ECG Test — Test Guide & Results Explained — Purpose, Procedure & Normal Values | MyMedicPlus

An electrocardiogram (ECG — also abbreviated EKG from the German Elektrokardiogramm) is a non-invasive, painless, and rapid diagnostic test that records the electrical impulses generated by the heart with each cardiac cycle. Every heartbeat originates in the sinoatrial (SA) node — the heart's natural pacemaker — and propagates through the atria, atrioventricular (AV) node, His–Purkinje system, and ventricular myocardium in a predictable, coordinated sequence. Ten adhesive electrodes placed at standardised anatomical positions on the skin capture the electrical wavefront from 12 different angles, called leads — six limb leads (I, II, III, aVR, aVL, aVF) and six precordial chest leads (V1–V6) — producing a 12-lead ECG. Each lead records a characteristic waveform: the P wave (atrial depolarisation), QRS complex (ventricular depolarisation), and T wave (ventricular repolarisation). The 12-lead ECG is one of the most widely performed diagnostic procedures in cardiology, emergency medicine, anaesthesia, and general practice, requiring no patient preparation and taking only 5–10 minutes from electrode placement to tracing acquisition.

An ECG is ordered as the first-line cardiac investigation for a wide range of symptoms and clinical scenarios. Acute presentations include chest pain (to differentiate ST-elevation myocardial infarction — STEMI — requiring emergency reperfusion, from NSTEMI, pericarditis, or musculoskeletal pain), palpitations, shortness of breath, syncope (fainting), presyncope, and dizziness. Specific diagnoses the ECG establishes include: acute and evolving myocardial infarction (ST elevation and Q waves), atrial fibrillation and flutter, supraventricular tachycardia (SVT), ventricular tachycardia and fibrillation, heart block (first, second, and third degree), bundle branch blocks (left and right), ventricular hypertrophy, Wolff–Parkinson–White syndrome, prolonged QT syndrome, Brugada syndrome, electrolyte disturbances (peaked T waves in hyperkalaemia; prolonged QT and U waves in hypokalaemia and hypocalcaemia), pericarditis (saddle-shaped ST elevation in multiple leads), and digoxin toxicity (reverse tick ST change). Routine ECGs are performed as pre-operative cardiac screens, in occupational health assessments, and as baseline monitoring in patients with known heart disease, hypertension, or diabetes beginning new cardiac medications.

No fasting, bowel preparation, or medication changes are required before an ECG. Wear clothing that allows easy exposure of the chest, wrists, and ankles — a two-piece outfit (separate top and trousers) or a loose-fitting shirt is ideal. Avoid applying body lotion, moisturiser, oil, or talcum powder to the chest, wrists, and ankle skin on the day of the test, as these substances impair electrical contact between the adhesive electrode and the skin surface and cause signal artefact. Inform the cardiac technician of all current medications, especially antiarrhythmic drugs (amiodarone, flecainide), rate-controlling agents (beta-blockers, verapamil, digoxin), and QT-prolonging medications. You do not need to stop any medication before the ECG. Remove metallic jewellery from the wrists and ankles to facilitate electrode placement and reduce electrical interference.

You lie supine and relaxed on an examination table. A cardiac technician or nurse places 10 self-adhesive electrodes at standardised anatomical positions: six precordial electrodes (V1 at the 4th intercostal space right sternal border, V2 left sternal border, V3 between V2 and V4, V4 at the 5th intercostal space mid-clavicular line, V5 anterior axillary line, V6 mid-axillary line) and four limb electrodes (right arm, left arm, right leg, left leg). A small amount of electrode gel may be applied to improve skin contact if signal quality is poor. The ECG machine acquires 10–30 seconds of continuous electrical recording simultaneously from all 12 leads. During acquisition, remain completely still, breathe normally, and avoid talking or swallowing — any movement or muscle activity causes electrical artefact that can obscure subtle findings. The electrodes are then removed; the test is entirely painless with no electrical current delivered to your body at any time. The tracing is printed or digitally stored for interpretation. The entire appointment takes 5–10 minutes.

ECG interpretation requires analysis of multiple parameters by a trained cardiologist or physician. Key quantitative measurements and their normal ranges include: Heart rate — 60–100 beats per minute (bpm); below 60 is bradycardia, above 100 is tachycardia. P wave — should be upright in leads I and II, with duration <120 ms and amplitude <2.5 mm. PR interval — 120–200 ms (0.12–0.20 seconds); prolongation indicates AV block; shortening in WPW syndrome or junctional rhythms. QRS complex duration — <120 ms (3 small squares); widening indicates bundle branch block, ventricular ectopy, or hyperkalaemia. QT/QTc interval — corrected QT (QTc using Bazett's formula) should be <440 ms in men and <460 ms in women; prolongation increases risk of torsades de pointes and sudden cardiac death. ST segment — normally isoelectric (on the baseline); ST elevation ≥1 mm in two contiguous limb leads or ≥2 mm in precordial leads indicates acute MI or pericarditis; ST depression indicates ischaemia or digoxin effect. T waves — should be upright in leads I, II, and V4–V6; inversion may indicate ischaemia, ventricular hypertrophy, or pericarditis. Results from the ECG machine's automated interpretation algorithm are available within seconds; the cardiologist's formal report is usually available within 24 hours for routine studies.

The ECG is completely safe — it emits no radiation, delivers no electrical current to the patient, and involves only passive detection of naturally occurring cardiac electrical signals through adhesive skin electrodes. The only conceivable adverse effect is mild, transient skin irritation or redness from the adhesive electrode gel at application sites, which resolves spontaneously. The primary clinical limitations of the standard 12-lead ECG are related to its very short recording duration (10–30 seconds). Paroxysmal arrhythmias — such as intermittent atrial fibrillation, SVT, or ventricular ectopy — that are not present during the brief recording window will not be captured, and a normal ECG does not exclude an arrhythmia that occurs only episodically. For patients with palpitations, syncope, or other intermittent symptoms, prolonged ambulatory ECG monitoring using a 24–48 hour Holter monitor, a 7–30 day event recorder, or an implantable loop recorder is used to detect infrequent arrhythmias. A normal resting ECG also does not exclude significant obstructive coronary artery disease — many patients with 70–90% stenoses have normal resting ECGs. Exercise ECG (stress test), stress echocardiography, or CT coronary angiography provides additional functional and anatomical information. The ECG measures cardiac electrical activity only; it cannot assess the mechanical pumping function of the heart — echocardiography or cardiac MRI is required for ejection fraction and structural assessment.