ECG (Electrocardiogram) — Purpose, Procedure & Normal Values | MyMedicPlus

An ECG (electrocardiogram, also written EKG from the German Elektrokardiogramm) is a non-invasive, entirely painless test that records the electrical activity of the heart over a short period, typically 10–30 seconds in its standard resting form. With each heartbeat, the sinoatrial (SA) node — the heart's intrinsic pacemaker located in the right atrial wall — generates an electrical impulse that spreads sequentially through the atria, crosses the atrioventricular (AV) node, travels rapidly down the His bundle and Purkinje fibres, and finally depolarises the ventricular myocardium. This reproducible electrical wavefront is detected by adhesive electrodes placed on the skin surface and displayed as characteristic waveforms — the P wave, QRS complex, and T wave — on a calibrated grid at 25 mm/second speed and 10 mm/mV amplitude. A standard 12-lead ECG acquires 12 simultaneous views (leads) of cardiac electrical activity from different spatial angles: six limb leads (I, II, III, aVR, aVL, aVF) and six precordial chest leads (V1–V6). Together, these 12 views allow precise identification of the rhythm, conduction pathway, and any regions of ischaemia, infarction, or hypertrophy.

An ECG is ordered when a patient presents with any symptom suggesting a cardiac or arrhythmic aetiology. Common presenting complaints include chest pain or tightness (to diagnose or exclude acute myocardial infarction), palpitations or awareness of an irregular heartbeat, shortness of breath (dyspnoea), dizziness, presyncope, or syncope (unexplained loss of consciousness). It is the essential, immediate diagnostic step in the emergency evaluation of suspected ST-elevation myocardial infarction (STEMI), where door-to-ECG time within 10 minutes is a key performance metric. Beyond acute presentations, an ECG is ordered routinely as a pre-operative cardiac screen before major surgery (especially in patients over 40 or those with known cardiovascular disease), to evaluate medication effects and toxicity (digoxin, antiarrhythmics, antidepressants causing QT prolongation), to screen athletes for channelopathies or hypertrophic cardiomyopathy, and to establish a cardiac baseline in patients with hypertension, diabetes, or established coronary artery disease.

No fasting, bowel preparation, or changes to medications are required before an ECG. Wear clothing that can be easily opened at the front to expose the chest, or a two-piece outfit that allows access to the wrists and ankles without requiring full undressing. Avoid applying skin lotions, creams, moisturisers, or oils to the chest, wrist, and ankle areas on the day of the test, as lipid-based substances impair the electrical conductance between the adhesive electrode pad and the skin, producing signal artefact and potentially unreliable tracings. Inform the cardiac physiologist or nurse of all current medications, particularly those with cardiac effects — beta-blockers (reducing heart rate), digoxin (producing the characteristic reverse-tick ST depression), antiarrhythmics, and QT-prolonging agents. Remove metallic jewellery (rings, bracelets, ankle chains) from the wrist and ankle electrode sites. Artificial pacemaker wires, implantable cardiac defibrillators, or recent cardiac surgery should be reported to the technician.

You lie flat and relaxed on an examination couch. A cardiac physiologist or nurse places 10 adhesive electrode pads (with conductive gel) at precisely defined anatomical positions: six precordial electrodes on the anterior chest wall at the sternal borders and along the left chest (V1–V6), and one electrode on each limb (right arm, left arm, right leg, left leg). The ECG machine simultaneously records all 12 electrical leads for 10–30 seconds. During acquisition, remain completely still, breathe gently and regularly, and avoid talking, swallowing, shivering, or muscle tensing — any voluntary or involuntary muscle activity introduces electrical interference called myogenic artefact that degrades recording quality and can mimic or mask real ECG abnormalities. Once the recording is complete, the electrodes are removed painlessly from the skin. No electrical current is delivered to your body at any time — the ECG machine passively detects naturally occurring bioelectric signals. The entire appointment, including positioning and electrode placement, takes 10–20 minutes.

ECG results are interpreted by a trained cardiologist, cardiac physiologist, or physician. Key parameters and their normal values: Heart rate — 60–100 bpm; rhythm should be regular (sinus rhythm). P wave — present and upright in leads I and II, with duration <120 ms and height <2.5 mm; absent P waves with chaotic baseline activity indicates atrial fibrillation. PR interval — 120–200 ms (3–5 small squares at 25 mm/s); prolonged in first-degree heart block, shortened in accessory pathway conduction. QRS complex duration — <120 ms; widened QRS (≥120 ms) indicates bundle branch block or ventricular ectopy. QTc interval — corrected for heart rate: normal QTc <440 ms in men, <460 ms in women; prolonged QTc increases the risk of potentially fatal torsades de pointes. ST segment — isoelectric (flat on the baseline) in normal sinus rhythm; ST elevation ≥1 mm in two contiguous limb leads or ≥2 mm in two contiguous precordial leads indicates STEMI or pericarditis; ST depression indicates subendocardial ischaemia, NSTEMI, or digoxin effect. T waves — normally upright in leads I, II, V4–V6; T wave inversion suggests ischaemia, left ventricular hypertrophy (strain pattern), or post-MI changes. The automated ECG machine interpretation algorithm provides an immediate preliminary analysis, but cardiologist review and clinical correlation are always required for definitive interpretation.

The ECG is entirely safe — it uses no ionising radiation and delivers no electrical current whatsoever to the patient. The only possible adverse effect is mild, transient skin irritation from adhesive electrode pads at application sites, occurring in a small number of patients with sensitive skin and resolving spontaneously within hours. The most important clinical limitation of the standard 12-lead ECG is its brevity: a 10-second recording captures only a snapshot of cardiac electrical activity. Paroxysmal (intermittent) arrhythmias that are not occurring at the precise moment of recording will not be detected, and a completely normal ECG does not exclude an arrhythmia that has been causing the patient's symptoms. For intermittent palpitations, syncope, or presyncope, prolonged ambulatory monitoring is required: a 24–48 hour Holter monitor records every heartbeat over one to two days; an external event recorder (patient-activated or auto-triggered) monitors for up to 30 days; an implantable loop recorder (ILR) inserted subcutaneously can monitor continuously for up to 3 years. A normal resting ECG does not exclude significant coronary artery disease. Furthermore, the ECG measures only electrical activity — it provides no information about the heart's mechanical pumping function, valve integrity, or structural anatomy, for which echocardiography or cardiac MRI is required.