infarctions, and repeating it at 15 to 30 minutes raises the yield. It detects atrial fibrillation, heart block and hypertrophy reliably.
Indications.
- Chest pain, suspected acute coronary syndrome and suspected pericarditis.
- Palpitations, syncope, presyncope and suspected arrhythmia.
- Breathlessness, suspected heart failure, pulmonary embolism or cardiomyopathy.
- Electrolyte disturbance (potassium, calcium, magnesium), poisoning and overdose.
- Drugs that prolong the QT interval, or other drugs with cardiac effects, at baseline and during treatment.
- Hypertension, diabetes, and before surgery in patients with heart disease or risk factors.
- Cardiac murmurs, family history of sudden death or inherited heart disease.
- Pacemaker and device follow up.
- Screening of athletes before sport, and of relatives of patients with inherited conditions.
- Monitoring in critical illness and after cardiac events.
Contraindications and precautions.
- There is no contraindication. Skin allergy to electrode gel or adhesive is rare.
- The skin should be clean and dry, and chest hair may need to be clipped.
- The patient lies supine and relaxed, and is warm and still, since shivering and movement cause artefact.
- Electrodes must not be placed on broken skin or burns.
- Medicines (digoxin, antiarrhythmics, tricyclics, antipsychotics) are recorded.
- In an emergency, the ECG should not delay treatment, and a first recording is obtained within 10 minutes.
- Clothing and metal on the chest are removed. Electrodes are placed beneath the breast, not over it.
- Pacemaker spikes may need a modified recording setting.
Preparation and technique.
The patient lies supine for 5 minutes. Limb electrodes are placed on the wrists and ankles (or the shoulders and hips in a monitored patient).
Chest electrodes are placed as follows:
- V1: fourth intercostal space, right of the sternum.
- V2: fourth intercostal space, left of the sternum.
- V3: midway between V2 and V4.
- V4: fifth intercostal space, midclavicular line.
- V5: same level as V4, in the anterior axillary line.
- V6: same level as V4, in the midaxillary line.
The recording is made for 10 seconds, with a rhythm strip of lead II for longer. Two checks are made: the time and patient name are correct, and there is no artefact.
Extra leads are added if needed: right sided leads (V3R to V6R) for inferior infarction, posterior leads (V7 to V9) for posterior infarction, and high chest leads for sodium channel disorders.
